diff --git a/compiler/rustc_attr/src/builtin.rs b/compiler/rustc_attr/src/builtin.rs
index 94f9727eb7fbe..c8928097eafda 100644
--- a/compiler/rustc_attr/src/builtin.rs
+++ b/compiler/rustc_attr/src/builtin.rs
@@ -49,12 +49,21 @@ pub enum InlineAttr {
     Never,
 }
 
-#[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq, HashStable_Generic)]
+#[derive(Copy, Clone, Encodable, Decodable, Debug, PartialEq, Eq, HashStable_Generic)]
 pub enum InstructionSetAttr {
     ArmA32,
     ArmT32,
 }
 
+impl InstructionSetAttr {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Self::ArmA32 => sym::a32.as_str(),
+            Self::ArmT32 => sym::t32.as_str(),
+        }
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum OptimizeAttr {
     None,
diff --git a/compiler/rustc_codegen_gcc/src/asm.rs b/compiler/rustc_codegen_gcc/src/asm.rs
index a1f9eab10e76b..415f8affab901 100644
--- a/compiler/rustc_codegen_gcc/src/asm.rs
+++ b/compiler/rustc_codegen_gcc/src/asm.rs
@@ -867,6 +867,13 @@ impl<'gcc, 'tcx> AsmCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         template_str.push_str("\n.popsection");
         self.context.add_top_level_asm(None, &template_str);
     }
+
+    fn mangled_name(&self, instance: Instance<'tcx>) -> String {
+        // TODO(@Amanieu): Additional mangling is needed on
+        // some targets to add a leading underscore (Mach-O)
+        // or byte count suffixes (x86 Windows).
+        self.tcx.symbol_name(instance).name.to_string()
+    }
 }
 
 fn modifier_to_gcc(
diff --git a/compiler/rustc_codegen_llvm/src/asm.rs b/compiler/rustc_codegen_llvm/src/asm.rs
index d1804cb49ad19..f257abe2a71af 100644
--- a/compiler/rustc_codegen_llvm/src/asm.rs
+++ b/compiler/rustc_codegen_llvm/src/asm.rs
@@ -442,6 +442,14 @@ impl<'tcx> AsmCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
             );
         }
     }
+
+    fn mangled_name(&self, instance: Instance<'tcx>) -> String {
+        let llval = self.get_fn(instance);
+        llvm::build_string(|s| unsafe {
+            llvm::LLVMRustGetMangledName(llval, s);
+        })
+        .expect("symbol is not valid UTF-8")
+    }
 }
 
 pub(crate) fn inline_asm_call<'ll>(
diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index cb958c1d4d771..5552a2410601d 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -395,17 +395,9 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
         to_add.push(MemoryEffects::None.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
-        to_add.push(AttributeKind::Naked.create_attr(cx.llcx));
-        // HACK(jubilee): "indirect branch tracking" works by attaching prologues to functions.
-        // And it is a module-level attribute, so the alternative is pulling naked functions into
-        // new LLVM modules. Otherwise LLVM's "naked" functions come with endbr prefixes per
-        // https://github.com/rust-lang/rust/issues/98768
-        to_add.push(AttributeKind::NoCfCheck.create_attr(cx.llcx));
-        if llvm_util::get_version() < (19, 0, 0) {
-            // Prior to LLVM 19, branch-target-enforcement was disabled by setting the attribute to
-            // the string "false". Now it is disabled by absence of the attribute.
-            to_add.push(llvm::CreateAttrStringValue(cx.llcx, "branch-target-enforcement", "false"));
-        }
+        // do nothing; a naked function is converted into an extern function
+        // and a global assembly block. LLVM's support for naked functions is
+        // not used.
     } else {
         // Do not set sanitizer attributes for naked functions.
         to_add.extend(sanitize_attrs(cx, codegen_fn_attrs.no_sanitize));
diff --git a/compiler/rustc_codegen_llvm/src/va_arg.rs b/compiler/rustc_codegen_llvm/src/va_arg.rs
index e4c3e748cb5df..8baa69cefe1e2 100644
--- a/compiler/rustc_codegen_llvm/src/va_arg.rs
+++ b/compiler/rustc_codegen_llvm/src/va_arg.rs
@@ -10,6 +10,15 @@ use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
 
+fn round_up_to_alignment<'ll>(
+    bx: &mut Builder<'_, 'll, '_>,
+    mut value: &'ll Value,
+    align: Align,
+) -> &'ll Value {
+    value = bx.add(value, bx.cx().const_i32(align.bytes() as i32 - 1));
+    return bx.and(value, bx.cx().const_i32(-(align.bytes() as i32)));
+}
+
 fn round_pointer_up_to_alignment<'ll>(
     bx: &mut Builder<'_, 'll, '_>,
     addr: &'ll Value,
@@ -17,8 +26,7 @@ fn round_pointer_up_to_alignment<'ll>(
     ptr_ty: &'ll Type,
 ) -> &'ll Value {
     let mut ptr_as_int = bx.ptrtoint(addr, bx.cx().type_isize());
-    ptr_as_int = bx.add(ptr_as_int, bx.cx().const_i32(align.bytes() as i32 - 1));
-    ptr_as_int = bx.and(ptr_as_int, bx.cx().const_i32(-(align.bytes() as i32)));
+    ptr_as_int = round_up_to_alignment(bx, ptr_as_int, align);
     bx.inttoptr(ptr_as_int, ptr_ty)
 }
 
@@ -270,6 +278,106 @@ fn emit_s390x_va_arg<'ll, 'tcx>(
     bx.load(val_type, val_addr, layout.align.abi)
 }
 
+fn emit_xtensa_va_arg<'ll, 'tcx>(
+    bx: &mut Builder<'_, 'll, 'tcx>,
+    list: OperandRef<'tcx, &'ll Value>,
+    target_ty: Ty<'tcx>,
+) -> &'ll Value {
+    // Implementation of va_arg for Xtensa. There doesn't seem to be an authoritative source for
+    // this, other than "what GCC does".
+    //
+    // The va_list type has three fields:
+    // struct __va_list_tag {
+    //   int32_t *va_stk; // Arguments passed on the stack
+    //   int32_t *va_reg; // Arguments passed in registers, saved to memory by the prologue.
+    //   int32_t va_ndx; // Offset into the arguments, in bytes
+    // };
+    //
+    // The first 24 bytes (equivalent to 6 registers) come from va_reg, the rest from va_stk.
+    // Thus if va_ndx is less than 24, the next va_arg *may* read from va_reg,
+    // otherwise it must come from va_stk.
+    //
+    // Primitive arguments are never split between registers and the stack. For example, if loading an 8 byte
+    // primitive value and va_ndx = 20, we instead bump the offset and read everything from va_stk.
+    let va_list_addr = list.immediate();
+    // FIXME: handle multi-field structs that split across regsave/stack?
+    let layout = bx.cx.layout_of(target_ty);
+    let from_stack = bx.append_sibling_block("va_arg.from_stack");
+    let from_regsave = bx.append_sibling_block("va_arg.from_regsave");
+    let end = bx.append_sibling_block("va_arg.end");
+
+    // (*va).va_ndx
+    let va_reg_offset = 4;
+    let va_ndx_offset = va_reg_offset + 4;
+    let offset_ptr =
+        bx.inbounds_gep(bx.type_i8(), va_list_addr, &[bx.cx.const_usize(va_ndx_offset)]);
+
+    let offset = bx.load(bx.type_i32(), offset_ptr, bx.tcx().data_layout.i32_align.abi);
+    let offset = round_up_to_alignment(bx, offset, layout.align.abi);
+
+    let slot_size = layout.size.align_to(Align::from_bytes(4).unwrap()).bytes() as i32;
+
+    // Update the offset in va_list, by adding the slot's size.
+    let offset_next = bx.add(offset, bx.const_i32(slot_size));
+
+    // Figure out where to look for our value. We do that by checking the end of our slot (offset_next).
+    // If that is within the regsave area, then load from there. Otherwise load from the stack area.
+    let regsave_size = bx.const_i32(24);
+    let use_regsave = bx.icmp(IntPredicate::IntULE, offset_next, regsave_size);
+    bx.cond_br(use_regsave, from_regsave, from_stack);
+
+    bx.switch_to_block(from_regsave);
+    // update va_ndx
+    bx.store(offset_next, offset_ptr, bx.tcx().data_layout.pointer_align.abi);
+
+    // (*va).va_reg
+    let regsave_area_ptr =
+        bx.inbounds_gep(bx.type_i8(), va_list_addr, &[bx.cx.const_usize(va_reg_offset)]);
+    let regsave_area =
+        bx.load(bx.type_ptr(), regsave_area_ptr, bx.tcx().data_layout.pointer_align.abi);
+    let regsave_value_ptr = bx.inbounds_gep(bx.type_i8(), regsave_area, &[offset]);
+    bx.br(end);
+
+    bx.switch_to_block(from_stack);
+
+    // The first time we switch from regsave to stack we needs to adjust our offsets a bit.
+    // va_stk is set up such that the first stack argument is always at va_stk + 32.
+    // The corrected offset is written back into the va_list struct.
+
+    // let offset_corrected = cmp::max(offset, 32);
+    let stack_offset_start = bx.const_i32(32);
+    let needs_correction = bx.icmp(IntPredicate::IntULE, offset, stack_offset_start);
+    let offset_corrected = bx.select(needs_correction, stack_offset_start, offset);
+
+    // let offset_next_corrected = offset_corrected + slot_size;
+    // va_ndx = offset_next_corrected;
+    let offset_next_corrected = bx.add(offset_next, bx.const_i32(slot_size));
+    // update va_ndx
+    bx.store(offset_next_corrected, offset_ptr, bx.tcx().data_layout.pointer_align.abi);
+
+    // let stack_value_ptr = unsafe { (*va).va_stk.byte_add(offset_corrected) };
+    let stack_area_ptr = bx.inbounds_gep(bx.type_i8(), va_list_addr, &[bx.cx.const_usize(0)]);
+    let stack_area = bx.load(bx.type_ptr(), stack_area_ptr, bx.tcx().data_layout.pointer_align.abi);
+    let stack_value_ptr = bx.inbounds_gep(bx.type_i8(), stack_area, &[offset_corrected]);
+    bx.br(end);
+
+    bx.switch_to_block(end);
+
+    // On big-endian, for values smaller than the slot size we'd have to align the read to the end
+    // of the slot rather than the start. While the ISA and GCC support big-endian, all the Xtensa
+    // targets supported by rustc are litte-endian so don't worry about it.
+
+    // if from_regsave {
+    //     unsafe { *regsave_value_ptr }
+    // } else {
+    //     unsafe { *stack_value_ptr }
+    // }
+    assert!(bx.tcx().sess.target.endian == Endian::Little);
+    let value_ptr =
+        bx.phi(bx.type_ptr(), &[regsave_value_ptr, stack_value_ptr], &[from_regsave, from_stack]);
+    return bx.load(layout.llvm_type(bx), value_ptr, layout.align.abi);
+}
+
 pub(super) fn emit_va_arg<'ll, 'tcx>(
     bx: &mut Builder<'_, 'll, 'tcx>,
     addr: OperandRef<'tcx, &'ll Value>,
@@ -302,6 +410,7 @@ pub(super) fn emit_va_arg<'ll, 'tcx>(
             let indirect: bool = target_ty_size > 8 || !target_ty_size.is_power_of_two();
             emit_ptr_va_arg(bx, addr, target_ty, indirect, Align::from_bytes(8).unwrap(), false)
         }
+        "xtensa" => emit_xtensa_va_arg(bx, addr, target_ty),
         // For all other architecture/OS combinations fall back to using
         // the LLVM va_arg instruction.
         // https://llvm.org/docs/LangRef.html#va-arg-instruction
diff --git a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
index a5acd8170ab81..01dd75c5ee85e 100644
--- a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
+++ b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
@@ -542,6 +542,13 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
         }
     });
 
+    // naked function MUST NOT be inlined! This attribute is required for the rust compiler itself,
+    // but not for the code generation backend because at that point the naked function will just be
+    // a declaration, with a definition provided in global assembly.
+    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
+        codegen_fn_attrs.inline = InlineAttr::Never;
+    }
+
     codegen_fn_attrs.optimize = attrs.iter().fold(OptimizeAttr::None, |ia, attr| {
         if !attr.has_name(sym::optimize) {
             return ia;
@@ -626,10 +633,6 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
         }
     }
 
-    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
-        codegen_fn_attrs.inline = InlineAttr::Never;
-    }
-
     // Weak lang items have the same semantics as "std internal" symbols in the
     // sense that they're preserved through all our LTO passes and only
     // strippable by the linker.
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 0cbc5c45736e8..62f69af3f2f75 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -20,6 +20,7 @@ mod coverageinfo;
 pub mod debuginfo;
 mod intrinsic;
 mod locals;
+mod naked_asm;
 pub mod operand;
 pub mod place;
 mod rvalue;
@@ -176,6 +177,11 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);
 
+    if cx.tcx().codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
+        crate::mir::naked_asm::codegen_naked_asm::<Bx>(cx, &mir, instance);
+        return;
+    }
+
     let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, mir);
 
     let start_llbb = Bx::append_block(cx, llfn, "start");
diff --git a/compiler/rustc_codegen_ssa/src/mir/naked_asm.rs b/compiler/rustc_codegen_ssa/src/mir/naked_asm.rs
new file mode 100644
index 0000000000000..0767c002c5e06
--- /dev/null
+++ b/compiler/rustc_codegen_ssa/src/mir/naked_asm.rs
@@ -0,0 +1,257 @@
+use rustc_attr::InstructionSetAttr;
+use rustc_middle::mir::mono::{Linkage, MonoItem, MonoItemData, Visibility};
+use rustc_middle::mir::{Body, InlineAsmOperand};
+use rustc_middle::ty::layout::{HasTyCtxt, HasTypingEnv, LayoutOf};
+use rustc_middle::ty::{Instance, TyCtxt};
+use rustc_middle::{bug, ty};
+use rustc_span::sym;
+
+use crate::common;
+use crate::traits::{AsmCodegenMethods, BuilderMethods, GlobalAsmOperandRef, MiscCodegenMethods};
+
+pub(crate) fn codegen_naked_asm<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
+    cx: &'a Bx::CodegenCx,
+    mir: &Body<'tcx>,
+    instance: Instance<'tcx>,
+) {
+    let rustc_middle::mir::TerminatorKind::InlineAsm {
+        asm_macro: _,
+        template,
+        ref operands,
+        options,
+        line_spans,
+        targets: _,
+        unwind: _,
+    } = mir.basic_blocks.iter().next().unwrap().terminator().kind
+    else {
+        bug!("#[naked] functions should always terminate with an asm! block")
+    };
+
+    let operands: Vec<_> =
+        operands.iter().map(|op| inline_to_global_operand::<Bx>(cx, instance, op)).collect();
+
+    let item_data = cx.codegen_unit().items().get(&MonoItem::Fn(instance)).unwrap();
+    let name = cx.mangled_name(instance);
+    let (begin, end) = prefix_and_suffix(cx.tcx(), instance, &name, item_data);
+
+    let mut template_vec = Vec::new();
+    template_vec.push(rustc_ast::ast::InlineAsmTemplatePiece::String(begin.into()));
+    template_vec.extend(template.iter().cloned());
+    template_vec.push(rustc_ast::ast::InlineAsmTemplatePiece::String(end.into()));
+
+    cx.codegen_global_asm(&template_vec, &operands, options, line_spans);
+}
+
+fn inline_to_global_operand<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
+    cx: &'a Bx::CodegenCx,
+    instance: Instance<'tcx>,
+    op: &InlineAsmOperand<'tcx>,
+) -> GlobalAsmOperandRef<'tcx> {
+    match op {
+        InlineAsmOperand::Const { value } => {
+            let const_value = instance
+                .instantiate_mir_and_normalize_erasing_regions(
+                    cx.tcx(),
+                    cx.typing_env(),
+                    ty::EarlyBinder::bind(value.const_),
+                )
+                .eval(cx.tcx(), cx.typing_env(), value.span)
+                .expect("erroneous constant missed by mono item collection");
+
+            let mono_type = instance.instantiate_mir_and_normalize_erasing_regions(
+                cx.tcx(),
+                cx.typing_env(),
+                ty::EarlyBinder::bind(value.ty()),
+            );
+
+            let string = common::asm_const_to_str(
+                cx.tcx(),
+                value.span,
+                const_value,
+                cx.layout_of(mono_type),
+            );
+
+            GlobalAsmOperandRef::Const { string }
+        }
+        InlineAsmOperand::SymFn { value } => {
+            let mono_type = instance.instantiate_mir_and_normalize_erasing_regions(
+                cx.tcx(),
+                cx.typing_env(),
+                ty::EarlyBinder::bind(value.ty()),
+            );
+
+            let instance = match mono_type.kind() {
+                &ty::FnDef(def_id, args) => Instance::new(def_id, args),
+                _ => bug!("asm sym is not a function"),
+            };
+
+            GlobalAsmOperandRef::SymFn { instance }
+        }
+        InlineAsmOperand::SymStatic { def_id } => {
+            GlobalAsmOperandRef::SymStatic { def_id: *def_id }
+        }
+        InlineAsmOperand::In { .. }
+        | InlineAsmOperand::Out { .. }
+        | InlineAsmOperand::InOut { .. }
+        | InlineAsmOperand::Label { .. } => {
+            bug!("invalid operand type for naked_asm!")
+        }
+    }
+}
+
+enum AsmBinaryFormat {
+    Elf,
+    Macho,
+    Coff,
+}
+
+impl AsmBinaryFormat {
+    fn from_target(target: &rustc_target::spec::Target) -> Self {
+        if target.is_like_windows {
+            Self::Coff
+        } else if target.is_like_osx {
+            Self::Macho
+        } else {
+            Self::Elf
+        }
+    }
+}
+
+fn prefix_and_suffix<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    instance: Instance<'tcx>,
+    asm_name: &str,
+    item_data: &MonoItemData,
+) -> (String, String) {
+    use std::fmt::Write;
+
+    let is_arm = tcx.sess.target.arch == "arm";
+    let is_thumb = tcx.sess.unstable_target_features.contains(&sym::thumb_mode);
+
+    let attrs = tcx.codegen_fn_attrs(instance.def_id());
+    let link_section = attrs.link_section.map(|symbol| symbol.as_str().to_string());
+    let align = attrs.alignment.map(|a| a.bytes()).unwrap_or(4);
+
+    // See https://sourceware.org/binutils/docs/as/ARM-Directives.html for info on these directives.
+    // In particular, `.arm` can also be written `.code 32` and `.thumb` as `.code 16`.
+    let (arch_prefix, arch_suffix) = if is_arm {
+        (
+            match attrs.instruction_set {
+                None => match is_thumb {
+                    true => ".thumb\n.thumb_func",
+                    false => ".arm",
+                },
+                Some(InstructionSetAttr::ArmT32) => ".thumb\n.thumb_func",
+                Some(InstructionSetAttr::ArmA32) => ".arm",
+            },
+            match is_thumb {
+                true => ".thumb",
+                false => ".arm",
+            },
+        )
+    } else {
+        ("", "")
+    };
+
+    let emit_fatal = |msg| tcx.dcx().span_fatal(tcx.def_span(instance.def_id()), msg);
+
+    // see https://godbolt.org/z/cPK4sxKor.
+    // None means the default, which corresponds to internal linkage
+    let linkage = match item_data.linkage {
+        Linkage::External => Some(".globl"),
+        Linkage::LinkOnceAny => Some(".weak"),
+        Linkage::LinkOnceODR => Some(".weak"),
+        Linkage::WeakAny => Some(".weak"),
+        Linkage::WeakODR => Some(".weak"),
+        Linkage::Internal => None,
+        Linkage::Private => None,
+        Linkage::Appending => emit_fatal("Only global variables can have appending linkage!"),
+        Linkage::Common => emit_fatal("Functions may not have common linkage"),
+        Linkage::AvailableExternally => {
+            // this would make the function equal an extern definition
+            emit_fatal("Functions may not have available_externally linkage")
+        }
+        Linkage::ExternalWeak => {
+            // FIXME: actually this causes a SIGILL in LLVM
+            emit_fatal("Functions may not have external weak linkage")
+        }
+    };
+
+    let mut begin = String::new();
+    let mut end = String::new();
+    match AsmBinaryFormat::from_target(&tcx.sess.target) {
+        AsmBinaryFormat::Elf => {
+            let section = link_section.unwrap_or(format!(".text.{asm_name}"));
+
+            let progbits = match is_arm {
+                true => "%progbits",
+                false => "@progbits",
+            };
+
+            let function = match is_arm {
+                true => "%function",
+                false => "@function",
+            };
+
+            writeln!(begin, ".pushsection {section},\"ax\", {progbits}").unwrap();
+            writeln!(begin, ".balign {align}").unwrap();
+            if let Some(linkage) = linkage {
+                writeln!(begin, "{linkage} {asm_name}").unwrap();
+            }
+            if let Visibility::Hidden = item_data.visibility {
+                writeln!(begin, ".hidden {asm_name}").unwrap();
+            }
+            writeln!(begin, ".type {asm_name}, {function}").unwrap();
+            if !arch_prefix.is_empty() {
+                writeln!(begin, "{}", arch_prefix).unwrap();
+            }
+            writeln!(begin, "{asm_name}:").unwrap();
+
+            writeln!(end).unwrap();
+            writeln!(end, ".size {asm_name}, . - {asm_name}").unwrap();
+            writeln!(end, ".popsection").unwrap();
+            if !arch_suffix.is_empty() {
+                writeln!(end, "{}", arch_suffix).unwrap();
+            }
+        }
+        AsmBinaryFormat::Macho => {
+            let section = link_section.unwrap_or("__TEXT,__text".to_string());
+            writeln!(begin, ".pushsection {},regular,pure_instructions", section).unwrap();
+            writeln!(begin, ".balign {align}").unwrap();
+            if let Some(linkage) = linkage {
+                writeln!(begin, "{linkage} {asm_name}").unwrap();
+            }
+            if let Visibility::Hidden = item_data.visibility {
+                writeln!(begin, ".private_extern {asm_name}").unwrap();
+            }
+            writeln!(begin, "{asm_name}:").unwrap();
+
+            writeln!(end).unwrap();
+            writeln!(end, ".popsection").unwrap();
+            if !arch_suffix.is_empty() {
+                writeln!(end, "{}", arch_suffix).unwrap();
+            }
+        }
+        AsmBinaryFormat::Coff => {
+            let section = link_section.unwrap_or(format!(".text.{asm_name}"));
+            writeln!(begin, ".pushsection {},\"xr\"", section).unwrap();
+            writeln!(begin, ".balign {align}").unwrap();
+            if let Some(linkage) = linkage {
+                writeln!(begin, "{linkage} {asm_name}").unwrap();
+            }
+            writeln!(begin, ".def {asm_name}").unwrap();
+            writeln!(begin, ".scl 2").unwrap();
+            writeln!(begin, ".type 32").unwrap();
+            writeln!(begin, ".endef {asm_name}").unwrap();
+            writeln!(begin, "{asm_name}:").unwrap();
+
+            writeln!(end).unwrap();
+            writeln!(end, ".popsection").unwrap();
+            if !arch_suffix.is_empty() {
+                writeln!(end, "{}", arch_suffix).unwrap();
+            }
+        }
+    }
+
+    (begin, end)
+}
diff --git a/compiler/rustc_codegen_ssa/src/mono_item.rs b/compiler/rustc_codegen_ssa/src/mono_item.rs
index 038c5857face1..6749bc63327e4 100644
--- a/compiler/rustc_codegen_ssa/src/mono_item.rs
+++ b/compiler/rustc_codegen_ssa/src/mono_item.rs
@@ -1,4 +1,5 @@
 use rustc_hir as hir;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::mir::mono::{Linkage, MonoItem, Visibility};
 use rustc_middle::ty::Instance;
@@ -135,7 +136,13 @@ impl<'a, 'tcx: 'a> MonoItemExt<'a, 'tcx> for MonoItem<'tcx> {
                 cx.predefine_static(def_id, linkage, visibility, symbol_name);
             }
             MonoItem::Fn(instance) => {
-                cx.predefine_fn(instance, linkage, visibility, symbol_name);
+                let attrs = cx.tcx().codegen_fn_attrs(instance.def_id());
+
+                if attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
+                    // do not define this function; it will become a global assembly block
+                } else {
+                    cx.predefine_fn(instance, linkage, visibility, symbol_name);
+                };
             }
             MonoItem::GlobalAsm(..) => {}
         }
diff --git a/compiler/rustc_codegen_ssa/src/traits/asm.rs b/compiler/rustc_codegen_ssa/src/traits/asm.rs
index f4853da115648..7767bffbfbfd6 100644
--- a/compiler/rustc_codegen_ssa/src/traits/asm.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/asm.rs
@@ -68,4 +68,11 @@ pub trait AsmCodegenMethods<'tcx> {
         options: InlineAsmOptions,
         line_spans: &[Span],
     );
+
+    /// The mangled name of this instance
+    ///
+    /// Additional mangling is used on
+    /// some targets to add a leading underscore (Mach-O)
+    /// or byte count suffixes (x86 Windows).
+    fn mangled_name(&self, instance: Instance<'tcx>) -> String;
 }
diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 161716610fe63..1f50b67cb50d5 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -19,6 +19,7 @@ use rustc_target::spec::SymbolVisibility;
 use tracing::debug;
 
 use crate::dep_graph::{DepNode, WorkProduct, WorkProductId};
+use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use crate::ty::{GenericArgs, Instance, InstanceKind, SymbolName, TyCtxt};
 
 /// Describes how a monomorphization will be instantiated in object files.
@@ -104,7 +105,9 @@ impl<'tcx> MonoItem<'tcx> {
                 let entry_def_id = tcx.entry_fn(()).map(|(id, _)| id);
                 // If this function isn't inlined or otherwise has an extern
                 // indicator, then we'll be creating a globally shared version.
-                if tcx.codegen_fn_attrs(instance.def_id()).contains_extern_indicator()
+                let codegen_fn_attrs = tcx.codegen_fn_attrs(instance.def_id());
+                if codegen_fn_attrs.contains_extern_indicator()
+                    || codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED)
                     || !instance.def.generates_cgu_internal_copy(tcx)
                     || Some(instance.def_id()) == entry_def_id
                 {
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index df110fdc20aea..8861f0814856a 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -916,6 +916,12 @@ rustc_queries! {
         cache_on_disk_if { true }
     }
 
+    /// Checks well-formedness of tail calls (`become f()`).
+    query check_tail_calls(key: LocalDefId) -> Result<(), rustc_errors::ErrorGuaranteed> {
+        desc { |tcx| "tail-call-checking `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { true }
+    }
+
     /// Returns the types assumed to be well formed while "inside" of the given item.
     ///
     /// Note that we've liberated the late bound regions of function signatures, so
diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/build/mod.rs
index 3317f3b7f8acb..f43c29d8f5d68 100644
--- a/compiler/rustc_mir_build/src/build/mod.rs
+++ b/compiler/rustc_mir_build/src/build/mod.rs
@@ -50,6 +50,10 @@ pub(crate) fn mir_build<'tcx>(tcx: TyCtxtAt<'tcx>, def: LocalDefId) -> Body<'tcx
         return construct_error(tcx, def, e);
     }
 
+    if let Err(err) = tcx.check_tail_calls(def) {
+        return construct_error(tcx, def, err);
+    }
+
     let body = match tcx.thir_body(def) {
         Err(error_reported) => construct_error(tcx, def, error_reported),
         Ok((thir, expr)) => {
diff --git a/compiler/rustc_mir_build/src/check_tail_calls.rs b/compiler/rustc_mir_build/src/check_tail_calls.rs
new file mode 100644
index 0000000000000..b1f46d37d5062
--- /dev/null
+++ b/compiler/rustc_mir_build/src/check_tail_calls.rs
@@ -0,0 +1,386 @@
+use rustc_abi::ExternAbi;
+use rustc_errors::Applicability;
+use rustc_hir::LangItem;
+use rustc_hir::def::DefKind;
+use rustc_middle::span_bug;
+use rustc_middle::thir::visit::{self, Visitor};
+use rustc_middle::thir::{BodyTy, Expr, ExprId, ExprKind, Thir};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::def_id::{DefId, LocalDefId};
+use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
+
+pub(crate) fn check_tail_calls(tcx: TyCtxt<'_>, def: LocalDefId) -> Result<(), ErrorGuaranteed> {
+    let (thir, expr) = tcx.thir_body(def)?;
+    let thir = &thir.borrow();
+
+    // If `thir` is empty, a type error occurred, skip this body.
+    if thir.exprs.is_empty() {
+        return Ok(());
+    }
+
+    let is_closure = matches!(tcx.def_kind(def), DefKind::Closure);
+    let caller_ty = tcx.type_of(def).skip_binder();
+
+    let mut visitor = TailCallCkVisitor {
+        tcx,
+        thir,
+        found_errors: Ok(()),
+        // FIXME(#132279): we're clearly in a body here.
+        typing_env: ty::TypingEnv::non_body_analysis(tcx, def),
+        is_closure,
+        caller_ty,
+    };
+
+    visitor.visit_expr(&thir[expr]);
+
+    visitor.found_errors
+}
+
+struct TailCallCkVisitor<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    thir: &'a Thir<'tcx>,
+    typing_env: ty::TypingEnv<'tcx>,
+    /// Whatever the currently checked body is one of a closure
+    is_closure: bool,
+    /// The result of the checks, `Err(_)` if there was a problem with some
+    /// tail call, `Ok(())` if all of them were fine.
+    found_errors: Result<(), ErrorGuaranteed>,
+    /// Type of the caller function.
+    caller_ty: Ty<'tcx>,
+}
+
+impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
+    fn check_tail_call(&mut self, call: &Expr<'_>, expr: &Expr<'_>) {
+        if self.is_closure {
+            self.report_in_closure(expr);
+            return;
+        }
+
+        let BodyTy::Fn(caller_sig) = self.thir.body_type else {
+            span_bug!(
+                call.span,
+                "`become` outside of functions should have been disallowed by hit_typeck"
+            )
+        };
+
+        let ExprKind::Scope { value, .. } = call.kind else {
+            span_bug!(call.span, "expected scope, found: {call:?}")
+        };
+        let value = &self.thir[value];
+
+        if matches!(
+            value.kind,
+            ExprKind::Binary { .. }
+                | ExprKind::Unary { .. }
+                | ExprKind::AssignOp { .. }
+                | ExprKind::Index { .. }
+        ) {
+            self.report_builtin_op(call, expr);
+            return;
+        }
+
+        let ExprKind::Call { ty, fun, ref args, from_hir_call, fn_span } = value.kind else {
+            self.report_non_call(value, expr);
+            return;
+        };
+
+        if !from_hir_call {
+            self.report_op(ty, args, fn_span, expr);
+        }
+
+        // Closures in thir look something akin to
+        // `for<'a> extern "rust-call" fn(&'a [closure@...], ()) -> <[closure@...] as FnOnce<()>>::Output {<[closure@...] as Fn<()>>::call}`
+        // So we have to check for them in this weird way...
+        if let &ty::FnDef(did, args) = ty.kind() {
+            let parent = self.tcx.parent(did);
+            if self.tcx.fn_trait_kind_from_def_id(parent).is_some()
+                && args.first().and_then(|arg| arg.as_type()).is_some_and(Ty::is_closure)
+            {
+                self.report_calling_closure(&self.thir[fun], args[1].as_type().unwrap(), expr);
+
+                // Tail calling is likely to cause unrelated errors (ABI, argument mismatches),
+                // skip them, producing an error about calling a closure is enough.
+                return;
+            };
+        }
+
+        // Erase regions since tail calls don't care about lifetimes
+        let callee_sig =
+            self.tcx.normalize_erasing_late_bound_regions(self.typing_env, ty.fn_sig(self.tcx));
+
+        if caller_sig.abi != callee_sig.abi {
+            self.report_abi_mismatch(expr.span, caller_sig.abi, callee_sig.abi);
+        }
+
+        if caller_sig.inputs_and_output != callee_sig.inputs_and_output {
+            if caller_sig.inputs() != callee_sig.inputs() {
+                self.report_arguments_mismatch(expr.span, caller_sig, callee_sig);
+            }
+
+            // FIXME(explicit_tail_calls): this currenly fails for cases where opaques are used.
+            // e.g.
+            // ```
+            // fn a() -> impl Sized { become b() } // ICE
+            // fn b() -> u8 { 0 }
+            // ```
+            // we should think what is the expected behavior here.
+            // (we should probably just accept this by revealing opaques?)
+            if caller_sig.output() != callee_sig.output() {
+                span_bug!(expr.span, "hir typeck should have checked the return type already");
+            }
+        }
+
+        {
+            let caller_needs_location = self.needs_location(self.caller_ty);
+            let callee_needs_location = self.needs_location(ty);
+
+            if caller_needs_location != callee_needs_location {
+                self.report_track_caller_mismatch(expr.span, caller_needs_location);
+            }
+        }
+
+        if caller_sig.c_variadic {
+            self.report_c_variadic_caller(expr.span);
+        }
+
+        if callee_sig.c_variadic {
+            self.report_c_variadic_callee(expr.span);
+        }
+    }
+
+    /// Returns true if function of type `ty` needs location argument
+    /// (i.e. if a function is marked as `#[track_caller]`)
+    fn needs_location(&self, ty: Ty<'tcx>) -> bool {
+        if let &ty::FnDef(did, substs) = ty.kind() {
+            let instance =
+                ty::Instance::expect_resolve(self.tcx, self.typing_env, did, substs, DUMMY_SP);
+
+            instance.def.requires_caller_location(self.tcx)
+        } else {
+            false
+        }
+    }
+
+    fn report_in_closure(&mut self, expr: &Expr<'_>) {
+        let err = self.tcx.dcx().span_err(expr.span, "`become` is not allowed in closures");
+        self.found_errors = Err(err);
+    }
+
+    fn report_builtin_op(&mut self, value: &Expr<'_>, expr: &Expr<'_>) {
+        let err = self
+            .tcx
+            .dcx()
+            .struct_span_err(value.span, "`become` does not support operators")
+            .with_note("using `become` on a builtin operator is not useful")
+            .with_span_suggestion(
+                value.span.until(expr.span),
+                "try using `return` instead",
+                "return ",
+                Applicability::MachineApplicable,
+            )
+            .emit();
+        self.found_errors = Err(err);
+    }
+
+    fn report_op(&mut self, fun_ty: Ty<'_>, args: &[ExprId], fn_span: Span, expr: &Expr<'_>) {
+        let mut err =
+            self.tcx.dcx().struct_span_err(fn_span, "`become` does not support operators");
+
+        if let &ty::FnDef(did, _substs) = fun_ty.kind()
+            && let parent = self.tcx.parent(did)
+            && matches!(self.tcx.def_kind(parent), DefKind::Trait)
+            && let Some(method) = op_trait_as_method_name(self.tcx, parent)
+        {
+            match args {
+                &[arg] => {
+                    let arg = &self.thir[arg];
+
+                    err.multipart_suggestion(
+                        "try using the method directly",
+                        vec![
+                            (fn_span.shrink_to_lo().until(arg.span), "(".to_owned()),
+                            (arg.span.shrink_to_hi(), format!(").{method}()")),
+                        ],
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                &[lhs, rhs] => {
+                    let lhs = &self.thir[lhs];
+                    let rhs = &self.thir[rhs];
+
+                    err.multipart_suggestion(
+                        "try using the method directly",
+                        vec![
+                            (lhs.span.shrink_to_lo(), format!("(")),
+                            (lhs.span.between(rhs.span), format!(").{method}(")),
+                            (rhs.span.between(expr.span.shrink_to_hi()), ")".to_owned()),
+                        ],
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                _ => span_bug!(expr.span, "operator with more than 2 args? {args:?}"),
+            }
+        }
+
+        self.found_errors = Err(err.emit());
+    }
+
+    fn report_non_call(&mut self, value: &Expr<'_>, expr: &Expr<'_>) {
+        let err = self
+            .tcx
+            .dcx()
+            .struct_span_err(value.span, "`become` requires a function call")
+            .with_span_note(value.span, "not a function call")
+            .with_span_suggestion(
+                value.span.until(expr.span),
+                "try using `return` instead",
+                "return ",
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+        self.found_errors = Err(err);
+    }
+
+    fn report_calling_closure(&mut self, fun: &Expr<'_>, tupled_args: Ty<'_>, expr: &Expr<'_>) {
+        let underscored_args = match tupled_args.kind() {
+            ty::Tuple(tys) if tys.is_empty() => "".to_owned(),
+            ty::Tuple(tys) => std::iter::repeat("_, ").take(tys.len() - 1).chain(["_"]).collect(),
+            _ => "_".to_owned(),
+        };
+
+        let err = self
+            .tcx
+            .dcx()
+            .struct_span_err(expr.span, "tail calling closures directly is not allowed")
+            .with_multipart_suggestion(
+                "try casting the closure to a function pointer type",
+                vec![
+                    (fun.span.shrink_to_lo(), "(".to_owned()),
+                    (fun.span.shrink_to_hi(), format!(" as fn({underscored_args}) -> _)")),
+                ],
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+        self.found_errors = Err(err);
+    }
+
+    fn report_abi_mismatch(&mut self, sp: Span, caller_abi: ExternAbi, callee_abi: ExternAbi) {
+        let err = self
+            .tcx
+            .dcx()
+            .struct_span_err(sp, "mismatched function ABIs")
+            .with_note("`become` requires caller and callee to have the same ABI")
+            .with_note(format!("caller ABI is `{caller_abi}`, while callee ABI is `{callee_abi}`"))
+            .emit();
+        self.found_errors = Err(err);
+    }
+
+    fn report_arguments_mismatch(
+        &mut self,
+        sp: Span,
+        caller_sig: ty::FnSig<'_>,
+        callee_sig: ty::FnSig<'_>,
+    ) {
+        let err = self
+            .tcx
+            .dcx()
+            .struct_span_err(sp, "mismatched signatures")
+            .with_note("`become` requires caller and callee to have matching signatures")
+            .with_note(format!("caller signature: `{caller_sig}`"))
+            .with_note(format!("callee signature: `{callee_sig}`"))
+            .emit();
+        self.found_errors = Err(err);
+    }
+
+    fn report_track_caller_mismatch(&mut self, sp: Span, caller_needs_location: bool) {
+        let err = match caller_needs_location {
+            true => self
+                .tcx
+                .dcx()
+                .struct_span_err(
+                    sp,
+                    "a function marked with `#[track_caller]` cannot tail-call one that is not",
+                )
+                .emit(),
+            false => self
+                .tcx
+                .dcx()
+                .struct_span_err(
+                    sp,
+                    "a function mot marked with `#[track_caller]` cannot tail-call one that is",
+                )
+                .emit(),
+        };
+
+        self.found_errors = Err(err);
+    }
+
+    fn report_c_variadic_caller(&mut self, sp: Span) {
+        let err = self
+            .tcx
+            .dcx()
+            // FIXME(explicit_tail_calls): highlight the `...`
+            .struct_span_err(sp, "tail-calls are not allowed in c-variadic functions")
+            .emit();
+
+        self.found_errors = Err(err);
+    }
+
+    fn report_c_variadic_callee(&mut self, sp: Span) {
+        let err = self
+            .tcx
+            .dcx()
+            // FIXME(explicit_tail_calls): highlight the function or something...
+            .struct_span_err(sp, "c-variadic functions can't be tail-called")
+            .emit();
+
+        self.found_errors = Err(err);
+    }
+}
+
+impl<'a, 'tcx> Visitor<'a, 'tcx> for TailCallCkVisitor<'a, 'tcx> {
+    fn thir(&self) -> &'a Thir<'tcx> {
+        &self.thir
+    }
+
+    fn visit_expr(&mut self, expr: &'a Expr<'tcx>) {
+        if let ExprKind::Become { value } = expr.kind {
+            let call = &self.thir[value];
+            self.check_tail_call(call, expr);
+        }
+
+        visit::walk_expr(self, expr);
+    }
+}
+
+fn op_trait_as_method_name(tcx: TyCtxt<'_>, trait_did: DefId) -> Option<&'static str> {
+    let m = match tcx.as_lang_item(trait_did)? {
+        LangItem::Add => "add",
+        LangItem::Sub => "sub",
+        LangItem::Mul => "mul",
+        LangItem::Div => "div",
+        LangItem::Rem => "rem",
+        LangItem::Neg => "neg",
+        LangItem::Not => "not",
+        LangItem::BitXor => "bitxor",
+        LangItem::BitAnd => "bitand",
+        LangItem::BitOr => "bitor",
+        LangItem::Shl => "shl",
+        LangItem::Shr => "shr",
+        LangItem::AddAssign => "add_assign",
+        LangItem::SubAssign => "sub_assign",
+        LangItem::MulAssign => "mul_assign",
+        LangItem::DivAssign => "div_assign",
+        LangItem::RemAssign => "rem_assign",
+        LangItem::BitXorAssign => "bitxor_assign",
+        LangItem::BitAndAssign => "bitand_assign",
+        LangItem::BitOrAssign => "bitor_assign",
+        LangItem::ShlAssign => "shl_assign",
+        LangItem::ShrAssign => "shr_assign",
+        LangItem::Index => "index",
+        LangItem::IndexMut => "index_mut",
+        _ => return None,
+    };
+
+    Some(m)
+}
diff --git a/compiler/rustc_mir_build/src/lib.rs b/compiler/rustc_mir_build/src/lib.rs
index 3dbb552cdbbef..833e5019865e8 100644
--- a/compiler/rustc_mir_build/src/lib.rs
+++ b/compiler/rustc_mir_build/src/lib.rs
@@ -12,6 +12,7 @@
 // tidy-alphabetical-end
 
 mod build;
+mod check_tail_calls;
 mod check_unsafety;
 mod errors;
 pub mod lints;
@@ -28,6 +29,7 @@ pub fn provide(providers: &mut Providers) {
     providers.closure_saved_names_of_captured_variables =
         build::closure_saved_names_of_captured_variables;
     providers.check_unsafety = check_unsafety::check_unsafety;
+    providers.check_tail_calls = check_tail_calls::check_tail_calls;
     providers.thir_body = thir::cx::thir_body;
     providers.hooks.thir_tree = thir::print::thir_tree;
     providers.hooks.thir_flat = thir::print::thir_flat;
diff --git a/compiler/rustc_target/src/spec/targets/armv4t_unknown_linux_gnueabi.rs b/compiler/rustc_target/src/spec/targets/armv4t_unknown_linux_gnueabi.rs
index 3a2fb753ba147..7ca2babffe9fc 100644
--- a/compiler/rustc_target/src/spec/targets/armv4t_unknown_linux_gnueabi.rs
+++ b/compiler/rustc_target/src/spec/targets/armv4t_unknown_linux_gnueabi.rs
@@ -7,7 +7,7 @@ pub(crate) fn target() -> Target {
             description: Some("Armv4T Linux".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/armv5te_unknown_linux_uclibceabi.rs b/compiler/rustc_target/src/spec/targets/armv5te_unknown_linux_uclibceabi.rs
index e0bc1936e221c..154103c36f79c 100644
--- a/compiler/rustc_target/src/spec/targets/armv5te_unknown_linux_uclibceabi.rs
+++ b/compiler/rustc_target/src/spec/targets/armv5te_unknown_linux_uclibceabi.rs
@@ -7,7 +7,7 @@ pub(crate) fn target() -> Target {
             description: Some("Armv5TE Linux with uClibc".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/m68k_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/m68k_unknown_linux_gnu.rs
index 1515d8fe00abc..47c6d80548359 100644
--- a/compiler/rustc_target/src/spec/targets/m68k_unknown_linux_gnu.rs
+++ b/compiler/rustc_target/src/spec/targets/m68k_unknown_linux_gnu.rs
@@ -12,7 +12,7 @@ pub(crate) fn target() -> Target {
             description: Some("Motorola 680x0 Linux".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "E-m:e-p:32:16:32-i8:8:8-i16:16:16-i32:16:32-n8:16:32-a:0:16-S16".into(),
diff --git a/compiler/rustc_target/src/spec/targets/mips64_openwrt_linux_musl.rs b/compiler/rustc_target/src/spec/targets/mips64_openwrt_linux_musl.rs
index b7415bf683de2..cc288e042d987 100644
--- a/compiler/rustc_target/src/spec/targets/mips64_openwrt_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/mips64_openwrt_linux_musl.rs
@@ -17,7 +17,7 @@ pub(crate) fn target() -> Target {
             description: Some("MIPS64 for OpenWrt Linux musl 1.2.3".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "E-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/mipsisa32r6_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/mipsisa32r6_unknown_linux_gnu.rs
index 2ff06dc6669bb..3ef0988d23570 100644
--- a/compiler/rustc_target/src/spec/targets/mipsisa32r6_unknown_linux_gnu.rs
+++ b/compiler/rustc_target/src/spec/targets/mipsisa32r6_unknown_linux_gnu.rs
@@ -8,7 +8,7 @@ pub(crate) fn target() -> Target {
             description: Some("32-bit MIPS Release 6 Big Endian".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "E-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/mipsisa32r6el_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/mipsisa32r6el_unknown_linux_gnu.rs
index 05b246698482a..c75023385c220 100644
--- a/compiler/rustc_target/src/spec/targets/mipsisa32r6el_unknown_linux_gnu.rs
+++ b/compiler/rustc_target/src/spec/targets/mipsisa32r6el_unknown_linux_gnu.rs
@@ -7,7 +7,7 @@ pub(crate) fn target() -> Target {
             description: Some("32-bit MIPS Release 6 Little Endian".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "e-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/mipsisa64r6_unknown_linux_gnuabi64.rs b/compiler/rustc_target/src/spec/targets/mipsisa64r6_unknown_linux_gnuabi64.rs
index c5948b745ff59..d0d0ed1acc3f0 100644
--- a/compiler/rustc_target/src/spec/targets/mipsisa64r6_unknown_linux_gnuabi64.rs
+++ b/compiler/rustc_target/src/spec/targets/mipsisa64r6_unknown_linux_gnuabi64.rs
@@ -8,7 +8,7 @@ pub(crate) fn target() -> Target {
             description: Some("64-bit MIPS Release 6 Big Endian".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "E-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/powerpc64_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/powerpc64_unknown_linux_musl.rs
index 5da891cc13f6d..62c30aebc518e 100644
--- a/compiler/rustc_target/src/spec/targets/powerpc64_unknown_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/powerpc64_unknown_linux_musl.rs
@@ -14,7 +14,7 @@ pub(crate) fn target() -> Target {
             description: Some("64-bit PowerPC Linux with musl 1.2.3".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "E-m:e-Fn32-i64:64-n32:64-S128-v256:256:256-v512:512:512".into(),
diff --git a/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_freebsd.rs b/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_freebsd.rs
index e194f5f8a36f3..0c1218bd05939 100644
--- a/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_freebsd.rs
+++ b/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_freebsd.rs
@@ -12,8 +12,8 @@ pub(crate) fn target() -> Target {
         metadata: crate::spec::TargetMetadata {
             description: Some("PPC64LE FreeBSD".into()),
             tier: Some(3),
-            host_tools: Some(false),
-            std: Some(false),
+            host_tools: Some(true),
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "e-m:e-Fn32-i64:64-n32:64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_linux_musl.rs
index e2921aa17fe64..50946ae4ce66d 100644
--- a/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/powerpc64le_unknown_linux_musl.rs
@@ -13,7 +13,7 @@ pub(crate) fn target() -> Target {
             description: Some("64-bit PowerPC Linux with musl 1.2.3, Little Endian".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "e-m:e-Fn32-i64:64-n32:64-S128-v256:256:256-v512:512:512".into(),
diff --git a/compiler/rustc_target/src/spec/targets/powerpc_unknown_freebsd.rs b/compiler/rustc_target/src/spec/targets/powerpc_unknown_freebsd.rs
index 393946980f8b3..9cfc2153ac170 100644
--- a/compiler/rustc_target/src/spec/targets/powerpc_unknown_freebsd.rs
+++ b/compiler/rustc_target/src/spec/targets/powerpc_unknown_freebsd.rs
@@ -17,7 +17,7 @@ pub(crate) fn target() -> Target {
             description: Some("PowerPC FreeBSD".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "E-m:e-p:32:32-Fn32-i64:64-n32".into(),
diff --git a/compiler/rustc_target/src/spec/targets/powerpc_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/powerpc_unknown_linux_musl.rs
index 9859e8ced4f46..5372a83e29a22 100644
--- a/compiler/rustc_target/src/spec/targets/powerpc_unknown_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/powerpc_unknown_linux_musl.rs
@@ -13,7 +13,7 @@ pub(crate) fn target() -> Target {
             description: Some("PowerPC Linux with musl 1.2.3".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "E-m:e-p:32:32-Fn32-i64:64-n32".into(),
diff --git a/compiler/rustc_target/src/spec/targets/powerpc_unknown_openbsd.rs b/compiler/rustc_target/src/spec/targets/powerpc_unknown_openbsd.rs
index a8254af9736c0..7aedc2d0665cb 100644
--- a/compiler/rustc_target/src/spec/targets/powerpc_unknown_openbsd.rs
+++ b/compiler/rustc_target/src/spec/targets/powerpc_unknown_openbsd.rs
@@ -13,7 +13,7 @@ pub(crate) fn target() -> Target {
             description: None,
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "E-m:e-p:32:32-Fn32-i64:64-n32".into(),
diff --git a/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_gnu.rs
index 8b0a140c9ade5..e9c57b99b9279 100644
--- a/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_gnu.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_gnu.rs
@@ -9,7 +9,7 @@ pub(crate) fn target() -> Target {
             description: Some("RISC-V Linux (kernel 5.4, glibc 2.33)".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "e-m:e-p:32:32-i64:64-n32-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_musl.rs
index 212de791e499a..08871d9d72bfb 100644
--- a/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv32gc_unknown_linux_musl.rs
@@ -11,7 +11,7 @@ pub(crate) fn target() -> Target {
             ),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "e-m:e-p:32:32-i64:64-n32-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs b/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs
index 8b5d4751a5733..f694a1cb60dd2 100644
--- a/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs
@@ -9,7 +9,7 @@ pub(crate) fn target() -> Target {
             description: Some("RISC-V 64-bit Android".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_freebsd.rs b/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_freebsd.rs
index d1b0d801fd60b..905bed76db4d0 100644
--- a/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_freebsd.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_freebsd.rs
@@ -7,7 +7,7 @@ pub(crate) fn target() -> Target {
             description: Some("RISC-V FreeBSD".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_fuchsia.rs b/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_fuchsia.rs
index 6de51f9b07e2a..7a887b604c507 100644
--- a/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_fuchsia.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_fuchsia.rs
@@ -7,7 +7,7 @@ pub(crate) fn target() -> Target {
             description: Some("RISC-V Fuchsia".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_linux_musl.rs
index 2e6fce91d4c75..397c202ec1a57 100644
--- a/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv64gc_unknown_linux_musl.rs
@@ -9,7 +9,7 @@ pub(crate) fn target() -> Target {
             description: Some("RISC-V Linux (kernel 4.20, musl 1.2.3)".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128".into(),
diff --git a/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_musl.rs
index 4bde0fb729c75..7a78004927b10 100644
--- a/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_musl.rs
+++ b/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_musl.rs
@@ -19,7 +19,7 @@ pub(crate) fn target() -> Target {
             description: Some("S390x Linux (kernel 3.2, musl 1.2.3)".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: Some(false),
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout: "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-v128:64-a:8:16-n32:64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/thumbv7neon_unknown_linux_musleabihf.rs b/compiler/rustc_target/src/spec/targets/thumbv7neon_unknown_linux_musleabihf.rs
index 5c43ca69bd852..2f539f4f6004d 100644
--- a/compiler/rustc_target/src/spec/targets/thumbv7neon_unknown_linux_musleabihf.rs
+++ b/compiler/rustc_target/src/spec/targets/thumbv7neon_unknown_linux_musleabihf.rs
@@ -16,7 +16,7 @@ pub(crate) fn target() -> Target {
             description: Some("Thumb2-mode ARMv7-A Linux with NEON, musl 1.2.3".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None, // ?
+            std: Some(true),
         },
         pointer_width: 32,
         data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".into(),
diff --git a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_none.rs b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_none.rs
index 91eb7b53fea8a..900dbed205c8e 100644
--- a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_none.rs
+++ b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_none.rs
@@ -14,7 +14,7 @@ pub(crate) fn target() -> Target {
             description: None,
             tier: None,
             host_tools: None,
-            std: None,
+            std: Some(true),
         },
         pointer_width: 64,
         data_layout:
diff --git a/library/core/Cargo.toml b/library/core/Cargo.toml
index cace4582b489a..46c55c437cce5 100644
--- a/library/core/Cargo.toml
+++ b/library/core/Cargo.toml
@@ -43,6 +43,7 @@ check-cfg = [
     'cfg(bootstrap)',
     'cfg(no_fp_fmt_parse)',
     'cfg(stdarch_intel_sde)',
+    'cfg(target_arch, values("xtensa"))',
     # core use #[path] imports to portable-simd `core_simd` crate
     # and to stdarch `core_arch` crate which messes-up with Cargo list
     # of declared features, we therefor expect any feature cfg
diff --git a/library/core/src/ffi/va_list.rs b/library/core/src/ffi/va_list.rs
index 3a224e4d8fe5f..f67c592d8d8f7 100644
--- a/library/core/src/ffi/va_list.rs
+++ b/library/core/src/ffi/va_list.rs
@@ -15,6 +15,7 @@ use crate::ops::{Deref, DerefMut};
         not(target_arch = "aarch64"),
         not(target_arch = "powerpc"),
         not(target_arch = "s390x"),
+        not(target_arch = "xtensa"),
         not(target_arch = "x86_64")
     ),
     all(target_arch = "aarch64", target_vendor = "apple"),
@@ -37,6 +38,7 @@ pub struct VaListImpl<'f> {
         not(target_arch = "aarch64"),
         not(target_arch = "powerpc"),
         not(target_arch = "s390x"),
+        not(target_arch = "xtensa"),
         not(target_arch = "x86_64")
     ),
     all(target_arch = "aarch64", target_vendor = "apple"),
@@ -113,6 +115,18 @@ pub struct VaListImpl<'f> {
     _marker: PhantomData<&'f mut &'f c_void>,
 }
 
+/// Xtensa ABI implementation of a `va_list`.
+#[cfg(target_arch = "xtensa")]
+#[repr(C)]
+#[derive(Debug)]
+#[lang = "va_list"]
+pub struct VaListImpl<'f> {
+    stk: *mut i32,
+    reg: *mut i32,
+    ndx: i32,
+    _marker: PhantomData<&'f mut &'f c_void>,
+}
+
 /// A wrapper for a `va_list`
 #[repr(transparent)]
 #[derive(Debug)]
@@ -124,6 +138,7 @@ pub struct VaList<'a, 'f: 'a> {
             not(target_arch = "s390x"),
             not(target_arch = "x86_64")
         ),
+        target_arch = "xtensa",
         all(target_arch = "aarch64", target_vendor = "apple"),
         target_family = "wasm",
         target_os = "uefi",
@@ -138,6 +153,7 @@ pub struct VaList<'a, 'f: 'a> {
             target_arch = "s390x",
             target_arch = "x86_64"
         ),
+        not(target_arch = "xtensa"),
         any(not(target_arch = "aarch64"), not(target_vendor = "apple")),
         not(target_family = "wasm"),
         not(target_os = "uefi"),
@@ -155,6 +171,7 @@ pub struct VaList<'a, 'f: 'a> {
         not(target_arch = "s390x"),
         not(target_arch = "x86_64")
     ),
+    target_arch = "xtensa",
     all(target_arch = "aarch64", target_vendor = "apple"),
     target_family = "wasm",
     target_os = "uefi",
@@ -173,8 +190,10 @@ impl<'f> VaListImpl<'f> {
         target_arch = "aarch64",
         target_arch = "powerpc",
         target_arch = "s390x",
+        target_arch = "xtensa",
         target_arch = "x86_64"
     ),
+    not(target_arch = "xtensa"),
     any(not(target_arch = "aarch64"), not(target_vendor = "apple")),
     not(target_family = "wasm"),
     not(target_os = "uefi"),
diff --git a/library/core/src/intrinsics/simd.rs b/library/core/src/intrinsics/simd.rs
index 0d24b0558c594..f80a60d471c04 100644
--- a/library/core/src/intrinsics/simd.rs
+++ b/library/core/src/intrinsics/simd.rs
@@ -619,7 +619,8 @@ extern "rust-intrinsic" {
     /// set has support for a fused operation, and that the fused operation is more efficient
     /// than the equivalent, separate pair of mul and add instructions. It is unspecified
     /// whether or not a fused operation is selected, and that may depend on optimization
-    /// level and context, for example.
+    /// level and context, for example. It may even be the case that some SIMD lanes get fused
+    /// and others do not.
     ///
     /// `T` must be a vector of floats.
     #[cfg(not(bootstrap))]
diff --git a/src/bootstrap/bootstrap.py b/src/bootstrap/bootstrap.py
index 762f4e653e91b..9ef9ccec1937f 100644
--- a/src/bootstrap/bootstrap.py
+++ b/src/bootstrap/bootstrap.py
@@ -1184,6 +1184,8 @@ def bootstrap(args):
     args = [build.bootstrap_binary()]
     args.extend(sys.argv[1:])
     env = os.environ.copy()
+    # The Python process ID is used when creating a Windows job object
+    # (see src\bootstrap\src\utils\job.rs)
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable
     run(args, env=env, verbose=build.verbose, is_bootstrap=True)
diff --git a/src/bootstrap/mk/Makefile.in b/src/bootstrap/mk/Makefile.in
index a1f38b9ac147f..a29c96ff78ceb 100644
--- a/src/bootstrap/mk/Makefile.in
+++ b/src/bootstrap/mk/Makefile.in
@@ -97,13 +97,18 @@ tidy:
 prepare:
 	$(Q)$(BOOTSTRAP) build --stage 2 --dry-run
 
+STAGE_2_TEST_SET1 := test --stage 2 --skip=compiler --skip=src
+STAGE_2_TEST_SET2 := test --stage 2 --skip=tests --skip=coverage-map --skip=coverage-run --skip=library --skip=tidyselftest
+
 ## MSVC native builders
 
+# Set of tests that should represent half of the time of the test suite.
+# Used to split tests across multiple CI runners.
 # this intentionally doesn't use `$(BOOTSTRAP)` so we can test the shebang on Windows
 ci-msvc-py:
-	$(Q)$(CFG_SRC_DIR)/x.py test --stage 2 tidy
+	$(Q)$(CFG_SRC_DIR)/x.py $(STAGE_2_TEST_SET1)
 ci-msvc-ps1:
-	$(Q)$(CFG_SRC_DIR)/x.ps1 test --stage 2 --skip tidy
+	$(Q)$(CFG_SRC_DIR)/x.ps1 $(STAGE_2_TEST_SET2)
 ci-msvc: ci-msvc-py ci-msvc-ps1
 
 ## MingW native builders
@@ -112,9 +117,9 @@ ci-msvc: ci-msvc-py ci-msvc-ps1
 # Used to split tests across multiple CI runners.
 # Test both x and bootstrap entrypoints.
 ci-mingw-x:
-	$(Q)$(CFG_SRC_DIR)/x test --stage 2 --skip=compiler --skip=src
+	$(Q)$(CFG_SRC_DIR)/x $(STAGE_2_TEST_SET1)
 ci-mingw-bootstrap:
-	$(Q)$(BOOTSTRAP) test --stage 2 --skip=tests --skip=coverage-map --skip=coverage-run --skip=library --skip=tidyselftest
+	$(Q)$(BOOTSTRAP) $(STAGE_2_TEST_SET2)
 ci-mingw: ci-mingw-x ci-mingw-bootstrap
 
 .PHONY: dist
diff --git a/src/bootstrap/src/utils/job.rs b/src/bootstrap/src/utils/job.rs
index c5e892450c4c5..fb69d331d27e1 100644
--- a/src/bootstrap/src/utils/job.rs
+++ b/src/bootstrap/src/utils/job.rs
@@ -15,11 +15,12 @@ pub unsafe fn setup(build: &mut crate::Build) {
 ///
 /// Most of the time when you're running a build system (e.g., make) you expect
 /// Ctrl-C or abnormal termination to actually terminate the entire tree of
-/// process in play, not just the one at the top. This currently works "by
+/// processes in play. This currently works "by
 /// default" on Unix platforms because Ctrl-C actually sends a signal to the
-/// *process group* rather than the parent process, so everything will get torn
-/// down. On Windows, however, this does not happen and Ctrl-C just kills the
-/// parent process.
+/// *process group* so everything will get torn
+/// down. On Windows, however, Ctrl-C is only sent to processes in the same console.
+/// If a process is detached or attached to another console, it won't receive the
+/// signal.
 ///
 /// To achieve the same semantics on Windows we use Job Objects to ensure that
 /// all processes die at the same time. Job objects have a mode of operation
@@ -87,15 +88,7 @@ mod for_windows {
         );
         assert!(r.is_ok(), "{}", io::Error::last_os_error());
 
-        // Assign our process to this job object. Note that if this fails, one very
-        // likely reason is that we are ourselves already in a job object! This can
-        // happen on the build bots that we've got for Windows, or if just anyone
-        // else is instrumenting the build. In this case we just bail out
-        // immediately and assume that they take care of it.
-        //
-        // Also note that nested jobs (why this might fail) are supported in recent
-        // versions of Windows, but the version of Windows that our bots are running
-        // at least don't support nested job objects.
+        // Assign our process to this job object.
         let r = AssignProcessToJobObject(job, GetCurrentProcess());
         if r.is_err() {
             CloseHandle(job).ok();
@@ -124,14 +117,19 @@ mod for_windows {
                 // (only when wrongly setting the environmental variable),
                 // it might be better to improve the experience of the second case
                 // when users have interrupted the parent process and we haven't finish
-                // duplicating the handle yet. We just need close the job object if that occurs.
-                CloseHandle(job).ok();
+                // duplicating the handle yet.
                 return;
             }
         };
 
         let mut parent_handle = HANDLE::default();
-        let r = DuplicateHandle(
+        // If this fails, well at least we tried! An example of DuplicateHandle
+        // failing in the past has been when the wrong python2 package spawned this
+        // build system (e.g., the `python2` package in MSYS instead of
+        // `mingw-w64-x86_64-python2`). Not sure why it failed, but the "failure
+        // mode" here is that we only clean everything up when the build system
+        // dies, not when the python parent does, so not too bad.
+        let _ = DuplicateHandle(
             GetCurrentProcess(),
             job,
             parent,
@@ -140,15 +138,6 @@ mod for_windows {
             false,
             DUPLICATE_SAME_ACCESS,
         );
-
-        // If this failed, well at least we tried! An example of DuplicateHandle
-        // failing in the past has been when the wrong python2 package spawned this
-        // build system (e.g., the `python2` package in MSYS instead of
-        // `mingw-w64-x86_64-python2`). Not sure why it failed, but the "failure
-        // mode" here is that we only clean everything up when the build system
-        // dies, not when the python parent does, so not too bad.
-        if r.is_err() {
-            CloseHandle(job).ok();
-        }
+        CloseHandle(parent).ok();
     }
 }
diff --git a/src/ci/docker/host-x86_64/i686-gnu-nopt/Dockerfile b/src/ci/docker/host-x86_64/i686-gnu-nopt/Dockerfile
index e2b66c2cff1d8..e273672060732 100644
--- a/src/ci/docker/host-x86_64/i686-gnu-nopt/Dockerfile
+++ b/src/ci/docker/host-x86_64/i686-gnu-nopt/Dockerfile
@@ -27,5 +27,5 @@ RUN echo "[rust]" > /config/nopt-std-config.toml
 RUN echo "optimize = false" >> /config/nopt-std-config.toml
 
 ENV RUST_CONFIGURE_ARGS --build=i686-unknown-linux-gnu --disable-optimize-tests
-ENV SCRIPT python3 ../x.py test --stage 0 --config /config/nopt-std-config.toml library/std \
-  && python3 ../x.py --stage 2 test
+ARG SCRIPT_ARG
+ENV SCRIPT=${SCRIPT_ARG}
diff --git a/src/ci/docker/host-x86_64/i686-gnu/Dockerfile b/src/ci/docker/host-x86_64/i686-gnu/Dockerfile
index 61811c41904c4..dec25461bb4e8 100644
--- a/src/ci/docker/host-x86_64/i686-gnu/Dockerfile
+++ b/src/ci/docker/host-x86_64/i686-gnu/Dockerfile
@@ -24,10 +24,5 @@ COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
 ENV RUST_CONFIGURE_ARGS --build=i686-unknown-linux-gnu
-# Skip some tests that are unlikely to be platform specific, to speed up
-# this slow job.
-ENV SCRIPT python3 ../x.py --stage 2 test \
-  --skip src/bootstrap \
-  --skip tests/rustdoc-js \
-  --skip src/tools/error_index_generator \
-  --skip src/tools/linkchecker
+ARG SCRIPT_ARG
+ENV SCRIPT=${SCRIPT_ARG}
diff --git a/src/ci/docker/run.sh b/src/ci/docker/run.sh
index d554186df4cfe..a0adf60b6b2c9 100755
--- a/src/ci/docker/run.sh
+++ b/src/ci/docker/run.sh
@@ -105,6 +105,23 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
     # It seems that it cannot be the same as $IMAGE_TAG, otherwise it overwrites the cache
     CACHE_IMAGE_TAG=${REGISTRY}/${REGISTRY_USERNAME}/rust-ci-cache:${cksum}
 
+    # Docker build arguments.
+    build_args=(
+        "build"
+        "--rm"
+        "-t" "rust-ci"
+        "-f" "$dockerfile"
+        "$context"
+    )
+
+    # If the environment variable DOCKER_SCRIPT is defined,
+    # set the build argument SCRIPT_ARG to DOCKER_SCRIPT.
+    # In this way, we run the script defined in CI,
+    # instead of the one defined in the Dockerfile.
+    if [ -n "${DOCKER_SCRIPT+x}" ]; then
+      build_args+=("--build-arg" "SCRIPT_ARG=${DOCKER_SCRIPT}")
+    fi
+
     # On non-CI jobs, we try to download a pre-built image from the rust-lang-ci
     # ghcr.io registry. If it is not possible, we fall back to building the image
     # locally.
@@ -115,7 +132,7 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
             docker tag "${IMAGE_TAG}" rust-ci
         else
             echo "Building local Docker image"
-            retry docker build --rm -t rust-ci -f "$dockerfile" "$context"
+            retry docker "${build_args[@]}"
         fi
     # On PR CI jobs, we don't have permissions to write to the registry cache,
     # but we can still read from it.
@@ -127,13 +144,9 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
         # Build the image using registry caching backend
         retry docker \
           buildx \
-          build \
-          --rm \
-          -t rust-ci \
-          -f "$dockerfile" \
+          "${build_args[@]}" \
           --cache-from type=registry,ref=${CACHE_IMAGE_TAG} \
-          --output=type=docker \
-          "$context"
+          --output=type=docker
     # On auto/try builds, we can also write to the cache.
     else
         # Log into the Docker registry, so that we can read/write cache and the final image
@@ -147,14 +160,10 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
         # Build the image using registry caching backend
         retry docker \
           buildx \
-          build \
-          --rm \
-          -t rust-ci \
-          -f "$dockerfile" \
+          "${build_args[@]}" \
           --cache-from type=registry,ref=${CACHE_IMAGE_TAG} \
           --cache-to type=registry,ref=${CACHE_IMAGE_TAG},compression=zstd \
-          --output=type=docker \
-          "$context"
+          --output=type=docker
 
         # Print images for debugging purposes
         docker images
diff --git a/src/ci/docker/scripts/rfl-build.sh b/src/ci/docker/scripts/rfl-build.sh
index f07515f7784f4..8776e0f0be901 100755
--- a/src/ci/docker/scripts/rfl-build.sh
+++ b/src/ci/docker/scripts/rfl-build.sh
@@ -2,7 +2,7 @@
 
 set -euo pipefail
 
-LINUX_VERSION=28e848386b92645f93b9f2fdba5882c3ca7fb3e2
+LINUX_VERSION=v6.13-rc1
 
 # Build rustc, rustdoc, cargo, clippy-driver and rustfmt
 ../x.py build --stage 2 library rustdoc clippy rustfmt
@@ -64,7 +64,7 @@ make -C linux LLVM=1 -j$(($(nproc) + 1)) \
 
 BUILD_TARGETS="
     samples/rust/rust_minimal.o
-    samples/rust/rust_print.o
+    samples/rust/rust_print_main.o
     drivers/net/phy/ax88796b_rust.o
     rust/doctests_kernel_generated.o
 "
diff --git a/src/ci/github-actions/jobs.yml b/src/ci/github-actions/jobs.yml
index 2ea37c168dd3c..555a8f2448d3f 100644
--- a/src/ci/github-actions/jobs.yml
+++ b/src/ci/github-actions/jobs.yml
@@ -58,6 +58,22 @@ envs:
     NO_DEBUG_ASSERTIONS: 1
     NO_OVERFLOW_CHECKS: 1
 
+  # Different set of tests to run tests in parallel in multiple jobs.
+  stage_2_test_set1: &stage_2_test_set1
+    DOCKER_SCRIPT: >-
+      python3 ../x.py --stage 2 test
+      --skip compiler
+      --skip src
+
+  stage_2_test_set2: &stage_2_test_set2
+    DOCKER_SCRIPT: >-
+      python3 ../x.py --stage 2 test
+      --skip tests
+      --skip coverage-map
+      --skip coverage-run
+      --skip library
+      --skip tidyselftest
+
   production:
     &production
     DEPLOY_BUCKET: rust-lang-ci2
@@ -212,11 +228,42 @@ auto:
   - image: dist-x86_64-netbsd
     <<: *job-linux-4c
 
-  - image: i686-gnu
-    <<: *job-linux-8c
+  # The i686-gnu job is split into multiple jobs to run tests in parallel.
+  # i686-gnu-1 skips tests that run in i686-gnu-2.
+  - image: i686-gnu-1
+    env:
+      IMAGE: i686-gnu
+      <<: *stage_2_test_set1
+    <<: *job-linux-4c
 
-  - image: i686-gnu-nopt
-    <<: *job-linux-8c
+  # Skip tests that run in i686-gnu-1
+  - image: i686-gnu-2
+    env:
+      IMAGE: i686-gnu
+      <<: *stage_2_test_set2
+    <<: *job-linux-4c
+
+  # The i686-gnu-nopt job is split into multiple jobs to run tests in parallel.
+  # i686-gnu-nopt-1 skips tests that run in i686-gnu-nopt-2
+  - image: i686-gnu-nopt-1
+    env:
+      IMAGE: i686-gnu-nopt
+      <<: *stage_2_test_set1
+    <<: *job-linux-4c
+
+  # Skip tests that run in i686-gnu-nopt-1
+  - image: i686-gnu-nopt-2
+    env:
+      IMAGE: i686-gnu-nopt
+      DOCKER_SCRIPT: >-
+        python3 ../x.py test --stage 0 --config /config/nopt-std-config.toml library/std &&
+        python3 ../x.py --stage 2 test
+        --skip tests
+        --skip coverage-map
+        --skip coverage-run
+        --skip library
+        --skip tidyselftest
+    <<: *job-linux-4c
 
   - image: mingw-check
     <<: *job-linux-4c
@@ -373,11 +420,18 @@ auto:
   #  Windows Builders  #
   ######################
 
-  - image: x86_64-msvc
+  # x86_64-msvc is split into two jobs to run tests in parallel.
+  - image: x86_64-msvc-1
     env:
       RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-msvc --enable-profiler
-      SCRIPT: make ci-msvc
-    <<: *job-windows-8c
+      SCRIPT: make ci-msvc-py
+    <<: *job-windows
+
+  - image: x86_64-msvc-2
+    env:
+      RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-msvc --enable-profiler
+      SCRIPT: make ci-msvc-ps1
+    <<: *job-windows
 
   - image: i686-msvc
     env:
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 243cb3b2fc832..3b82d123276d9 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -344,15 +344,15 @@ target | std | host | notes
 [`powerpc-wrs-vxworks-spe`](platform-support/vxworks.md) | ✓ |  |
 [`powerpc-wrs-vxworks`](platform-support/vxworks.md) | ✓ |  |
 `powerpc64-unknown-freebsd` | ✓ | ✓ | PPC64 FreeBSD (ELFv1 and ELFv2)
-`powerpc64le-unknown-freebsd` |   |   | PPC64LE FreeBSD
-`powerpc-unknown-freebsd` |   |   | PowerPC FreeBSD
+`powerpc64le-unknown-freebsd` | ✓ | ✓ | PPC64LE FreeBSD
+`powerpc-unknown-freebsd` | ? |   | PowerPC FreeBSD
 `powerpc64-unknown-linux-musl` | ? |  | 64-bit PowerPC Linux with musl 1.2.3
 [`powerpc64-wrs-vxworks`](platform-support/vxworks.md) | ✓ |  |
 `powerpc64le-unknown-linux-musl` | ? |  | 64-bit PowerPC Linux with musl 1.2.3, Little Endian
 [`powerpc64-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | OpenBSD/powerpc64
 [`powerpc64-ibm-aix`](platform-support/aix.md) | ? |  | 64-bit AIX (7.2 and newer)
-`riscv32gc-unknown-linux-gnu` |   |   | RISC-V Linux (kernel 5.4, glibc 2.33)
-`riscv32gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 5.4, musl 1.2.3 + RISCV32 support patches)
+`riscv32gc-unknown-linux-gnu` | ✓ |   | RISC-V Linux (kernel 5.4, glibc 2.33)
+`riscv32gc-unknown-linux-musl` | ? |   | RISC-V Linux (kernel 5.4, musl 1.2.3 + RISCV32 support patches)
 [`riscv32im-risc0-zkvm-elf`](platform-support/riscv32im-risc0-zkvm-elf.md) | ? |  | RISC Zero's zero-knowledge Virtual Machine (RV32IM ISA)
 [`riscv32ima-unknown-none-elf`](platform-support/riscv32-unknown-none-elf.md) | * |  | Bare RISC-V (RV32IMA ISA)
 [`riscv32imac-unknown-xous-elf`](platform-support/riscv32imac-unknown-xous-elf.md) | ? |  | RISC-V Xous (RV32IMAC ISA)
@@ -361,13 +361,13 @@ target | std | host | notes
 [`riscv32imafc-esp-espidf`](platform-support/esp-idf.md) | ✓ |  | RISC-V ESP-IDF
 [`riscv32-wrs-vxworks`](platform-support/vxworks.md) | ✓ |  |
 [`riscv64gc-unknown-hermit`](platform-support/hermit.md) | ✓ |   | RISC-V Hermit
-`riscv64gc-unknown-freebsd` |   |   | RISC-V FreeBSD
-`riscv64gc-unknown-fuchsia` |   |   | RISC-V Fuchsia
+`riscv64gc-unknown-freebsd` | ? |   | RISC-V FreeBSD
+`riscv64gc-unknown-fuchsia` | ? |   | RISC-V Fuchsia
 [`riscv64gc-unknown-netbsd`](platform-support/netbsd.md) | ✓ | ✓ | RISC-V NetBSD
 [`riscv64gc-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | OpenBSD/riscv64
-[`riscv64-linux-android`](platform-support/android.md) |   |   | RISC-V 64-bit Android
+[`riscv64-linux-android`](platform-support/android.md) | ? |   | RISC-V 64-bit Android
 [`riscv64-wrs-vxworks`](platform-support/vxworks.md) | ✓ |  |
-[`s390x-unknown-linux-musl`](platform-support/s390x-unknown-linux-musl.md) |  |  | S390x Linux (kernel 3.2, musl 1.2.3)
+[`s390x-unknown-linux-musl`](platform-support/s390x-unknown-linux-musl.md) | ✓ |  | S390x Linux (kernel 3.2, musl 1.2.3)
 `sparc-unknown-linux-gnu` | ✓ |  | 32-bit SPARC Linux
 [`sparc-unknown-none-elf`](./platform-support/sparc-unknown-none-elf.md) | * |  | Bare 32-bit SPARC V7+
 [`sparc64-unknown-netbsd`](platform-support/netbsd.md) | ✓ | ✓ | NetBSD/sparc64
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index 8c747a1d93f61..1cf6b55accd26 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -22,7 +22,7 @@ use crate::common::{
     UI_STDERR, UI_STDOUT, UI_SVG, UI_WINDOWS_SVG, Ui, expected_output_path, incremental_dir,
     output_base_dir, output_base_name, output_testname_unique,
 };
-use crate::compute_diff::{write_diff, write_filtered_diff};
+use crate::compute_diff::{DiffLine, make_diff, write_diff, write_filtered_diff};
 use crate::errors::{self, Error, ErrorKind};
 use crate::header::TestProps;
 use crate::read2::{Truncated, read2_abbreviated};
@@ -2295,17 +2295,31 @@ impl<'test> TestCx<'test> {
         match output_kind {
             TestOutput::Compile => {
                 if !self.props.dont_check_compiler_stdout {
-                    errors +=
-                        self.compare_output(stdout_kind, &normalized_stdout, &expected_stdout);
+                    errors += self.compare_output(
+                        stdout_kind,
+                        &normalized_stdout,
+                        &proc_res.stdout,
+                        &expected_stdout,
+                    );
                 }
                 if !self.props.dont_check_compiler_stderr {
-                    errors +=
-                        self.compare_output(stderr_kind, &normalized_stderr, &expected_stderr);
+                    errors += self.compare_output(
+                        stderr_kind,
+                        &normalized_stderr,
+                        &stderr,
+                        &expected_stderr,
+                    );
                 }
             }
             TestOutput::Run => {
-                errors += self.compare_output(stdout_kind, &normalized_stdout, &expected_stdout);
-                errors += self.compare_output(stderr_kind, &normalized_stderr, &expected_stderr);
+                errors += self.compare_output(
+                    stdout_kind,
+                    &normalized_stdout,
+                    &proc_res.stdout,
+                    &expected_stdout,
+                );
+                errors +=
+                    self.compare_output(stderr_kind, &normalized_stderr, &stderr, &expected_stderr);
             }
         }
         errors
@@ -2533,7 +2547,13 @@ impl<'test> TestCx<'test> {
         }
     }
 
-    fn compare_output(&self, stream: &str, actual: &str, expected: &str) -> usize {
+    fn compare_output(
+        &self,
+        stream: &str,
+        actual: &str,
+        actual_unnormalized: &str,
+        expected: &str,
+    ) -> usize {
         let are_different = match (self.force_color_svg(), expected.find('\n'), actual.find('\n')) {
             // FIXME: We ignore the first line of SVG files
             // because the width parameter is non-deterministic.
@@ -2590,28 +2610,14 @@ impl<'test> TestCx<'test> {
             if expected.is_empty() {
                 eprintln!("normalized {}:\n{}\n", stream, actual);
             } else {
-                eprintln!("diff of {stream}:\n");
-                if let Some(diff_command) = self.config.diff_command.as_deref() {
-                    let mut args = diff_command.split_whitespace();
-                    let name = args.next().unwrap();
-                    match Command::new(name)
-                        .args(args)
-                        .args([&expected_path, &actual_path])
-                        .output()
-                    {
-                        Err(err) => {
-                            self.fatal(&format!(
-                                "failed to call custom diff command `{diff_command}`: {err}"
-                            ));
-                        }
-                        Ok(output) => {
-                            let output = String::from_utf8_lossy(&output.stdout);
-                            print!("{output}");
-                        }
-                    }
-                } else {
-                    print!("{}", write_diff(expected, actual, 3));
-                }
+                self.show_diff(
+                    stream,
+                    &expected_path,
+                    &actual_path,
+                    expected,
+                    actual,
+                    actual_unnormalized,
+                );
             }
         } else {
             // Delete non-revision .stderr/.stdout file if revisions are used.
@@ -2633,6 +2639,76 @@ impl<'test> TestCx<'test> {
         if self.config.bless { 0 } else { 1 }
     }
 
+    /// Returns whether to show the full stderr/stdout.
+    fn show_diff(
+        &self,
+        stream: &str,
+        expected_path: &Path,
+        actual_path: &Path,
+        expected: &str,
+        actual: &str,
+        actual_unnormalized: &str,
+    ) {
+        eprintln!("diff of {stream}:\n");
+        if let Some(diff_command) = self.config.diff_command.as_deref() {
+            let mut args = diff_command.split_whitespace();
+            let name = args.next().unwrap();
+            match Command::new(name).args(args).args([expected_path, actual_path]).output() {
+                Err(err) => {
+                    self.fatal(&format!(
+                        "failed to call custom diff command `{diff_command}`: {err}"
+                    ));
+                }
+                Ok(output) => {
+                    let output = String::from_utf8_lossy(&output.stdout);
+                    eprint!("{output}");
+                }
+            }
+        } else {
+            eprint!("{}", write_diff(expected, actual, 3));
+        }
+
+        // NOTE: argument order is important, we need `actual` to be on the left so the line number match up when we compare it to `actual_unnormalized` below.
+        let diff_results = make_diff(actual, expected, 0);
+
+        let (mut mismatches_normalized, mut mismatch_line_nos) = (String::new(), vec![]);
+        for hunk in diff_results {
+            let mut line_no = hunk.line_number;
+            for line in hunk.lines {
+                // NOTE: `Expected` is actually correct here, the argument order is reversed so our line numbers match up
+                if let DiffLine::Expected(normalized) = line {
+                    mismatches_normalized += &normalized;
+                    mismatches_normalized += "\n";
+                    mismatch_line_nos.push(line_no);
+                    line_no += 1;
+                }
+            }
+        }
+        let mut mismatches_unnormalized = String::new();
+        let diff_normalized = make_diff(actual, actual_unnormalized, 0);
+        for hunk in diff_normalized {
+            if mismatch_line_nos.contains(&hunk.line_number) {
+                for line in hunk.lines {
+                    if let DiffLine::Resulting(unnormalized) = line {
+                        mismatches_unnormalized += &unnormalized;
+                        mismatches_unnormalized += "\n";
+                    }
+                }
+            }
+        }
+
+        let normalized_diff = make_diff(&mismatches_normalized, &mismatches_unnormalized, 0);
+        // HACK: instead of checking if each hunk is empty, this only checks if the whole input is empty. we should be smarter about this so we don't treat added or removed output as normalized.
+        if !normalized_diff.is_empty()
+            && !mismatches_unnormalized.is_empty()
+            && !mismatches_normalized.is_empty()
+        {
+            eprintln!("Note: some mismatched output was normalized before being compared");
+            // FIXME: respect diff_command
+            eprint!("{}", write_diff(&mismatches_unnormalized, &mismatches_normalized, 0));
+        }
+    }
+
     fn check_and_prune_duplicate_outputs(
         &self,
         proc_res: &ProcRes,
diff --git a/src/tools/compiletest/src/runtest/coverage.rs b/src/tools/compiletest/src/runtest/coverage.rs
index 961a160298631..030ca5ebb2474 100644
--- a/src/tools/compiletest/src/runtest/coverage.rs
+++ b/src/tools/compiletest/src/runtest/coverage.rs
@@ -39,8 +39,12 @@ impl<'test> TestCx<'test> {
         let expected_coverage_dump = self.load_expected_output(kind);
         let actual_coverage_dump = self.normalize_output(&proc_res.stdout, &[]);
 
-        let coverage_dump_errors =
-            self.compare_output(kind, &actual_coverage_dump, &expected_coverage_dump);
+        let coverage_dump_errors = self.compare_output(
+            kind,
+            &actual_coverage_dump,
+            &proc_res.stdout,
+            &expected_coverage_dump,
+        );
 
         if coverage_dump_errors > 0 {
             self.fatal_proc_rec(
@@ -135,8 +139,12 @@ impl<'test> TestCx<'test> {
                 self.fatal_proc_rec(&err, &proc_res);
             });
 
-        let coverage_errors =
-            self.compare_output(kind, &normalized_actual_coverage, &expected_coverage);
+        let coverage_errors = self.compare_output(
+            kind,
+            &normalized_actual_coverage,
+            &proc_res.stdout,
+            &expected_coverage,
+        );
 
         if coverage_errors > 0 {
             self.fatal_proc_rec(
diff --git a/src/tools/compiletest/src/runtest/ui.rs b/src/tools/compiletest/src/runtest/ui.rs
index 414f1a6adb33d..bc860bf18c741 100644
--- a/src/tools/compiletest/src/runtest/ui.rs
+++ b/src/tools/compiletest/src/runtest/ui.rs
@@ -100,7 +100,7 @@ impl TestCx<'_> {
                 )
             });
 
-            errors += self.compare_output("fixed", &fixed_code, &expected_fixed);
+            errors += self.compare_output("fixed", &fixed_code, &fixed_code, &expected_fixed);
         } else if !expected_fixed.is_empty() {
             panic!(
                 "the `//@ run-rustfix` directive wasn't found but a `*.fixed` \
diff --git a/tests/codegen/naked-asan.rs b/tests/codegen/naked-asan.rs
index bcaa60baeffd8..ac6b0ffec74c3 100644
--- a/tests/codegen/naked-asan.rs
+++ b/tests/codegen/naked-asan.rs
@@ -8,14 +8,16 @@
 #![no_std]
 #![feature(abi_x86_interrupt, naked_functions)]
 
-// CHECK: define x86_intrcc void @page_fault_handler(ptr {{.*}}%0, i64 {{.*}}%1){{.*}}#[[ATTRS:[0-9]+]] {
+pub fn caller() {
+    page_fault_handler(1, 2);
+}
+
+// CHECK: declare x86_intrcc void @page_fault_handler(ptr {{.*}}, i64{{.*}}){{.*}}#[[ATTRS:[0-9]+]]
 // CHECK-NOT: memcpy
 #[naked]
 #[no_mangle]
 pub extern "x86-interrupt" fn page_fault_handler(_: u64, _: u64) {
-    unsafe {
-        core::arch::naked_asm!("ud2");
-    }
+    unsafe { core::arch::naked_asm!("ud2") };
 }
 
 // CHECK: #[[ATTRS]] =
diff --git a/tests/codegen/naked-fn/aligned.rs b/tests/codegen/naked-fn/aligned.rs
index 3bbd67981e5bf..d9dcd7f6c3ef7 100644
--- a/tests/codegen/naked-fn/aligned.rs
+++ b/tests/codegen/naked-fn/aligned.rs
@@ -6,15 +6,12 @@
 #![feature(naked_functions, fn_align)]
 use std::arch::naked_asm;
 
-// CHECK: Function Attrs: naked
-// CHECK-NEXT: define{{.*}}void @naked_empty()
-// CHECK: align 16
+// CHECK: .balign 16
+// CHECK-LABEL: naked_empty:
 #[repr(align(16))]
 #[no_mangle]
 #[naked]
 pub unsafe extern "C" fn naked_empty() {
-    // CHECK-NEXT: start:
-    // CHECK-NEXT: call void asm
-    // CHECK-NEXT: unreachable
+    // CHECK: ret
     naked_asm!("ret");
 }
diff --git a/tests/codegen/naked-fn/generics.rs b/tests/codegen/naked-fn/generics.rs
new file mode 100644
index 0000000000000..23c7766203b9f
--- /dev/null
+++ b/tests/codegen/naked-fn/generics.rs
@@ -0,0 +1,115 @@
+//@ compile-flags: -O
+//@ only-x86_64
+
+#![crate_type = "lib"]
+#![feature(naked_functions, asm_const)]
+
+use std::arch::naked_asm;
+
+#[no_mangle]
+fn test(x: u64) {
+    // just making sure these symbols get used
+    using_const_generics::<1>(x);
+    using_const_generics::<2>(x);
+
+    generic_function::<i64>(x as i64);
+
+    let foo = Foo(x);
+
+    foo.method();
+    foo.trait_method();
+}
+
+// CHECK: .balign 4
+// CHECK: add rax, 2
+// CHECK: add rax, 42
+
+// CHECK: .balign 4
+// CHECK: add rax, 1
+// CHECK: add rax, 42
+
+#[naked]
+pub extern "C" fn using_const_generics<const N: u64>(x: u64) -> u64 {
+    const M: u64 = 42;
+
+    unsafe {
+        naked_asm!(
+            "xor rax, rax",
+            "add rax, rdi",
+            "add rax, {}",
+            "add rax, {}",
+            "ret",
+            const N,
+            const M,
+        )
+    }
+}
+
+trait Invert {
+    fn invert(self) -> Self;
+}
+
+impl Invert for i64 {
+    fn invert(self) -> Self {
+        -1 * self
+    }
+}
+
+// CHECK-LABEL: generic_function
+// CHECK: .balign 4
+// CHECK: call
+// CHECK: ret
+
+#[naked]
+pub extern "C" fn generic_function<T: Invert>(x: i64) -> i64 {
+    unsafe {
+        naked_asm!(
+            "call {}",
+            "ret",
+            sym <T as Invert>::invert,
+        )
+    }
+}
+
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+struct Foo(u64);
+
+// CHECK-LABEL: method
+// CHECK: .balign 4
+// CHECK: mov rax, rdi
+
+impl Foo {
+    #[naked]
+    #[no_mangle]
+    extern "C" fn method(self) -> u64 {
+        unsafe { naked_asm!("mov rax, rdi", "ret") }
+    }
+}
+
+// CHECK-LABEL: trait_method
+// CHECK: .balign 4
+// CHECK: mov rax, rdi
+
+trait Bar {
+    extern "C" fn trait_method(self) -> u64;
+}
+
+impl Bar for Foo {
+    #[naked]
+    #[no_mangle]
+    extern "C" fn trait_method(self) -> u64 {
+        unsafe { naked_asm!("mov rax, rdi", "ret") }
+    }
+}
+
+// CHECK-LABEL: naked_with_args_and_return
+// CHECK: .balign 4
+// CHECK: lea rax, [rdi + rsi]
+
+// this previously ICE'd, see https://github.com/rust-lang/rust/issues/124375
+#[naked]
+#[no_mangle]
+pub unsafe extern "C" fn naked_with_args_and_return(a: isize, b: isize) -> isize {
+    naked_asm!("lea rax, [rdi + rsi]", "ret");
+}
diff --git a/tests/codegen/naked-fn/instruction-set.rs b/tests/codegen/naked-fn/instruction-set.rs
new file mode 100644
index 0000000000000..5b790b2034c09
--- /dev/null
+++ b/tests/codegen/naked-fn/instruction-set.rs
@@ -0,0 +1,59 @@
+//@ revisions: arm-mode thumb-mode
+//@ [arm-mode] compile-flags: --target armv5te-none-eabi
+//@ [thumb-mode] compile-flags: --target thumbv5te-none-eabi
+//@ [arm-mode] needs-llvm-components: arm
+//@ [thumb-mode] needs-llvm-components: arm
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items, rustc_attrs, naked_functions)]
+#![no_core]
+
+#[rustc_builtin_macro]
+macro_rules! naked_asm {
+    () => {};
+}
+
+#[lang = "sized"]
+trait Sized {}
+#[lang = "copy"]
+trait Copy {}
+
+// arm-mode: .arm
+// thumb-mode: .thumb
+// CHECK-LABEL: test_unspecified:
+// CHECK: bx lr
+// CHECK: .popsection
+// arm-mode: .arm
+// thumb-mode: .thumb
+#[no_mangle]
+#[naked]
+unsafe extern "C" fn test_unspecified() {
+    naked_asm!("bx lr");
+}
+
+// CHECK: .thumb
+// CHECK: .thumb_func
+// CHECK-LABEL: test_thumb:
+// CHECK: bx lr
+// CHECK: .popsection
+// arm-mode: .arm
+// thumb-mode: .thumb
+#[no_mangle]
+#[naked]
+#[instruction_set(arm::t32)]
+unsafe extern "C" fn test_thumb() {
+    naked_asm!("bx lr");
+}
+
+// CHECK: .arm
+// CHECK-LABEL: test_arm:
+// CHECK: bx lr
+// CHECK: .popsection
+// arm-mode: .arm
+// thumb-mode: .thumb
+#[no_mangle]
+#[naked]
+#[instruction_set(arm::a32)]
+unsafe extern "C" fn test_arm() {
+    naked_asm!("bx lr");
+}
diff --git a/tests/codegen/naked-fn/naked-functions.rs b/tests/codegen/naked-fn/naked-functions.rs
index 3f7447af599ab..f505d27d48c62 100644
--- a/tests/codegen/naked-fn/naked-functions.rs
+++ b/tests/codegen/naked-fn/naked-functions.rs
@@ -1,29 +1,147 @@
-//@ compile-flags: -C no-prepopulate-passes -Copt-level=0
-//@ needs-asm-support
-//@ only-x86_64
+//@ revisions: linux win macos thumb
+//
+//@[linux] compile-flags: --target x86_64-unknown-linux-gnu
+//@[linux] needs-llvm-components: x86
+//@[win] compile-flags: --target x86_64-pc-windows-gnu
+//@[win] needs-llvm-components: x86
+//@[macos] compile-flags: --target aarch64-apple-darwin
+//@[macos] needs-llvm-components: arm
+//@[thumb] compile-flags: --target thumbv7em-none-eabi
+//@[thumb] needs-llvm-components: arm
 
 #![crate_type = "lib"]
-#![feature(naked_functions)]
-use std::arch::naked_asm;
+#![feature(no_core, lang_items, rustc_attrs, naked_functions)]
+#![no_core]
+
+#[rustc_builtin_macro]
+macro_rules! naked_asm {
+    () => {};
+}
+
+#[lang = "sized"]
+trait Sized {}
+#[lang = "copy"]
+trait Copy {}
+
+// linux,win: .intel_syntax
+//
+// linux:   .pushsection .text.naked_empty,\22ax\22, @progbits
+// macos:   .pushsection __TEXT,__text,regular,pure_instructions
+// win: .pushsection .text.naked_empty,\22xr\22
+// thumb:   .pushsection .text.naked_empty,\22ax\22, %progbits
+//
+// CHECK: .balign 4
+//
+// linux,win,thumb: .globl naked_empty
+// macos: .globl _naked_empty
+//
+// CHECK-NOT: .private_extern
+// CHECK-NOT: .hidden
+//
+// linux: .type naked_empty, @function
+//
+// win: .def naked_empty
+// win: .scl 2
+// win: .type 32
+// win: .endef naked_empty
+//
+// thumb: .type naked_empty, %function
+// thumb: .thumb
+// thumb: .thumb_func
+//
+// CHECK-LABEL: naked_empty:
+//
+// linux,macos,win: ret
+// thumb: bx lr
+//
+// CHECK: .popsection
+//
+// thumb: .thumb
+//
+// linux,win: .att_syntax
 
-// CHECK: Function Attrs: naked
-// CHECK-NEXT: define{{.*}}void @naked_empty()
 #[no_mangle]
 #[naked]
 pub unsafe extern "C" fn naked_empty() {
-    // CHECK-NEXT: {{.+}}:
-    // CHECK-NEXT: call void asm
-    // CHECK-NEXT: unreachable
+    #[cfg(not(all(target_arch = "arm", target_feature = "thumb-mode")))]
     naked_asm!("ret");
+
+    #[cfg(all(target_arch = "arm", target_feature = "thumb-mode"))]
+    naked_asm!("bx lr");
 }
 
-// CHECK: Function Attrs: naked
-// CHECK-NEXT: define{{.*}}i{{[0-9]+}} @naked_with_args_and_return(i64 %0, i64 %1)
+// linux,win: .intel_syntax
+//
+// linux:   .pushsection .text.naked_with_args_and_return,\22ax\22, @progbits
+// macos:   .pushsection __TEXT,__text,regular,pure_instructions
+// win: .pushsection .text.naked_with_args_and_return,\22xr\22
+// thumb:   .pushsection .text.naked_with_args_and_return,\22ax\22, %progbits
+//
+// CHECK: .balign 4
+//
+// linux,win,thumb: .globl naked_with_args_and_return
+// macos: .globl _naked_with_args_and_return
+//
+// CHECK-NOT: .private_extern
+// CHECK-NOT: .hidden
+//
+// linux: .type naked_with_args_and_return, @function
+//
+// win: .def naked_with_args_and_return
+// win: .scl 2
+// win: .type 32
+// win: .endef naked_with_args_and_return
+//
+// thumb: .type naked_with_args_and_return, %function
+// thumb: .thumb
+// thumb: .thumb_func
+//
+// CHECK-LABEL: naked_with_args_and_return:
+//
+// linux, win: lea rax, [rdi + rsi]
+// macos: add x0, x0, x1
+// thumb: adds r0, r0, r1
+//
+// linux,macos,win: ret
+// thumb: bx lr
+//
+// CHECK: .popsection
+//
+// thumb: .thumb
+//
+// linux,win: .att_syntax
+
 #[no_mangle]
 #[naked]
 pub unsafe extern "C" fn naked_with_args_and_return(a: isize, b: isize) -> isize {
-    // CHECK-NEXT: {{.+}}:
-    // CHECK-NEXT: call void asm
-    // CHECK-NEXT: unreachable
-    naked_asm!("lea rax, [rdi + rsi]", "ret");
+    #[cfg(any(target_os = "windows", target_os = "linux"))]
+    {
+        naked_asm!("lea rax, [rdi + rsi]", "ret")
+    }
+
+    #[cfg(target_os = "macos")]
+    {
+        naked_asm!("add x0, x0, x1", "ret")
+    }
+
+    #[cfg(all(target_arch = "arm", target_feature = "thumb-mode"))]
+    {
+        naked_asm!("adds r0, r0, r1", "bx lr")
+    }
+}
+
+// linux:   .pushsection .text.some_different_name,\22ax\22, @progbits
+// macos:   .pushsection .text.some_different_name,regular,pure_instructions
+// win: .pushsection .text.some_different_name,\22xr\22
+// thumb:   .pushsection .text.some_different_name,\22ax\22, %progbits
+// CHECK-LABEL: test_link_section:
+#[no_mangle]
+#[naked]
+#[link_section = ".text.some_different_name"]
+pub unsafe extern "C" fn test_link_section() {
+    #[cfg(not(all(target_arch = "arm", target_feature = "thumb-mode")))]
+    naked_asm!("ret");
+
+    #[cfg(all(target_arch = "arm", target_feature = "thumb-mode"))]
+    naked_asm!("bx lr");
 }
diff --git a/tests/codegen/naked-fn/naked-nocoverage.rs b/tests/codegen/naked-fn/naked-nocoverage.rs
deleted file mode 100644
index f63661bcd3a7a..0000000000000
--- a/tests/codegen/naked-fn/naked-nocoverage.rs
+++ /dev/null
@@ -1,19 +0,0 @@
-// Checks that naked functions are not instrumented by -Cinstrument-coverage.
-// Regression test for issue #105170.
-//
-//@ needs-asm-support
-//@ compile-flags: -Zno-profiler-runtime
-//@ compile-flags: -Cinstrument-coverage
-#![crate_type = "lib"]
-#![feature(naked_functions)]
-use std::arch::naked_asm;
-
-#[naked]
-#[no_mangle]
-pub unsafe extern "C" fn f() {
-    // CHECK:       define {{(dso_local )?}}void @f()
-    // CHECK-NEXT:  start:
-    // CHECK-NEXT:    call void asm
-    // CHECK-NEXT:    unreachable
-    naked_asm!("");
-}
diff --git a/tests/codegen/naked-fn/naked-noinline.rs b/tests/codegen/naked-fn/naked-noinline.rs
deleted file mode 100644
index 6ea36d9678315..0000000000000
--- a/tests/codegen/naked-fn/naked-noinline.rs
+++ /dev/null
@@ -1,31 +0,0 @@
-// Checks that naked functions are never inlined.
-//@ compile-flags: -O -Zmir-opt-level=3
-//@ needs-asm-support
-//@ ignore-wasm32
-#![crate_type = "lib"]
-#![feature(naked_functions)]
-
-use std::arch::naked_asm;
-
-#[naked]
-#[no_mangle]
-pub unsafe extern "C" fn f() {
-    // Check that f has naked and noinline attributes.
-    //
-    // CHECK:       define {{(dso_local )?}}void @f() unnamed_addr [[ATTR:#[0-9]+]]
-    // CHECK-NEXT:  start:
-    // CHECK-NEXT:    call void asm
-    naked_asm!("");
-}
-
-#[no_mangle]
-pub unsafe fn g() {
-    // Check that call to f is not inlined.
-    //
-    // CHECK-LABEL: define {{(dso_local )?}}void @g()
-    // CHECK-NEXT:  start:
-    // CHECK-NEXT:    call void @f()
-    f();
-}
-
-// CHECK: attributes [[ATTR]] = { naked{{.*}}noinline{{.*}} }
diff --git a/tests/crashes/124375.rs b/tests/crashes/124375.rs
deleted file mode 100644
index 1d877caeb8bc1..0000000000000
--- a/tests/crashes/124375.rs
+++ /dev/null
@@ -1,11 +0,0 @@
-//@ known-bug: #124375
-//@ compile-flags: -Zmir-opt-level=0
-//@ only-x86_64
-#![crate_type = "lib"]
-#![feature(naked_functions)]
-use std::arch::naked_asm;
-
-#[naked]
-pub unsafe extern "C" fn naked_with_args_and_return(a: isize, b: isize) -> isize {
-    naked_asm!("lea rax, [rdi + rsi]", "ret");
-}
diff --git a/tests/ui/asm/naked-functions-instruction-set.rs b/tests/ui/asm/naked-functions-instruction-set.rs
index 37c7b52c191cd..3a6e7a46ce5a7 100644
--- a/tests/ui/asm/naked-functions-instruction-set.rs
+++ b/tests/ui/asm/naked-functions-instruction-set.rs
@@ -24,7 +24,7 @@ unsafe extern "C" fn test_thumb() {
 
 #[no_mangle]
 #[naked]
-#[instruction_set(arm::t32)]
+#[instruction_set(arm::a32)]
 unsafe extern "C" fn test_arm() {
     naked_asm!("bx lr");
 }
diff --git a/tests/ui/asm/naked-functions.rs b/tests/ui/asm/naked-functions.rs
index 5c58f1498cc97..586645edac969 100644
--- a/tests/ui/asm/naked-functions.rs
+++ b/tests/ui/asm/naked-functions.rs
@@ -183,6 +183,12 @@ pub unsafe extern "C" fn invalid_asm_syntax(a: u32) -> u32 {
     //~^ ERROR asm template must be a string literal
 }
 
+// this previously ICE'd, see https://github.com/rust-lang/rust/issues/124375
+#[naked]
+pub unsafe extern "C" fn naked_with_args_and_return(a: isize, b: isize) -> isize {
+    naked_asm!("lea rax, [rdi + rsi]", "ret");
+}
+
 #[cfg(target_arch = "x86_64")]
 #[cfg_attr(target_pointer_width = "64", no_mangle)]
 #[naked]
diff --git a/tests/ui/explicit-tail-calls/become-macro.rs b/tests/ui/explicit-tail-calls/become-macro.rs
new file mode 100644
index 0000000000000..0a9d069ceb3cb
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-macro.rs
@@ -0,0 +1,13 @@
+//@ check-pass
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls, decl_macro)]
+
+macro call($f:expr $(, $args:expr)* $(,)?) {
+    ($f)($($args),*)
+}
+
+fn main() {
+    become call!(f);
+}
+
+fn f() {}
diff --git a/tests/ui/explicit-tail-calls/become-operator.fixed b/tests/ui/explicit-tail-calls/become-operator.fixed
new file mode 100644
index 0000000000000..24baef42d69a3
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-operator.fixed
@@ -0,0 +1,42 @@
+//@ run-rustfix
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+#![allow(unused)]
+use std::num::Wrapping;
+use std::ops::{Not, Add, BitXorAssign};
+
+// built-ins and overloaded operators are handled differently
+
+fn f(a: u64, b: u64) -> u64 {
+    return a + b; //~ error: `become` does not support operators
+}
+
+fn g(a: String, b: &str) -> String {
+    become (a).add(b); //~ error: `become` does not support operators
+}
+
+fn h(x: u64) -> u64 {
+    return !x; //~ error: `become` does not support operators
+}
+
+fn i_do_not_know_any_more_letters(x: Wrapping<u32>) -> Wrapping<u32> {
+    become (x).not(); //~ error: `become` does not support operators
+}
+
+fn builtin_index(x: &[u8], i: usize) -> u8 {
+    return x[i] //~ error: `become` does not support operators
+}
+
+// FIXME(explicit_tail_calls): overloaded index is represented like `[&]*x.index(i)`,
+//                             and so need additional handling
+
+fn a(a: &mut u8, _: u8) {
+    return *a ^= 1; //~ error: `become` does not support operators
+}
+
+fn b(b: &mut Wrapping<u8>, _: u8) {
+    become (*b).bitxor_assign(1); //~ error: `become` does not support operators
+}
+
+
+fn main() {}
diff --git a/tests/ui/explicit-tail-calls/become-operator.rs b/tests/ui/explicit-tail-calls/become-operator.rs
new file mode 100644
index 0000000000000..e547e979b6485
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-operator.rs
@@ -0,0 +1,42 @@
+//@ run-rustfix
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+#![allow(unused)]
+use std::num::Wrapping;
+use std::ops::{Not, Add, BitXorAssign};
+
+// built-ins and overloaded operators are handled differently
+
+fn f(a: u64, b: u64) -> u64 {
+    become a + b; //~ error: `become` does not support operators
+}
+
+fn g(a: String, b: &str) -> String {
+    become a + b; //~ error: `become` does not support operators
+}
+
+fn h(x: u64) -> u64 {
+    become !x; //~ error: `become` does not support operators
+}
+
+fn i_do_not_know_any_more_letters(x: Wrapping<u32>) -> Wrapping<u32> {
+    become !x; //~ error: `become` does not support operators
+}
+
+fn builtin_index(x: &[u8], i: usize) -> u8 {
+    become x[i] //~ error: `become` does not support operators
+}
+
+// FIXME(explicit_tail_calls): overloaded index is represented like `[&]*x.index(i)`,
+//                             and so need additional handling
+
+fn a(a: &mut u8, _: u8) {
+    become *a ^= 1; //~ error: `become` does not support operators
+}
+
+fn b(b: &mut Wrapping<u8>, _: u8) {
+    become *b ^= 1; //~ error: `become` does not support operators
+}
+
+
+fn main() {}
diff --git a/tests/ui/explicit-tail-calls/become-operator.stderr b/tests/ui/explicit-tail-calls/become-operator.stderr
new file mode 100644
index 0000000000000..26e4343faaea7
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-operator.stderr
@@ -0,0 +1,75 @@
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:11:12
+   |
+LL |     become a + b;
+   |     -------^^^^^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+   = note: using `become` on a builtin operator is not useful
+
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:15:12
+   |
+LL |     become a + b;
+   |            ^^^^^
+   |
+help: try using the method directly
+   |
+LL |     become (a).add(b);
+   |            + ~~~~~~ +
+
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:19:12
+   |
+LL |     become !x;
+   |     -------^^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+   = note: using `become` on a builtin operator is not useful
+
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:23:12
+   |
+LL |     become !x;
+   |            ^^
+   |
+help: try using the method directly
+   |
+LL |     become (x).not();
+   |            ~ +++++++
+
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:27:12
+   |
+LL |     become x[i]
+   |     -------^^^^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+   = note: using `become` on a builtin operator is not useful
+
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:34:12
+   |
+LL |     become *a ^= 1;
+   |     -------^^^^^^^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+   = note: using `become` on a builtin operator is not useful
+
+error: `become` does not support operators
+  --> $DIR/become-operator.rs:38:12
+   |
+LL |     become *b ^= 1;
+   |            ^^^^^^^
+   |
+help: try using the method directly
+   |
+LL |     become (*b).bitxor_assign(1);
+   |            +  ~~~~~~~~~~~~~~~~ +
+
+error: aborting due to 7 previous errors
+
diff --git a/tests/ui/explicit-tail-calls/become-outside.rs b/tests/ui/explicit-tail-calls/become-outside.rs
index 9c90d929111e1..29738bb9a299f 100644
--- a/tests/ui/explicit-tail-calls/become-outside.rs
+++ b/tests/ui/explicit-tail-calls/become-outside.rs
@@ -1,5 +1,5 @@
 //@ revisions: constant array
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 #[cfg(constant)]
diff --git a/tests/ui/explicit-tail-calls/become-uncallable.fixed b/tests/ui/explicit-tail-calls/become-uncallable.fixed
new file mode 100644
index 0000000000000..b77c46ea43549
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-uncallable.fixed
@@ -0,0 +1,18 @@
+//@ run-rustfix
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+#![allow(unused)]
+
+fn f() -> u64 {
+    return 1; //~ error: `become` requires a function call
+}
+
+fn g() {
+    return { h() }; //~ error: `become` requires a function call
+}
+
+fn h() {
+    return *&g(); //~ error: `become` requires a function call
+}
+
+fn main() {}
diff --git a/tests/ui/explicit-tail-calls/become-uncallable.rs b/tests/ui/explicit-tail-calls/become-uncallable.rs
new file mode 100644
index 0000000000000..a73b9258aaabb
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-uncallable.rs
@@ -0,0 +1,18 @@
+//@ run-rustfix
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+#![allow(unused)]
+
+fn f() -> u64 {
+    become 1; //~ error: `become` requires a function call
+}
+
+fn g() {
+    become { h() }; //~ error: `become` requires a function call
+}
+
+fn h() {
+    become *&g(); //~ error: `become` requires a function call
+}
+
+fn main() {}
diff --git a/tests/ui/explicit-tail-calls/become-uncallable.stderr b/tests/ui/explicit-tail-calls/become-uncallable.stderr
new file mode 100644
index 0000000000000..90f10b05d2afc
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/become-uncallable.stderr
@@ -0,0 +1,44 @@
+error: `become` requires a function call
+  --> $DIR/become-uncallable.rs:7:12
+   |
+LL |     become 1;
+   |     -------^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+note: not a function call
+  --> $DIR/become-uncallable.rs:7:12
+   |
+LL |     become 1;
+   |            ^
+
+error: `become` requires a function call
+  --> $DIR/become-uncallable.rs:11:12
+   |
+LL |     become { h() };
+   |     -------^^^^^^^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+note: not a function call
+  --> $DIR/become-uncallable.rs:11:12
+   |
+LL |     become { h() };
+   |            ^^^^^^^
+
+error: `become` requires a function call
+  --> $DIR/become-uncallable.rs:15:12
+   |
+LL |     become *&g();
+   |     -------^^^^^
+   |     |
+   |     help: try using `return` instead: `return`
+   |
+note: not a function call
+  --> $DIR/become-uncallable.rs:15:12
+   |
+LL |     become *&g();
+   |            ^^^^^
+
+error: aborting due to 3 previous errors
+
diff --git a/tests/ui/explicit-tail-calls/closure.fixed b/tests/ui/explicit-tail-calls/closure.fixed
new file mode 100644
index 0000000000000..4af71c5d4cc95
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/closure.fixed
@@ -0,0 +1,31 @@
+//@ run-rustfix
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn a() {
+    become ((|| ()) as fn() -> _)();
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn aa((): ()) {
+    become ((|()| ()) as fn(_) -> _)(());
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn aaa((): (), _: i32) {
+    become ((|(), _| ()) as fn(_, _) -> _)((), 1);
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn v((): (), ((), ()): ((), ())) -> (((), ()), ()) {
+    let f = |(), ((), ())| (((), ()), ());
+    become (f as fn(_, _) -> _)((), ((), ()));
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn main() {
+    a();
+    aa(());
+    aaa((), 1);
+    v((), ((), ()));
+}
diff --git a/tests/ui/explicit-tail-calls/closure.rs b/tests/ui/explicit-tail-calls/closure.rs
new file mode 100644
index 0000000000000..79a1b5296b0ae
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/closure.rs
@@ -0,0 +1,31 @@
+//@ run-rustfix
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn a() {
+    become (|| ())();
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn aa((): ()) {
+    become (|()| ())(());
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn aaa((): (), _: i32) {
+    become (|(), _| ())((), 1);
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn v((): (), ((), ()): ((), ())) -> (((), ()), ()) {
+    let f = |(), ((), ())| (((), ()), ());
+    become f((), ((), ()));
+    //~^ ERROR: tail calling closures directly is not allowed
+}
+
+fn main() {
+    a();
+    aa(());
+    aaa((), 1);
+    v((), ((), ()));
+}
diff --git a/tests/ui/explicit-tail-calls/closure.stderr b/tests/ui/explicit-tail-calls/closure.stderr
new file mode 100644
index 0000000000000..5d57bece68e2c
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/closure.stderr
@@ -0,0 +1,46 @@
+error: tail calling closures directly is not allowed
+  --> $DIR/closure.rs:6:5
+   |
+LL |     become (|| ())();
+   |     ^^^^^^^^^^^^^^^^
+   |
+help: try casting the closure to a function pointer type
+   |
+LL |     become ((|| ()) as fn() -> _)();
+   |            +        +++++++++++++
+
+error: tail calling closures directly is not allowed
+  --> $DIR/closure.rs:11:5
+   |
+LL |     become (|()| ())(());
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+help: try casting the closure to a function pointer type
+   |
+LL |     become ((|()| ()) as fn(_) -> _)(());
+   |            +          ++++++++++++++
+
+error: tail calling closures directly is not allowed
+  --> $DIR/closure.rs:16:5
+   |
+LL |     become (|(), _| ())((), 1);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try casting the closure to a function pointer type
+   |
+LL |     become ((|(), _| ()) as fn(_, _) -> _)((), 1);
+   |            +             +++++++++++++++++
+
+error: tail calling closures directly is not allowed
+  --> $DIR/closure.rs:22:5
+   |
+LL |     become f((), ((), ()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try casting the closure to a function pointer type
+   |
+LL |     become (f as fn(_, _) -> _)((), ((), ()));
+   |            +  +++++++++++++++++
+
+error: aborting due to 4 previous errors
+
diff --git a/tests/ui/explicit-tail-calls/constck.rs b/tests/ui/explicit-tail-calls/constck.rs
index 938f15f12c091..36fc3ef6f995b 100644
--- a/tests/ui/explicit-tail-calls/constck.rs
+++ b/tests/ui/explicit-tail-calls/constck.rs
@@ -1,4 +1,4 @@
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 const fn f() {
diff --git a/tests/ui/explicit-tail-calls/ctfe-arg-bad-borrow.rs b/tests/ui/explicit-tail-calls/ctfe-arg-bad-borrow.rs
index 5a105ee4eb59d..0a61c90bd00a7 100644
--- a/tests/ui/explicit-tail-calls/ctfe-arg-bad-borrow.rs
+++ b/tests/ui/explicit-tail-calls/ctfe-arg-bad-borrow.rs
@@ -1,4 +1,4 @@
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 pub const fn test(_: &Type) {
diff --git a/tests/ui/explicit-tail-calls/ctfe-arg-good-borrow.rs b/tests/ui/explicit-tail-calls/ctfe-arg-good-borrow.rs
index 50bf6c946ca8f..a34482a352725 100644
--- a/tests/ui/explicit-tail-calls/ctfe-arg-good-borrow.rs
+++ b/tests/ui/explicit-tail-calls/ctfe-arg-good-borrow.rs
@@ -1,5 +1,5 @@
 //@ check-pass
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 pub const fn test(x: &Type) {
diff --git a/tests/ui/explicit-tail-calls/ctfe-arg-move.rs b/tests/ui/explicit-tail-calls/ctfe-arg-move.rs
index 88ff3a4a5ad13..610eb74a91e37 100644
--- a/tests/ui/explicit-tail-calls/ctfe-arg-move.rs
+++ b/tests/ui/explicit-tail-calls/ctfe-arg-move.rs
@@ -1,5 +1,5 @@
 //@ check-pass
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 pub const fn test(s: String) -> String {
diff --git a/tests/ui/explicit-tail-calls/ctfe-collatz-multi-rec.rs b/tests/ui/explicit-tail-calls/ctfe-collatz-multi-rec.rs
index 86041b669601d..53d46d92bd7b7 100644
--- a/tests/ui/explicit-tail-calls/ctfe-collatz-multi-rec.rs
+++ b/tests/ui/explicit-tail-calls/ctfe-collatz-multi-rec.rs
@@ -1,5 +1,5 @@
 //@ run-pass
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 /// A very unnecessarily complicated "implementation" of the Collatz conjecture.
diff --git a/tests/ui/explicit-tail-calls/ctfe-id-unlimited.rs b/tests/ui/explicit-tail-calls/ctfe-id-unlimited.rs
index 54e68b2b7f793..2a04d4893e68a 100644
--- a/tests/ui/explicit-tail-calls/ctfe-id-unlimited.rs
+++ b/tests/ui/explicit-tail-calls/ctfe-id-unlimited.rs
@@ -1,6 +1,6 @@
 //@ revisions: become return
 //@ [become] run-pass
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 // This is an identity function (`|x| x`), but implemented using recursion.
diff --git a/tests/ui/explicit-tail-calls/ctfe-tail-call-panic.rs b/tests/ui/explicit-tail-calls/ctfe-tail-call-panic.rs
index 3d69cde29895a..fba4a2692afe3 100644
--- a/tests/ui/explicit-tail-calls/ctfe-tail-call-panic.rs
+++ b/tests/ui/explicit-tail-calls/ctfe-tail-call-panic.rs
@@ -1,4 +1,4 @@
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 pub const fn f() {
diff --git a/tests/ui/explicit-tail-calls/drop-order.rs b/tests/ui/explicit-tail-calls/drop-order.rs
index e20730446ec55..242336be4845e 100644
--- a/tests/ui/explicit-tail-calls/drop-order.rs
+++ b/tests/ui/explicit-tail-calls/drop-order.rs
@@ -1,7 +1,7 @@
 // FIXME(explicit_tail_calls): enable this test once rustc_codegen_ssa supports tail calls
 //@ ignore-test: tail calls are not implemented in rustc_codegen_ssa yet, so this causes 🧊
 //@ run-pass
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 use std::cell::RefCell;
 
diff --git a/tests/ui/explicit-tail-calls/in-closure.rs b/tests/ui/explicit-tail-calls/in-closure.rs
new file mode 100644
index 0000000000000..b23ff5a1e05b4
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/in-closure.rs
@@ -0,0 +1,8 @@
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn main() {
+    || become f(); //~ error: `become` is not allowed in closures
+}
+
+fn f() {}
diff --git a/tests/ui/explicit-tail-calls/in-closure.stderr b/tests/ui/explicit-tail-calls/in-closure.stderr
new file mode 100644
index 0000000000000..d221f732e938b
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/in-closure.stderr
@@ -0,0 +1,8 @@
+error: `become` is not allowed in closures
+  --> $DIR/in-closure.rs:5:8
+   |
+LL |     || become f();
+   |        ^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/explicit-tail-calls/return-lifetime-sub.rs b/tests/ui/explicit-tail-calls/return-lifetime-sub.rs
index 1243fba9b5883..df0982cc02bdc 100644
--- a/tests/ui/explicit-tail-calls/return-lifetime-sub.rs
+++ b/tests/ui/explicit-tail-calls/return-lifetime-sub.rs
@@ -1,5 +1,5 @@
 //@ check-pass
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 fn _f<'a>() -> &'a [u8] {
diff --git a/tests/ui/explicit-tail-calls/return-mismatches.rs b/tests/ui/explicit-tail-calls/return-mismatches.rs
index 935a1a1d28b02..e48766b89b00c 100644
--- a/tests/ui/explicit-tail-calls/return-mismatches.rs
+++ b/tests/ui/explicit-tail-calls/return-mismatches.rs
@@ -1,4 +1,4 @@
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 fn _f0<'a>() -> &'static [u8] {
diff --git a/tests/ui/explicit-tail-calls/signature-mismatch.rs b/tests/ui/explicit-tail-calls/signature-mismatch.rs
new file mode 100644
index 0000000000000..a32ac9d8bfee3
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/signature-mismatch.rs
@@ -0,0 +1,33 @@
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+#![feature(c_variadic)]
+
+fn _f0((): ()) {
+    become _g0(); //~ error: mismatched signatures
+}
+
+fn _g0() {}
+
+
+fn _f1() {
+    become _g1(()); //~ error: mismatched signatures
+}
+
+fn _g1((): ()) {}
+
+
+extern "C" fn _f2() {
+    become _g2(); //~ error: mismatched function ABIs
+}
+
+fn _g2() {}
+
+
+fn _f3() {
+    become _g3(); //~ error: mismatched function ABIs
+}
+
+extern "C" fn _g3() {}
+
+
+fn main() {}
diff --git a/tests/ui/explicit-tail-calls/signature-mismatch.stderr b/tests/ui/explicit-tail-calls/signature-mismatch.stderr
new file mode 100644
index 0000000000000..ba9e9dcb98483
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/signature-mismatch.stderr
@@ -0,0 +1,40 @@
+error: mismatched signatures
+  --> $DIR/signature-mismatch.rs:6:5
+   |
+LL |     become _g0();
+   |     ^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have matching signatures
+   = note: caller signature: `fn(())`
+   = note: callee signature: `fn()`
+
+error: mismatched signatures
+  --> $DIR/signature-mismatch.rs:13:5
+   |
+LL |     become _g1(());
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have matching signatures
+   = note: caller signature: `fn()`
+   = note: callee signature: `fn(())`
+
+error: mismatched function ABIs
+  --> $DIR/signature-mismatch.rs:20:5
+   |
+LL |     become _g2();
+   |     ^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have the same ABI
+   = note: caller ABI is `"C"`, while callee ABI is `"Rust"`
+
+error: mismatched function ABIs
+  --> $DIR/signature-mismatch.rs:27:5
+   |
+LL |     become _g3();
+   |     ^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have the same ABI
+   = note: caller ABI is `"Rust"`, while callee ABI is `"C"`
+
+error: aborting due to 4 previous errors
+
diff --git a/tests/ui/explicit-tail-calls/unsafeck.rs b/tests/ui/explicit-tail-calls/unsafeck.rs
index 872a70ca3a0a7..f8a8140cedc33 100644
--- a/tests/ui/explicit-tail-calls/unsafeck.rs
+++ b/tests/ui/explicit-tail-calls/unsafeck.rs
@@ -1,4 +1,4 @@
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
 const fn f() {