Auto merge of rust-lang#104862 - saethlin:mir-niche-checks, r=<try>

Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`

compiler/rustc_codegen_cranelift/src/base.rs

@@ -368,6 +368,29 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                             source_info.span,
                         );
                     }
+                    AssertKind::OccupiedNiche {
+                        ref found,
+                        ref start,
+                        ref end,
+                        ref type_name,
+                        ref offset,
+                        ref niche_ty,
+                    } => {
+                        let found = codegen_operand(fx, found).load_scalar(fx);
+                        let start = codegen_operand(fx, start).load_scalar(fx);
+                        let end = codegen_operand(fx, end).load_scalar(fx);
+                        let type_name = fx.anonymous_str(type_name);
+                        let offset = codegen_operand(fx, offset).load_scalar(fx);
+                        let niche_ty = fx.anonymous_str(niche_ty);
+                        let location = fx.get_caller_location(source_info).load_scalar(fx);
+
+                        codegen_panic_inner(
+                            fx,
+                            rustc_hir::LangItem::PanicOccupiedNiche,
+                            &[found, start, end, type_name, offset, niche_ty, location],
+                            source_info.span,
+                        )
+                    }
                     _ => {
                         let msg_str = msg.description();
                         codegen_panic(fx, msg_str, source_info);

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -624,6 +624,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 // and `#[track_caller]` adds an implicit third argument.
                 (LangItem::PanicMisalignedPointerDereference, vec![required, found, location])
             }
+            AssertKind::OccupiedNiche { ref found, ref start, ref end } => {
+                let found = self.codegen_operand(bx, found).immediate();
+                let start = self.codegen_operand(bx, start).immediate();
+                let end = self.codegen_operand(bx, end).immediate();
+                (LangItem::PanicOccupiedNiche, vec![found, start, end, location])
+            }
             _ => {
                 let msg = bx.const_str(msg.description());
                 // It's `pub fn panic(expr: &str)`, with the wide reference being passed

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -542,6 +542,11 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                     found: eval_to_int(found)?,
                 }
             }
+            OccupiedNiche { ref found, ref start, ref end } => OccupiedNiche {
+                found: eval_to_int(found)?,
+                start: eval_to_int(start)?,
+                end: eval_to_int(end)?,
+            },
         };
         Err(ConstEvalErrKind::AssertFailure(err).into())
     }

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -6,6 +6,7 @@ use rustc_middle::mir::interpret::{InterpResult, PointerArithmetic, Scalar};
 use rustc_middle::mir::CastKind;
 use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::layout::{IntegerExt, LayoutOf, TyAndLayout};
+use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{self, FloatTy, Ty, TypeAndMut};
 use rustc_target::abi::Integer;
 use rustc_type_ir::TyKind::*;
@@ -147,8 +148,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 if src.layout.size != dest.layout.size {
                     let src_bytes = src.layout.size.bytes();
                     let dest_bytes = dest.layout.size.bytes();
-                    let src_ty = format!("{}", src.layout.ty);
-                    let dest_ty = format!("{}", dest.layout.ty);
+                    let src_ty = with_no_trimmed_paths!(format!("{}", src.layout.ty));
+                    let dest_ty = with_no_trimmed_paths!(format!("{}", dest.layout.ty));
                     throw_ub_custom!(
                         fluent::const_eval_invalid_transmute,
                         src_bytes = src_bytes,

compiler/rustc_hir/src/lang_items.rs

@@ -234,6 +234,7 @@ language_item_table! {
     ConstPanicFmt,           sym::const_panic_fmt,     const_panic_fmt,            Target::Fn,             GenericRequirement::None;
     PanicBoundsCheck,        sym::panic_bounds_check,  panic_bounds_check_fn,      Target::Fn,             GenericRequirement::Exact(0);
     PanicMisalignedPointerDereference,        sym::panic_misaligned_pointer_dereference,  panic_misaligned_pointer_dereference_fn,      Target::Fn,             GenericRequirement::Exact(0);
+    PanicOccupiedNiche,      sym::panic_occupied_niche, panic_occupied_niche_fn,      Target::Fn,             GenericRequirement::Exact(0);
     PanicInfo,               sym::panic_info,          panic_info,                 Target::Struct,         GenericRequirement::None;
     PanicLocation,           sym::panic_location,      panic_location,             Target::Struct,         GenericRequirement::None;
     PanicImpl,               sym::panic_impl,          panic_impl,                 Target::Fn,             GenericRequirement::None;

compiler/rustc_middle/messages.ftl

@@ -15,6 +15,9 @@ middle_assert_divide_by_zero =
 middle_assert_misaligned_ptr_deref =
     misaligned pointer dereference: address must be a multiple of {$required} but is {$found}
 
+middle_assert_occupied_niche =
+    occupied niche: {$found} must be in {$start}..={$end}
+
 middle_assert_op_overflow =
     attempt to compute `{$left} {$op} {$right}`, which would overflow
 

compiler/rustc_middle/src/mir/syntax.rs

@@ -886,6 +886,7 @@ pub enum AssertKind<O> {
     ResumedAfterReturn(CoroutineKind),
     ResumedAfterPanic(CoroutineKind),
     MisalignedPointerDereference { required: O, found: O },
+    OccupiedNiche { found: O, start: O, end: O },
 }
 
 #[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]

compiler/rustc_middle/src/mir/terminator.rs

@@ -150,7 +150,7 @@ impl<O> AssertKind<O> {
             ResumedAfterReturn(CoroutineKind::Async(_)) => "`async fn` resumed after completion",
             ResumedAfterPanic(CoroutineKind::Coroutine) => "coroutine resumed after panicking",
             ResumedAfterPanic(CoroutineKind::Async(_)) => "`async fn` resumed after panicking",
-            BoundsCheck { .. } | MisalignedPointerDereference { .. } => {
+            BoundsCheck { .. } | MisalignedPointerDereference { .. } | OccupiedNiche { .. } => {
                 bug!("Unexpected AssertKind")
             }
         }
@@ -213,6 +213,13 @@ impl<O> AssertKind<O> {
                     "\"misaligned pointer dereference: address must be a multiple of {{}} but is {{}}\", {required:?}, {found:?}"
                 )
             }
+            OccupiedNiche { found, start, end } => {
+                write!(
+                    f,
+                    "\"occupied niche: {{}} must be in {{}}..={{}}\" {:?} {:?} {:?}",
+                    found, start, end
+                )
+            }
             _ => write!(f, "\"{}\"", self.description()),
         }
     }
@@ -243,8 +250,8 @@ impl<O> AssertKind<O> {
             ResumedAfterPanic(CoroutineKind::Coroutine) => {
                 middle_assert_coroutine_resume_after_panic
             }
-
             MisalignedPointerDereference { .. } => middle_assert_misaligned_ptr_deref,
+            OccupiedNiche { .. } => middle_assert_occupied_niche,
         }
     }
 
@@ -281,6 +288,11 @@ impl<O> AssertKind<O> {
                 add!("required", format!("{required:#?}"));
                 add!("found", format!("{found:#?}"));
             }
+            OccupiedNiche { found, start, end } => {
+                add!("found", format!("{found:?}"));
+                add!("start", format!("{start:?}"));
+                add!("end", format!("{end:?}"));
+            }
         }
     }
 }

compiler/rustc_middle/src/mir/visit.rs

@@ -625,6 +625,11 @@ macro_rules! make_mir_visitor {
                         self.visit_operand(required, location);
                         self.visit_operand(found, location);
                     }
+                    OccupiedNiche { found, start, end } => {
+                        self.visit_operand(found, location);
+                        self.visit_operand(start, location);
+                        self.visit_operand(end, location);
+                    }
                 }
             }