Auto merge of rust-lang#80290 - RalfJung:less-intrinsic-write, r=lcnr

implement ptr::write without dedicated intrinsic

This makes `ptr::write` more consistent with `ptr::write_unaligned`, `ptr::read`, `ptr::read_unaligned`, all of which are implemented in terms of `copy_nonoverlapping`.

This means we can also remove `move_val_init` implementations in codegen and Miri, and its special handling in the borrow checker.

Also see [this Zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/ptr.3A.3Aread.20vs.20ptr.3A.3Awrite).

compiler/rustc_mir/src/transform/check_unsafety.rs

@@ -223,21 +223,13 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
         // Check for raw pointer `Deref`.
         for (base, proj) in place.iter_projections() {
             if proj == ProjectionElem::Deref {
-                let source_info = self.source_info; // Backup source_info so we can restore it later.
-                if base.projection.is_empty() && decl.internal {
-                    // Internal locals are used in the `move_val_init` desugaring.
-                    // We want to check unsafety against the source info of the
-                    // desugaring, rather than the source info of the RHS.
-                    self.source_info = self.body.local_decls[place.local].source_info;
-                }
                 let base_ty = base.ty(self.body, self.tcx).ty;
                 if base_ty.is_unsafe_ptr() {
                     self.require_unsafe(
                         UnsafetyViolationKind::GeneralAndConstFn,
                         UnsafetyViolationDetails::DerefOfRawPointer,
                     )
                 }
-                self.source_info = source_info; // Restore backed-up source_info.
             }
         }
 

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -10,9 +10,7 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir as hir;
 use rustc_middle::middle::region;
 use rustc_middle::mir::*;
-use rustc_middle::ty::{self, CanonicalUserTypeAnnotation};
-use rustc_span::symbol::sym;
-use rustc_target::spec::abi::Abi;
+use rustc_middle::ty::{CanonicalUserTypeAnnotation};
 
 use std::slice;
 
@@ -219,79 +217,41 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     },
                 )
             }
-            ExprKind::Call { ty, fun, args, from_hir_call, fn_span } => {
-                let intrinsic = match *ty.kind() {
-                    ty::FnDef(def_id, _) => {
-                        let f = ty.fn_sig(this.hir.tcx());
-                        if f.abi() == Abi::RustIntrinsic || f.abi() == Abi::PlatformIntrinsic {
-                            Some(this.hir.tcx().item_name(def_id))
-                        } else {
-                            None
-                        }
-                    }
-                    _ => None,
-                };
+            ExprKind::Call { ty: _, fun, args, from_hir_call, fn_span } => {
                 let fun = unpack!(block = this.as_local_operand(block, fun));
-                if let Some(sym::move_val_init) = intrinsic {
-                    // `move_val_init` has "magic" semantics - the second argument is
-                    // always evaluated "directly" into the first one.
-
-                    let mut args = args.into_iter();
-                    let ptr = args.next().expect("0 arguments to `move_val_init`");
-                    let val = args.next().expect("1 argument to `move_val_init`");
-                    assert!(args.next().is_none(), ">2 arguments to `move_val_init`");
-
-                    let ptr = this.hir.mirror(ptr);
-                    let ptr_ty = ptr.ty;
-                    // Create an *internal* temp for the pointer, so that unsafety
-                    // checking won't complain about the raw pointer assignment.
-                    let ptr_temp = this
-                        .local_decls
-                        .push(LocalDecl::with_source_info(ptr_ty, source_info).internal());
-                    let ptr_temp = Place::from(ptr_temp);
-                    // No need for a scope, ptr_temp doesn't need drop
-                    let block = unpack!(this.into(ptr_temp, None, block, ptr));
-                    // Maybe we should provide a scope here so that
-                    // `move_val_init` wouldn't leak on panic even with an
-                    // arbitrary `val` expression, but `schedule_drop`,
-                    // borrowck and drop elaboration all prevent us from
-                    // dropping `ptr_temp.deref()`.
-                    this.into(this.hir.tcx().mk_place_deref(ptr_temp), None, block, val)
-                } else {
-                    let args: Vec<_> = args
-                        .into_iter()
-                        .map(|arg| unpack!(block = this.as_local_call_operand(block, arg)))
-                        .collect();
-
-                    let success = this.cfg.start_new_block();
-
-                    this.record_operands_moved(&args);
-
-                    debug!("into_expr: fn_span={:?}", fn_span);
-
-                    this.cfg.terminate(
-                        block,
-                        source_info,
-                        TerminatorKind::Call {
-                            func: fun,
-                            args,
-                            cleanup: None,
-                            // FIXME(varkor): replace this with an uninhabitedness-based check.
-                            // This requires getting access to the current module to call
-                            // `tcx.is_ty_uninhabited_from`, which is currently tricky to do.
-                            destination: if expr.ty.is_never() {
-                                None
-                            } else {
-                                Some((destination, success))
-                            },
-                            from_hir_call,
-                            fn_span,
+                let args: Vec<_> = args
+                    .into_iter()
+                    .map(|arg| unpack!(block = this.as_local_call_operand(block, arg)))
+                    .collect();
+
+                let success = this.cfg.start_new_block();
+
+                this.record_operands_moved(&args);
+
+                debug!("into_expr: fn_span={:?}", fn_span);
+
+                this.cfg.terminate(
+                    block,
+                    source_info,
+                    TerminatorKind::Call {
+                        func: fun,
+                        args,
+                        cleanup: None,
+                        // FIXME(varkor): replace this with an uninhabitedness-based check.
+                        // This requires getting access to the current module to call
+                        // `tcx.is_ty_uninhabited_from`, which is currently tricky to do.
+                        destination: if expr.ty.is_never() {
+                            None
+                        } else {
+                            Some((destination, success))
                         },
-                    );
-                    this.diverge_from(block);
-                    schedule_drop(this);
-                    success.unit()
-                }
+                        from_hir_call,
+                        fn_span,
+                    },
+                );
+                this.diverge_from(block);
+                schedule_drop(this);
+                success.unit()
             }
             ExprKind::Use { source } => this.into(destination, scope, block, source),
             ExprKind::Borrow { arg, borrow_kind } => {

compiler/rustc_span/src/symbol.rs

@@ -716,7 +716,6 @@ symbols! {
         more_struct_aliases,
         movbe_target_feature,
         move_ref_pattern,
-        move_val_init,
         mul,
         mul_assign,
         mul_with_overflow,

compiler/rustc_typeck/src/check/intrinsic.rs

@@ -157,7 +157,6 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
             }
             sym::forget => (1, vec![param(0)], tcx.mk_unit()),
             sym::transmute => (2, vec![param(0)], param(1)),
-            sym::move_val_init => (1, vec![tcx.mk_mut_ptr(param(0)), param(0)], tcx.mk_unit()),
             sym::prefetch_read_data
             | sym::prefetch_write_data
             | sym::prefetch_read_instruction

library/core/src/intrinsics.rs

@@ -768,13 +768,6 @@ extern "rust-intrinsic" {
     #[rustc_const_stable(feature = "const_size_of", since = "1.40.0")]
     pub fn size_of<T>() -> usize;
 
-    /// Moves a value to an uninitialized memory location.
-    ///
-    /// Drop glue is not run on the destination.
-    ///
-    /// The stabilized version of this intrinsic is [`core::ptr::write`](crate::ptr::write).
-    pub fn move_val_init<T>(dst: *mut T, src: T);
-
     /// The minimum alignment of a type.
     ///
     /// The stabilized version of this intrinsic is [`core::mem::align_of`](crate::mem::align_of).

library/core/src/ptr/mod.rs

@@ -883,12 +883,19 @@ pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn write<T>(dst: *mut T, src: T) {
-    if cfg!(debug_assertions) && !is_aligned_and_not_null(dst) {
-        // Not panicking to keep codegen impact smaller.
-        abort();
+    // We are calling the intrinsics directly to avoid function calls in the generated code
+    // as `intrinsics::copy_nonoverlapping` is a wrapper function.
+    extern "rust-intrinsic" {
+        fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
+    }
+
+    // SAFETY: the caller must guarantee that `dst` is valid for writes.
+    // `dst` cannot overlap `src` because the caller has mutable access
+    // to `dst` while `src` is owned by this function.
+    unsafe {
+        copy_nonoverlapping(&src as *const T, dst, 1);
+        intrinsics::forget(src);
     }
-    // SAFETY: the caller must uphold the safety contract for `move_val_init`.
-    unsafe { intrinsics::move_val_init(&mut *dst, src) }
 }
 
 /// Overwrites a memory location with the given value without reading or