Auto merge of #78068 - RalfJung:union-safe-assign, r=nikomatsakis

consider assignments of union field of ManuallyDrop type safe

Assigning to `Copy` union fields is safe because that assignment will never drop anything. However, with https://github.com/rust-lang/rust/pull/77547, unions may also have `ManuallyDrop` fields, and their assignments are currently still unsafe. That seems unnecessary though, as assigning `ManuallyDrop` does not drop anything either, and is thus safe even for union fields.

I assume this will at least require FCP.

compiler/rustc_middle/src/mir/mod.rs

@@ -1756,6 +1756,21 @@ impl<'tcx> Place<'tcx> {
     pub fn as_ref(&self) -> PlaceRef<'tcx> {
         PlaceRef { local: self.local, projection: &self.projection }
     }
+
+    /// Iterate over the projections in evaluation order, i.e., the first element is the base with
+    /// its projection and then subsequently more projections are added.
+    /// As a concrete example, given the place a.b.c, this would yield:
+    /// - (a, .b)
+    /// - (a.b, .c)
+    /// Given a place without projections, the iterator is empty.
+    pub fn iter_projections(
+        self,
+    ) -> impl Iterator<Item = (PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator {
+        self.projection.iter().enumerate().map(move |(i, proj)| {
+            let base = PlaceRef { local: self.local, projection: &self.projection[..i] };
+            (base, proj)
+        })
+    }
 }
 
 impl From<Local> for Place<'_> {

compiler/rustc_middle/src/mir/query.rs

@@ -46,7 +46,7 @@ pub enum UnsafetyViolationDetails {
     UseOfMutableStatic,
     UseOfExternStatic,
     DerefOfRawPointer,
-    AssignToNonCopyUnionField,
+    AssignToDroppingUnionField,
     AccessToUnionField,
     MutationOfLayoutConstrainedField,
     BorrowOfLayoutConstrainedField,
@@ -94,8 +94,8 @@ impl UnsafetyViolationDetails {
                 "raw pointers may be NULL, dangling or unaligned; they can violate aliasing rules \
                  and cause data races: all of these are undefined behavior",
             ),
-            AssignToNonCopyUnionField => (
-                "assignment to non-`Copy` union field",
+            AssignToDroppingUnionField => (
+                "assignment to union field that might need dropping",
                 "the previous content of the field will be dropped, which causes undefined \
                  behavior if the field was not properly initialized",
             ),

compiler/rustc_middle/src/mir/tcx.rs

@@ -136,6 +136,15 @@ impl<'tcx> Place<'tcx> {
     }
 }
 
+impl<'tcx> PlaceRef<'tcx> {
+    pub fn ty<D>(&self, local_decls: &D, tcx: TyCtxt<'tcx>) -> PlaceTy<'tcx>
+    where
+        D: HasLocalDecls<'tcx>,
+    {
+        Place::ty_from(self.local, &self.projection, local_decls, tcx)
+    }
+}
+
 pub enum RvalueInitializationState {
     Shallow,
     Deep,