Auto merge of rust-lang#96326 - JakobDegen:relax-operand, r=oli-obk

Relax restrictions for copy operands

This was [discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Removing.20requirement.20that.20.60Copy.60.20operands.20have.20.60Copy.60.20types/near/279102313). Details about motivation and such can be found there

r? `@oli-obk`

Author: bors

compiler/rustc_const_eval/src/transform/validate.rs

@@ -214,7 +214,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
 
     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
         // This check is somewhat expensive, so only run it when -Zvalidate-mir is passed.
-        if self.tcx.sess.opts.debugging_opts.validate_mir {
+        if self.tcx.sess.opts.debugging_opts.validate_mir && self.mir_phase < MirPhase::DropsLowered
+        {
             // `Operand::Copy` is only supposed to be used with `Copy` types.
             if let Operand::Copy(place) = operand {
                 let ty = place.ty(&self.body.local_decls, self.tcx).ty;

compiler/rustc_middle/src/mir/mod.rs

@@ -166,7 +166,8 @@ pub enum MirPhase {
     /// * [`StatementKind::Retag`]
     ///
     /// Furthermore, `Drop` now uses explicit drop flags visible in the MIR and reaching a `Drop`
-    /// terminator means that the auto-generated drop glue will be invoked.
+    /// terminator means that the auto-generated drop glue will be invoked. Also, `Copy` operands
+    /// are allowed for non-`Copy` types.
     DropsLowered = 3,
     /// Beginning with this phase, the following variant is disallowed:
     /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array`
@@ -2330,7 +2331,10 @@ pub struct SourceScopeLocalData {
 /// validator.
 #[derive(Clone, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
 pub enum Operand<'tcx> {
-    /// Creates a value by loading the given place. The type of the place must be `Copy`
+    /// Creates a value by loading the given place.
+    ///
+    /// Before drop elaboration, the type of the place must be `Copy`. After drop elaboration there
+    /// is no such requirement.
     Copy(Place<'tcx>),
 
     /// Creates a value by performing loading the place, just like the `Copy` operand.