Auto merge of #106340 - saethlin:propagate-operands, r=oli-obk

Always permit ConstProp to exploit arithmetic identities

Fixes https://github.com/rust-lang/rust/issues/72751

Initially, I thought I would need to enable operand propagation then do something else, but actually https://github.com/rust-lang/rust/pull/74491 already has the fix for the issue in question! It looks like this optimization was put under MIR opt level 3 due to possible soundness/stability implications, then demoted further to MIR opt level 4 when MIR opt level 2 became associated with `--release`.

Perhaps in the past we were doing CTFE on optimized MIR? We aren't anymore, so this optimization has no stability implications.

r? `@oli-obk`

1 files changed, 1 insertions, 5 deletions

diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index e384cfe1659..5c45abc5a17 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -655,11 +655,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             return None;
         }
 
-        if self.tcx.sess.mir_opt_level() >= 4 {
-            self.eval_rvalue_with_identities(rvalue, place)
-        } else {
-            self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
-        }
+        self.eval_rvalue_with_identities(rvalue, place)
     }
 
     // Attempt to use algebraic identities to eliminate constant expressions