Explain that this is UB catching instead of malformed MIR.

Author: cjgillot

compiler/rustc_const_eval/src/transform/validate.rs

@@ -756,6 +756,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             StatementKind::StorageLive(local) => {
+                // We check that the local is not live when entering a `StorageLive` for it.
+                // Technically, violating this restriction is only UB and not actually indicative
+                // of not well-formed MIR. This means that an optimization which turns MIR that
+                // already has UB into MIR that fails this check is not necessarily wrong. However,
+                // we have no such optimizations at the moment, and so we include this check anyway
+                // to help us catch bugs. If you happen to write an optimization that might cause
+                // this to incorrectly fire, feel free to remove this check.
                 if self.reachable_blocks.contains(location.block) {
                     self.storage_liveness.seek_before_primary_effect(location);
                     let locals_with_storage = self.storage_liveness.get();