Rollup merge of #108786 - saethlin:free-regions-check, r=oli-obk

Check for free regions in MIR validation

This turns https://github.com/rust-lang/rust/issues/108720 into a MIR validation failure that will reproduce without debug-assertions enabled.

```
error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:296 ~ futures_util[3805]::future::future::remote_handle::{impl#3}::poll), const_param_did: None }) (after pass ScalarReplacementOfAggregates) at bb0[0]:
                                Free regions in optimized runtime-post-cleanup MIR
  --> /home/ben/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.26/src/future/future/remote_handle.rs:96:13
   |
96 |         let this = self.project();
   |             ^^^^
```

1 files changed, 11 insertions, 0 deletions

diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index fb37eb79a33..272fe3d1b31 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -72,6 +72,17 @@ impl<'tcx> MirPass<'tcx> for Validator {
         };
         checker.visit_body(body);
         checker.check_cleanup_control_flow();
+
+        if let MirPhase::Runtime(_) = body.phase {
+            if let ty::InstanceDef::Item(_) = body.source.instance {
+                if body.has_free_regions() {
+                    checker.fail(
+                        Location::START,
+                        format!("Free regions in optimized {} MIR", body.phase.name()),
+                    );
+                }
+            }
+        }
     }
 }