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

Check for free regions in MIR validation

This turns rust-lang#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();
   |             ^^^^
```

Author: matthiaskrgr

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()),
+                    );
+                }
+            }
+        }
     }
 }
 

compiler/rustc_mir_transform/src/lib.rs

@@ -416,8 +416,6 @@ fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -
 
     pm::run_passes(tcx, &mut body, &[&ctfe_limit::CtfeLimit], None);
 
-    debug_assert!(!body.has_free_regions(), "Free regions in MIR for CTFE");
-
     body
 }
 
@@ -626,8 +624,6 @@ fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
     debug!("body: {:#?}", body);
     run_optimization_passes(tcx, &mut body);
 
-    debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");
-
     body
 }
 
@@ -651,7 +647,5 @@ fn promoted_mir(
         run_analysis_to_runtime_passes(tcx, body);
     }
 
-    debug_assert!(!promoted.has_free_regions(), "Free regions in promoted MIR");
-
     tcx.arena.alloc(promoted)
 }