Only remove unreachable blocks

Only remove unreachable blocks after drop elaboration but
avoid merging blocks, as that sometimes confuses borrowck
precomputation of borrows_out_of_scope.
See issue rust-lang#80949 for more details.

Author: zeegomo

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -1,3 +1,4 @@
+use crate::simplify::remove_dead_blocks;
 use crate::MirPass;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_index::bit_set::BitSet;
@@ -68,6 +69,9 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
             .elaborate()
         };
         elaborate_patch.apply(body);
+
+        remove_dead_blocks(tcx, body);
+        body.basic_blocks_mut().raw.shrink_to_fit();
     }
 }
 

compiler/rustc_mir_transform/src/lib.rs

@@ -342,7 +342,6 @@ fn mir_promoted(
             // These next passes must be executed together
             &add_call_guards::CriticalCallEdges,
             &elaborate_drops::ElaborateDrops,
-            &simplify::SimplifyCfg::new("elaborate-drops"),
             &coverage::InstrumentCoverage,
         ],
         Some(MirPhase::Analysis(AnalysisPhase::Initial)),