make MoveDataBuilder a bit less strict so it can run in copy_prop, fix copy_prop's taut remover

Author: ohadravid

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -1,9 +1,9 @@
 use std::mem;
 
 use rustc_index::IndexVec;
+use rustc_middle::bug;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
-use rustc_middle::{bug, span_bug};
 use smallvec::{SmallVec, smallvec};
 use tracing::debug;
 
@@ -344,13 +344,14 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
         match &stmt.kind {
             StatementKind::Assign(box (place, Rvalue::CopyForDeref(reffed))) => {
                 let local = place.as_local().unwrap();
-                assert!(self.body.local_decls[local].is_deref_temp());
+                if self.body.local_decls[local].is_deref_temp() {
+                    let rev_lookup = &mut self.data.rev_lookup;
 
-                let rev_lookup = &mut self.data.rev_lookup;
-
-                rev_lookup.un_derefer.insert(local, reffed.as_ref());
-                let base_local = rev_lookup.un_derefer.deref_chain(local).first().unwrap().local;
-                rev_lookup.locals[local] = rev_lookup.locals[base_local];
+                    rev_lookup.un_derefer.insert(local, reffed.as_ref());
+                    let base_local =
+                        rev_lookup.un_derefer.deref_chain(local).first().unwrap().local;
+                    rev_lookup.locals[local] = rev_lookup.locals[base_local];
+                }
             }
             StatementKind::Assign(box (place, rval)) => {
                 self.create_move_path(*place);
@@ -375,13 +376,9 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
                     self.gather_move(Place::from(*local));
                 }
             }
-            StatementKind::SetDiscriminant { .. } | StatementKind::Deinit(..) => {
-                span_bug!(
-                    stmt.source_info.span,
-                    "SetDiscriminant/Deinit should not exist during borrowck"
-                );
-            }
-            StatementKind::Retag { .. }
+            StatementKind::SetDiscriminant { .. }
+            | StatementKind::Deinit(..)
+            | StatementKind::Retag { .. }
             | StatementKind::AscribeUserType(..)
             | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)

compiler/rustc_mir_transform/src/copy_prop.rs

@@ -52,8 +52,12 @@ impl<'tcx> crate::MirPass<'tcx> for CopyProp {
         let storage_to_remove = if any_replacement {
             // The replacer will remove tautological moves from the head to the local,
             // so we remove them before running `MaybeUninitializedPlaces`.
-            TautologicalMoveAssignmentRemover { tcx, copy_classes: ssa.copy_classes() }
-                .visit_body_preserves_cfg(body);
+            TautologicalMoveAssignmentRemover {
+                tcx,
+                copy_classes: ssa.copy_classes(),
+                borrowed_locals: ssa.borrowed_locals(),
+            }
+            .visit_body_preserves_cfg(body);
 
             let move_data = MoveData::gather_moves(body, tcx, |_| true);
 
@@ -134,6 +138,7 @@ fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
 struct TautologicalMoveAssignmentRemover<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     copy_classes: &'a IndexSlice<Local, Local>,
+    borrowed_locals: &'a DenseBitSet<Local>,
 }
 
 impl<'tcx> MutVisitor<'tcx> for TautologicalMoveAssignmentRemover<'_, 'tcx> {
@@ -142,10 +147,17 @@ impl<'tcx> MutVisitor<'tcx> for TautologicalMoveAssignmentRemover<'_, 'tcx> {
     }
 
     fn visit_statement(&mut self, stmt: &mut Statement<'tcx>, loc: Location) {
+        self.super_statement(stmt, loc);
+
         // Similar to `Replacer::visit_statement`, but only handle moves.
         if let StatementKind::Assign(box (lhs, ref rhs)) = stmt.kind
             && let Rvalue::Use(Operand::Move(rhs)) = *rhs
         {
+            if self.borrowed_locals.contains(lhs.local) || self.borrowed_locals.contains(rhs.local)
+            {
+                return;
+            }
+
             let new_lhs_local = self.copy_classes[lhs.local];
             let new_rhs_local = self.copy_classes[rhs.local];