Avoid Operand::Copy with &mut T

Author: jonas-schievink

src/librustc_mir/shim.rs

@@ -700,7 +700,7 @@ fn build_call_shim<'tcx>(
 
     let rcvr = rcvr_adjustment.map(|rcvr_adjustment| match rcvr_adjustment {
         Adjustment::Identity => Operand::Move(rcvr_place()),
-        Adjustment::Deref => Operand::Copy(tcx.mk_place_deref(rcvr_place())),
+        Adjustment::Deref => Operand::Move(tcx.mk_place_deref(rcvr_place())), // Can't copy `&mut`
         Adjustment::DerefMove => Operand::Move(tcx.mk_place_deref(rcvr_place())),
         Adjustment::RefMut => {
             // let rcvr = &mut rcvr;

src/librustc_mir/transform/instcombine.rs

@@ -1,11 +1,11 @@
 //! Performs various peephole optimizations.
 
 use crate::transform::{MirPass, MirSource};
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::FxHashMap;
 use rustc_index::vec::Idx;
 use rustc_middle::mir::visit::{MutVisitor, Visitor};
 use rustc_middle::mir::{
-    Body, Constant, Local, Location, Operand, Place, PlaceRef, ProjectionElem, Rvalue,
+    Body, Constant, Local, Location, Mutability, Operand, Place, PlaceRef, ProjectionElem, Rvalue,
 };
 use rustc_middle::ty::{self, TyCtxt};
 use std::mem;
@@ -39,7 +39,7 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
     }
 
     fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
-        if self.optimizations.and_stars.remove(&location) {
+        if let Some(mtbl) = self.optimizations.and_stars.remove(&location) {
             debug!("replacing `&*`: {:?}", rvalue);
             let new_place = match rvalue {
                 Rvalue::Ref(_, _, place) => {
@@ -57,7 +57,10 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
                 }
                 _ => bug!("Detected `&*` but didn't find `&*`!"),
             };
-            *rvalue = Rvalue::Use(Operand::Copy(new_place))
+            *rvalue = Rvalue::Use(match mtbl {
+                Mutability::Mut => Operand::Move(new_place),
+                Mutability::Not => Operand::Copy(new_place),
+            });
         }
 
         if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
@@ -88,8 +91,10 @@ impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
             if let PlaceRef { local, projection: &[ref proj_base @ .., ProjectionElem::Deref] } =
                 place.as_ref()
             {
-                if Place::ty_from(local, proj_base, self.body, self.tcx).ty.is_region_ptr() {
-                    self.optimizations.and_stars.insert(location);
+                // The dereferenced place must have type `&_`.
+                let ty = Place::ty_from(local, proj_base, self.body, self.tcx).ty;
+                if let ty::Ref(_, _, mtbl) = ty.kind {
+                    self.optimizations.and_stars.insert(location, mtbl);
                 }
             }
         }
@@ -109,6 +114,6 @@ impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
 
 #[derive(Default)]
 struct OptimizationList<'tcx> {
-    and_stars: FxHashSet<Location>,
+    and_stars: FxHashMap<Location, Mutability>,
     arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
 }

src/test/mir-opt/inline/issue-58867-inline-as-ref-as-mut/rustc.a.Inline.after.mir

@@ -15,7 +15,7 @@ fn a(_1: &mut [T]) -> &mut [T] {
         StorageLive(_3);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
         StorageLive(_4);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:6
         _4 = &mut (*_1);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:6
-        _3 = _4;                         // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
+        _3 = move _4;                    // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
         _2 = &mut (*_3);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
         StorageDead(_4);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:14: 3:15
         _0 = &mut (*_2);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15

src/test/mir-opt/inline/issue-58867-inline-as-ref-as-mut/rustc.b.Inline.after.mir

@@ -18,7 +18,7 @@ fn b(_1: &mut std::boxed::Box<T>) -> &mut T {
         _4 = &mut (*_1);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:6
         StorageLive(_5);                 // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
         _5 = &mut (*(*_4));              // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
-        _3 = _5;                         // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
+        _3 = move _5;                    // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
         StorageDead(_5);                 // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
         _2 = &mut (*_3);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:15
         StorageDead(_4);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:14: 8:15