Rollup merge of rust-lang#107662 - cjgillot:copy-projection, r=oli-obk

Turn projections into copies in CopyProp.

The current implementation can leave behind projections that are moved out several times.

This PR widens the check to turn such moves into copies: a move out of a projection of a copy is equivalent to a copy of the original projection.

Author: matthiaskrgr

compiler/rustc_mir_transform/src/copy_prop.rs

@@ -153,8 +153,9 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
 
     fn visit_operand(&mut self, operand: &mut Operand<'tcx>, loc: Location) {
         if let Operand::Move(place) = *operand
-            && let Some(local) = place.as_local()
-            && !self.fully_moved.contains(local)
+            // A move out of a projection of a copy is equivalent to a copy of the original projection.
+            && !place.has_deref()
+            && !self.fully_moved.contains(place.local)
         {
             *operand = Operand::Copy(place);
         }

tests/mir-opt/copy-prop/move_projection.f.CopyProp.diff

@@ -0,0 +1,31 @@
+- // MIR for `f` before CopyProp
++ // MIR for `f` after CopyProp
+  
+  fn f(_1: Foo) -> bool {
+      let mut _0: bool;                    // return place in scope 0 at $DIR/move_projection.rs:+0:17: +0:21
+      let mut _2: Foo;                     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+      let mut _3: u8;                      // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+  
+      bb0: {
+-         _2 = _1;                         // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+-         _3 = move (_2.0: u8);            // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+-         _0 = opaque::<Foo>(move _1) -> bb1; // scope 0 at $DIR/move_projection.rs:+6:13: +6:44
++         _3 = (_1.0: u8);                 // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
++         _0 = opaque::<Foo>(_1) -> bb1;   // scope 0 at $DIR/move_projection.rs:+6:13: +6:44
+                                           // mir::Constant
+                                           // + span: $DIR/move_projection.rs:19:28: 19:34
+                                           // + literal: Const { ty: fn(Foo) -> bool {opaque::<Foo>}, val: Value(<ZST>) }
+      }
+  
+      bb1: {
+          _0 = opaque::<u8>(move _3) -> bb2; // scope 0 at $DIR/move_projection.rs:+9:13: +9:44
+                                           // mir::Constant
+                                           // + span: $DIR/move_projection.rs:22:28: 22:34
+                                           // + literal: Const { ty: fn(u8) -> bool {opaque::<u8>}, val: Value(<ZST>) }
+      }
+  
+      bb2: {
+          return;                          // scope 0 at $DIR/move_projection.rs:+12:13: +12:21
+      }
+  }
+  

tests/mir-opt/copy-prop/move_projection.rs

@@ -0,0 +1,34 @@
+// unit-test: CopyProp
+
+#![feature(custom_mir, core_intrinsics)]
+#![allow(unused_assignments)]
+extern crate core;
+use core::intrinsics::mir::*;
+
+fn opaque(_: impl Sized) -> bool { true }
+
+struct Foo(u8);
+
+#[custom_mir(dialect = "analysis", phase = "post-cleanup")]
+fn f(a: Foo) -> bool {
+    mir!(
+        {
+            let b = a;
+            // This is a move out of a copy, so must become a copy of `a.0`.
+            let c = Move(b.0);
+            Call(RET, bb1, opaque(Move(a)))
+        }
+        bb1 = {
+            Call(RET, ret, opaque(Move(c)))
+        }
+        ret = {
+            Return()
+        }
+    )
+}
+
+fn main() {
+    assert!(f(Foo(0)));
+}
+
+// EMIT_MIR move_projection.f.CopyProp.diff

tests/mir-opt/simple_option_map_e2e.ezmap.PreCodegen.after.mir

@@ -34,7 +34,7 @@ fn ezmap(_1: Option<i32>) -> Option<i32> {
     }
 
     bb3: {
-        _4 = move ((_1 as Some).0: i32); // scope 1 at $DIR/simple_option_map_e2e.rs:7:14: 7:15
+        _4 = ((_1 as Some).0: i32);      // scope 1 at $DIR/simple_option_map_e2e.rs:7:14: 7:15
         StorageLive(_5);                 // scope 2 at $DIR/simple_option_map_e2e.rs:7:25: 7:29
         _5 = Add(_4, const 1_i32);       // scope 3 at $DIR/simple_option_map_e2e.rs:+1:16: +1:21
         _0 = Option::<i32>::Some(move _5); // scope 2 at $DIR/simple_option_map_e2e.rs:7:20: 7:30