Add a repro example for not propagating constants of partially const …

…initialized variables

src/test/mir-opt/const_prop/mutable_variable_aggregate_partial_read.rs

@@ -0,0 +1,14 @@
+// compile-flags: -O
+
+// EMIT_MIR rustc.main.ConstProp.diff
+fn main() {
+    let mut x: (i32, i32) = foo();
+    x.1 = 99;
+    x.0 = 42;
+    let y = x.1;
+}
+
+#[inline(never)]
+fn foo() -> (i32, i32) {
+    unimplemented!()
+}

src/test/mir-opt/const_prop/mutable_variable_aggregate_partial_read/rustc.main.ConstProp.diff

@@ -0,0 +1,55 @@
+- // MIR for `main` before ConstProp
++ // MIR for `main` after ConstProp
+
+  fn main() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:4:11: 4:11
+      let mut _1: (i32, i32) as UserTypeProjection { base: UserType(0), projs: [] }; // in scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:5:9: 5:14
+      scope 1 {
+          debug x => _1;                   // in scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:5:9: 5:14
+          let _2: i32;                     // in scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:9: 8:10
+          scope 2 {
+              debug y => _2;               // in scope 2 at $DIR/mutable_variable_aggregate_partial_read.rs:8:9: 8:10
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);                 // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:5:9: 5:14
+          _1 = const foo() -> bb1;         // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:5:29: 5:34
+                                           // ty::Const
+                                           // + ty: fn() -> (i32, i32) {foo}
+                                           // + val: Value(Scalar(<ZST>))
+                                           // mir::Constant
+                                           // + span: $DIR/mutable_variable_aggregate_partial_read.rs:5:29: 5:32
+                                           // + literal: Const { ty: fn() -> (i32, i32) {foo}, val: Value(Scalar(<ZST>)) }
+      }
+
+      bb1: {
+          (_1.1: i32) = const 99i32;       // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:6:5: 6:13
+                                           // ty::Const
+                                           // + ty: i32
+                                           // + val: Value(Scalar(0x00000063))
+                                           // mir::Constant
+                                           // + span: $DIR/mutable_variable_aggregate_partial_read.rs:6:11: 6:13
+                                           // + literal: Const { ty: i32, val: Value(Scalar(0x00000063)) }
+          (_1.0: i32) = const 42i32;       // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:7:5: 7:13
+                                           // ty::Const
+                                           // + ty: i32
+                                           // + val: Value(Scalar(0x0000002a))
+                                           // mir::Constant
+                                           // + span: $DIR/mutable_variable_aggregate_partial_read.rs:7:11: 7:13
+                                           // + literal: Const { ty: i32, val: Value(Scalar(0x0000002a)) }
+          StorageLive(_2);                 // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:9: 8:10
+          _2 = (_1.1: i32);                // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:13: 8:16
+          _0 = const ();                   // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:4:11: 9:2
+                                           // ty::Const
+                                           // + ty: ()
+                                           // + val: Value(Scalar(<ZST>))
+                                           // mir::Constant
+                                           // + span: $DIR/mutable_variable_aggregate_partial_read.rs:4:11: 9:2
+                                           // + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
+          StorageDead(_2);                 // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:9:1: 9:2
+          StorageDead(_1);                 // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:9:1: 9:2
+          return;                          // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:9:2: 9:2
+      }
+  }
+