Add some more sanity tests and add a debug log message for it

3 files changed, 94 insertions, 0 deletions

diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index d66c909fe6c..88dfa90ac21 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -893,6 +893,11 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
                     // ```
                     // FIXME: we overzealously erase the entire local, because that's easier to
                     // implement.
+                    trace!(
+                        "propagation into {:?} failed.
+                        Nuking the entire site from orbit, it's the only way to be sure",
+                        place,
+                    );
                     Self::remove_const(&mut self.ecx, place.local);
                 }
             }
diff --git a/src/test/mir-opt/const_prop/mutable_variable_unprop_assign.rs b/src/test/mir-opt/const_prop/mutable_variable_unprop_assign.rs
new file mode 100644
index 00000000000..40f801b1b5e
--- /dev/null
+++ b/src/test/mir-opt/const_prop/mutable_variable_unprop_assign.rs
@@ -0,0 +1,15 @@
+// compile-flags: -O
+
+// EMIT_MIR rustc.main.ConstProp.diff
+fn main() {
+    let a = foo();
+    let mut x: (i32, i32) = (1, 2);
+    x.1 = a;
+    let y = x.1;
+    let z = x.0; // this could theoretically be allowed, but we can't handle it right now
+}
+
+#[inline(never)]
+fn foo() -> i32 {
+    unimplemented!()
+}
diff --git a/src/test/mir-opt/const_prop/mutable_variable_unprop_assign/rustc.main.ConstProp.diff b/src/test/mir-opt/const_prop/mutable_variable_unprop_assign/rustc.main.ConstProp.diff
new file mode 100644
index 00000000000..738343c655e
--- /dev/null
+++ b/src/test/mir-opt/const_prop/mutable_variable_unprop_assign/rustc.main.ConstProp.diff
@@ -0,0 +1,74 @@
+- // MIR for `main` before ConstProp
++ // MIR for `main` after ConstProp
+  
+  fn main() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/mutable_variable_unprop_assign.rs:4:11: 4:11
+      let _1: i32;                         // in scope 0 at $DIR/mutable_variable_unprop_assign.rs:5:9: 5:10
+      let mut _3: i32;                     // in scope 0 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
+      scope 1 {
+          debug a => _1;                   // in scope 1 at $DIR/mutable_variable_unprop_assign.rs:5:9: 5:10
+          let mut _2: (i32, i32) as UserTypeProjection { base: UserType(0), projs: [] }; // in scope 1 at $DIR/mutable_variable_unprop_assign.rs:6:9: 6:14
+          scope 2 {
+              debug x => _2;               // in scope 2 at $DIR/mutable_variable_unprop_assign.rs:6:9: 6:14
+              let _4: i32;                 // in scope 2 at $DIR/mutable_variable_unprop_assign.rs:8:9: 8:10
+              scope 3 {
+                  debug y => _4;           // in scope 3 at $DIR/mutable_variable_unprop_assign.rs:8:9: 8:10
+                  let _5: i32;             // in scope 3 at $DIR/mutable_variable_unprop_assign.rs:9:9: 9:10
+                  scope 4 {
+                      debug z => _5;       // in scope 4 at $DIR/mutable_variable_unprop_assign.rs:9:9: 9:10
+                  }
+              }
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);                 // scope 0 at $DIR/mutable_variable_unprop_assign.rs:5:9: 5:10
+          _1 = const foo() -> bb1;         // scope 0 at $DIR/mutable_variable_unprop_assign.rs:5:13: 5:18
+                                           // ty::Const
+                                           // + ty: fn() -> i32 {foo}
+                                           // + val: Value(Scalar(<ZST>))
+                                           // mir::Constant
+                                           // + span: $DIR/mutable_variable_unprop_assign.rs:5:13: 5:16
+                                           // + literal: Const { ty: fn() -> i32 {foo}, val: Value(Scalar(<ZST>)) }
+      }
+  
+      bb1: {
+          StorageLive(_2);                 // scope 1 at $DIR/mutable_variable_unprop_assign.rs:6:9: 6:14
+          _2 = (const 1i32, const 2i32);   // scope 1 at $DIR/mutable_variable_unprop_assign.rs:6:29: 6:35
+                                           // ty::Const
+                                           // + ty: i32
+                                           // + val: Value(Scalar(0x00000001))
+                                           // mir::Constant
+-                                          // + span: $DIR/mutable_variable_unprop_assign.rs:6:30: 6:31
++                                          // + span: $DIR/mutable_variable_unprop_assign.rs:6:29: 6:35
+                                           // + literal: Const { ty: i32, val: Value(Scalar(0x00000001)) }
+                                           // ty::Const
+                                           // + ty: i32
+                                           // + val: Value(Scalar(0x00000002))
+                                           // mir::Constant
+-                                          // + span: $DIR/mutable_variable_unprop_assign.rs:6:33: 6:34
++                                          // + span: $DIR/mutable_variable_unprop_assign.rs:6:29: 6:35
+                                           // + literal: Const { ty: i32, val: Value(Scalar(0x00000002)) }
+          StorageLive(_3);                 // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
+          _3 = _1;                         // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
+          (_2.1: i32) = move _3;           // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:5: 7:12
+          StorageDead(_3);                 // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
+          StorageLive(_4);                 // scope 2 at $DIR/mutable_variable_unprop_assign.rs:8:9: 8:10
+          _4 = (_2.1: i32);                // scope 2 at $DIR/mutable_variable_unprop_assign.rs:8:13: 8:16
+          StorageLive(_5);                 // scope 3 at $DIR/mutable_variable_unprop_assign.rs:9:9: 9:10
+          _5 = (_2.0: i32);                // scope 3 at $DIR/mutable_variable_unprop_assign.rs:9:13: 9:16
+          _0 = const ();                   // scope 0 at $DIR/mutable_variable_unprop_assign.rs:4:11: 10:2
+                                           // ty::Const
+                                           // + ty: ()
+                                           // + val: Value(Scalar(<ZST>))
+                                           // mir::Constant
+                                           // + span: $DIR/mutable_variable_unprop_assign.rs:4:11: 10:2
+                                           // + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
+          StorageDead(_5);                 // scope 3 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
+          StorageDead(_4);                 // scope 2 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
+          StorageDead(_2);                 // scope 1 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
+          StorageDead(_1);                 // scope 0 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
+          return;                          // scope 0 at $DIR/mutable_variable_unprop_assign.rs:10:2: 10:2
+      }
+  }
+