ConstProp: Correctly remove const if unknown value assigned to it.

compiler/rustc_mir_transform/src/const_prop.rs

@@ -439,6 +439,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
         // FIXME we need to revisit this for #67176
         if rvalue.has_param() {
+            trace!("skipping, has param");
             return None;
         }
         if !rvalue
@@ -707,7 +708,11 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
     fn visit_assign(&mut self, place: &Place<'tcx>, rvalue: &Rvalue<'tcx>, location: Location) {
         self.super_assign(place, rvalue, location);
 
-        let Some(()) = self.check_rvalue(rvalue) else { return };
+        let Some(()) = self.check_rvalue(rvalue) else {
+            trace!("rvalue check failed, removing const");
+            Self::remove_const(&mut self.ecx, place.local);
+            return;
+        };
 
         match self.ecx.machine.can_const_prop[place.local] {
             // Do nothing if the place is indirect.

tests/mir-opt/const_prop/overwrite_with_const_with_params.rs

@@ -0,0 +1,26 @@
+// unit-test: ConstProp
+// compile-flags: -O
+
+// Regression test for https://github.com/rust-lang/rust/issues/118328
+
+#![allow(unused_assignments)]
+
+struct SizeOfConst<T>(std::marker::PhantomData<T>);
+impl<T> SizeOfConst<T> {
+    const SIZE: usize = std::mem::size_of::<T>();
+}
+
+// EMIT_MIR overwrite_with_const_with_params.size_of.ConstProp.diff
+fn size_of<T>() -> usize {
+    // CHECK-LABEL: fn size_of(
+    // CHECK: _1 = const 0_usize;
+    // CHECK-NEXT: _1 = const _;
+    // CHECK-NEXT: _0 = _1;
+    let mut a = 0;
+    a = SizeOfConst::<T>::SIZE;
+    a
+}
+
+fn main() {
+    assert_eq!(size_of::<u32>(), std::mem::size_of::<u32>());
+}

tests/mir-opt/const_prop/overwrite_with_const_with_params.size_of.ConstProp.diff

@@ -0,0 +1,20 @@
+- // MIR for `size_of` before ConstProp
++ // MIR for `size_of` after ConstProp
+
+  fn size_of() -> usize {
+      let mut _0: usize;
+      let mut _1: usize;
+      scope 1 {
+          debug a => _1;
+      }
+
+      bb0: {
+          StorageLive(_1);
+          _1 = const 0_usize;
+          _1 = const _;
+          _0 = _1;
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/ui/const_prop/overwrite_with_const_with_params.rs

@@ -0,0 +1,21 @@
+// compile-flags: -O
+// run-pass
+
+// Regression test for https://github.com/rust-lang/rust/issues/118328
+
+#![allow(unused_assignments)]
+
+struct SizeOfConst<T>(std::marker::PhantomData<T>);
+impl<T> SizeOfConst<T> {
+    const SIZE: usize = std::mem::size_of::<T>();
+}
+
+fn size_of<T>() -> usize {
+    let mut a = 0;
+    a = SizeOfConst::<T>::SIZE;
+    a
+}
+
+fn main() {
+    assert_eq!(size_of::<u32>(), std::mem::size_of::<u32>());
+}