always evaluate constant operands in const-prop

Author: b-naber

compiler/rustc_mir_transform/src/const_prop.rs

@@ -468,11 +468,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
     /// Returns the value, if any, of evaluating `c`.
     fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
-        // FIXME we need to revisit this for #67176
-        if c.needs_subst() {
-            return None;
-        }
-
         self.ecx.mir_const_to_op(&c.literal, None).ok()
     }
 

src/test/ui/consts/const-eval/issue-50814.rs

@@ -14,12 +14,15 @@ struct Sum<A,B>(A,B);
 impl<A: Unsigned, B: Unsigned> Unsigned for Sum<A,B> {
     const MAX: u8 = A::MAX + B::MAX;
     //~^ ERROR any use of this value will cause an error [const_err]
+    //~| ERROR any use of this value will cause an error
     //~| WARN this was previously accepted by the compiler but is being phased out
+    //~| WARNING this was previously accepted by the compiler
 }
 
 fn foo<T>(_: T) -> &'static u8 {
     &Sum::<U8,U8>::MAX
     //~^ ERROR E0080
+    //~| ERROR evaluation of `foo::<i32>` failed
 }
 
 fn main() {

src/test/ui/consts/const-eval/issue-50814.stderr

@@ -8,19 +8,36 @@ LL |     const MAX: u8 = A::MAX + B::MAX;
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
+error[E0080]: evaluation of `foo::<T>` failed
+  --> $DIR/issue-50814.rs:23:6
+   |
+LL |     &Sum::<U8,U8>::MAX
+   |      ^^^^^^^^^^^^^^^^^ referenced constant has errors
+
+error: any use of this value will cause an error
+  --> $DIR/issue-50814.rs:15:21
+   |
+LL |     const MAX: u8 = A::MAX + B::MAX;
+   |     ----------------^^^^^^^^^^^^^^^-
+   |                     |
+   |                     attempt to compute `u8::MAX + u8::MAX`, which would overflow
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
 error[E0080]: evaluation of `foo::<i32>` failed
-  --> $DIR/issue-50814.rs:21:6
+  --> $DIR/issue-50814.rs:23:6
    |
 LL |     &Sum::<U8,U8>::MAX
    |      ^^^^^^^^^^^^^^^^^ referenced constant has errors
 
 note: the above error was encountered while instantiating `fn foo::<i32>`
-  --> $DIR/issue-50814.rs:26:5
+  --> $DIR/issue-50814.rs:29:5
    |
 LL |     foo(0);
    |     ^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0080`.
 Future incompatibility report: Future breakage diagnostic: