Fix missing value for constant ICE

Author: BoxyUwU

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -141,6 +141,8 @@ pub fn is_const_evaluatable<'tcx>(
                     NotConstEvaluatable::MentionsInfer
                 } else if uv.has_non_region_param() {
                     NotConstEvaluatable::MentionsParam
+                } else if tcx.instantiate_and_check_impossible_predicates((uv.def, uv.args)) {
+                    return Ok(());
                 } else {
                     let guar = infcx.dcx().span_delayed_bug(
                         span,

tests/crashes/135617.rs

@@ -0,0 +1,31 @@
+//@ check-pass
+
+// Split out from `impossible_preds_repeat_expr_hack` to allow for delay span bugs to ICE
+
+pub trait Unimplemented<'a> {}
+
+trait Trait<T>
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize;
+}
+
+impl<T> Trait<T> for u8
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize = 1;
+}
+
+pub fn impossible_preds_repeat_expr_count()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    // This won't actually be an error in the future but we can't really determine that
+    let _b = [(); 1 + 1];
+    //~^ WARN: cannot use constants which depend on trivially-false where clauses
+    //~| WARN: this was previously accepted by the compiler
+}
+
+fn main() {}

tests/ui/const-generics/backcompat/impossible_preds_repeat_expr_hack-2.rs

@@ -0,0 +1,12 @@
+warning: cannot use constants which depend on trivially-false where clauses
+  --> $DIR/impossible_preds_repeat_expr_hack-2.rs:26:19
+   |
+LL |     let _b = [(); 1 + 1];
+   |                   ^^^^^
+   |
+   = 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 #76200 <https://github.com/rust-lang/rust/issues/76200>
+   = note: `#[warn(const_evaluatable_unchecked)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/const-generics/backcompat/impossible_preds_repeat_expr_hack-2.stderr

@@ -22,10 +22,26 @@ pub fn impossible_preds_repeat_expr_count()
 where
     for<'a> (): Unimplemented<'a>,
 {
+    // I expect this to potentially compile with MGCA (maybe)
+    let _a = [(); <u8 as Trait<()>>::ASSOC];
+    //~^ WARN: cannot use constants which depend on trivially-false where clauses
+    //~| WARN: this was previously accepted by the compiler
+
+    // evaluating the anon const requires evaluating an assoc const which fails even though
+    // we "best effort" try to allow evaluating repeat exprs still. this prevents equating
+    // `1` with the anon const.
+    //
+    // I expect this to potentially compile with MGCA (maybe)
     let _a: [(); 1] = [(); <u8 as Trait<()>>::ASSOC];
     //~^ WARN: cannot use constants which depend on trivially-false where clauses
     //~| WARN: this was previously accepted by the compiler
     //~^^^ ERROR: mismatched types
+
+    // This will just compiling in the future and not be an error but it's hard to
+    // detect this until making the FCW a hard error
+    let _c: [(); 2] = [(); 1 + 1];
+    //~^ WARN: cannot use constants which depend on trivially-false where clauses
+    //~| WARN: this was previously accepted by the compiler
 }
 
 struct Foo<const N: usize>;

tests/ui/const-generics/backcompat/impossible_preds_repeat_expr_hack.rs

@@ -1,23 +1,41 @@
 warning: cannot use constants which depend on trivially-false where clauses
-  --> $DIR/impossible_preds_repeat_expr_hack.rs:25:28
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:26:19
+   |
+LL |     let _a = [(); <u8 as Trait<()>>::ASSOC];
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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 #76200 <https://github.com/rust-lang/rust/issues/76200>
+   = note: `#[warn(const_evaluatable_unchecked)]` on by default
+
+warning: cannot use constants which depend on trivially-false where clauses
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:35:28
    |
 LL |     let _a: [(); 1] = [(); <u8 as Trait<()>>::ASSOC];
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = 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 #76200 <https://github.com/rust-lang/rust/issues/76200>
-   = note: `#[warn(const_evaluatable_unchecked)]` on by default
 
 error[E0308]: mismatched types
-  --> $DIR/impossible_preds_repeat_expr_hack.rs:25:23
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:35:23
    |
 LL |     let _a: [(); 1] = [(); <u8 as Trait<()>>::ASSOC];
    |             -------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an array with a size of 1, found one with a size of <u8 as Trait<()>>::ASSOC
    |             |
    |             expected due to this
 
+warning: cannot use constants which depend on trivially-false where clauses
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:42:28
+   |
+LL |     let _c: [(); 2] = [(); 1 + 1];
+   |                            ^^^^^
+   |
+   = 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 #76200 <https://github.com/rust-lang/rust/issues/76200>
+
 error[E0277]: the trait bound `for<'a> (): Unimplemented<'a>` is not satisfied
-  --> $DIR/impossible_preds_repeat_expr_hack.rs:37:43
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:53:43
    |
 LL |     let _a: Foo<1> = Foo::<{ <u8 as Trait<()>>::ASSOC }>;
    |                                           ^^ the trait `for<'a> Unimplemented<'a>` is not implemented for `()`
@@ -37,7 +55,7 @@ LL |     const ASSOC: usize;
    |           ----- required by a bound in this associated constant
 
 error[E0277]: the trait bound `for<'a> (): Unimplemented<'a>` is not satisfied
-  --> $DIR/impossible_preds_repeat_expr_hack.rs:37:31
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:53:31
    |
 LL |     let _a: Foo<1> = Foo::<{ <u8 as Trait<()>>::ASSOC }>;
    |                               ^^ the trait `for<'a> Unimplemented<'a>` is not implemented for `()`
@@ -56,7 +74,7 @@ LL | where
 LL |     for<'a> T: Unimplemented<'a>,
    |                ----------------- unsatisfied trait bound introduced here
 
-error: aborting due to 3 previous errors; 1 warning emitted
+error: aborting due to 3 previous errors; 3 warnings emitted
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.