Avoid ICE when handling const auto traits in the next-gen solver

Author: lapla-cogito

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -189,10 +189,11 @@ where
     }
 
     fn consider_auto_trait_candidate(
-        _ecx: &mut EvalCtxt<'_, D>,
+        ecx: &mut EvalCtxt<'_, D>,
         _goal: Goal<I, Self>,
     ) -> Result<Candidate<I>, NoSolution> {
-        unreachable!("auto traits are never const")
+        ecx.cx().delay_bug("auto traits are never const");
+        Err(NoSolution)
     }
 
     fn consider_trait_alias_candidate(

tests/ui/traits/const-traits/const-auto-trait.rs

@@ -1,6 +1,13 @@
+//@ compile-flags: -Znext-solver
 #![feature(auto_traits, const_trait_impl)]
 
 const auto trait Marker {}
 //~^ ERROR: auto traits cannot be const
 
+fn scope() {
+    fn check<T: const Marker>() {}
+    check::<()>();
+    //~^ ERROR: the trait bound `(): const Marker` is not satisfied
+}
+
 fn main() {}

tests/ui/traits/const-traits/const-auto-trait.stderr

@@ -1,10 +1,23 @@
 error: auto traits cannot be const
-  --> $DIR/const-auto-trait.rs:3:1
+  --> $DIR/const-auto-trait.rs:4:1
    |
 LL | const auto trait Marker {}
    | ^^^^^
    |
    = help: remove the `const` keyword
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `(): const Marker` is not satisfied
+  --> $DIR/const-auto-trait.rs:9:13
+   |
+LL |     check::<()>();
+   |             ^^
+   |
+note: required by a bound in `check`
+  --> $DIR/const-auto-trait.rs:8:17
+   |
+LL |     fn check<T: const Marker>() {}
+   |                 ^^^^^^^^^^^^ required by this bound in `check`
+
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0277`.