Constrain const vars to error if const types are mismatched

compiler/rustc_infer/src/infer/combine.rs

@@ -189,10 +189,19 @@ impl<'tcx> InferCtxt<'tcx> {
         // the expected const's type. Specifically, we don't want const infer vars
         // to do any type shapeshifting before and after resolution.
         if let Err(guar) = compatible_types {
-            return Ok(self.tcx.const_error_with_guaranteed(
+            // HACK: equating both sides with `[const error]` eagerly prevents us
-                if relation.a_is_expected() { a.ty() } else { b.ty() },
+            // from leaving unconstrained inference vars during things like impl
-                guar,
+            // matching in the solver.
-            ));
+            let a_error = self.tcx.const_error_with_guaranteed(a.ty(), guar);
+            if let ty::ConstKind::Infer(InferConst::Var(vid)) = a.kind() {
+                return self.unify_const_variable(vid, a_error);
+            }
+            let b_error = self.tcx.const_error_with_guaranteed(b.ty(), guar);
+            if let ty::ConstKind::Infer(InferConst::Var(vid)) = b.kind() {
+                return self.unify_const_variable(vid, b_error);
+            }
+
+            return Ok(if relation.a_is_expected() { a_error } else { b_error });
         }
 
         match (a.kind(), b.kind()) {

tests/ui/const-generics/bad-subst-const-kind.rs

@@ -0,0 +1,13 @@
+// incremental
+#![crate_type = "lib"]
+
+trait Q {
+    const ASSOC: usize;
+}
+
+impl<const N: u64> Q for [u8; N] {
+    //~^ ERROR mismatched types
+    const ASSOC: usize = 1;
+}
+
+pub fn test() -> [u8; <[u8; 13] as Q>::ASSOC] { todo!() }

tests/ui/const-generics/bad-subst-const-kind.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/bad-subst-const-kind.rs:8:31
+   |
+LL | impl<const N: u64> Q for [u8; N] {
+   |                               ^ expected `usize`, found `u64`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs

@@ -2,11 +2,11 @@
 
 // An impl that has an erroneous const substitution should not specialize one
 // that is well-formed.
-
+#[derive(Clone)]
 struct S<const L: usize>;
 
 impl<const N: i32> Copy for S<N> {}
 impl<const M: usize> Copy for S<M> {}
-//~^ ERROR conflicting implementations of trait `Copy` for type `S<_>`
+//~^ ERROR conflicting implementations of trait `Copy` for type `S<[const error]>`
 
 fn main() {}

tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr

@@ -1,10 +1,10 @@
-error[E0119]: conflicting implementations of trait `Copy` for type `S<_>`
+error[E0119]: conflicting implementations of trait `Copy` for type `S<[const error]>`
   --> $DIR/bad-const-wf-doesnt-specialize.rs:9:1
    |
 LL | impl<const N: i32> Copy for S<N> {}
    | -------------------------------- first implementation here
 LL | impl<const M: usize> Copy for S<M> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `S<_>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `S<[const error]>`
 
 error: aborting due to previous error