Change:
<ins> error: const `impl` for trait `Foo` which is not marked with `#[const_trait]` --> $DIR/spec-effectvar-ice.rs:12:15 | LL | trait Foo {} | - help: mark `Foo` as const: `#[const_trait]` LL | LL | impl<T> const Foo for T {} |               ^^^ | = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const` = note: adding a non-const method body in the future would be a breaking change error: const `impl` for trait `Foo` which is not marked with `#[const_trait]` --> $DIR/spec-effectvar-ice.rs:16:15 | LL | trait Foo {} | - help: mark `Foo` as const: `#[const_trait]` ... LL | impl<T> const Foo for T where T: const Specialize {} |               ^^^ | = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const` = note: adding a non-const method body in the future would be a breaking change error: `const` can only be applied to `#[const_trait]` traits --> $DIR/spec-effectvar-ice.rs:16:40 | LL | impl<T> const Foo for T where T: const Specialize {} |                                        ^^^^^^^^^^ error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates --> $DIR/spec-effectvar-ice.rs:12:9 | LL | impl<T> const Foo for T {} |         ^^^^^ unconstrained const parameter | = note: expressions using a const parameter must map each value to a distinct output value = note: proving the result of expressions other than the parameter are unique is not supported error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates --> $DIR/spec-effectvar-ice.rs:16:9 | LL | impl<T> const Foo for T where T: const Specialize {} |         ^^^^^ unconstrained const parameter | = note: expressions using a const parameter must map each value to a distinct output value = note: proving the result of expressions other than the parameter are unique is not supported error: specialization impl does not specialize any associated items --> $DIR/spec-effectvar-ice.rs:16:1 | LL | impl<T> const Foo for T where T: const Specialize {} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | note: impl is a specialization of this impl --> $DIR/spec-effectvar-ice.rs:12:1 | LL | impl<T> const Foo for T {} | ^^^^^^^^^^^^^^^^^^^^^^^ error: could not resolve generic parameters on overridden impl --> $DIR/spec-effectvar-ice.rs:16:1 | LL | impl<T> const Foo for T where T: const Specialize {} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ error: aborting due to 7 previous errors For more information about this error, try `rustc --explain E0207`. </ins>

Discussion:
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This is caused by the fact that  does not check for const effect vars when visiting . It should be relatively simple to match on in the folder.
Working on this one.