rustc_typeck::check::compare_method::compare_const_impl doesn't drain its fullfillment context nor invoke regionck.

[eddyb]

UPDATE: Mentoring instructions for fixing this bug can be found here.

---

There are some calls to add obligations to a fulfillment context but it's never checked.

Some examples of code that shouldn't compile:

trait TransmuteTarget: Sized {
    type Out: Sized;
}

// Can't use generic impl here.
impl TransmuteTarget for String {
    type Out = String;
}

trait Transmute: TransmuteTarget {
    const FROM: Self::Out;
}

impl<T: TransmuteTarget> Transmute for T {
    // Associated type projections aren't checked (obligation fulfillment).
    const FROM: &'static str = "foo";
}

trait Foo {
    const NAME: &'static str;
}

impl<'a> Foo for &'a () {
    // Lifetimes aren't checked (regionck).
    const NAME: &'a str = "unit";
}

cc @rust-lang/compiler

[nikomatsakis]

triage: P-medium

[withoutboats]

@eddyb does this block stabilization of associated consts?

[eddyb]

@withoutboats Yes, as it allows const types in impls that don't match the trait definition.

[withoutboats]

This can be exploited to elongate lifetimes:

#![feature(associated_consts)]

trait Evil<'a, T: 'static> {
    const EVIL: fn(&'a T) -> &'static T;
}

impl<'a, T: 'static> Evil<'a, T> for () {
    const EVIL: fn(&'a T) -> &'a T = identity;
}

fn identity<T: 'static>(s: &T) -> &T { s }

fn evil() -> &'static Box<i32> {
    let b = Box::new(0);
    <()>::EVIL(&b)
}


fn main() {
    println!("{}", **evil())
}