RPITIT with generic const constraint requires implied lifetime bounds to be explicitly specified in impls

[alice-emerson]

I tried this code:

#![feature(generic_const_exprs)]

trait Trait {}
struct Concrete;
impl Trait for Concrete {}

trait Demo<const N: usize> {
    fn neither() -> Concrete;
    fn bound_only() -> Concrete where [(); N-1]:;
    fn rpit_only() -> impl Trait;
    fn rpit_and_bound() -> impl Trait where [(); N-1]:;
}

impl<'a, T, const N: usize> Demo<N> for &'a T {
    // Compiles fine
    fn neither() -> Concrete { Concrete }
    fn bound_only() -> Concrete where [(); N-1]: { Concrete }
    fn rpit_only() -> impl Trait { Concrete }
    
    // Compile error
    fn rpit_and_bound() -> impl Trait where [(); N-1]: { Concrete }
}

I expected to see this happen: The code compiles without errors

Instead, this happened:

The [(); N-1]: bound in the trait definition is highlighted as an error complaining that T does not outlive 'a, and suggests adding T: 'a to the impl block.

Meta

rustc --version --verbose:

rustc 1.87.0-nightly (665025243 2025-03-11)
binary: rustc
commit-hash: 6650252439d4e03368b305c42a10006e36f1545e
commit-date: 2025-03-11
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

[ShE3py]

Minimized:

#![feature(generic_const_exprs)]
// ^ compiles fine without this

trait Foo {
    fn foo() -> impl ToString where [(); 0 + 0]:;
}

impl<T> Foo for &'_ T {
    fn foo() -> impl ToString where [(); 0 + 0]: { "foo" }
}

error[E0311]: the parameter type `T` may not live long enough
 --> src/lib.rs:5:37
  |
5 |     fn foo() -> impl ToString where [(); 0 + 0]:;
  |                                     ^^^^^^^^^^^ ...so that the reference type `&T` does not outlive the data it points at
...
8 | impl<T> Foo for &'_ T {
  |                  -- the parameter type `T` must be valid for the anonymous lifetime as defined here...
  |
help: consider adding an explicit lifetime bound
  |
8 - impl<T> Foo for &'_ T {
8 + impl<'a, T: 'a> Foo for &'a T {
  |

@rustbot label +T-compiler +F-generic_const_exprs +requires-incomplete-features