#[marker]: overlapping impls misinteract with lifetimes

[danielhenrymantilla]

I tried this code:

#![feature(marker_trait_attr)]

#[marker] trait Marker {}

impl Marker for &'_ () {}
impl Marker for &'_ () {}

I expected to see this happen:

• Code compiles fine, and for any 'lt, &'lt () : Marker does hold.

Instead, this happened:

• error[E0283]: type annotations needed
   --> src/lib.rs:5:6
    |
  5 | impl Marker for &'_ () {}
    |      ^^^^^^ cannot infer type for reference `&()`
    |
    = note: cannot satisfy `&(): Marker`
  note: required by a bound in `Marker`
   --> src/lib.rs:3:11
    |
  3 | #[marker] trait Marker {}
    |           ^^^^^^^^^^^^ required by this bound in `Marker`
  
  error[E0283]: type annotations needed
   --> src/lib.rs:6:6
    |
  6 | impl Marker for &'_ () {}
    |      ^^^^^^ cannot infer type for reference `&()`
    |
    = note: cannot satisfy `&(): Marker`
  note: required by a bound in `Marker`
   --> src/lib.rs:3:11
    |
  3 | #[marker] trait Marker {}
    |           ^^^^^^^^^^^^ required by this bound in `Marker`
  
  For more information about this error, try `rustc --explain E0283`.

  • FWIW, here is the one-sentence description of that error code:

    An implementation cannot be chosen unambiguously because of lack of information.

Meta

Happens as of the latest nightly available on the Playground:

• 1.57.0-nightly — 2021-10-03

Context

This comes from #89357 (review), once reduced to:

trait A {}
trait B {}

impl A for &'_ () {}
impl B for &'_ () {}

/// `A ∪ B`
#[marker]
trait Marker {}

impl<T : A> Marker for T {}
impl<T : B> Marker for T {}

const _: () = {
    fn assert_impls_Marker<T : Marker> ()
    {}
    
    let _ = assert_impls_Marker::<&()>; // Fails
    let _ = assert_impls_Marker::<&'static ()>; // Fails as well
};

which was then further reduced to the initial snippet.

---

@rustbot modify labels +requires-nightly +F-marker_trait_attr +A-traits +A-lifetimes

• Tracking issue: Tracking issue for allowing overlapping implementations for marker trait #29864

[rustbot]

Error: Label B-unstable can only be set by Rust team members

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[crlf0710]

Seems this is just a diagnostics issue? Better error message is needed here, and "type annotations needed" is not actually correct.

Edit: No, sorry.