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Incoherent error when a method and the containing impl require the same trait with different lifetimes #78258

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elidupree opened this issue Oct 23, 2020 · 2 comments
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A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. A-lifetimes Area: lifetime related A-traits Area: Trait system C-bug Category: This is a bug. D-confusing Confusing diagnostic error that should be reworked T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@elidupree
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elidupree commented Oct 23, 2020

While trying to make the lifetimes work in a -> impl Iterator method, I ran into a weird error. Minimal test case:

trait Trait<'a> {}

struct Struct<'a, T>(&'a T);

impl<'a, T: Trait<'a>> Struct<'a, T> {
    pub fn method<'b>(&self)
    where
        T: Trait<'b>,
    {
    }
}

As far as I can tell, this should be valid code. However, it gives this bizarre error message:

error[E0283]: type annotations needed
 --> src/lib.rs:5:13
  |
1 | trait Trait<'a> {}
  | --------------- required by this bound in `Trait`
...
5 | impl<'a, T: Trait<'a>> Struct<'a, T> {
  |             ^^^^^^^^^ cannot infer type for type parameter `T`
  |
  = note: cannot satisfy `T: Trait<'a>`

This occurs on both stable and nightly.

More details

The error still occurs if we replace one lifetime with 'static - either

trait Trait<'a> {}

struct Struct<T>(T);

impl<'a, T: Trait<'a>> Struct<T> {
    pub fn method(&self)
    where
        T: Trait<'static>,
    {
    }
}

or

trait Trait<'a> {}

struct Struct<T>(T);

impl<T: Trait<'static>> Struct<T> {
    pub fn method<'b>(&self)
    where
        T: Trait<'b>,
    {
    }
}

But it doesn't occur if either bound is removed, or if the same lifetime is used in both bounds, or if the bounds differ by a type parameter rather than a lifetime parameter.

On stable, the error message is even less helpful if the trait has an associated type:

trait Trait<'a> {
    type A;
}

struct Struct<'a, T>(&'a T);

impl<'a, T: Trait<'a>> Struct<'a, T> {
    pub fn method<'b>(&self)
    where
        T: Trait<'b>,
    {
    }
}
error[E0282]: type annotations needed
 --> src/lib.rs:7:13
  |
7 | impl<'a, T: Trait<'a>> Struct<'a, T> {
  |             ^^^^^^^^^ cannot infer type

But on nightly, the error is the same either way.

@elidupree elidupree added the C-bug Category: This is a bug. label Oct 23, 2020
@ohsayan
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ohsayan commented Oct 23, 2020

@rustbot modify labels to +A-traits +A-impl-trait +A-lifetimes +T-compiler
Edit: oops, I accidentally tagged T-lang (seems like it's for T-compiler)

@rustbot rustbot added A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. A-lifetimes Area: lifetime related A-traits Area: Trait system T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Oct 23, 2020
@elidupree
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For reference, in my actual code, I wrote this to work around #62849 – the Self type requires associated types from the trait, so I have to have a lifetime parameter on the impl, but the Item type in the returned iterator also requires associated types from the trait, so I have to have a second lifetime parameter on the method or I run into the error `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope.

@estebank estebank added D-confusing Confusing diagnostic error that should be reworked T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. and removed T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Oct 23, 2020
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Labels
A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. A-lifetimes Area: lifetime related A-traits Area: Trait system C-bug Category: This is a bug. D-confusing Confusing diagnostic error that should be reworked T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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