dyn Trait<'a> shouldn't carry 'a lifetime

[yshui]

I tried this code:

fn check<'a, T: Trait<'a> + ?Sized>() {}
trait Trait<'a>: 'static {}

fn check2<'a>() {
    check::<dyn Trait<'a> + 'static>();
}

I expected to see this happen: It compiles

dyn Trait<'a> should have an lifetime independent from 'a. i.e. dyn Trait<'a> + 'b should have lifetime 'b. in this particular case, 'b is 'static.

Instead, this happened:

error: lifetime may not live long enough
 --> src/lib.rs:5:5
  |
4 | fn check2<'a>() {
  |           -- lifetime `'a` defined here
5 |     check::<dyn Trait<'a> + 'static>();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`

Related

#88904. Since this is another way of creating dyn Trait: !Trait, except in this case, I think the unsized coercion should be allowed, and dyn Trait does impl Trait.

[yshui]

Besides giving dyn Trait<'a> a lifetime independent of 'a, I think if trait Trait<'a>: 'static, then dyn Trait<'a> should implicitly be : 'static

[Jules-Bertholet]

It's an invariant of the type system that projections from a type can't have a lesser lifetime than that type. In other words, if T: 'static, then T::Some::Arbitary::Path: 'static must also hold (unless the path involves some non-'static lifetime parameter). If dyn Trait<'a> was 'static, this rule could be violated:

trait Foo {
    type Assoc;
}

impl<'a> Foo for dyn Trait<'a> {
    type Assoc = &'a u32;
}

// `<dyn Trait<'a> as Foo>::Assoc` is not `'static`, so `dyn Trait<'a>` cannot be `'static` either

@rustbot label -C-bug C-discussion A-lifetimes A-trait-objects T-types

[yshui]

@Jules-Bertholet hmm, ok.

What if we explicitly apply this rule and disallow the impl in this case?

impl<'a> Foo for dyn Trait<'a> + 'static {
    type Assoc = &'a u32; // error: <dyn Trait<'a> + 'static as Foo>::Assoc must outlive 'static
}

I would expect this at least needs an RFC, but does this make sense?

[Jules-Bertholet]

@yshui That would be a breaking change, as the impl for dyn Trait<'a> is currently accepted on stable Rust.

[yshui]

true, but that only affects impls explicitly for dyn Trait<'a> + 'lifetime, currently this + 'lifetime does nothing, so I doubt there's a lot of code out there written like this.