Lifetime is unnecessarily captured in nested functions returning impl trait via async

[sfackler]

When trying to create some futures that do a bit of work up-front to avoid borrowing some inputs, I ran into this strange issue where it seems like an input lifetime is being unnecessarily captured by the returned impl trait even though the value isn't:

#![feature(async_await)]
use std::future::Future;

fn query<'a, I>(params: I) -> impl Future<Output = Vec<i32>> + 'static
where
    I: IntoIterator<Item = &'a i32>,
{
    let start = start(params);
    
    async {
        start.await
    }
}

fn start<'a, I>(params: I) -> impl Future<Output = Vec<i32>> + 'static
where
    I: IntoIterator<Item = &'a i32>,
{
    let buf = encode(params);
    
    async {
        buf
    }
}

fn encode<'a, I>(params: I) -> Vec<i32>
where
    I: IntoIterator<Item = &'a i32>,
{
    params.into_iter().cloned().collect()
}

error[E0310]: the parameter type `I` may not live long enough
 --> src/lib.rs:4:31
  |
4 | fn query<'a, I>(params: I) -> impl Future<Output = Vec<i32>> + 'static
  |              -                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |              |
  |              help: consider adding an explicit lifetime bound `I: 'static`...
  |
note: ...so that the type `impl std::future::Future` will meet its required lifetime bounds
 --> src/lib.rs:4:31
  |
4 | fn query<'a, I>(params: I) -> impl Future<Output = Vec<i32>> + 'static
  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0310`.

It also seems really weird that this only happens with 2 layers of "nesting" - start compiles just fine, but query doesn't even though it's pretty much just a pass-through.

[nikomatsakis]

Check-in from async-await foundations wg: marking as blocking for now and self-assigning to do a bit of investigation. Not entirely sure what's going on.

[nikomatsakis]

OK, so, first of all -- removing the + 'static bound (which is not necessary) makes the code compile.

[nikomatsakis]

I think that this bug is a duplicate of #42940. What happens is that the return type of start is something like Opaque<I> -- i.e., it captures all the type parameters from the calling function. Now, we know that type Opaque<I> = impl Future<Output=Vec<i32>> + 'static, so it ought to compile, but because of #42940, we wind up requiring that I: 'static in order to compile.

[nikomatsakis]

As this is a duplicate of #42940, it doesn't need to block stabilization. In particular, I believe that improving our ability to figure out that the type is 'static would be forwards compatible -- also, this is really a bug about impl Trait, more than async itself.

[steveklabnik]

Triage: to get this to compile on stable, you only need to remove the feature flag, the code still "works." It fails to compile, and removing 'static lets it still compile.