How can I get impl Trait to use the appropriate lifetime for a mutable reference to a value with another lifetime in it?

Question

I have a struct with a lifetime:

struct HasLifetime<'a>( /* ... */ );

There is there is an implementation of the trait Foo:

impl<'a, 'b: 'a> Foo for &'a mut HasLifetime<'b> { }

I want to implement the following function:

fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut Lifetime<'b>) -> impl Foo {
    bar
}

This won't compile because the returned impl is only valid for 'a. However, specifying impl Foo + 'a results in:

error[E0909]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
 --> src/main.rs:7:60
  |
7 | fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut HasLifetime<'b>) -> impl Trait + 'a {
  |                                                            ^^^^^^^^^^^^^^^
  |
note: hidden type `&'a mut HasLifetime<'b>` captures the lifetime 'b as defined on the function body at 7:1
 --> src/main.rs:7:1
  |
7 | fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut HasLifetime<'b>) -> impl Trait + 'a {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The seemingly equivalent function with a boxed trait object compiles:

fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut Lifetime<'b>) -> Box<Foo + 'a> {
    Box::new(bar)
}

How can I define bar_to_foo with impl Trait?

Playground link

Answer 1

You need to indicate that the returned value is built upon multiple lifetimes. However, you can't use multiple lifetime bounds with impl Trait, and attempting to do so doesn't have a useful error message.

There's a trick you can use that involves creating a dummy trait that has a lifetime parameter:

trait Captures<'a> {}
impl<'a, T: ?Sized> Captures<'a> for T {}

fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut HasLifetime<'b>) -> impl Trait + Captures<'b> + 'a {
    bar
}

Thankfully, this only occurs when the "hidden" lifetime is invariant, which occurs because the reference is mutable.