impl Trait return value infers 'static lifetime

[HMPerson1]

This code (playground):

struct Foo {
    v: Vec<u8>
}

impl Foo {
    fn bar(&self) -> impl Iterator<Item=u8> {
        self.v.iter().cloned()
    }
}

fails to compile with:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements
 --> src/main.rs:7:16
  |
7 |         self.v.iter().cloned()
  |                ^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 6:5...
 --> src/main.rs:6:5
  |
6 | /     fn bar(&self) -> impl Iterator {
7 | |         self.v.iter().cloned()
8 | |     }
  | |_____^
note: ...so that reference does not outlive borrowed content
 --> src/main.rs:7:9
  |
7 |         self.v.iter().cloned()
  |         ^^^^^^
  = note: but, the lifetime must be valid for the static lifetime...
note: ...so that return value is valid for the call
 --> src/main.rs:6:22
  |
6 |     fn bar(&self) -> impl Iterator<Item=u8> {
  |                      ^^^^^^^^^^^^^^^^^^^^^^

Replacing impl Iterator with an explicit type works:

fn bar(&self) -> std::iter::Cloned<std::slice::Iter<u8>> {
    self.v.iter().cloned()
}

Explicitly bounding the output lifetime also works:

fn bar<'a>(&'a self) -> impl Iterator<Item=u8> + 'a {
    self.v.iter().cloned()
}

[estebank]

I don't know what the implications are of starting to infer 'self instead of 'static, but we could definitely improve the error message to help you fix the code.

The following code also works:

struct Foo {
    v: Vec<u8>
}

impl Foo {
    fn bar1(&self) -> impl Iterator<Item=u8> + '_ {
        self.v.iter().cloned()
    }

    fn bar2(&self) -> std::iter::Cloned<std::slice::Iter<u8>> {
        self.v.iter().cloned()
    }

    fn bar3<'a>(&'a self) -> impl Iterator<Item=u8> + 'a {
        self.v.iter().cloned()
    }
}

fn main() {}

@nikomatsakis what do you think would break by modifying the behavior to infer impl Trait + '_? The thing that was worrying me mostly (impl Trait + '_ + 'a), works.

[HMPerson1]

Oh, I didn't realize that lifetimes had to be brought into scope first. 😅

So then this works:

fn bar(&self) -> impl Iterator<Item=u8> + '_ {
    self.v.iter().cloned()
}

[estebank]

cc #43719, #38615

[estebank]

Current output:

error: cannot infer an appropriate lifetime
 --> src/main.rs:7:16
  |
6 |     fn bar1(&self) -> impl Iterator<Item=u8> {
  |                       ---------------------- this return type evaluates to the `'static` lifetime...
7 |         self.v.iter().cloned()
  |         ------ ^^^^
  |         |
  |         ...but this borrow...
  |
note: ...can't outlive the anonymous lifetime #1 defined on the method body at 6:5
 --> src/main.rs:6:5
  |
6 | /     fn bar1(&self) -> impl Iterator<Item=u8> {
7 | |         self.v.iter().cloned()
8 | |     }
  | |_____^
help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the method body at 6:5
  |
6 |     fn bar1(&self) -> impl Iterator<Item=u8> + '_ {
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^