False positive by redundant_closure to remove a closure that is required for lifetime reasons

[weiznich]

Lint name: redundant_closure

I tried this code:

pub trait Insertable<T> {
    type Values;

    fn values(self) -> Self::Values;
}


pub trait AsValueIterator<Tab> {
    type Item;

    fn as_value_iter<'a>(
        &'a self,
    ) -> Box<dyn Iterator<Item = <&'a Self::Item as Insertable<Tab>>::Values> + 'a>
    where
        Self::Item: 'a,
        &'a Self::Item: Insertable<Tab>;
}

impl<T, Tab, const N: usize> AsValueIterator<Tab> for [T; N] {
    type Item = T;

    fn as_value_iter<'a>(&'a self) -> Box<dyn Iterator<Item = <&'a T as Insertable<Tab>>::Values> + 'a>
    where
        Self::Item: 'a,
        &'a T: Insertable<Tab>,
    {
        Box::new(IntoIterator::into_iter(self).map(|v| Insertable::values(v)))
    }
}

impl<'b, T, Tab, const N: usize> AsValueIterator<Tab> for &'b [T; N] {
    type Item = T;

    fn as_value_iter<'a>(
        &'a self,
    ) -> Box<dyn Iterator<Item = <&'a Self::Item as Insertable<Tab>>::Values> + 'a>
    where
        Self::Item: 'a,
        &'a T: Insertable<Tab>,
    {
        Box::new(IntoIterator::into_iter(*self).map(|v| Insertable::values(v)))
    }
}

impl<T, Tab, const N: usize> AsValueIterator<Tab> for Box<[T; N]> {
    type Item = T;

    fn as_value_iter<'a>(
        &'a self,
    ) -> Box<dyn Iterator<Item = <&'a Self::Item as Insertable<Tab>>::Values> + 'a>
    where
        Self::Item: 'a,
        &'a T: Insertable<Tab>,
    {
        Box::new(IntoIterator::into_iter(&**self).map(|v| Insertable::values(v)))
    }
}

impl<T, Tab> AsValueIterator<Tab> for Vec<T> {
    type Item = T;

    fn as_value_iter<'a>(
        &'a self,
    ) -> Box<dyn Iterator<Item = <&'a Self::Item as Insertable<Tab>>::Values> + 'a>
    where
        Self::Item: 'a,
        &'a T: Insertable<Tab>,
    {
        Box::new(IntoIterator::into_iter(self).map(|v| Insertable::values(v)))
    }
}

impl<'b, T, Tab> AsValueIterator<Tab> for &'b [T] {
    type Item = T;

    fn as_value_iter<'a>(
        &'a self,
    ) -> Box<dyn Iterator<Item = <&'a Self::Item as Insertable<Tab>>::Values> + 'a>
    where
        Self::Item: 'a,
        &'a T: Insertable<Tab>,
    {
        Box::new(IntoIterator::into_iter(*self).map(|v| Insertable::values(v)))
    }
}

This is a minimized version from real code inside of diesel. See this playground for an interactive version.

This results in the following output:

warning: redundant closure
  --> src/main.rs:30:52
   |
30 |         Box::new(IntoIterator::into_iter(self).map(|v| Insertable::values(v)))
   |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `Insertable::values`
   |
   = note: `#[warn(clippy::redundant_closure)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

warning: redundant closure
  --> src/main.rs:44:53
   |
44 |         Box::new(IntoIterator::into_iter(*self).map(|v| Insertable::values(v)))
   |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `Insertable::values`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

warning: redundant closure
  --> src/main.rs:58:55
   |
58 |         Box::new(IntoIterator::into_iter(&**self).map(|v| Insertable::values(v)))
   |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `Insertable::values`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

warning: redundant closure
  --> src/main.rs:72:52
   |
72 |         Box::new(IntoIterator::into_iter(self).map(|v| Insertable::values(v)))
   |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `Insertable::values`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

warning: redundant closure
  --> src/main.rs:86:53
   |
86 |         Box::new(IntoIterator::into_iter(*self).map(|v| Insertable::values(v)))
   |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `Insertable::values`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

I expected to see this happen: No warning or a suggestion that won't cause a compiler error after applying.
Instead, this happened: Clippy suggested to remove the map closure in every trait implementation. This results in a compiler error (See this playground for a changed version)

Meta

• cargo clippy -V: 0.1.55 (2021-07-26 08095fc)
• rustc -Vv:

1.56.0-nightly (2021-07-26 08095fc1f875c89e507f)
(Playground version)

[Jarcho]

This was fixed by #7661.