Warn on unused for<'a> lifetime

What it does

Given:

struct Foo(Box<dyn for<'a> Fn(&u32)>);

The for<'a> is redundant, since the lifetime is not mentioned.

Advantage

• Remove dead code
• Consistency with clippy::extra_unused_lifetimes which can already detect dead lifetimes such as:

fn foo<'a>(x: u32) {}

In fact, this suggestion may be reasonable as an extension to the existing unused_lifetimes code.

Drawbacks

None I can think of

Example

struct Foo(Box<dyn for<'a> Fn(&u32)>);

Could be written as:

struct Foo(Box<dyn Fn(&u32)>);

The issue does not name implementation files or tests. Start by locating Clippy's existing unused_lifetimes lint and compare how it detects redundant lifetimes; the work is complete when the shown for<'a> lifetime produces an appropriate warning and removing it is covered by a test.