Function pointer does not fulfill the required lifetime #80317
Description
Function pointers have no lifetime of their own, and essentially are 'static (Unsafe Code Guidelines Reference), so I would expect fn(T) -> U: 'a to hold for any lifetime 'a and types T, U. A function pointer that lived longer than its argument type would be fine, since it won't be possible to make an instance of the argument type after it's expired, and so you cannot call it then. Similarly, there's nothing wrong with a function pointer that lives longer than its return type, as outside the return type's lifetime you either won't be able to call it or it will never return.
Yet the following example fails to compile.
pub fn foo<'a, T: 'a>() {}
pub fn bar<'a, 'b>() {
foo::<'a, fn(&'b u32)>(); // Errors
}error[E0477]: the type `fn(&'b u32)` does not fulfill the required lifetime
--> src/lib.rs:5:5
|
5 | foo::<'a, fn(&'b u32)>(|_x: &'b u32| {}); // Errors
| ^^^^^^^^^^^^^^^^^^^^^^
|
note: type must outlive the lifetime `'a` as defined on the function body at 3:12
--> src/lib.rs:3:12
|
3 | pub fn bar<'a, 'b>() {
| ^^
The compiler seems to think that a function pointer is only valid for as long as the types it uses are, rather than for the duration of the program.
I have also run into a similar issue when using Box<dyn Fn(&'b u32)> (and also Box<dyn Fn(&'b u32) + 'a>) instead of fn(&'b u32), but I thought that the function pointer case would be the clearest.