Static lifetimes and variance: Associated const treated differently (and perhaps incorrectly?)

[steffahn]

If the line after this does not work is commented out, the remaining code does compile.
If there is any other reason why the const can’t work, while the function can, then the error message is misleading.

struct SomethingStatic<T: 'static>(T);

trait Foo<'a> {
    // this does work
    const FUN: fn(&&'a ());
}
trait Bar<T /* <- not 'static */> {
    // this does work
    const FUN: fn(&T);
}

trait Qux<'a> {
    // this does work
    fn fun() -> SomethingStatic<fn(&&'a ())>;
    // only this does not work:
    const FUN: SomethingStatic<fn(&&'a ())>;
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0477]: the type `for<'r> fn(&'r &'a ())` does not fulfill the required lifetime
  --> src/lib.rs:14:5
   |
17 |     const FUN: SomethingStatic<fn(&&'a ())>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: type must satisfy the static lifetime

error: aborting due to previous error

For more information about this error, try `rustc --explain E0477`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

Applies to stable as well as the current nightly (1.50.0-nightly (fa4163942 2020-12-14))

⟶ originally from this discussion on URLO

@rustbot modify labels: C-bug, T-lang, T-compiler, A-associated-items, A-lifetimes, A-typesystem

[steffahn]

I just noticed that return types like this are not really an indication of the type being valid. E.g. this compiles:

trait Foo<T /* not static */> {
    fn foo() -> &'static T;
}

I’m not quite 100% sure yet as to why fn(&T) isn’t static regardless of T being static. (Or perhaps even: why fn(S) isn’t always 'static. How can an fn contain non-static reference -- or contain anything for that matter?)

Perhaps I should close this issue or re-label to something about confusing diagnostics (since undestanding what’s going on here is apparently incredibly hard).

[BGR360]

@rustbot label +A-variance