Lifetime of fn() converted to &Fn() isn't long enough

[eefriedman]

fn double(x: i32) -> i32 { x * 2 }
fn get_double_func() -> &'static (Fn(i32) -> i32)
{
    // Works
    const GET_DOUBLE_FUNC: &'static (Fn(i32) -> i32) = &double;
    GET_DOUBLE_FUNC
}
fn get_double_func_2() -> &'static (Fn(i32) -> i32)
{
    // Error message
    &double
}
fn main() {}

It would be nice if the lifetime of &double were the same in all contexts.

[arielb1]

An &'static Fn(i32) -> i32 is represented as a (shim vtable, pointer-to-pointer-to-function)

[krdln]

arielb: Note that double's type is fn(i32) -> i32 { double }, not just fn(i32) -> i32. In this case both the vtable and pointer-to-function are allocated in a static memory and the only change needed to fix that bug is making the lifetime 'static.

Casting arbitrary fn() function pointers to a &'static Fn() is a harder case, but it could be done too.

Discussion on reddit

[nikomatsakis]

Seems related to rust-lang/rfcs#1414. I agree it'd probably be nice for this to work.

[brson]

Tracking issue #38865

[nikomatsakis]

Closing in favor of #38865. I'll copy the example over there.