Indexing via [v] does not cast fn items to fn pointers

[aidanhs]

use std::ops::Index;
fn bar() {}
static UNIT: () = ();
struct S;
impl Index<fn()> for S {
    type Output = ();
    fn index(&self, f: fn()) -> &() { f(); &UNIT }
}
fn main() {
    *S.index(bar);
    // S[bar];
    // ^^^^^^ expected fn pointer, found fn item
}

https://is.gd/R97Dc7

The docs for Index (https://doc.rust-lang.org/std/ops/trait.Index.html) say:

container[index] is actually syntactic sugar for *container.index(index)

but the error above indicates that this isn't true.

[est31]

Seems its not doing the coercion. This works:

    let b :fn() = bar;
    S[b];

[aidanhs]

Suspect it has something to do with this line - https://github.com/rust-lang/rust/blob/1.15.1/src/librustc_typeck/check/mod.rs#L3978

Going to try and work on a patch.