unsafe fn coerces to &fn without an unsafe block

[lilyball]

The following code compiles, surprisingly:

let x: Option<&[u8]> = Some("foo").map(std::cast::transmute);

The coercion of std::cast::transmute to &fn should only be allowed inside an unsafe block.

[thestinger]

Nominating for the backwards compatible milestone. This doesn't seem like it would be very hard to fix though, just an oversight.

[nikomatsakis]

cc me

This seems like it's a bug in the type system impl of coercion

[catamorphism]

1.0, high