clone of fn has odd lifetime issues that copy of fn does not

[pnkfelix]

Code:

#![crate_type="lib"]

#[cfg(v1)]
pub fn foo(f: fn<'a>(x: &'a())) -> fn<'b>(y: &'b()) { f }

#[cfg(v2)]
pub fn foo(f: fn<'a>(x: &'a())) -> fn<'b>(y: &'b()) { f.clone() }

Transcript of compile attempts:

% rustc --cfg v1 /tmp/cf.rs
% rustc --cfg v2 /tmp/cf.rs
/tmp/cf.rs:7:55: 7:64 error: mismatched types: expected `fn(&'b ())` but found `fn(&())` (expected concrete lifetime, but found bound lifetime parameter 'b)
/tmp/cf.rs:7 pub fn foo(f: fn<'a>(x: &'a())) -> fn<'b>(y: &'b()) { f.clone() }
                                                                   ^~~~~~~~~
note: expected concrete lifetime is lifetime ReInfer(ReVar(middle::ty::RegionVid{id: 43u}))
error: aborting due to previous error
% 

[pnkfelix]

cc @nikomatsakis

[pnkfelix]

Note that this bug can also manifest itself more subtly if one attempts to do #[deriving(Clone)] on a struct that has a field of type fn (...) -> ..

[nikomatsakis]

I think this has to do with the implementation of Clone for fn types, which is kinda broken because it loses bound lifetimes. After #5527 lands, we'll be able to do:

impl<T:Copy> Clone for T { ... }

and then remove the impls for bare fns, which should solve this problem, at least in this case.

Closing as a dupe of #12741.