Fix traits implemented for function items

This fixes the set of traits implemented for function items. `Fn` family
traits are not implemented in some cases. This is roughly implemented in
[`assemble_fn_pointer_candidates`](https://github.com/rust-lang/rust/blob/8c32e313cccf7df531e2d49ffb8227bb92304aee/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L553-L585).

I did not include "has variadic", since that also requires unsafe and
extern "C" and is thus covered by the other rules, AFAIK. Could also add
that to CYA, though.

Fixes rust-lang#1968

Author: ehuss

src/types/function-item.md

@@ -50,8 +50,13 @@ let foo_ptr_2 = if want_i32 {
 ```
 
 r[type.fn-item.traits]
-All function items implement [`Fn`], [`FnMut`], [`FnOnce`], [`Copy`],
-[`Clone`], [`Send`], and [`Sync`].
+All function items implement [`Copy`], [`Clone`], [`Send`], and [`Sync`].
+
+[`Fn`], [`FnMut`], and [`FnOnce`] are implemented unless the function has any of the following:
+
+- an [`unsafe`][unsafe.fn] qualifier
+- a [`target_feature` attribute][attributes.codegen.target_feature]
+- an [ABI][items.fn.extern] other than `"Rust"`
 
 [`Clone`]: ../special-types-and-traits.md#clone
 [`Copy`]: ../special-types-and-traits.md#copy