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 #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