Improve shallow `Clone` deriving
`Copy` unions now support `#[derive(Clone)]`.
Less code is generated for `#[derive(Clone, Copy)]`.
+
Unions now support `#[derive(Eq)]`.
Less code is generated for `#[derive(Eq)]`.
---
Example of code reduction:
```
enum E {
A { a: u8, b: u16 },
B { c: [u8; 100] },
}
```
Before:
```
fn clone(&self) -> E {
match (&*self,) {
(&E::A { a: ref __self_0, b: ref __self_1 },) => {
::std::clone::assert_receiver_is_clone(&(*__self_0));
::std::clone::assert_receiver_is_clone(&(*__self_1));
*self
}
(&E::B { c: ref __self_0 },) => {
::std::clone::assert_receiver_is_clone(&(*__self_0));
*self
}
}
}
```
After:
```
fn clone(&self) -> E {
{
let _: ::std::clone::AssertParamIsClone<u8>;
let _: ::std::clone::AssertParamIsClone<u16>;
let _: ::std::clone::AssertParamIsClone<[u8; 100]>;
*self
}
}
```
All the matches are removed, bound assertions are more lightweight.
`let _: Checker<CheckMe>;`, unlike `checker(&check_me);`, doesn't have to be translated by rustc_trans and then inlined by LLVM, it doesn't even exist in MIR, this means faster compilation.
---
Union impls are generated like this:
```
union U {
a: u8,
b: u16,
c: [u8; 100],
}
```
```
fn clone(&self) -> U {
{
let _: ::std::clone::AssertParamIsCopy<Self>;
*self
}
}
```
Fixes https://github.com/rust-lang/rust/issues/36043
cc @durka
r? @alexcrichton
726850170d