Derives can conflict with constants

[Manishearth]

// pub const __self_0: u8 = 1;
pub const __cmp: u8 = 1;

#[derive(PartialOrd, PartialEq)]
pub enum Foo {
    A(u8), B(u8)
}

fn main() {
}

(playpen)

With __cmp uncommented we get errors like:

error[E0308]: mismatched types
 --> src/main.rs:7:7
  |
7 |     A(u8), B(u8)
  |       ^^^ expected enum `std::option::Option`, found u8
  |
  = note: expected type `std::option::Option<std::cmp::Ordering>`
             found type `u8`

error[E0308]: match arms have incompatible types
 --> src/main.rs:7:7
  |
7 |     A(u8), B(u8)
  |       ^^^
  |       |
  |       expected enum `std::option::Option`, found u8
  |       match arm with an incompatible type
  |
  = note: expected type `std::option::Option<std::cmp::Ordering>`
             found type `u8`

With __self_0 uncommented we get errors like:

error[E0530]: match bindings cannot shadow constants
 --> src/main.rs:7:7
  |
2 | pub const __self_0: u8 = 1;
  | --------------------------- a constant `__self_0` is defined here
...
7 |     A(u8), B(u8)
  |       ^^^ cannot be named the same as a constant

This is basically because they generate matches on variables named __cmp and __self_# and __arg_#_#, and rustc doesn't like it when you conflict those with constants.

[Manishearth]

I first noticed the __cmp one -- that comes from a match that does:

match cmp(...) {
   Equal => ...
   __cmp => __cmp
}

This could be rewritten to do __cmp @ _ => __cmp but that hits the shadow error. It can also be rewritten to bind the result of cmp() to a temporary, which should work.

However there's no way at all of fixing the __self stuff.

The reason we have the match bindings shadow error is #33118 (comment), which we hopefully can fix? cc @petrochenkov

[eddyb]

Note the difference between unhygienic old macros and hygienic new macros.

[jonas-schievink]

This appears to be fixed (the binding names are now different, but those still don't cause conflicts, presumably because they're gensym'd, or whatever our equivalent to that is)