compiletest: we should not allow revision names that collide with builtin cfg names

[jieyouxu]

Example: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/error.3A.20unexpected.20.60--cfg.20windows.60.20flag/near/459869068

--- stderr -------------------------------
error: unexpected `--cfg windows` flag
  |
  = note: config `windows` is only supposed to be controlled by `--target`
  = note: manually setting a built-in cfg can and does create incoherent behaviors
  = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default

error: aborting due to 1 previous error

compiletest will try to set a --cfg <revision_name> that matches the revision name, but this goes poorly if the <revision_name> collides with a builtin cfg. Builtin cfgs can be found at

rust/compiler/rustc_session/src/config/cfg.rs

Line 90 in c9bd03c

pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {

, but we should also deny test revision name because it has special meaning cfg(test).

[Urgau]

This could (and should IMO) be accomplished using the (currently unstable) --print=check-cfg option, which list all the well known cfgs.

[heysujal]

Hi, I would like to work on this issue and will be going through the contribution docs. I don't have prior experience using Rust so I will be a good motivation for me to learn rust.