Rollup merge of #132993 - jieyouxu:i_am_very_stable, r=chenyukang

Make rustc consider itself a stable compiler when `RUSTC_BOOTSTRAP=-1`

Addresses https://github.com/rust-lang/rust/issues/123404 to allow test writers to specify `//@ rustc-env:RUSTC_BOOTSTRAP=-1` to have a given rustc consider itself a stable rustc. This is only intended for testing usages.

I did not use `RUSTC_BOOTSTRAP=0` because that can be confusing, i.e. one might think that means "not bootstrapping", but "forcing a given rustc to consider itself a stable compiler" is a different use case.

I also added a specific test to check `RUSTC_BOOTSTRAP`'s various values and how that interacts with rustc's stability story w.r.t. features and cli flags.

Noticed when trying to write a test for enabling ICE file dumping on stable.

Dunno if this needs a compiler FCP or MCP, but I can file an MCP or ask someone to start an FCP if needed. Note that `RUSTC_BOOTSTRAP` is a perma-unstable env var and has no stability guarantees (heh) whatsoever. This does not affect bootstrapping because bootstrap never sets `RUSTC_BOOTSTRAP=-1`. If someone does set that when bootstrapping, it is considered PEBKAC.

Accompanying dev-guide PR: https://github.com/rust-lang/rustc-dev-guide/pull/2136

cc `@estebank` and `@rust-lang/wg-diagnostics` for FYI

0 files changed, 0 insertions, 0 deletions