Add a ui test for RUSTC_BOOTSTRAP vs rustc's stability

tests/ui/bootstrap/rustc_bootstap.force_stable.stderr

@@ -0,0 +1,10 @@
+error: the option `Z` is only accepted on the nightly compiler
+
+help: consider switching to a nightly toolchain: `rustup default nightly`
+
+note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>
+
+note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>
+
+error: 1 nightly option were parsed
+

tests/ui/bootstrap/rustc_bootstap.rs

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+//! Check `RUSTC_BOOTSTRAP`'s behavior in relation to feature stability and what rustc considers
+//! itself to be (stable vs non-stable ).
+//!
+//! `RUSTC_BOOTSTRAP` accepts:
+//!
+//! - `1`: cheat, allow usage of unstable features even if rustc thinks it is a stable compiler.
+//! - `x,y,z`: comma-delimited list of crates, basically allow unstable features for specific
+//!   crates, this is for use by cargo.
+//! - `-1`: force rustc to think it is a stable compiler.
+
+// ignore-tidy-linelength
+
+//@ revisions: default_nightly cheat cheat_single_crate cheat_multi_crate force_stable invalid_zero invalid_junk
+//@ only-nightly
+
+//@[default_nightly] unset-rustc-env:RUSTC_BOOTSTRAP
+//@[default_nightly] check-pass
+
+// For a nightly compiler, this is same as `default_nightly` as if `RUSTC_BOOTSTRAP` was unset.
+//@[invalid_zero] rustc-env:RUSTC_BOOTSTRAP=0
+//@[invalid_zero] check-pass
+
+// Invalid values are silently discarded, same as `default_nightly`, i.e. as if `RUSTC_BOOTSTRAP`
+// was unset.
+//@[invalid_junk] rustc-env:RUSTC_BOOTSTRAP=*
+//@[invalid_junk] check-pass
+
+//@[cheat] rustc-env:RUSTC_BOOTSTRAP=1
+//@[cheat] check-pass
+
+//@[cheat_single_crate] rustc-env:RUSTC_BOOTSTRAP=x
+//@[cheat_single_crate] check-pass
+
+//@[cheat_multi_crate] rustc-env:RUSTC_BOOTSTRAP=x,y,z
+//@[cheat_multi_crate] check-pass
+
+// Note: compiletest passes some `-Z` flags to the compiler for ui testing purposes, so here we
+// instead abuse the fact that `-Z unstable-options` is also part of rustc's stability story and is
+// also affected by `RUSTC_BOOTSTRAP`.
+//@[force_stable] rustc-env:RUSTC_BOOTSTRAP=-1
+//@[force_stable] compile-flags: -Z unstable-options
+//@[force_stable] regex-error-pattern: error: the option `Z` is only accepted on the nightly compiler
+
+#![crate_type = "lib"]
+
+// Note: `rustc_attrs` is a perma-unstable internal feature that is unlikely to change, which is
+// used as a proxy to check `RUSTC_BOOTSTRAP` versus stability checking logic.
+#![feature(rustc_attrs)]