bootstrap: RUSTFLAGS_BOOTSTRAP does not affect building bootstrap

[jonhoo]

I'll prefix this with: I don't know if this is a bug, or just confusing behavior.

It appears that setting RUSTFLAGS_BOOTSTRAP does not affect the RUSTFLAGS used to build the Rust-based bootstrap binary (src/bootstrap). Instead, when x.py builds src/bootstrap, it just uses RUSTFLAGS directly:

rust/src/bootstrap/bootstrap.py

Lines 1012 to 1013 in 8769f4e

	# preserve existing RUSTFLAGS
	env.setdefault("RUSTFLAGS", "")

Given the name of RUSTFLAGS_BOOTSTRAP, this seems counterintuitive. Even if this is the desired behavior, it should probably be documented. Or perhaps x.py could detect if RUSTFLAGS_BOOTSTRAP is set and warn about the fact that it is ignored?

Meta

rustc --version --verbose:

rustc 1.58.1 (db9d1b20b 2022-01-20)
binary: rustc
commit-hash: db9d1b20bba1968c1ec1fc49616d4742c1725b4b
commit-date: 2022-01-20
host: x86_64-unknown-linux-gnu
release: 1.58.1
LLVM version: 13.0.0

[jyn514]

RUSTFLAGS_BOOTSTRAP is set only when compiling rustc for the first time. If you want a flag to affect rustbuild, use RUSTFLAGS. There's no way to set flags for rustbuild and not rustc that I know of.

[Mark-Simulacrum]

I would expect RUSTFLAGS_BOOTSTRAP to affect all builds with the stage0 (beta) compiler -- it's probably an oversight if that's not the case and could be corrected.