unable to build rust on tiers-3 platform

[semarie]

with the introduction of #65939, the bootstrap.py mandatories the use of downloaded rustfmt. but all platforms doesn't have binary distribution, mainly tiers-3 platforms like OpenBSD.

For rustc or cargo dependencies, it is possible to use config.toml (with rustc and cargo key in [build] section) to point to alternate binaries. this way, bootstrap.py doesn't download them and use local supplied binaries.

For rustfmt it is currently not possible: if bootstrap.py is able to deal with it, rustbuild will complains that rustfmt isn't a valid key:

failed to parse TOML configuration 'config.toml': unknown field `rustfmt`, expected one of `build`, `host`, `target`, `cargo`, `rustc`, `docs`, `
compiler-docs`, `submodules`, `fast-submodules`, `gdb`, `nodejs`, `python`, `locked-deps`, `vendor`, `full-bootstrap`, `extended`, `tools`, `verb
ose`, `sanitizers`, `profiler`, `cargo-native-static`, `low-priority`, `configure-args`, `local-rebuild`, `print-step-timings` for key `build` at
 line 26 column 1

What would be the right way to use a local rustfmt binary in order to copte with #65939, and not force the use of downloaded binary ?

Would adding a rustfmt key in [build] section of config.toml would be accepted (I could provide a patch of it, if yes) ? or should another method used ?

Cc @anp @Mark-Simulacrum

[Mark-Simulacrum]

Yes, that was on oversight on my part. I would be happy to review a patch adding it to config.toml.

You should also be able to comment out the line in stage0.txt as a (possibly) temporary fix, this is my plan for beta and stable branches FWIW.