add `rustflags` option to `bootstrap.toml`

[karolzwolak]

Add a rustflags like you can do in cargo configuration.
This option should just append it's contents at the end of the end of rustflags passed to the compiler.

This consists of implementing:

• global rust.rustflags and per target rustflags (add rust.rustflags and per target rustflags options to bootstrap.toml #148795)

[weihanglo]

If we want bootstrap to pick up and append rustflags from .cargo/config.toml, should we switch to CARGO_BUILD_RUSTFLAGS or --config 'build.rustflags = ["…", "…"]' so they can merge with configuration?

[Kobzol]

--config 'build.rustflags = ["…", "…"]' might be an interesting alternative, although I always found this a bit tricky w.r.t. which "root" config to actually choose, i.e. should the rustflags be put into build or the target? I remember running into some annoying edge-cases with that, with target overriding the build, IIRC.

[karolzwolak]

If we want bootstrap to pick up and append rustflags from .cargo/config.toml, should we switch to CARGO_BUILD_RUSTFLAGS or --config 'build.rustflags = ["…", "…"]' so they can merge with configuration?

I don't get what you're suggesting. Did you mean from bootstrap.toml?

[Kobzol]

Instead of passing RUSTFLAGS to Cargo from bootstrap, we could pass --config build.rustflags. Then Cargo would append whatever you have in .cargo/config.toml to what bootstrap sets.

But in that case setting RUSTFLAGS manually would override (and break) everything, which seems a bit annoying.

[karolzwolak]

Ohh okay I get it. That comes back to what you said in #148795 (comment):

Right. .cargo/config.toml is actually used, but since bootstrap also unconditionally sets RUSTFLAGS, and that has priority over the config file, rustflags specifically won't be applied.

If we want bootstrap to pick up and append rustflags from .cargo/config.toml, should we switch to CARGO_BUILD_RUSTFLAGS or --config 'build.rustflags = ["…", "…"]' so they can merge with configuration?

This might be indeed a more elegant solution!
I'd allow us to have the same exact setup for rustflags as any other cargo project (well almost because of rustflags precedence etc).
The tradeoff is that's it's kinda weird to have the config split into so many files. It's nothing new though right? Right now you can configure something by using the .cargo/config.toml but it's just very limited. Rustc dev guide should do a better job of telling you how to configure if we decide to go that route.

But in that case setting RUSTFLAGS manually would override (and break) everything, which seems a bit annoying.

Would clearing the variable and appending it's contents to the build.rustflags resolve this issue? The rustflags passed as an --config arg should come after ones in .cargo/config.toml right?

[weihanglo]

The environment variable CARGO_BUILD_RUSTFLAGS will got appended as well

[Kobzol]

After thinking about this a bit more, I'm not sure if I like the idea of taking rustflags from .cargo/config.toml (which can be also e.g. in the user's HOME directory btw, which makes it harder to notice that it is being applied), because it makes rustc builds less self-contained, and could also lead to misleading bootstrap bug reports where someone has a rustflag set in their config that is silently affecting the build. Explicitly setting the flags through bootstrap.toml makes it much more explicit what is going on.