config.toml parsing error improvements

[osa1]

Improve error messages for musl-libdir and wasi-root keys. Previously
the parser would panic with unwrap(). Now it prints

  Target "wasm32-wasi" does not have a "wasi-root" key

(and similar for the musl-libdir field, which is used in target that
use musl)

Also update comments around wasi-root field to make it clear that the
field is only valid in wasm32-wasi target and needs to be moved to a
[target.wasm32-wasi] section to be valid.

Fixes #82317

---

r? @Mark-Simulacrum

[osa1]

configure.py looks for an llvm-config key in all target sections, and it doesn't exist in target.wasm32-wasi section.

I think specifying a different llvm-config for different targets doesn't make sense, right? So perhaps this key should be in [target] rather than in target subsections like target.x86_64-....

[Mark-Simulacrum]

llvm-config can be different for different targets, if e.g. you are targeting 64-bit and 32-bit systems. I think adding a commented one to the wasi section is probably easiest - even if compiling LLVM for wasm32-wasi seems unlikely :)

[osa1]

llvm-config can be different for different targets, if e.g. you are targeting 64-bit and 32-bit systems

I'm surprised that this works. Does LLVM allow linking multiple version in the single binary? If not, I don't see how this can work.

I think adding a commented one to the wasi section is probably easiest - even if compiling LLVM for wasm32-wasi seems unlikely :)

config.toml.example is already quite large. I'm thinking of reverting the changes to change.toml.example and just committing the improved errors messages. Would that work?

[Mark-Simulacrum]

I'm surprised that this works. Does LLVM allow linking multiple version in the single binary? If not, I don't see how this can work.

Note that we produce a single rustc per target, so there's not actually multiple LLVMs getting linked in. But if I'm producing compilers for, say, both 32-bit and 64-bit windows, those'll need two different LLVMs (just like the Rust code needs to be built twice).

I would be happy to just add the improved error messages, but we can also add a note on the wasi-root in config.toml.example indicating that it likely wants to get set for the wasi target.

[osa1]

@Mark-Simulacrum OK, that makes sense. I updated the PR.

@rustbot label -S-waiting-on-author +S-waiting-on-review

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 81cfa98 has been approved by Mark-Simulacrum