parallel_compiler: hide dependencies behind feature

[klensy]

Separate dependencies for parallel_compiler feature, so they will not be compiled if feature not selected, reducing number of compiled crates from 238 to 224.

[rust-highfive]

r? @wesleywiser

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[cjgillot]

Could you make two closures? This would allow to avoid the cfg-if dependency:

#[cfg(parallel_compiler)]
let pre_compile_cgus = |cgu_reuse: &[CguReuse]| { /* ... */ };

#[cfg(not(parallel_compiler))]
let pre_compile_cgus = |_: &[CguReuse]| (FxHashMap::default(), Duration::new(0, 0));

[klensy]

Done.

[klensy]

Only thing that depends on rustc-rayon* crates for non parallel_compiler is indexmap. Before #90842 it wasn't used rustc-rayon feature. Should this feature for indexmap be gated for parallel_compiler too?

[klensy]

Err, looks like that cfg in cargo don't work?

I'm expecting, that this definition will work, Cargo.toml:

...
[target.'cfg(parallel_compiler)'.dependencies]
rayon = { version = "0.3.2", package = "rustc-rayon" }
rayon-core = { version = "0.3.2", package = "rustc-rayon-core" }
indexmap = { version = "1.8.0", features = ["rustc-rayon"] }

[target.'cfg(not(parallel_compiler))'.dependencies]
indexmap = { version = "1.8.0" }

and compile indexmap with different features, depending on cfg, but it always builded with indexmap = { version = "1.8.0", features = ["rustc-rayon"] }

[cuviper]

I don't think you can use cfg that way, since that's not a builtin from rustc --print cfg:
https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies

It could be a Cargo feature, something like:

[features]
parallel_compiler = ["indexmap/rustc-rayon", "rayon", "rayon-core"]

[dependencies]
indexmap = "1.8.0"
rayon = { version = "0.3.2", package = "rustc-rayon", optional = true }
rayon-core = { version = "0.3.2", package = "rustc-rayon-core", optional = true }

Plumbing that feature from rustbuild through the crate tree might be challenging, or at least tedious.

[klensy]

Looks like that should work, but propagating feature through rustc crates that way not fun at all.

[bors]

☔ The latest upstream changes (presumably #94350) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

Triage:
@klensy can you please rebase this PR?

[wesleywiser]

Tested locally both with and without parallel-compiler = true and builds succeed.

[wesleywiser]

Thanks @klensy!

@bors r+

[bors]

📌 Commit 008fc79 has been approved by wesleywiser

[bors]

⌛ Testing commit 008fc79 with merge 6809057bc8259f310e1eb6d51d81018dab5010fc...

[Dylan-DPC]

@bors retry (yield to rollup containing this pr )

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[bors]

⌛ Testing commit 008fc79 with merge ee915c3...