Add --compile-time-deps argument for x check

[bjorn3]

Together with skipping building C++ code in rustc_llvm for check, this reduces the amount of time it takes to do the x check for rust-analyzer analysis from 12m16s to 3m06s when the bootstrap compiler is already downloaded.

[rustbot]

r? @clubby789

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[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[Kobzol]

r? @Kobzol

I don't use RA much, so could you please walk me through what exactly this does? I know that RA/RustRover wanted this flag to only compile proc macros and compile + run build scripts to speed up the initial analysis of the crate. In RustRover it's not that critical, because it caches stuff heavily. If I understand it correctly, RA doesn't cache stuff between executions, so if you reopen the rustc project, it has to do x check from scratch to prime its analysis engine? And this will cause bootstrap to only do the minimum amount of work necessary to achieve that? So it only helps for the initial startup, but then if you make changes to rustc and RA runs x check in the background, it won't be applied?

[ShoyuVanilla]

r? @Kobzol

I don't use RA much, so could you please walk me through what exactly this does? I know that RA/RustRover wanted this flag to only compile proc macros and compile + run build scripts to speed up the initial analysis of the crate. In RustRover it's not that critical, because it caches stuff heavily. If I understand it correctly, RA doesn't cache stuff between executions, so if you reopen the rustc project, it has to do x check from scratch to prime its analysis engine? And this will cause bootstrap to only do the minimum amount of work necessary to achieve that? So it only helps for the initial startup, but then if you make changes to rustc and RA runs x check in the background, it won't be applied?

Basically you're correct but there are two things to clarify:

• RA doesn't have persistent cache (yet) but it runs cargo check (--compile-time-deps) to build the necessary artifacts to the filesystem. So this is not directly related to cache priming and RA and RR can equally benefit from this.
• And it only helps the very first run wrt build scripts and proc macro dependency changes. As mentioned above, all artifacts needed are built into filesystem and as you said, further interactions inside RA trigger cargo check without --compile-time-deps option and also leave artifacts to filesystem. Since then, any new runs won't benefit from this as there isn't anything to build when starting RA (though it might help slightly by reducing fingerprint checks)

[Kobzol]

I see. So it only helps if you open RA after clearing the build directory or changing the checked out rustc SHA a lot? 12 minutes sounds quite brutal, I wonder if the RA build script command should be changed to x check compiler instead of x check or something.

[bjorn3]

So it only helps if you open RA after clearing the build directory or changing the checked out rustc SHA a lot?

Yes

I wonder if the RA build script command should be changed to x check compiler instead of x check or something.

That would hurt working on things other than rustc, like clippy, miri or rustfmt.

[Kobzol]

Could you please add a change_tracker.rs entry with Info level about this?

[Kobzol]

Could you please add a brief comment here, explaining what the flag does. Also I would move this into Builder::cargo, in stream_cargo we shouldn't be doing large modifications to the Cargo command anymore, we only configure output diagnostics and message format (which are actually related to streaming).