handle ci-rustc incompatible options during config parse

[onur-ozkan]

This PR ensures that config.toml does not use CI rustc incompatible options when CI rustc is enabled (just like ci-llvm checks). Some options can change compiler's behavior in certain scenarios. If we don't check these incompatible options, CI runners using CI rustc might ignore options we have explicitly set. This could be dangerous as we might think a rustc test passed with option T but in fact it wasn't tested with option T.

Later in #122709, I will disable CI rustc if any of those options were used (similar to this approach). If CI runners fail because of these checks, it means the logic in run.sh isn't covering the incompatible options correctly (since any incompatible option should turn off CI rustc).

The list may not be complete, but should be a good first step as it's better than nothing!

Blocker for #122709

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

This PR modifies config.example.toml.

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

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

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

[Mark-Simulacrum]

Are these really all incompatible? I'd have assumed that you still get to use all of these for a full build (i.e., basically stage 2 build). Those options related to std definitely feel like they still are fine -- they would certainly apply to the standard library built by the downloaded rustc, right?

[onur-ozkan]

Building std from source is not currently allowed with download-rustc:

rust/src/bootstrap/src/core/build_steps/compile.rs

Lines 155 to 171 in 51917ba

	// When using `download-rustc`, we already have artifacts for the host available. Don't
	// recompile them.
	if builder.download_rustc() && target == builder.build.build
	// NOTE: the beta compiler may generate different artifacts than the downloaded compiler, so
	// its artifacts can't be reused.
	&& compiler.stage != 0
	// This check is specific to testing std itself; see `test::Std` for more details.
	&& !self.force_recompile
	{
	let sysroot = builder.ensure(Sysroot { compiler, force_recompile: false });
	cp_rustc_component_to_ci_sysroot(
	builder,
	&sysroot,
	builder.config.ci_rust_std_contents(),
	);
	return;
	}

But it will eventually be possible with redesign stage 0 std (I will make it possible before landing use precompiled rustc for non-dist builders), so I will remove the std-related options from the list.

[onur-ozkan]

I got some ideas to improve the current approach. I will work on them later today.

@rustbot author

[onur-ozkan]

@rustbot ready

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 335bdd4 has been approved by Mark-Simulacrum

It is now in the queue for this repository.