Make new symbol mangling scheme default for compiler itself.

[michaelwoerister]

As suggest in #89917 (comment), this PR enables the new symbol mangling scheme for the compiler itself. The standard library is still compiled using the legacy mangling scheme so that the new symbol format does not show up in user code (yet).

r? @Mark-Simulacrum

[Mark-Simulacrum]

Is it permissible to mix the mangling like this? I guess that makes sense -- presumably we're encoding a bit somewhere indicating the mangling for -Zshare-generics and the like?

r=me

[michaelwoerister]

Yes, we re-construct which version is needed:

rust/compiler/rustc_symbol_mangling/src/lib.rs

Lines 231 to 247 in d45ed75

	let instantiating_crate =
	if avoid_cross_crate_conflicts { Some(compute_instantiating_crate()) } else { None };
	// Pick the crate responsible for the symbol mangling version, which has to:
	// 1. be stable for each instance, whether it's being defined or imported
	// 2. obey each crate's own `-Z symbol-mangling-version`, as much as possible
	// We solve these as follows:
	// 1. because symbol names depend on both `def_id` and `instantiating_crate`,
	// both their `CrateNum`s are stable for any given instance, so we can pick
	// either and have a stable choice of symbol mangling version
	// 2. we favor `instantiating_crate` where possible (i.e. when `Some`)
	let mangling_version_crate = instantiating_crate.unwrap_or(def_id.krate);
	let mangling_version = if mangling_version_crate == LOCAL_CRATE {
	tcx.sess.opts.debugging_opts.get_symbol_mangling_version()
	} else {
	tcx.symbol_mangling_version(mangling_version_crate)
	};

It's not particularly pretty.

[michaelwoerister]

@bors r=Mark-Simulacrum

[bors]

📌 Commit 456283c has been approved by Mark-Simulacrum

[jackh726]

Given that #89917 had a perf effect. I think this shouldn't be rolled up. @bors rollup=never

[wesleywiser]

It looks like most of the effects in #89917 are due to increased linker time which, while rustc will take longer to link, shouldn't make the compiler itself slower. However, no reason not to be cautious here as well 🙂

[bors]

⌛ Testing commit 456283c with merge a3f7c4d...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing a3f7c4d to master...

[rust-timer]

Finished benchmarking commit (a3f7c4d): comparison url.

Summary: This change led to moderate relevant mixed results 🤷 in compiler performance.

• Moderate improvement in instruction counts (up to -0.8% on incr-unchanged builds of deeply-nested-async)
• Moderate regression in instruction counts (up to 0.4% on incr-unchanged builds of deep-vector)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression