test(pgo): ensure PGO works

[weihanglo]

What does this PR try to resolve?

This is a regression test to prevent issues like #7416.

The test only run on Linux,
as other platforms have different requirements for PGO,
or emit different PGO function missing warnings.

How should we test and review this PR?

Not sure how brittle it is. We can optionally run it only on Cargo's CI?

cc #14830

[rustbot]

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[weihanglo]

This is how LLVM warnings look like if we hash RUSTFLAGS into -Cmetadata

---- expected: tests/testsuite/pgo.rs:98:27
++++ actual:   stderr
   1    1 | [COMPILING] foo v0.0.0 ([ROOT]/foo)
   2    2 | [RUNNING] `rustc [..]-Cprofile-use=[ROOT]/foo/target/merged.profdata -Cllvm-args=-pgo-warn-missing-function`
        3 + [WARNING] foo.6d8fa1cc59b84deb-cgu.0: no profile data available for function _ZN3std2rt10lang_start17h326bc87852528165E Hash = 742261418966908927 up to 0 count discarded
        4 + 
        5 + [WARNING] foo.6d8fa1cc59b84deb-cgu.0: no profile data available for function _ZN3std2rt10lang_start28_$u7b$$u7b$closure$u7d$$u7d$17hf49388a32a52a388E Hash = 7422[614](https://github.com/weihanglo/cargo/actions/runs/12032501015/job/33544505332#step:11:615)18966908927 up to 0 count discarded
        6 + 
        7 + [WARNING] foo.6d8fa1cc59b84deb-cgu.0: no profile data available for function _ZN3std3sys9backtrace28__rust_begin_short_backtrace17hfbc657bfed1d0a0fE Hash = 170957022131388415 up to 0 count discarded
        8 + 
        9 + [WARNING] foo.6d8fa1cc59b84deb-cgu.0: no profile data available for function _ZN4core3ops8function6FnOnce40call_once$u7b$$u7b$vtable.shim$u7d$$u7d$17h392723b7040dbd77E Hash = 742261418966908927 up to 0 count discarded
       10 + 
       11 + [WARNING] foo.6d8fa1cc59b84deb-cgu.0: no profile data available for function _ZN3foo9fibonacci17he8ee274359179b8dE Hash = 1063705160746808702 up to 0 count discarded
       12 + 
       13 + [WARNING] foo.6d8fa1cc59b84deb-cgu.0: no profile data available for function _ZN3foo4main17hc72aa038f08b88faE Hash = 1124680652719534771 up to 0 count discarded
       14 + 
       15 + [WARNING] `foo` (bin "foo") generated 6 warnings
   3   16 | [FINISHED] `release` profile [optimized] target(s) in [ELAPSED]s

[ehuss]

Would it be possible to enforce that this exists in CI? Otherwise we don't know if this test is ever actually running.

[weihanglo]

Yeah I've considered that and found it needs more work than the current one.

I'll try to improve it!

[ehuss]

Can you make sure to use the ignore attribute when disabling a test? Otherwise it can be confusing when tests don't seem to change when you expect them to, are say that they are passing when you expect them to fail.

I realize there's still a big hole when llvm-profdata is missing. We could maybe add that to our requires mechanism?

Suggested change

	#[cargo_test]
	fn pgo_works() {
	if cfg!(not(target_os = "linux")) {
	// macOS may emit different LLVM PGO warnings.
	// Windows LLVM has different requirements.
	return;
	}
	#[cargo_test]
	// macOS may emit different LLVM PGO warnings.
	// Windows LLVM has different requirements.
	#[cfg_attr(not(target_os = "linux"), ignore = "linux only")]
	fn pgo_works() {