rustbuild: LLVM is not partially rebuilt anymore

[petrochenkov]

Even tiny changes like #68570 cause a full rebuild.

This is a regression from the few recent months (?).
Before that partial rebuilds worked, I remember it because I implemented them back in 2017 (#40329).

I didn't yet investigate what happens exactly.

[cuviper]

I think it's because the git hash is set in LLVM_VERSION_SUFFIX here:

rust/src/bootstrap/native.rs

Lines 240 to 253 in cd1ef39

	if let Some(ref suffix) = builder.config.llvm_version_suffix {
	// Allow version-suffix="" to not define a version suffix at all.
	if !suffix.is_empty() {
	cfg.define("LLVM_VERSION_SUFFIX", suffix);
	}
	} else {
	let mut default_suffix =
	format!("-rust-{}-{}", channel::CFG_RELEASE_NUM, builder.config.channel,);
	if let Some(sha) = llvm_info.sha_short() {
	default_suffix.push_str("-");
	default_suffix.push_str(sha);
	}
	cfg.define("LLVM_VERSION_SUFFIX", default_suffix);
	}

This propagates to PACKAGE_VERSION in llvm/Config/config.h, which affects the entire build.

[cuviper]

There's a cmake variable LLVM_APPEND_VC_REV that would probably behave better for this.

[tmiasko]

The LLVM_VERSION_SUFFIX is indeed responsible for complete rebuilds.
The intent behind PR that introduced it was to disambiguate between versions of
LLVM library that are incompatible yet previously could share the same name.

Unfortunately LLVM_APPEND_VC_REV doesn't help, since it is about stashing
version control revision information somewhere inside the library, rather than
changing the library name itself.

Removing commit SHA from default version suffix should be enough to address
the original issue while resolving this one.