Option `--thinlto-emit-index-files` potentially generates huge filenames

[tarcisiofischer]

While building chromium with lld_emit_indexes_and_imports=true (That's converted into -Wl,--thinlto-emit-index-files), I'm getting

ld.lld: error: cannot open obj/third_party/rust/serde_json_lenient/v0_2/wrapper/libthird_uparty_srust_sserde_ujson_ulenient_sv0_u2_swrapper_cwrapper.rlib(libthird_uparty_srust_sserde_ujson_ulenient_sv0_u2_swrapper_cw
rapper.third_uparty_srust_sserde_ujson_ulenient_sv0_u2_swrapper_cwrapper.6e3eaf7b964e238a-cgu.0.rcgu.o at 67896).thinlto.bc: File name too long

On my linux with BTRFS. This is due to (1) The name mangling mechanism in Chromium: https://source.chromium.org/chromium/chromium/src/+/main:build/rust/rust_target.gni;l=44;drc=15d204f8bf5cc292f592792948823a2bbd77d82c and (2) Lack of handling of files with long filenames in LLVM:

llvm-project/llvm/lib/LTO/LTO.cpp

Lines 1402 to 1405 in 665299e

	raw_fd_ostream OS(NewModulePath + ".thinlto.bc", EC,
	sys::fs::OpenFlags::OF_None);
	if (EC)
	return createFileError("cannot open " + NewModulePath + ".thinlto.bc", EC);

I've created an issue on Chromium's issue tracker and fix proposals in a CL there. But we are wondering if it'd be possible to change code in LLVM's side in a way that it creates shorter file names in the first place. Please see the discussion in the issue tracker on Chromium further information.