warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC

[Zeckmathederg]

On my x86_64-unknown-linux-gnu system when compiling Rustc-1.84.1, I get the following warnings:

   Compiling rustc_parse v0.0.0 (/sources/mesa/rustc-1.84.1-src/compiler/rustc_parse)
   Compiling rustc_middle v0.0.0 (/sources/mesa/rustc-1.84.1-src/compiler/rustc_middle)
   Compiling rustc_ast_passes v0.0.0 (/sources/mesa/rustc-1.84.1-src/compiler/rustc_ast_passes)
   Compiling rustc_expand v0.0.0 (/sources/mesa/rustc-1.84.1-src/compiler/rustc_expand)
warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-fembed-bitcode=all" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-fembed-bitcode=all" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-fembed-bitcode=all" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-fembed-bitcode=all" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-fembed-bitcode=all" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-flto=thin" is not supported by the currently used CC
warning: rustc_llvm@0.0.0: Inherited flag "-fembed-bitcode=all" is not supported by the currently used CC
   Compiling rustc_builtin_macros v0.0.0 (/sources/mesa/rustc-1.84.1-src/compiler/rustc_builtin_macros)
   Compiling rustc_infer v0.0.0 (/sources/mesa/rustc-1.84.1-src/compiler/rustc_infer)

This occurs a few more times. This seems similar to issue #134657, although that issue involved MSVC, whereas I am using system LLVM-19.1.6 compiled by GCC-14.2.0 on a glibc-2.40 system. From what I understand of the linked issue, these warnings are harmless and have to do with the cc version in Cargo.lock and other similar files. Rustc compiled just fine and compiles Rust packages fine.

Thanks in advance!

[jieyouxu]

I believe this is due to trying to pass clang-specific flags via cc-the-crate, but the underlying C++ compiler is actually gcc (well, g++). For instance,

• -flto=thin is a LLVM thing. gcc only supports -flto plus additional flag to specify -fno-fat-lto-objects. This is almost certainly not 1-to-1.
• -fembed-bitcode=all is a LLVM-thing again (in that it's embedding LLVM bitcode). gcc doesn't have this; one of the -fdump-tree* flags could be used, but that'd be dumping GIMPLE et al. and not LLVM bitcode anyway.

[jieyouxu]

Hang on, that might not be it No that is it, in cc https://github.com/rust-lang/cc-rs/blob/8533e8cdb73a07ef4fc694c0c9c5b5d67f491ebe/src/flags.rs#L219-L221 here we try to inherit lto flags, but that flag interface is only available for clang, not gcc.

[Zeckmathederg]

Yeah I talked with another person and he said that GCC is the underlying compiler and doesn't support those flags, that Clang would have to be used to support them.

[jieyouxu]

cc-side PR: rust-lang/cc-rs#1379

[jieyouxu]

rust-lang/cc-rs#1379 was merged, so rust-lang/rust side we just need to wait for the next cc version to bump the bootstrap cc (er maybe also compiler cc, can't quite remember).