Function lto_release_c_flags 

pub fn lto_release_c_flags(arch: &str, rustc_cpu: Option<&str>) -> String

CMAKE_C_FLAGS_RELEASE under cross-language LTO. LLVM inlines a C function into a Rust caller only when the C target features are a subset of Rust’s, so C targets exactly the CPU rustc does (rustc_cpu, from -Ctarget-cpu; None keeps both on the target default).