I am trying to use the mold linker with the Rust compiler, but I am having a hard time with it. I am trying this method specified in the mold repository README:
[target.x86_64-unknown-linux-gnu]
rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold"]
I added this to the .config/cargo.toml file, but it deosn't seem to work. When I set the mold path as /usr/bin/mold, I get errors saying that the linker was not found. When I set the path as mold, it compiles successfully. When running readelf -p .comment target/release/binary, I can see the mold comment there. But while compiling, I am not seeing any difference in the time spent on linking. Also, when monitoring my CPU I can see that only one core is used when linking. What is going wrong here?
EDIT: If it matters, I noticed the exact same behaviour when trying to use the lld linker.
EDIT2: Turns out, I'm a complete idiot. Mold is working as it should, what's taking the time is the fat LTO. After switching to thin LTO, the build time dropped from 2 minutes to 1 minute 15 seconds. Still, I'm not sure if I want to use thin LTO as it produces slightly bigger binaries and may not be as performant.