From their repo (github.com/rui314/mold#how-to-use)

Create .cargo/config.toml in your project directory with the following:

[target.x86_64-unknown-linux-gnu] linker = “clang” rustflags = [“-C”, “link-arg=-fuse-ld=/path/to/mold”] where /path/to/mold is an absolute path to the mold executable. In the example above, we use clang as a linker driver since it always accepts the -fuse-ld option. If your GCC is recent enough to recognize the option, you may be able to remove the linker = “clang” line.

[target.x86_64-unknown-linux-gnu] rustflags = [“-C”, “link-arg=-fuse-ld=/path/to/mold”] If you want to use mold for all projects, add the above snippet to ~/.cargo/config.toml