I use wsl with Windows and the path to the standard Rust libraries is this:
\\wsl$\kali-linux\home\myUserName\.rustup\toolchains\stable-x86_64-unknown-linux-gnu\lib\rustlib\src\rust
Only to get this I searched because right after installation it didn't work either.
I had to delete all my toolchains directory from wsl
with
rm -frv ~/.rustup/toolchains/* directory
and then install it again by doing
rustup update
and
rustup -v component add rust-src
otherwise I didn't have the standard Rust libraries in my installation and I got a
/lib/rustlib/src': No such file or directory
error
when testing
ls ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src