If you hack on the Rust compiler or libraries you may find the DXR index of Rust useful: https://dxr.mozilla.org/rust/source/src. DXR is a cross-reference tool (think ‘go to definition’ from your favourite IDE, but with more stuff). If you find bugs (you will, it is pretty untested), please report them here.
You can run DXR on your own projects too, although it is not super-easy, see the code is here if you’re keen.