This directory contains the source code of the rust project, including:

• rustc and its tests
• libstd
• Various submodules for tools, like rustdoc, rls, etc.

For more information on how various parts of the compiler work, see the rustc guide.

There is also useful content in this README: https://github.com/rust-lang/rust/tree/master/src/librustc/infer/lexical_region_resolve.