This is a stylistic change to libstd to reduce the number of checks of `feature = "backtrace"` now that we unconditionally depend on the `backtrace` crate and rely on it having an empty implementation. otherwise.
This directory contains the source code of the rust project, including:
rustcand its testslibstd- 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.