Restrict where in the tree platform-specific cfgs may be mentioned
With the ports of Rust never ending, it's important that we keep things tidy. The main thing this PR does is introduce a new "pal" (platform abstraction layer) tidy check that limits where platform-specific CFGs may appear.
This is intended to maintain existing standards of code organization
in hopes that the standard library will continue to be refactored to
isolate platform-specific bits, making porting easier; where "standard
library" roughly means "all the dependencies of the std and test
crates".
This generally means placing restrictions on where `cfg(unix)`,
`cfg(windows)`, `cfg(target_os)` and `cfg(target_env)` may appear,
the basic objective being to isolate platform-specific code to the
platform-specific `std::sys` modules, and to the allocation,
unwinding, and libc crates.
Following are the basic rules, though there are currently
exceptions:
- core may not have platform-specific code
- liballoc_system may have platform-specific code
- liballoc_jemalloc may have platform-specific code
- libpanic_abort may have platform-specific code
- libpanic_unwind may have platform-specific code
- other crates in the std facade may not
- std may have platform-specific code in the following places
- sys/unix/
- sys/windows/
- os/
There are plenty of exceptions today though, noted in the whitelist.
The end-state, IMO, is for the standard library to be portable by porting only `std::sys` (possibly extracted to its own crate), an allocator crate, an unwinder crate, and possibly a libc crate (if std depends on it); but that outcome is far off and independent of the utility of enforcing where such code lives today.
cc @rust-lang/libs
144af3e97a