1928: Support `#[cfg(..)]` r=matklad a=oxalica
This PR implement `#[cfg(..)]` conditional compilation. It read default cfg options from `rustc --print cfg` with also hard-coded `test` and `debug_assertion` enabled.
Front-end settings are **not** included in this PR.
There is also a known issue that inner control attributes are totally ignored. I think it is **not** a part of `cfg` and create a separated issue for it. #1949Fixes#1920
Related: #1073
Co-authored-by: uHOOCCOOHu <hooccooh1896@gmail.com>
Co-authored-by: oxalica <oxalicc@pm.me>
ae6305b90c