Pre-RFC: Allow packages to specify a set of supported targets

[newpavlov]

Somewhat relevant proposal (see the "Integration with Cargo.toml" section):

[Pre-Pre-RFC] Target restriction contexts

The following text is braindump-like description of how I would like to see Rust work with various target options. (target_arch, target_feature, target_os and others) This approach aims to provide stronger compile time guarantees compared to current cfg setups, remove most of the unsafe in the stdsimd crate and integrate target parameters closer with Cargo.toml. Also it could be useful for improving Rust portability story. While RFC 2045 is an important step towards stabilization of SIMD, I don…