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Add the `cfg_match!` macro # Movitation Adds a match-like version of the `cfg_if` crate without a RFC [for the same reasons that caused `matches!` to be included in the standard library](https://github.com/rust-lang/rust/pull/65479). * General-purpose (not domain-specific) * Simple (the implementation is short) and useful (things can become difficult with several `cfg`s) * Very popular [on crates.io ](https://crates.io/crates/cfg-if) (currently 3th in all-time downloads) * The two previous points combined make it number three in [left-pad index](https://twitter.com/bascule/status/1184523027888988160) score ```rust match_cfg! { cfg(unix) => { fn foo() { /* unix specific functionality */ } } cfg(target_pointer_width = "32") => { fn foo() { /* non-unix, 32-bit functionality */ } } _ => { fn foo() { /* fallback implementation */ } } } ``` # Considerations A match-like syntax feels more natural in the sense that each macro fragment resembles an arm but I personally don't mind switching to any other desired syntax. The lack of `#[ ... ]` is intended to reduce typing, nevertheless, the same reasoning described above can also be applied to this aspect. Since blocks are intended to only contain items, anything but `cfg` is not expected to be supported at the current or future time. ~~Credits goes to `@gnzlbg` because most of the code was shamelessly copied from https://github.com/gnzlbg/match_cfg.~~ Credits goes to `@alexcrichton` because most of the code was shamelessly copied from https://github.com/rust-lang/cfg-if. |
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