cfg_select! macro
#2103
cfg_select! macro
#2103
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| r[cfg.cfg_select] | ||
| ### The `cfg_select` macro | ||
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| The built-in `cfg_select` macro expands to the right-hand side of the first configuration predicate that evaluates to `true`. | ||
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| For example: | ||
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| ```rust | ||
| cfg_select! { | ||
| unix => { | ||
| fn foo() { /* unix specific functionality */ } | ||
| } | ||
| target_pointer_width = "32" => { | ||
| fn foo() { /* non-unix, 32-bit functionality */ } | ||
| } | ||
| _ => { | ||
| fn foo() { /* fallback implementation */ } | ||
| } | ||
| } | ||
| ``` | ||
| The `cfg_select` macro can also be used in expression position: | ||
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| ```rust | ||
| let is_unix_str = cfg_select! { | ||
| unix => "unix", | ||
| _ => "not unix", | ||
| }; | ||
| ``` | ||
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| A `_` can be used to write a configuration predicate that always evaluates to `true`. |
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Thanks for the Reference PR. It'd be worth filling in the details you provided in rust-lang/rust#149783 (comment), e.g. with the grammar, the removal of one level of braces, the compile error on fallthrough, etc.
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This can discuss as well how the bodies of each branch must be able to parse.
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I think all the raw information is here now, but it probably needs some editing/clarification.
Also should the lint on branches that come after a wildcard be mentioned?
| r[cfg.cfg_select.general] | ||
| The built-in `cfg_select` macro expands to the `TokenTree` on the right-hand side of the first configuration predicate that evaluates to `true`. |
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is it clear enough from the grammer and the language here that the { /* ... */ } are dropped?
| r[cfg.cfg_select.positions] | ||
| The `cfg_select!` macro is accepted in the following macro expansion positions | ||
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| - items | ||
| - statements | ||
| - expression | ||
| - impl items | ||
| - trait impl items | ||
| - trait items | ||
| - foreign items |
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I can't find the positions in which macro expansion is allowed in the reference, actually.
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Stabilization report: rust-lang/rust#149783