cfg

Configuration conditional checks are possible through two different operators:

• the cfg attribute: #[cfg(...)] in attribute position
• the cfg! macro: cfg!(...) in boolean expressions

While the former enables conditional compilation, the latter conditionally evaluates to true or false literals allowing for checks at run-time. Both utilize identical argument syntax.

cfg!, unlike #[cfg], does not remove any code and only evaluates to true or false. For example, all blocks in an if/else expression need to be valid when cfg! is used for the condition, regardless of what cfg! is evaluating.

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See also:

the reference, cfg!, and macros.