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## UNSTABLE_SYNTAX_PRE_EXPANSION

# Static UNSTABLE_SYNTAX_PRE_EXPANSION

Source

    pub static UNSTABLE_SYNTAX_PRE_EXPANSION: &'static Lint

Expand description

The `unstable_syntax_pre_expansion` lint detects the use of unstable syntax that is discarded during attribute expansion.

#### §Example

    #[cfg(feature = "nightly")]
    macro foo() {}

{{produces}}

#### §Explanation

The input to active attributes such as `#[cfg]` or procedural macro attributes is required to be valid syntax. Previously, the compiler only gated the use of unstable syntax features after resolving `#[cfg]` gates and expanding procedural macros.

To avoid relying on unstable syntax, move the use of unstable syntax into a position where the compiler does not parse the syntax, such as a functionlike macro.

    macro_rules! identity {
       ( $($tokens:tt)* ) => { $($tokens)* }
    }

    #[cfg(feature = "nightly")]
    identity! {
       macro foo() {}
    }

This is a future-incompatible lint to transition this to a hard error in the future. See issue #154045 for more details.