pub_if

A Rust procedural macro that conditionally makes struct fields public based on cfg attributes. syn-free implementation using only proc_macro APIs.

Usage

Add the #[pub_if(...)] attribute to a struct to generate two versions:

• One with #[cfg(...)] where all fields are public
• One with #[cfg(not(...))] where fields retain their original visibility

use pub_if::pub_if;

#[pub_if(feature = "foo")]
pub struct Struct<F, B> {
    field: F,
    bar: B,
}

This expands to:

#[cfg(feature = "foo")]
pub struct Struct<F, B> {
    pub field: F,
    pub bar: B,
}

#[cfg(not(feature = "foo"))]
pub struct Struct<F, B> {
    field: F,
    bar: B,
}

Features

• Fields already marked pub remain public in both versions
• Supports generic types
• Works with any cfg condition (features, target_os, etc.)
• Implemented without the syn crate using only proc_macro APIs

Examples

Mixed visibility

#[pub_if(feature = "expose_internals")]
pub struct Config {
    private_setting: i32,
    pub public_setting: String,
}

When the feature is enabled, both fields are public. When disabled, only public_setting remains public.

Testing

The project includes compile-time tests using trybuild to verify that:

• Fields are public when the cfg condition is enabled
• Fields remain private when the cfg condition is disabled
• Fields already marked pub stay public in both cases

# Run tests without feature (verifies private fields are not accessible)
cargo test

# Run tests with feature (verifies all fields become public)
cargo test --features foo