fieldx_aux

Helper crate for fieldx and any third-party crates that extend its functionality.

fieldx is heavily based on the darling crate, which greatly simplifies proc-macro development, but also imposes some constraints on attribute argument syntax. This crate overcomes these limitations and provides support for attribute kinds required to implement fieldx.

Here is a brief breakdown of what is provided:

• Support for nested arguments, i.e. those that look like arg1("value", trigger, subarg(...)).
• Support for syntax elements not covered by the darling crate, such as some_type(crate::types::Foo) and error(crate::error::Error, crate::error::Error::SomeProblem("with details"))[^tuple].
• A set of types implementing standard fieldx arguments like helpers or literal values.

[^tuple]: Here, the first argument of error()—Error—is an enum, and SomeProblem is one of its variants.

Usage

Imagine we are implementing a field-level attribute foo using the darling::FromField trait, and we want it to accept the following arguments:

• trigger: enables or disables certain functionality
• action: specifies a method with special meaning
• comment: accepts arbitrary text
• vis: indicates whether field-related code should be public, and if so, which kind of pub modifier to use

A field declaration may take the following form with the attribute:

    #[foo(
        trigger,
        action("method_name", private),
        comment("Whatever we consider useful."),
        vis(pub(crate))
    )]
    bar: usize,

For this, you'll need the following declaration somewhere in your proc-macro implementation:

#derive(FromField)
#[darling(attributes(foo))]
struct FooField {
    // ... skipping some darling default fields ...

    trigger: Option<FXBool>,
    action: Option<FXHelper>,
    comment: Option<FXString>,
    vis: Option<FXSynValue<syn::Visibility>>,
}

That's all; this crate will take care of implementing the arguments for you!