darling_core 0.23.0

//! Functions to use with `#[darling(with = "...")]` that control how quoted values
//! in [`Meta`] instances are parsed into [`Expr`] fields.
//!
//! Version 1 of syn did not permit expressions on the right-hand side of the `=` in a
//! [`MetaNameValue`](syn::MetaNameValue), so darling accepted string literals and then
//! parsed their contents as expressions.
//! Passing a string literal in this version would have required the use of a raw string
//! to add quotation marks inside the literal.
//!
//! Version 2 of syn removes the requirement that the right-hand side be a literal.
//! For most types, such as [`Path`](syn::Path), the [`FromMeta`] impl can accept the
//! version without quotation marks without causing ambiguity; a path cannot start and
//! end with quotation marks, so removal is automatic.
//!
//! [`Expr`] is the one type where this ambiguity is new and unavoidable. To address this,
//! this module provides different functions for different expected behaviors.

use syn::{Expr, Meta};

use crate::{Error, FromMeta};

/// Parse a [`Meta`] to an [`Expr`]; if the value is a string literal, the emitted
/// expression will be a string literal.
pub fn preserve_str_literal(meta: &Meta) -> crate::Result<Expr> {
    match meta {
        Meta::Path(_) => Err(Error::unsupported_format("path").with_span(meta)),
        Meta::List(_) => Err(Error::unsupported_format("list").with_span(meta)),
        Meta::NameValue(nv) => Ok(nv.value.clone()),
    }
}

/// Parse a [`Meta`] to an [`Expr`]; if the value is a string literal, the string's
/// contents will be parsed as an expression and emitted.
pub fn parse_str_literal(meta: &Meta) -> crate::Result<Expr> {
    match meta {
        Meta::Path(_) => Err(Error::unsupported_format("path").with_span(meta)),
        Meta::List(_) => Err(Error::unsupported_format("list").with_span(meta)),
        Meta::NameValue(nv) => {
            if let Expr::Lit(expr_lit) = &nv.value {
                Expr::from_value(&expr_lit.lit)
            } else {
                Ok(nv.value.clone())
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use syn::parse_quote;

    use super::*;

    macro_rules! meta {
        ($body:expr) => {
            {
                let attr: ::syn::Attribute = ::syn::parse_quote!(#[ignore = $body]);
                attr.meta
            }
        };
    }

    #[test]
    fn preserve_str() {
        assert_eq!(
            preserve_str_literal(&meta!("World")).unwrap(),
            parse_quote!("World")
        );
    }

    #[test]
    fn preserve_binary_exp() {
        assert_eq!(
            preserve_str_literal(&meta!("World" + 5)).unwrap(),
            parse_quote!("World" + 5)
        )
    }

    #[test]
    fn parse_ident() {
        assert_eq!(
            parse_str_literal(&meta!("world")).unwrap(),
            parse_quote!(world)
        )
    }
}