darling_core/util/
parse_expr.rs

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1//! Functions to use with `#[darling(with = "...")]` that control how quoted values
2//! in [`Meta`] instances are parsed into [`Expr`] fields.
3//!
4//! Version 1 of syn did not permit expressions on the right-hand side of the `=` in a
5//! [`MetaNameValue`](syn::MetaNameValue), so darling accepted string literals and then
6//! parsed their contents as expressions.
7//! Passing a string literal in this version would have required the use of a raw string
8//! to add quotation marks inside the literal.
9//!
10//! Version 2 of syn removes the requirement that the right-hand side be a literal.
11//! For most types, such as [`Path`](syn::Path), the [`FromMeta`] impl can accept the
12//! version without quotation marks without causing ambiguity; a path cannot start and
13//! end with quotation marks, so removal is automatic.
14//!
15//! [`Expr`] is the one type where this ambiguity is new and unavoidable. To address this,
16//! this module provides different functions for different expected behaviors.
17
18use syn::{Expr, Meta};
19
20use crate::{Error, FromMeta};
21
22/// Parse a [`Meta`] to an [`Expr`]; if the value is a string literal, the emitted
23/// expression will be a string literal.
24pub fn preserve_str_literal(meta: &Meta) -> crate::Result<Expr> {
25    match meta {
26        Meta::Path(_) => Err(Error::unsupported_format("path").with_span(meta)),
27        Meta::List(_) => Err(Error::unsupported_format("list").with_span(meta)),
28        Meta::NameValue(nv) => Ok(nv.value.clone()),
29    }
30}
31
32/// Parse a [`Meta`] to an [`Expr`]; if the value is a string literal, the string's
33/// contents will be parsed as an expression and emitted.
34pub fn parse_str_literal(meta: &Meta) -> crate::Result<Expr> {
35    match meta {
36        Meta::Path(_) => Err(Error::unsupported_format("path").with_span(meta)),
37        Meta::List(_) => Err(Error::unsupported_format("list").with_span(meta)),
38        Meta::NameValue(nv) => {
39            if let Expr::Lit(expr_lit) = &nv.value {
40                Expr::from_value(&expr_lit.lit)
41            } else {
42                Ok(nv.value.clone())
43            }
44        }
45    }
46}
47
48#[cfg(test)]
49mod tests {
50    use syn::parse_quote;
51
52    use super::*;
53
54    macro_rules! meta {
55        ($body:expr) => {
56            {
57                let attr: ::syn::Attribute = ::syn::parse_quote!(#[ignore = $body]);
58                attr.meta
59            }
60        };
61    }
62
63    #[test]
64    fn preserve_str() {
65        assert_eq!(
66            preserve_str_literal(&meta!("World")).unwrap(),
67            parse_quote!("World")
68        );
69    }
70
71    #[test]
72    fn preserve_binary_exp() {
73        assert_eq!(
74            preserve_str_literal(&meta!("World" + 5)).unwrap(),
75            parse_quote!("World" + 5)
76        )
77    }
78
79    #[test]
80    fn parse_ident() {
81        assert_eq!(
82            parse_str_literal(&meta!("world")).unwrap(),
83            parse_quote!(world)
84        )
85    }
86}
darling_core/util/parse_expr.rs

darling_core/util/
parse_expr.rs
