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
18 use syn::{Expr, Meta};
19
20 use 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.
preserve_str_literal(meta: &Meta) -> crate::Result<Expr>24 pub 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.
parse_str_literal(meta: &Meta) -> crate::Result<Expr>34 pub 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)]
49 mod 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]
preserve_str()64 fn preserve_str() {
65 assert_eq!(
66 preserve_str_literal(&meta!("World")).unwrap(),
67 parse_quote!("World")
68 );
69 }
70
71 #[test]
preserve_binary_exp()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]
parse_ident()80 fn parse_ident() {
81 assert_eq!(
82 parse_str_literal(&meta!("world")).unwrap(),
83 parse_quote!(world)
84 )
85 }
86 }
87