syn/
custom_keyword.rs

1/// Define a type that supports parsing and printing a given identifier as if it
2/// were a keyword.
3///
4/// # Usage
5///
6/// As a convention, it is recommended that this macro be invoked within a
7/// module called `kw` or `keyword` and that the resulting parser be invoked
8/// with a `kw::` or `keyword::` prefix.
9///
10/// ```
11/// mod kw {
12///     syn::custom_keyword!(whatever);
13/// }
14/// ```
15///
16/// The generated syntax tree node supports the following operations just like
17/// any built-in keyword token.
18///
19/// - [Peeking] — `input.peek(kw::whatever)`
20///
21/// - [Parsing] — `input.parse::<kw::whatever>()?`
22///
23/// - [Printing] — `quote!( ... #whatever_token ... )`
24///
25/// - Construction from a [`Span`] — `let whatever_token = kw::whatever(sp)`
26///
27/// - Field access to its span — `let sp = whatever_token.span`
28///
29/// [Peeking]: parse::ParseBuffer::peek
30/// [Parsing]: parse::ParseBuffer::parse
31/// [Printing]: quote::ToTokens
32/// [`Span`]: proc_macro2::Span
33///
34/// # Example
35///
36/// This example parses input that looks like `bool = true` or `str = "value"`.
37/// The key must be either the identifier `bool` or the identifier `str`. If
38/// `bool`, the value may be either `true` or `false`. If `str`, the value may
39/// be any string literal.
40///
41/// The symbols `bool` and `str` are not reserved keywords in Rust so these are
42/// not considered keywords in the `syn::token` module. Like any other
43/// identifier that is not a keyword, these can be declared as custom keywords
44/// by crates that need to use them as such.
45///
46/// ```
47/// use syn::{LitBool, LitStr, Result, Token};
48/// use syn::parse::{Parse, ParseStream};
49///
50/// mod kw {
51///     syn::custom_keyword!(bool);
52///     syn::custom_keyword!(str);
53/// }
54///
55/// enum Argument {
56///     Bool {
57///         bool_token: kw::bool,
58///         eq_token: Token![=],
59///         value: LitBool,
60///     },
61///     Str {
62///         str_token: kw::str,
63///         eq_token: Token![=],
64///         value: LitStr,
65///     },
66/// }
67///
68/// impl Parse for Argument {
69///     fn parse(input: ParseStream) -> Result<Self> {
70///         let lookahead = input.lookahead1();
71///         if lookahead.peek(kw::bool) {
72///             Ok(Argument::Bool {
73///                 bool_token: input.parse::<kw::bool>()?,
74///                 eq_token: input.parse()?,
75///                 value: input.parse()?,
76///             })
77///         } else if lookahead.peek(kw::str) {
78///             Ok(Argument::Str {
79///                 str_token: input.parse::<kw::str>()?,
80///                 eq_token: input.parse()?,
81///                 value: input.parse()?,
82///             })
83///         } else {
84///             Err(lookahead.error())
85///         }
86///     }
87/// }
88/// ```
89#[macro_export(local_inner_macros)]
90macro_rules! custom_keyword {
91    ($ident:ident) => {
92        #[allow(non_camel_case_types)]
93        pub struct $ident {
94            pub span: $crate::export::Span,
95        }
96
97        #[doc(hidden)]
98        #[allow(non_snake_case)]
99        pub fn $ident<__S: $crate::export::IntoSpans<[$crate::export::Span; 1]>>(
100            span: __S,
101        ) -> $ident {
102            $ident {
103                span: $crate::export::IntoSpans::into_spans(span)[0],
104            }
105        }
106
107        impl $crate::export::Default for $ident {
108            fn default() -> Self {
109                $ident {
110                    span: $crate::export::Span::call_site(),
111                }
112            }
113        }
114
115        impl_parse_for_custom_keyword!($ident);
116        impl_to_tokens_for_custom_keyword!($ident);
117        impl_clone_for_custom_keyword!($ident);
118        impl_extra_traits_for_custom_keyword!($ident);
119    };
120}
121
122// Not public API.
123#[cfg(feature = "parsing")]
124#[doc(hidden)]
125#[macro_export]
126macro_rules! impl_parse_for_custom_keyword {
127    ($ident:ident) => {
128        // For peek.
129        impl $crate::token::CustomToken for $ident {
130            fn peek(cursor: $crate::buffer::Cursor) -> $crate::export::bool {
131                if let Some((ident, _rest)) = cursor.ident() {
132                    ident == stringify!($ident)
133                } else {
134                    false
135                }
136            }
137
138            fn display() -> &'static $crate::export::str {
139                concat!("`", stringify!($ident), "`")
140            }
141        }
142
143        impl $crate::parse::Parse for $ident {
144            fn parse(input: $crate::parse::ParseStream) -> $crate::parse::Result<$ident> {
145                input.step(|cursor| {
146                    if let $crate::export::Some((ident, rest)) = cursor.ident() {
147                        if ident == stringify!($ident) {
148                            return $crate::export::Ok(($ident { span: ident.span() }, rest));
149                        }
150                    }
151                    $crate::export::Err(cursor.error(concat!(
152                        "expected `",
153                        stringify!($ident),
154                        "`"
155                    )))
156                })
157            }
158        }
159    };
160}
161
162// Not public API.
163#[cfg(not(feature = "parsing"))]
164#[doc(hidden)]
165#[macro_export]
166macro_rules! impl_parse_for_custom_keyword {
167    ($ident:ident) => {};
168}
169
170// Not public API.
171#[cfg(feature = "printing")]
172#[doc(hidden)]
173#[macro_export]
174macro_rules! impl_to_tokens_for_custom_keyword {
175    ($ident:ident) => {
176        impl $crate::export::ToTokens for $ident {
177            fn to_tokens(&self, tokens: &mut $crate::export::TokenStream2) {
178                let ident = $crate::Ident::new(stringify!($ident), self.span);
179                $crate::export::TokenStreamExt::append(tokens, ident);
180            }
181        }
182    };
183}
184
185// Not public API.
186#[cfg(not(feature = "printing"))]
187#[doc(hidden)]
188#[macro_export]
189macro_rules! impl_to_tokens_for_custom_keyword {
190    ($ident:ident) => {};
191}
192
193// Not public API.
194#[cfg(feature = "clone-impls")]
195#[doc(hidden)]
196#[macro_export]
197macro_rules! impl_clone_for_custom_keyword {
198    ($ident:ident) => {
199        impl $crate::export::Copy for $ident {}
200
201        impl $crate::export::Clone for $ident {
202            fn clone(&self) -> Self {
203                *self
204            }
205        }
206    };
207}
208
209// Not public API.
210#[cfg(not(feature = "clone-impls"))]
211#[doc(hidden)]
212#[macro_export]
213macro_rules! impl_clone_for_custom_keyword {
214    ($ident:ident) => {};
215}
216
217// Not public API.
218#[cfg(feature = "extra-traits")]
219#[doc(hidden)]
220#[macro_export]
221macro_rules! impl_extra_traits_for_custom_keyword {
222    ($ident:ident) => {
223        impl $crate::export::Debug for $ident {
224            fn fmt(&self, f: &mut $crate::export::Formatter) -> $crate::export::fmt::Result {
225                $crate::export::Formatter::write_str(
226                    f,
227                    concat!("Keyword [", stringify!($ident), "]"),
228                )
229            }
230        }
231
232        impl $crate::export::Eq for $ident {}
233
234        impl $crate::export::PartialEq for $ident {
235            fn eq(&self, _other: &Self) -> $crate::export::bool {
236                true
237            }
238        }
239
240        impl $crate::export::Hash for $ident {
241            fn hash<__H: $crate::export::Hasher>(&self, _state: &mut __H) {}
242        }
243    };
244}
245
246// Not public API.
247#[cfg(not(feature = "extra-traits"))]
248#[doc(hidden)]
249#[macro_export]
250macro_rules! impl_extra_traits_for_custom_keyword {
251    ($ident:ident) => {};
252}