Macro syn::custom_keyword [−][src]
Define a type that supports parsing and printing a given identifier as if it were a keyword.
Usage
As a convention, it is recommended that this macro be invoked within a
module called kw
or keyword
and that the resulting parser be invoked
with a kw::
or keyword::
prefix.
mod kw { syn::custom_keyword!(whatever); }
The generated syntax tree node supports the following operations just like any built-in keyword token.
-
Peeking —
input.peek(kw::whatever)
-
Parsing —
input.parse::<kw::whatever>()?
-
Printing —
quote!( ... #whatever_token ... )
-
Construction from a
Span
—let whatever_token = kw::whatever(sp)
-
Field access to its span —
let sp = whatever_token.span
Example
This example parses input that looks like bool = true
or str = "value"
.
The key must be either the identifier bool
or the identifier str
. If
bool
, the value may be either true
or false
. If str
, the value may
be any string literal.
The symbols bool
and str
are not reserved keywords in Rust so these are
not considered keywords in the syn::token
module. Like any other
identifier that is not a keyword, these can be declared as custom keywords
by crates that need to use them as such.
use syn::{LitBool, LitStr, Result, Token}; use syn::parse::{Parse, ParseStream}; mod kw { syn::custom_keyword!(bool); syn::custom_keyword!(str); } enum Argument { Bool { bool_token: kw::bool, eq_token: Token![=], value: LitBool, }, Str { str_token: kw::str, eq_token: Token![=], value: LitStr, }, } impl Parse for Argument { fn parse(input: ParseStream) -> Result<Self> { let lookahead = input.lookahead1(); if lookahead.peek(kw::bool) { Ok(Argument::Bool { bool_token: input.parse::<kw::bool>()?, eq_token: input.parse()?, value: input.parse()?, }) } else if lookahead.peek(kw::str) { Ok(Argument::Str { str_token: input.parse::<kw::str>()?, eq_token: input.parse()?, value: input.parse()?, }) } else { Err(lookahead.error()) } } }