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< main > < div class = "width-limiter" > < nav class = "sub" > < form class = "search-form" > < span > < / span > < div id = "sidebar-button" tabindex = "-1" > < a href = "../../syn/all.html" title = "show sidebar" > < / a > < / div > < input class = "search-input" name = "search" aria-label = "Run search in the documentation" autocomplete = "off" spellcheck = "false" placeholder = "Click or press ‘ ’ ‘ ’ type = "search" > < div id = "help-button" tabindex = "-1" > < a href = "../../help.html" title = "help" > ?< / a > < / div > < div id = "settings-menu" tabindex = "-1" > < a href = "../../settings.html" title = "settings" > < img width = "22" height = "22" alt = "Change settings" src = "../../static.files/wheel-7b819b6101059cd0.svg" > < / a > < / div > < / form > < / nav > < section id = "main-content" class = "content" > < div class = "main-heading" > < h1 > Function < a href = "../index.html" > syn< / a > ::< wbr > < a href = "index.html" > meta< / a > ::< wbr > < a class = "fn" href = "#" > parser< / a > < button id = "copy-path" title = "Copy item path to clipboard" > < img src = "../../static.files/clipboard-7571035ce49a181d.svg" width = "19" height = "18" alt = "Copy item path" > < / button > < / h1 > < span class = "out-of-band" > < a class = "src" href = "../../src/syn/meta.rs.html#132-140" > source< / a > · < button id = "toggle-all-docs" title = "collapse all docs" > [< span > − < / span > ]< / button > < / span > < / div > < pre class = "rust item-decl" > < code > pub fn parser(
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logic: impl < a class = "trait" href = "https://doc.rust-lang.org/1.78.0/core/ops/function/trait.FnMut.html" title = "trait core::ops::function::FnMut" > FnMut< / a > (< a class = "struct" href = "struct.ParseNestedMeta.html" title = "struct syn::meta::ParseNestedMeta" > ParseNestedMeta< / a > < '_> ) -> < a class = "type" href = "../parse/type.Result.html" title = "type syn::parse::Result" > Result< / a > < < a class = "primitive" href = "https://doc.rust-lang.org/1.78.0/std/primitive.unit.html" > ()< / a > >
) -> impl < a class = "trait" href = "../parse/trait.Parser.html" title = "trait syn::parse::Parser" > Parser< / a > < Output = < a class = "primitive" href = "https://doc.rust-lang.org/1.78.0/std/primitive.unit.html" > ()< / a > > < / code > < / pre > < details class = "toggle top-doc" open > < summary class = "hideme" > < span > Expand description< / span > < / summary > < div class = "docblock" > < p > Make a parser that is usable with < code > parse_macro_input!< / code > in a
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< code > #[proc_macro_attribute]< / code > macro.< / p >
< p > < em > Warning:< / em > When parsing attribute args < strong > other than< / strong > the
< code > proc_macro::TokenStream< / code > input of a < code > proc_macro_attribute< / code > , you do < strong > not< / strong >
need this function. In several cases your callers will get worse error
messages if you use this function, because the surrounding delimiter’
is concealed from attribute macros by rustc. Use
< a href = "../struct.Attribute.html#method.parse_nested_meta" title = "method syn::Attribute::parse_nested_meta" > < code > Attribute::parse_nested_meta< / code > < / a > instead.< / p >
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< h2 id = "example" > < a class = "doc-anchor" href = "#example" > §< / a > Example< / h2 >
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< p > This example implements an attribute macro whose invocations look like this:< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[tea(kind = < span class = "string" > "EarlGrey"< / span > , hot)]
< / span > < span class = "kw" > struct < / span > Picard {...}< / code > < / pre > < / div >
< p > The “parameters” supported by the attribute are:< / p >
< ul >
< li > < code > kind = " ..." < / code > < / li >
< li > < code > hot< / code > < / li >
< li > < code > with(sugar, milk, ...)< / code > , a comma-separated list of ingredients< / li >
< / ul >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "kw" > use < / span > proc_macro::TokenStream;
< span class = "kw" > use < / span > syn::{parse_macro_input, LitStr, Path};
< span class = "attr" > #[proc_macro_attribute]
< / span > < span class = "kw" > pub fn < / span > tea(args: TokenStream, input: TokenStream) -> TokenStream {
< span class = "kw" > let < / span > < span class = "kw-2" > mut < / span > kind: < span class = "prelude-ty" > Option< / span > < LitStr> = < span class = "prelude-val" > None< / span > ;
< span class = "kw" > let < / span > < span class = "kw-2" > mut < / span > hot: bool = < span class = "bool-val" > false< / span > ;
< span class = "kw" > let < / span > < span class = "kw-2" > mut < / span > with: Vec< Path> = Vec::new();
< span class = "kw" > let < / span > tea_parser = syn::meta::parser(|meta| {
< span class = "kw" > if < / span > meta.path.is_ident(< span class = "string" > "kind"< / span > ) {
kind = < span class = "prelude-val" > Some< / span > (meta.value()< span class = "question-mark" > ?< / span > .parse()< span class = "question-mark" > ?< / span > );
< span class = "prelude-val" > Ok< / span > (())
} < span class = "kw" > else if < / span > meta.path.is_ident(< span class = "string" > "hot"< / span > ) {
hot = < span class = "bool-val" > true< / span > ;
< span class = "prelude-val" > Ok< / span > (())
} < span class = "kw" > else if < / span > meta.path.is_ident(< span class = "string" > "with"< / span > ) {
meta.parse_nested_meta(|meta| {
with.push(meta.path);
< span class = "prelude-val" > Ok< / span > (())
})
} < span class = "kw" > else < / span > {
< span class = "prelude-val" > Err< / span > (meta.error(< span class = "string" > "unsupported tea property"< / span > ))
}
});
< span class = "macro" > parse_macro_input!< / span > (args with tea_parser);
< span class = "macro" > eprintln!< / span > (< span class = "string" > "kind={kind:?} hot={hot} with={with:?}"< / span > );
< span class = "comment" > /* ... */
< / span > }< / code > < / pre > < / div >
< p > The < code > syn::meta< / code > library will take care of dealing with the commas including
trailing commas, and producing sensible error messages on unexpected input.< / p >
< div class = "example-wrap" > < pre class = "language-console" > < code > error: expected `,`
--> src/main.rs:3:37
|
3 | #[tea(kind = " EarlGrey" , with(sugar = " lol" , milk))]
| ^
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< / code > < / pre > < / div > < h2 id = "example-1" > < a class = "doc-anchor" href = "#example-1" > §< / a > Example< / h2 >
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< p > Same as above but we factor out most of the logic into a separate function.< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "kw" > use < / span > proc_macro::TokenStream;
< span class = "kw" > use < / span > syn::meta::ParseNestedMeta;
< span class = "kw" > use < / span > syn::parse::{Parser, < span class = "prelude-ty" > Result< / span > };
< span class = "kw" > use < / span > syn::{parse_macro_input, LitStr, Path};
< span class = "attr" > #[proc_macro_attribute]
< / span > < span class = "kw" > pub fn < / span > tea(args: TokenStream, input: TokenStream) -> TokenStream {
< span class = "kw" > let < / span > < span class = "kw-2" > mut < / span > attrs = TeaAttributes::default();
< span class = "kw" > let < / span > tea_parser = syn::meta::parser(|meta| attrs.parse(meta));
< span class = "macro" > parse_macro_input!< / span > (args with tea_parser);
< span class = "comment" > /* ... */
< / span > }
< span class = "attr" > #[derive(Default)]
< / span > < span class = "kw" > struct < / span > TeaAttributes {
kind: < span class = "prelude-ty" > Option< / span > < LitStr> ,
hot: bool,
with: Vec< Path> ,
}
< span class = "kw" > impl < / span > TeaAttributes {
< span class = "kw" > fn < / span > parse(< span class = "kw-2" > & mut < / span > < span class = "self" > self< / span > , meta: ParseNestedMeta) -> < span class = "prelude-ty" > Result< / span > < ()> {
< span class = "kw" > if < / span > meta.path.is_ident(< span class = "string" > "kind"< / span > ) {
< span class = "self" > self< / span > .kind = < span class = "prelude-val" > Some< / span > (meta.value()< span class = "question-mark" > ?< / span > .parse()< span class = "question-mark" > ?< / span > );
< span class = "prelude-val" > Ok< / span > (())
} < span class = "kw" > else < / span > < span class = "comment" > /* just like in last example */
< / span > }
}< / code > < / pre > < / div >
< / div > < / details > < / section > < / div > < / main > < / body > < / html >
