Struct End

pub struct End;

Available on crate feature parsing only.

Pseudo-token used for peeking the end of a parse stream.

This type is only useful as an argument to one of the following functions:

• ParseStream::peek
• ParseStream::peek2
• ParseStream::peek3
• Lookahead1::peek

The peek will return true if there are no remaining tokens after that point in the parse stream.

Example

Suppose we are parsing attributes containing core::fmt inspired formatting arguments:

• #[fmt("simple example")]
• #[fmt("interpolation e{}ample", self.x)]
• #[fmt("interpolation e{x}ample")]

and we want to recognize the cases where no interpolation occurs so that more efficient code can be generated.

The following implementation uses input.peek(Token![,]) && input.peek2(End) to recognize the case of a trailing comma without consuming the comma from the parse stream, because if it isn’t a trailing comma, that same comma needs to be parsed as part of args.

use proc_macro2::TokenStream;
use quote::quote;
use syn::parse::{End, Parse, ParseStream, Result};
use syn::{parse_quote, Attribute, LitStr, Token};

struct FormatArgs {
    template: LitStr,  // "...{}..."
    args: TokenStream, // , self.x
}

impl Parse for FormatArgs {
    fn parse(input: ParseStream) -> Result<Self> {
        let template: LitStr = input.parse()?;

        let args = if input.is_empty()
            || input.peek(Token![,]) && input.peek2(End)
        {
            input.parse::<Option<Token![,]>>()?;
            TokenStream::new()
        } else {
            input.parse()?
        };

        Ok(FormatArgs {
            template,
            args,
        })
    }
}

fn main() -> Result<()> {
    let attrs: Vec<Attribute> = parse_quote! {
        #[fmt("simple example")]
        #[fmt("interpolation e{}ample", self.x)]
        #[fmt("interpolation e{x}ample")]
    };

    for attr in &attrs {
        let FormatArgs { template, args } = attr.parse_args()?;
        let requires_fmt_machinery =
            !args.is_empty() || template.value().contains(['{', '}']);
        let out = if requires_fmt_machinery {
            quote! {
                ::core::write!(__formatter, #template #args)
            }
        } else {
            quote! {
                __formatter.write_str(#template)
            }
        };
        println!("{}", out);
    }
    Ok(())
}

Implementing this parsing logic without peek2(End) is more clumsy because we’d need a parse stream actually advanced past the comma before being able to find out whether there is anything after it. It would look something like:

use syn::parse::discouraged::Speculative as _;

let ahead = input.fork();
ahead.parse::<Option<Token![,]>>()?;
let args = if ahead.is_empty() {
    input.advance_to(&ahead);
    TokenStream::new()
} else {
    input.parse()?
};

or:

use quote::ToTokens as _;

let comma: Option<Token![,]> = input.parse()?;
let mut args = TokenStream::new();
if !input.is_empty() {
    comma.to_tokens(&mut args);
    input.parse::<TokenStream>()?.to_tokens(&mut args);
}

Trait Implementations

impl Clone for End

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for End

impl Peek for End

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 0 bytes