syn-error-experiment 0.0.0

use std;
use std::fmt::{self, Display};
use std::iter::FromIterator;

use proc_macro2::{
    Delimiter, Group, Ident, LexError, Literal, Punct, Spacing, Span, TokenStream, TokenTree,
};

use syn::buffer::Cursor;

/// The result of a Syn parser.
pub type Result<T> = std::result::Result<T, Error>;

/// Error returned when a Syn parser cannot parse the input tokens.
#[derive(Debug)]
pub struct Error {
    span: Span,
    message: String,
}

impl Error {
    pub fn new<T: Display>(span: Span, message: T) -> Self {
        Error {
            span: span,
            message: message.to_string(),
        }
    }

    /// Render the error as an invocation of [`compile_error!`].
    ///
    /// The [`parse_macro_input!`] macro provides a convenient way to invoke
    /// this method correctly in a procedural macro.
    ///
    /// [`compile_error!`]: https://doc.rust-lang.org/std/macro.compile_error.html
    /// [`parse_macro_input!`]: ../macro.parse_macro_input.html
    pub fn into_compile_error(self) -> TokenStream {
        // compile_error!($message)
        TokenStream::from_iter(vec![
            TokenTree::Ident(Ident::new("compile_error", self.span)),
            TokenTree::Punct({
                let mut punct = Punct::new('!', Spacing::Alone);
                punct.set_span(self.span);
                punct
            }),
            TokenTree::Group({
                let mut group = Group::new(Delimiter::Brace, {
                    TokenStream::from_iter(vec![TokenTree::Literal({
                        let mut string = Literal::string(&self.message);
                        string.set_span(self.span);
                        string
                    })])
                });
                group.set_span(self.span);
                group
            }),
        ])
    }
}

// Not public API.
#[doc(hidden)]
pub fn new_at<T: Display>(scope: Span, cursor: Cursor, message: T) -> Error {
    if cursor.eof() {
        Error::new(scope, format!("unexpected end of input, {}", message))
    } else {
        Error::new(cursor.span(), message)
    }
}

impl Display for Error {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str(&self.message)
    }
}

impl std::error::Error for Error {
    fn description(&self) -> &str {
        "parse error"
    }
}

impl From<LexError> for Error {
    fn from(err: LexError) -> Self {
        Error::new(Span::call_site(), format!("{:?}", err))
    }
}