src/error.rs

use proc_macro::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
use std::{fmt, iter::FromIterator};

pub(crate) struct Error {
    span: Span,
    msg: String,
}

impl Error {
    pub(crate) fn new(span: Span, msg: impl fmt::Display) -> Self {
        Error { span, msg: msg.to_string() }
    }

    // https://github.com/dtolnay/syn/blob/1.0.39/src/error.rs#L218-L237
    pub(crate) fn to_compile_error(&self) -> TokenStream {
        // compile_error!($msg)
        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.msg);
                        string.set_span(self.span);
                        string
                    })])
                });
                group.set_span(self.span);
                group
            }),
        ])
    }
}