efg 0.1.4

Conditional compilation using boolean expression syntax, rather than any(), all(), not().
Documentation 
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use proc_macro::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
use std::iter::FromIterator;

pub struct Error {
    span: Span,
    msg: String,
}

impl Error {
    pub fn new(span: Span, msg: impl Into<String>) -> Self {
        Error {
            span,
            msg: msg.into(),
        }
    }

    pub fn 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
            }),
        ])
    }
}