--- /dev/null
+use proc_macro::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
+use std::fmt::Display;
+use std::iter::FromIterator;
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;
+
+pub struct Error {
+ begin: Span,
+ end: Span,
+ msg: String,
+}
+
+impl Error {
+ pub fn new(span: Span, msg: impl Display) -> Self {
+ Self::new2(span, span, msg)
+ }
+
+ pub fn new2(begin: Span, end: Span, msg: impl Display) -> Self {
+ Error {
+ begin,
+ end,
+ msg: msg.to_string(),
+ }
+ }
+
+ pub fn group(group: Group, msg: impl Display) -> Self {
+ let mut iter = group.stream().into_iter();
+ let delimiter = group.span();
+ let begin = iter.next().map_or(delimiter, |t| t.span());
+ let end = iter.last().map_or(begin, |t| t.span());
+ Self::new2(begin, end, msg)
+ }
+
+ pub fn into_compile_error(self) -> TokenStream {
+ // compile_error! { $msg }
+ TokenStream::from_iter(vec![
+ TokenTree::Ident(Ident::new("compile_error", self.begin)),
+ TokenTree::Punct({
+ let mut punct = Punct::new('!', Spacing::Alone);
+ punct.set_span(self.begin);
+ 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.end);
+ string
+ })])
+ });
+ group.set_span(self.end);
+ group
+ }),
+ ])
+ }
+}