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Auto merge of #38268 - withoutboats:parse_where_higher_rank_hack, r=eddyb

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Prevent where < ident > from parsing.

In order to be forward compatible with `where<'a>` syntax for higher
rank parameters, prevent potential conflicts with UFCS from parsing
correctly for the near term.
This commit is contained in:
bors 2016-12-24 00:22:00 +00:00
commit 8d65c8d64e
2 changed files with 33 additions and 0 deletions
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17
src/libsyntax/parse/parser.rs
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@ -4368,6 +4368,23 @@ impl<'a> Parser<'a> {
return Ok(where_clause);
}
// This is a temporary hack.
//
// We are considering adding generics to the `where` keyword as an alternative higher-rank
// parameter syntax (as in `where<'a>` or `where<T>`. To avoid that being a breaking
// change, for now we refuse to parse `where < (ident | lifetime) (> | , | :)`.
if token::Lt == self.token {
let ident_or_lifetime = self.look_ahead(1, |t| t.is_ident() || t.is_lifetime());
if ident_or_lifetime {
let gt_comma_or_colon = self.look_ahead(2, |t| {
*t == token::Gt || *t == token::Comma || *t == token::Colon
});
if gt_comma_or_colon {
self.span_err(self.span, "syntax `where<T>` is reserved for future use");
}
}
}
let mut parsed_something = false;
loop {
let lo = self.span.lo;