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Prevent where < ident > from parsing. #38268

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Merged
merged 4 commits into from
Dec 24, 2016
Merged

Prevent where < ident > from parsing. #38268

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90f6219
Prevent where < ident > from parsing.
withoutboats Dec 9, 2016
ddae271
Improve error message.
withoutboats Dec 10, 2016
e93e00f
Add test.
withoutboats Dec 10, 2016
14e4b00
Fix mistake.
withoutboats Dec 10, 2016
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17 changes: 17 additions & 0 deletions src/libsyntax/parse/parser.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4377,6 +4377,23 @@ 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 {
return Err(self.fatal("TODO How to even explain this error?"));
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@petrochenkov petrochenkov Dec 9, 2016 •
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This is a recoverable error, no need to return Err.
self.err("syntax `where<T>` is reserved for future use"); would be more appropriate.

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Should we consume tokens until the the next Gt in that case?

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Nah, if it's indeed <A>::B: Tr (more likely), then it'll parse successfully without consuming extra tokens, otherwise (less likely) it doesn't matter.

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But don't we want it to parse as ::B: Tr1? I suppose it doesn't matter much since it errors anyway.

}
}
}

let mut parsed_something = false;
loop {
let lo = self.span.lo;
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