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Rollup merge of #70209 - Centril:recover-quant-closure, r=petrochenkov

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parser: recover on `for<'a> |...| body` closures

When encountering `for` and `<` is 1 token ahead, interpret this as an explicitly quantified generic closure and recover, rather than attempting to parse a `for` loop. This provides both improved diagnostics as well as an insurance policy for the ability to use this as the syntax for generic closures in the future.

As requested by r? @eddyb
This commit is contained in:
Dylan DPC 2020-03-22 15:48:32 +01:00 • committed by GitHub
commit ea44d71f9b
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5 changed files with 66 additions and 14 deletions
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33
src/librustc_parse/parser/expr.rs
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@ -925,8 +925,17 @@ impl<'a> Parser<'a> {
self.parse_closure_expr(attrs)
} else if self.eat_keyword(kw::If) {
self.parse_if_expr(attrs)
} else if self.eat_keyword(kw::For) {
self.parse_for_expr(None, self.prev_token.span, attrs)
} else if self.check_keyword(kw::For) {
if self.choose_generics_over_qpath(1) {
// NOTE(Centril, eddyb): DO NOT REMOVE! Beyond providing parser recovery,
// this is an insurance policy in case we allow qpaths in (tuple-)struct patterns.
// When `for <Foo as Bar>::Proj in $expr $block` is wanted,
// you can disambiguate in favor of a pattern with `(...)`.
self.recover_quantified_closure_expr(attrs)
} else {
assert!(self.eat_keyword(kw::For));
self.parse_for_expr(None, self.prev_token.span, attrs)
}
} else if self.eat_keyword(kw::While) {
self.parse_while_expr(None, self.prev_token.span, attrs)
} else if let Some(label) = self.eat_label() {
@ -1417,6 +1426,26 @@ impl<'a> Parser<'a> {
Ok(self.mk_expr(blk.span, ExprKind::Block(blk, opt_label), attrs))
}
/// Recover on an explicitly quantified closure expression, e.g., `for<'a> |x: &'a u8| *x + 1`.
fn recover_quantified_closure_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
let lo = self.token.span;
let _ = self.parse_late_bound_lifetime_defs()?;
let span_for = lo.to(self.prev_token.span);
let closure = self.parse_closure_expr(attrs)?;
self.struct_span_err(span_for, "cannot introduce explicit parameters for a closure")
.span_label(closure.span, "the parameters are attached to this closure")
.span_suggestion(
span_for,
"remove the parameters",
String::new(),
Applicability::MachineApplicable,
)
.emit();
Ok(self.mk_expr_err(lo.to(closure.span)))
}
/// Parses a closure expression (e.g., `move |args| expr`).
fn parse_closure_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
let lo = self.token.span;