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Recover param: Ty = EXPR #137396

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27 changes: 25 additions & 2 deletions compiler/rustc_parse/src/parser/ty.rs
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@fmease fmease Feb 22, 2025 •
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On a second glance, I would think parse_param_general is 'a better fit' for this compared to parse_ty_for_param (which should probably only concern itself with parsing types).

Mostly an organizational nitpick since it shouldn't be able to affect behavior (CC anon params in Rust 2015). I was only able to find trait T { fn f((x)=0 fn) {} } which will emit two errors under this PR in Rust 2015 (parameter defaults are not supported and expected `:`, found `=`) instead of one and would keep emitting a single one if the logic was moved into parse_param_general I think but that's hardly interesting.

nw, r=me then

r=me with or without moving the logic into parse_param_general. Sorry for the churn / wrong signal there.

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@fmease fmease Feb 22, 2025 •
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(preexisting: the patterns aren't allowed in methods without bodies parser recovery is fragile already: it suggests the more butchered _: )=0 for trait T { fn f((x)=0); } under Rust 2015)

Original file line number Diff line number Diff line change
Expand Up @@ -132,14 +132,37 @@ impl<'a> Parser<'a> {
/// The difference from `parse_ty` is that this version allows `...`
/// (`CVarArgs`) at the top level of the type.
pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, P<Ty>> {
self.parse_ty_common(
let ty = self.parse_ty_common(
AllowPlus::Yes,
AllowCVariadic::Yes,
RecoverQPath::Yes,
RecoverReturnSign::Yes,
None,
RecoverQuestionMark::Yes,
)
)?;

// Recover a trailing `= EXPR` if present.
if self.may_recover()
&& self.check_noexpect(&token::Eq)
&& self.look_ahead(1, |tok| tok.can_begin_expr())
{
let snapshot = self.create_snapshot_for_diagnostic();
self.bump();
let eq_span = self.prev_token.span;
match self.parse_expr() {
Ok(e) => {
self.dcx()
.struct_span_err(eq_span.to(e.span), "parameter defaults are not supported")
.emit();
}
Err(diag) => {
diag.cancel();
self.restore_snapshot(snapshot);
}
}
}

Ok(ty)
}

/// Parses a type in restricted contexts where `+` is not permitted.
Expand Down
4 changes: 4 additions & 0 deletions tests/ui/parser/fn-with-default-expr.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
fn foo(x: i32 = 1) {}
//~^ ERROR parameter defaults are not supported

fn main() {}
8 changes: 8 additions & 0 deletions tests/ui/parser/fn-with-default-expr.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
error: parameter defaults are not supported
--> $DIR/fn-with-default-expr.rs:1:15
|
LL | fn foo(x: i32 = 1) {}
| ^^^

error: aborting due to 1 previous error

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