Recover param: Ty = EXPR

Author: compiler-errors

compiler/rustc_parse/src/parser/ty.rs

@@ -132,14 +132,38 @@ 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() {
+            let snapshot = self.create_snapshot_for_diagnostic();
+            if self.eat_noexpect(&token::Eq) {
+                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.

tests/ui/parser/fn-with-default-expr.rs

@@ -0,0 +1,4 @@
+fn foo(x: i32 = 1) {}
+//~^ ERROR parameter defaults are not supported
+
+fn main() {}

tests/ui/parser/fn-with-default-expr.stderr

@@ -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
+