parser: Fix panic in 'const impl' recovery

The panic happens when in recovery parsing a full `impl`
(`parse_item_impl`) fails and we drop the `DiagnosticBuilder` for the
recovery suggestion and return the `parse_item_impl` error.

We now raise the original error "expected identifier found `impl`" when
parsing the `impl` fails.

Note that the regression test is slightly simplified version of the
original repro in #81806, to make the error output smaller and more
resilient to unrelated changes in parser error messages.

Fixes #81806

compiler/rustc_parse/src/parser/item.rs

@@ -1010,9 +1010,18 @@ impl<'a> Parser<'a> {
     ) -> PResult<'a, ItemInfo> {
         let impl_span = self.token.span;
         let mut err = self.expected_ident_found();
-        let mut impl_info = self.parse_item_impl(attrs, defaultness)?;
+
+        // Only try to recover if this is implementing a trait for a type
+        let mut impl_info = match self.parse_item_impl(attrs, defaultness) {
+            Ok(impl_info) => impl_info,
+            Err(mut recovery_error) => {
+                // Recovery failed, raise the "expected identifier" error
+                recovery_error.cancel();
+                return Err(err);
+            }
+        };
+
         match impl_info.1 {
-            // only try to recover if this is implementing a trait for a type
             ItemKind::Impl(box ImplKind {
                 of_trait: Some(ref trai), ref mut constness, ..
             }) => {
@@ -1030,6 +1039,7 @@ impl<'a> Parser<'a> {
             ItemKind::Impl { .. } => return Err(err),
             _ => unreachable!(),
         }
+
         Ok(impl_info)
     }
 

src/test/ui/parser/issue-81806.rs

@@ -0,0 +1,5 @@
+trait T { const
+impl //~ ERROR: expected identifier, found keyword `impl`
+}
+
+fn main() {}

src/test/ui/parser/issue-81806.stderr

@@ -0,0 +1,17 @@
+error: expected identifier, found keyword `impl`
+  --> $DIR/issue-81806.rs:2:1
+   |
+LL | trait T { const
+   |         - while parsing this item list starting here
+LL | impl
+   | ^^^^ expected identifier, found keyword
+LL | }
+   | - the item list ends here
+   |
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL | r#impl
+   | ^^^^^^
+
+error: aborting due to previous error
+