Fix parser panic on malformed tuple literals

CHANGELOG.md

@@ -11,6 +11,12 @@ Added a warning for unnecessary `let` bindings where a variable is
 bound and immediately returned (e.g. `let x = foo() x` can be
 simplified to `foo()`), with an autofix.
 
+## Parsing
+
+Fixed a parser panic on malformed tuple literals such as `(,{` where
+the parser failed to make forward progress after recovering from an
+invalid element.
+
 ## Commands
 
 Added :load to evaluate all definitions in a file and switch to that

src/parser.rs

@@ -266,10 +266,12 @@ fn parse_tuple_literal_or_parentheses(
 
             let start_idx = tokens.idx;
             exprs.push(parse_expression(tokens, id_gen, diagnostics));
-            assert!(
-                tokens.idx > start_idx,
-                "The parser should always make forward progress."
-            );
+            if tokens.idx == start_idx {
+                // `parse_expression` could not consume any tokens and
+                // has already emitted a diagnostic. Stop collecting
+                // tuple elements to avoid an infinite loop.
+                break;
+            }
         }
 
         let close_paren = require_token(tokens, diagnostics, ")");

src/test_files/parser/tuple_no_progress.gdn

@@ -0,0 +1,47 @@
+(,{
+
+// args: dump-ast
+// expected stdout:
+// TupleLiteral(
+//     [
+//         Expression {
+//             position: Position { ... },
+//             expr_: Invalid,
+//             value_is_used: true,
+//             id: SyntaxId(0),
+//         },
+//         Expression {
+//             position: Position { ... },
+//             expr_: Invalid,
+//             value_is_used: true,
+//             id: SyntaxId(1),
+//         },
+//     ],
+// )
+// Block(
+//     Block {
+//         open_brace: Position { ... },
+//         exprs: [],
+//         close_brace: Position { ... },
+//     },
+// )
+
+// expected stderr:
+// Error: Parse error: Expected an expression after this.
+// --| src/test_files/parser/tuple_no_progress.gdn:1:1
+//  1| (,{
+//   | ^
+//  2| 
+//  3| // args: dump-ast
+// Error: Parse error: Expected an expression after this.
+// --| src/test_files/parser/tuple_no_progress.gdn:1:2
+//  1| (,{
+//  2|  ^
+//  3| // args: dump-ast
+// Error: Parse error: Expected `)` after this.
+// --| src/test_files/parser/tuple_no_progress.gdn:1:2
+//  1| (,{
+//  2|  ^
+//  3| // args: dump-ast
+// Parse error (incomplete input): Expected `}`, but got EOF.
+