Err for comma after functional update syntax

crates/parser/src/grammar/expressions.rs

@@ -693,6 +693,17 @@ pub(crate) fn record_expr_field_list(p: &mut Parser<'_>) {
                 // We permit `.. }` on the left-hand side of a destructuring assignment.
                 if !p.at(T!['}']) {
                     expr(p);
+
+                    if p.at(T![,]) {
+                        // test_err comma_after_functional_update_syntax
+                        // fn foo() {
+                        //     S { ..x, };
+                        //     S { ..x, a: 0 }
+                        // }
+
+                        // Do not bump, so we can support additional fields after this comma.
+                        p.error("cannot use a comma after the base struct");
+                    }
                 }
             }
             T!['{'] => {

crates/parser/test_data/parser/inline/err/0024_comma_after_functional_update_syntax.rast

@@ -0,0 +1,66 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE "\n    "
+        EXPR_STMT
+          RECORD_EXPR
+            PATH
+              PATH_SEGMENT
+                NAME_REF
+                  IDENT "S"
+            WHITESPACE " "
+            RECORD_EXPR_FIELD_LIST
+              L_CURLY "{"
+              WHITESPACE " "
+              DOT2 ".."
+              PATH_EXPR
+                PATH
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "x"
+              COMMA ","
+              WHITESPACE " "
+              R_CURLY "}"
+          SEMICOLON ";"
+        WHITESPACE "\n    "
+        RECORD_EXPR
+          PATH
+            PATH_SEGMENT
+              NAME_REF
+                IDENT "S"
+          WHITESPACE " "
+          RECORD_EXPR_FIELD_LIST
+            L_CURLY "{"
+            WHITESPACE " "
+            DOT2 ".."
+            PATH_EXPR
+              PATH
+                PATH_SEGMENT
+                  NAME_REF
+                    IDENT "x"
+            COMMA ","
+            WHITESPACE " "
+            RECORD_EXPR_FIELD
+              NAME_REF
+                IDENT "a"
+              COLON ":"
+              WHITESPACE " "
+              LITERAL
+                INT_NUMBER "0"
+            WHITESPACE " "
+            R_CURLY "}"
+        WHITESPACE "\n"
+        R_CURLY "}"
+  WHITESPACE "\n"
+error 22: cannot use a comma after the base struct
+error 38: cannot use a comma after the base struct

crates/parser/test_data/parser/inline/err/0024_comma_after_functional_update_syntax.rs

@@ -0,0 +1,4 @@
+fn foo() {
+    S { ..x, };
+    S { ..x, a: 0 }
+}