Custom expression suggestion: tackle a dangling single/double quote (#19033)

If the expression is invalid due to an unterminated string (perhaps the
user is still not finished typing the expression), do not try to suggest
possible matches (functions etc) for an identifier following that
dangling quote.

frontend/src/metabase/lib/expressions/completer.js

@@ -13,6 +13,13 @@ export function partialMatch(expression) {
   const lastToken = _.last(tokens);
   if (lastToken && lastToken.type === TOKEN.Identifier) {
     if (lastToken.end === expression.length) {
+      const prevToken = tokens[tokens.length - 2];
+      if (prevToken && prevToken.type === TOKEN.String) {
+        if (prevToken.start + 1 === prevToken.end) {
+          // a dangling single- or double-quote
+          return null;
+        }
+      }
       return expression.slice(lastToken.start, lastToken.end);
     }
   }

frontend/test/metabase/lib/expressions/completer.unit.spec.js

@@ -28,6 +28,11 @@ describe("metabase/lib/expressions/completer", () => {
       expect(partialMatch("")).toEqual(null);
       expect(partialMatch(" ")).toEqual(null);
     });
+
+    it("should handle a dangling quote", () => {
+      expect(partialMatch("concat('s")).toEqual(null);
+      expect(partialMatch('length("c')).toEqual(null);
+    });
   });
 
   describe("enclosingFunction", () => {