Fix diagnostic of ternary operator missing only colon

Author: TTOzzi

Sources/SwiftParser/Expressions.swift

@@ -318,10 +318,8 @@ extension Parser {
       )
 
       let rhs: RawExprSyntax?
-      if colon.isMissing {
-        // If the colon is missing there's not much more structure we can
-        // expect out of this expression sequence. Emit a missing expression
-        // to end the parsing here.
+      if colon.isMissing,
+         currentToken.rawTokenKind != currentToken.cursor.previousTokenKind {
         rhs = RawExprSyntax(RawMissingExprSyntax(arena: self.arena))
       } else {
         rhs = nil

Tests/SwiftParserTest/translated/InvalidIfExprTests.swift

@@ -69,4 +69,15 @@ final class InvalidIfExprTests: XCTestCase {
     )
   }
 
+  func testInvalidIfExpr5() {
+    assertParse(
+      """
+      foo ? 1 1️⃣2
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "expected ':' after '? ...' in ternary expression", fixIts: ["insert ':'"])
+      ],
+      fixedSource: "foo ? 1 : 2"
+    )
+  }
 }