[Swift] Report InputMismatchException with original context information

Port 0803c74 from the Java runtime to Swift.  This was issue antlr#1922.

runtime-testsuite/test/org/antlr/v4/test/runtime/descriptors/ParserErrorsDescriptors.java

@@ -639,7 +639,7 @@ public static class ExtraneousInput extends BaseParserTestDescriptor {
 
 		@Override
 		public boolean ignore(String targetName) {
-			return !"Java".equals(targetName);
+			return !"Java".equals(targetName) && !"Swift".equals(targetName);
 		}
 	}
 }

runtime-testsuite/test/org/antlr/v4/test/runtime/descriptors/SemPredEvalParserDescriptors.java

@@ -291,7 +291,7 @@ public static class PredFromAltTestedInLoopBack_1 extends PredFromAltTestedInLoo
 
 		@Override
 		public boolean ignore(String targetName) {
-			return !"Java".equals(targetName);
+			return !"Java".equals(targetName) && !"Swift".equals(targetName);
 		}
 	}
 

runtime/Swift/Sources/Antlr4/DefaultErrorStrategy.swift

@@ -32,6 +32,22 @@ open class DefaultErrorStrategy: ANTLRErrorStrategy {
 
     open var lastErrorStates: IntervalSet?
 
+    /**
+     * This field is used to propagate information about the lookahead following
+     * the previous match. Since prediction prefers completing the current rule
+     * to error recovery efforts, error reporting may occur later than the
+     * original point where it was discoverable. The original context is used to
+     * compute the true expected sets as though the reporting occurred as early
+     * as possible.
+     */
+    open var nextTokensContext: ParserRuleContext?
+
+    /**
+     * @see #nextTokensContext
+     */
+    open var nextTokensState = ATNState.INVALID_STATE_NUMBER
+
+
     public init() {
     }
 
@@ -207,7 +223,20 @@ open class DefaultErrorStrategy: ANTLRErrorStrategy {
 
         // try cheaper subset first; might get lucky. seems to shave a wee bit off
         let nextToks = recognizer.getATN().nextTokens(s)
-        if nextToks.contains(CommonToken.EPSILON) || nextToks.contains(la) {
+        if nextToks.contains(la) {
+            // We are sure the token matches
+            nextTokensContext = nil
+            nextTokensState = ATNState.INVALID_STATE_NUMBER
+            return
+        }
+
+        if nextToks.contains(CommonToken.EPSILON) {
+            if nextTokensContext == nil {
+                    // It's possible the next token won't match; information tracked
+                    // by sync is restricted for performance.
+                    nextTokensContext = recognizer.getContext()
+                    nextTokensState = recognizer.getState()
+            }
             return
         }
 
@@ -423,7 +452,8 @@ open class DefaultErrorStrategy: ANTLRErrorStrategy {
             return try getMissingSymbol(recognizer)
         }
         // even that didn't work; must throw the exception
-        throw ANTLRException.recognition(e: InputMismatchException(recognizer))
+        let exn = InputMismatchException(recognizer, state: nextTokensState, ctx: nextTokensContext)
+        throw ANTLRException.recognition(e: exn)
     }
 
     /// 

runtime/Swift/Sources/Antlr4/InputMismatchException.swift

@@ -11,10 +11,14 @@
 /// 
 
 public class InputMismatchException: RecognitionException {
-    public init(_ recognizer: Parser) {
-        super.init(recognizer, recognizer.getInputStream()!, recognizer._ctx)
+    public init(_ recognizer: Parser, state: Int = ATNState.INVALID_STATE_NUMBER, ctx: ParserRuleContext? = nil) {
+        let bestCtx = ctx ?? recognizer._ctx
+
+        super.init(recognizer, recognizer.getInputStream()!, bestCtx)
+
         if let token = try? recognizer.getCurrentToken() {
             setOffendingToken(token)
         }
+        setOffendingState(state)
     }
 }