Fix parser performance: don't do the additional lookahead when looking for the comma in parameters

compiler/frontend/src/org/jetbrains/jet/lang/parsing/JetExpressionParsing.java

@@ -1003,10 +1003,8 @@ public class JetExpressionParsing extends AbstractJetParsing {
             //   {(a, b) -> ...}
 
             {
-                boolean preferParamsToExpressions = isConfirmedParametersByComma();
-
                 PsiBuilder.Marker rollbackMarker = mark();
-                parseFunctionLiteralParametersAndType();
+                boolean preferParamsToExpressions = parseFunctionLiteralParametersAndType();
 
                 paramsFound = preferParamsToExpressions ?
                               rollbackOrDrop(rollbackMarker, ARROW, "An -> is expected", RBRACE) :
@@ -1129,17 +1127,6 @@ public class JetExpressionParsing extends AbstractJetParsing {
         parameterList.done(VALUE_PARAMETER_LIST);
     }
 
-    // Check that position is followed by top level comma. It can't be expression and we want it be
-    // parsed as parameters in function literal
-    private boolean isConfirmedParametersByComma() {
-        assert _at(LPAR);
-        PsiBuilder.Marker lparMarker = mark();
-        advance(); // LPAR
-        int comma = matchTokenStreamPredicate(new FirstBefore(new At(COMMA), new AtSet(ARROW, RPAR)));
-        lparMarker.rollbackTo();
-        return comma > 0;
-    }
-
     private boolean parseFunctionTypeDotParametersAndType() {
         PsiBuilder.Marker rollbackMarker = mark();
 
@@ -1152,12 +1139,7 @@ public class JetExpressionParsing extends AbstractJetParsing {
             createTruncatedBuilder(lastDot).parseTypeRef();
             if (at(DOT)) {
                 advance(); // DOT
-
+                preferParamsToExpressions = parseFunctionLiteralParametersAndType();
-                if (at(LPAR)) {
-                    preferParamsToExpressions = isConfirmedParametersByComma();
-                }
-
-                parseFunctionLiteralParametersAndType();
             }
         }
 
@@ -1166,9 +1148,10 @@ public class JetExpressionParsing extends AbstractJetParsing {
                rollbackOrDropAt(rollbackMarker, ARROW);
     }
 
-    private void parseFunctionLiteralParametersAndType() {
+    private boolean parseFunctionLiteralParametersAndType() {
-        parseFunctionLiteralParameterList();
+        boolean hasCommaInParametersList = parseFunctionLiteralParameterList();
         parseOptionalFunctionLiteralType();
+        return hasCommaInParametersList;
     }
 
     /*
@@ -1186,13 +1169,17 @@ public class JetExpressionParsing extends AbstractJetParsing {
         }
     }
 
-    /*
+    /**
      * "(" (modifiers SimpleName (":" type)?){","} ")"
+     *
+     * @return true if at least one comma was found
      */
-    private void parseFunctionLiteralParameterList() {
+    private boolean parseFunctionLiteralParameterList() {
         PsiBuilder.Marker list = mark();
         expect(LPAR, "Expecting a parameter list in parentheses (...)", TokenSet.create(ARROW, COLON));
 
+        boolean hasComma = false;
+
         myBuilder.disableNewlines();
 
         if (!at(RPAR)) {
@@ -1213,6 +1200,8 @@ public class JetExpressionParsing extends AbstractJetParsing {
                 if (!at(COMMA)) break;
                 advance(); // COMMA
 
+                hasComma = true;
+
                 if (at(RPAR)) {
                     error("Expecting a parameter declaration");
                     break;
@@ -1224,6 +1213,8 @@ public class JetExpressionParsing extends AbstractJetParsing {
 
         expect(RPAR, "Expecting ')'", TokenSet.create(ARROW, COLON));
         list.done(VALUE_PARAMETER_LIST);
+
+        return hasComma;
     }
 
     /*

compiler/testData/psi/FunctionLiterals_ERR.kt

@@ -18,5 +18,6 @@ fun foo() {
 
     {(a, b) }
     {T.(a, b) }
+    {(a: Int, )}
     {a, }
 }

compiler/testData/psi/FunctionLiterals_ERR.txt

@@ -370,6 +370,30 @@ JetFile: FunctionLiterals_ERR.kt
             <empty list>
           PsiElement(RBRACE)('}')
       PsiWhiteSpace('\n    ')
+      FUNCTION_LITERAL_EXPRESSION
+        FUNCTION_LITERAL
+          PsiElement(LBRACE)('{')
+          VALUE_PARAMETER_LIST
+            PsiElement(LPAR)('(')
+            VALUE_PARAMETER
+              PsiElement(IDENTIFIER)('a')
+              PsiElement(COLON)(':')
+              PsiWhiteSpace(' ')
+              TYPE_REFERENCE
+                USER_TYPE
+                  REFERENCE_EXPRESSION
+                    PsiElement(IDENTIFIER)('Int')
+            PsiElement(COMMA)(',')
+            PsiErrorElement:Expecting a parameter declaration
+              <empty list>
+            PsiWhiteSpace(' ')
+            PsiElement(RPAR)(')')
+          PsiErrorElement:An -> is expected
+            <empty list>
+          BLOCK
+            <empty list>
+          PsiElement(RBRACE)('}')
+      PsiWhiteSpace('\n    ')
       FUNCTION_LITERAL_EXPRESSION
         FUNCTION_LITERAL
           PsiElement(LBRACE)('{')