[code-completion] Fix compiler crash on implicit member expression in string interpolation

Resetting Lexer to the start of the line result re-tokenizning whole
string literal as a single token, and go past artificial EOF.
This used to end up with assertion failure or infinite-loop in
no-assertion builds.

Quick fix for: rdar://problem/36881302

Author: rintaro

lib/Parse/ParseExpr.cpp

@@ -1632,20 +1632,25 @@ ParserResult<Expr> Parser::parseExprPrimary(Diag<> ID, bool isExprBasic) {
                     Context.getIdentifier("_"), /*implicit=*/false);
       auto Result = makeParserResult(Expr);
       if (CodeCompletion) {
-        std::vector<StringRef> Identifiers;
-
-        // Move lexer to the start of the current line.
-        L->backtrackToState(L->getStateForBeginningOfTokenLoc(
-          L->getLocForStartOfLine(SourceMgr, Tok.getLoc())));
 
+        // FIXME: Code-completion should be able to find the contextual type
+        // from AST.
+        std::vector<StringRef> Identifiers;
         bool HasReturn = false;
-
-        // Until we see the code completion token, collect identifiers.
-        for (L->lex(Tok); !Tok.is(tok::code_complete); consumeToken()) {
-          if (!HasReturn)
-            HasReturn = Tok.is(tok::kw_return);
-          if (Tok.is(tok::identifier)) {
-            Identifiers.push_back(Tok.getText());
+        {
+          ParserPositionRAII PPR(*this);
+          // Move lexer to the start of the current line.
+          L->backtrackToState(L->getStateForBeginningOfTokenLoc(
+            L->getLocForStartOfLine(SourceMgr, Tok.getLoc())));
+
+          // Until we see the code completion token, collect identifiers.
+          for (L->lex(Tok); !Tok.isAny(tok::code_complete, tok::eof);
+              consumeTokenWithoutFeedingReceiver()) {
+            if (!HasReturn)
+              HasReturn = Tok.is(tok::kw_return);
+            if (Tok.is(tok::identifier)) {
+              Identifiers.push_back(Tok.getText());
+            }
           }
         }
         CodeCompletion->completeUnresolvedMember(Expr, Identifiers, HasReturn);

test/IDE/complete_unresolved_members.swift

@@ -50,6 +50,8 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=NON_OPT_SET_2 | %FileCheck %s -check-prefix=NON_OPT_SET_1
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=NON_OPT_SET_3 | %FileCheck %s -check-prefix=NON_OPT_SET_1
 
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=STRING_INTERPOLATION_1 | %FileCheck %s -check-prefix=STRING_INTERPOLATION_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=STRING_INTERPOLATION_INVALID
 enum SomeEnum1 {
   case South
   case North
@@ -366,3 +368,14 @@ func testNonOptSet() {
 func testNonOptSet() -> NonOptSet {
   return .#^NON_OPT_SET_3^#
 }
+
+func testInStringInterpolation() {
+  enum MyEnum { case foo, bar }
+  func takeEnum(_ e: MyEnum) -> MyEnum { return e }
+  let x = "enum: \(takeEnum(.#^STRING_INTERPOLATION_1^#))"
+  let y = "enum: \(.#^STRING_INTERPOLATION_INVALID^#)" // Dont'crash.
+}
+// STRING_INTERPOLATION_1: Begin completions
+// STRING_INTERPOLATION_1-DAG: Decl[EnumElement]/ExprSpecific:     foo[#MyEnum#];
+// STRING_INTERPOLATION_1-DAG: Decl[EnumElement]/ExprSpecific:     bar[#MyEnum#];
+// STRING_INTERPOLATION_1: End completions