[ParseSIL] Return empty SILModule on error

Because we were previously performing SIL parsing
during `performSema`, we were relying on the
pipeline being stopped before reaching the SIL
pipeline passes.

However under a lazy evaluation model, we can't
rely on that. Instead, just return an empty
SILModule if we encounter a parsing error.

Author: hamishknight

include/swift/Parse/Parser.h

@@ -888,7 +888,8 @@ class Parser {
   void parseTopLevel(SmallVectorImpl<Decl *> &decls);
 
   /// Parse the top-level SIL decls into the SIL module.
-  void parseTopLevelSIL();
+  /// \returns \c true if there was a parsing error.
+  bool parseTopLevelSIL();
 
   /// Flags that control the parsing of declarations.
   enum ParseDeclFlags {

lib/Parse/ParseDecl.cpp

@@ -241,7 +241,7 @@ void Parser::parseTopLevel(SmallVectorImpl<Decl *> &decls) {
   TokReceiver->finalize();
 }
 
-void Parser::parseTopLevelSIL() {
+bool Parser::parseTopLevelSIL() {
   assert(SIL && isInSILMode());
 
   // Prime the lexer.
@@ -253,6 +253,7 @@ void Parser::parseTopLevelSIL() {
       skipSingle();
   };
 
+  auto hadError = false;
   while (!Tok.is(tok::eof)) {
     // If we run into a Swift decl, skip over until we find the next SIL decl.
     if (isStartOfSwiftDecl()) {
@@ -269,6 +270,7 @@ void Parser::parseTopLevelSIL() {
       if (SIL->parse##NAME(*this)) {                                           \
         Lexer::SILBodyRAII sbr(*L);                                            \
         skipToNextSILDecl();                                                   \
+        hadError = true;                                                       \
       }                                                                        \
       break;                                                                   \
     }
@@ -288,9 +290,11 @@ void Parser::parseTopLevelSIL() {
       // or a SIL decl. Emit an error and skip ahead to the next SIL decl.
       diagnose(Tok, diag::expected_sil_keyword);
       skipToNextSILDecl();
+      hadError = true;
       break;
     }
   }
+  return hadError;
 }
 
 ParserResult<AvailableAttr> Parser::parseExtendedAvailabilitySpecList(

lib/SIL/Parser/ParseSIL.cpp

@@ -121,7 +121,14 @@ ParseSILModuleRequest::evaluate(Evaluator &evaluator,
   SILParserState parserState(silMod.get());
   Parser parser(*bufferID, *SF, parserState.Impl.get());
   PrettyStackTraceParser StackTrace(parser);
-  parser.parseTopLevelSIL();
+
+  auto hadError = parser.parseTopLevelSIL();
+  if (hadError) {
+    // The rest of the SIL pipeline expects well-formed SIL, so if we encounter
+    // a parsing error, just return an empty SIL module.
+    return SILModule::createEmptyModule(mod, desc.conv, desc.opts,
+                                        desc.isWholeModule());
+  }
   return silMod;
 }