[Parse] Fix issue with incremental re-parsing for SwiftSyntax emittin…

…g bogus parsing error

rdar://60232712

include/swift/Basic/LangOptions.h

@@ -296,6 +296,11 @@ namespace swift {
     /// incrementals parsing.
     bool BuildSyntaxTree = false;
 
+    /// Whether parsing is occurring for creation of syntax tree only, and no typechecking will occur after
+    /// parsing e.g. when parsing for SwiftSyntax. This is intended to affect parsing, e.g. disable
+    /// unnecessary name lookups that are not useful for pure syntactic parsing.
+    bool ParseForSyntaxTreeOnly = false;
+
     /// Whether to verify the parsed syntax tree and emit related diagnostics.
     bool VerifySyntaxTree = false;
 

lib/Parse/ParseExpr.cpp

@@ -2264,7 +2264,10 @@ Expr *Parser::parseExprIdentifier() {
   }
 
   ValueDecl *D = nullptr;
-  if (!InPoundIfEnvironment) {
+  // When doing incremental re-parsing for SwiftSyntax this check may emit bogus
+  // diagnostic. Also really the syntactic parser should not be doing name
+  // lookups, so disable this check when parsing for SwiftSyntax.
+  if (!InPoundIfEnvironment && !Context.LangOpts.ParseForSyntaxTreeOnly) {
     D = lookupInScope(name);
     // FIXME: We want this to work: "var x = { x() }", but for now it's better
     // to disallow it than to crash.

test/incrParse/incremental-diagnostic.swift

@@ -0,0 +1,7 @@
+// RUN: %empty-directory(%t)
+// RUN: %validate-incrparse %s --test-case REPLACE
+
+let myFont: Font = {
+    let myFont = Font.body
+    return myFont.weight(.<<REPLACE<bold|||light>>>)
+}()

tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp

@@ -277,6 +277,7 @@ swiftparse_client_node_t SynParser::parse(const char *source) {
   TypeCheckerOptions tyckOpts;
   LangOptions langOpts;
   langOpts.BuildSyntaxTree = true;
+  langOpts.ParseForSyntaxTreeOnly = true;
   langOpts.CollectParsedToken = false;
   // Disable name lookups during parsing.
   // Not ready yet:

tools/swift-syntax-test/swift-syntax-test.cpp

@@ -606,6 +606,7 @@ int parseFile(
   // Set up the compiler invocation
   CompilerInvocation Invocation;
   Invocation.getLangOptions().BuildSyntaxTree = true;
+  Invocation.getLangOptions().ParseForSyntaxTreeOnly = true;
   Invocation.getLangOptions().VerifySyntaxTree = options::VerifySyntaxTree;
   Invocation.getLangOptions().RequestEvaluatorGraphVizPath = options::GraphVisPath;
   Invocation.getFrontendOptions().InputsAndOutputs.addInputFile(InputFileName);