[SyntaxColor] Respect keywords' syntax kind when they appear in conditions. (#5076)

Keywords like 'let' can serve as argument labels. When they do so, we should
highlight them as identifiers instead of keywords. However, the check for this
situation seems overly lenient so that when 'let', 'var' appear in conditions
of IfStmt or GuardStmt, they are wrongly highlighted as identifiers too. This
commit strengthens the checking to preserve keywords' identity in these statements.
rdar://28297337

Author: nkcsgexi

lib/IDE/SyntaxModel.cpp

@@ -46,6 +46,23 @@ struct SyntaxModelContext::Implementation {
       SrcMgr(SrcFile.getASTContext().SourceMgr) {}
 };
 
+static bool canFindSurroundingParens(ArrayRef<Token> Toks,
+                                    const unsigned Current) {
+  bool LeftOk = false;
+  bool RightOk = false;
+  for (unsigned Offset = 1; ; Offset ++) {
+    if (Current < Offset || Current + Offset >= Toks.size())
+      return false;
+    auto &L = Toks[Current - Offset];
+    auto &R = Toks[Current + Offset];
+    LeftOk |= L.getKind() == tok::l_paren;
+    RightOk |= R.getKind() == tok::r_paren;
+    if (LeftOk && RightOk)
+      return true;
+  }
+  llvm_unreachable("unreachable exit condition");
+}
+
 SyntaxModelContext::SyntaxModelContext(SourceFile &SrcFile)
   : Impl(*new Implementation(SrcFile)) {
   const bool IsPlayground = Impl.LangOpts.Playground;
@@ -103,7 +120,8 @@ SyntaxModelContext::SyntaxModelContext(SourceFile &SrcFile)
              Tokens[I-1].getKind() == tok::comma) &&
             (Tokens[I+1].getKind() == tok::colon ||
              Tokens[I+1].getKind() == tok::identifier ||
-             Tokens[I+1].isKeyword())) {
+             Tokens[I+1].isKeyword()) &&
+            canFindSurroundingParens(Tokens, I)) {
           // Keywords are allowed as argument labels and should be treated as
           // identifiers.  The exception is '_' which is not a name.
           Kind = SyntaxNodeKind::Identifier;

test/IDE/coloring.swift

@@ -545,6 +545,13 @@ func foo1() {
 // CHECK: <kw>_</kw> = (<kw>_</kw>: <int>0</int>, <kw>_</kw>: <int>2</int>)
 }
 
+func foo2(O1 : Int?, O2: Int?, O3: Int?) {
+  guard let _ = O1, var _ = O2, let _ = O3 else { }
+// CHECK:  <kw>guard</kw> <kw>let</kw> <kw>_</kw> = O1, <kw>var</kw> <kw>_</kw> = O2, <kw>let</kw> <kw>_</kw> = O3 <kw>else</kw> { }
+  if let _ = O1, var _ = O2, let _ = O3 {}
+// CHECK: <kw>if</kw> <kw>let</kw> <kw>_</kw> = O1, <kw>var</kw> <kw>_</kw> = O2, <kw>let</kw> <kw>_</kw> = O3 {}
+}
+
 // Keep this as the last test
 /**
   Trailing off ...