[CodeCompletion] Enable type completion at beginning of attribute

for 'VarDecl' or if we don't know the kind of the decl.
Property delegate allows arbitrary type name (with `@propertyDelegate`
attr).

Author: rintaro

include/swift/Parse/CodeCompletionCallbacks.h

@@ -186,7 +186,7 @@ class CodeCompletionCallbacks {
   virtual void completeAccessorBeginning(CodeCompletionExpr *E) {};
 
   /// Complete the keyword in attribute, for instance, @available.
-  virtual void completeDeclAttrKeyword(Decl *D, bool Sil, bool Param) {};
+  virtual void completeDeclAttrBeginning(Decl *D, bool Sil, bool Param) {};
 
   /// Complete the parameters in attribute, for instance, version specifier for
   /// @available.

lib/IDE/CodeCompletion.cpp

@@ -1367,7 +1367,7 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
   void completeCaseStmtKeyword() override;
   void completeCaseStmtBeginning() override;
   void completeCaseStmtDotPrefix() override;
-  void completeDeclAttrKeyword(Decl *D, bool Sil, bool Param) override;
+  void completeDeclAttrBeginning(Decl *D, bool Sil, bool Param) override;
   void completeDeclAttrParam(DeclAttrKind DK, int Index) override;
   void completeInPrecedenceGroup(SyntaxKind SK) override;
   void completeNominalMemberBeginning(
@@ -4601,9 +4601,8 @@ void CodeCompletionCallbacksImpl::completeDeclAttrParam(DeclAttrKind DK,
   CurDeclContext = P.CurDeclContext;
 }
 
-void CodeCompletionCallbacksImpl::completeDeclAttrKeyword(Decl *D,
-                                                          bool Sil,
-                                                          bool Param) {
+void CodeCompletionCallbacksImpl::completeDeclAttrBeginning(Decl *D, bool Sil,
+                                                            bool Param) {
   Kind = CompletionKind::AttributeBegin;
   IsInSil = Sil;
   if (Param) {
@@ -5378,6 +5377,11 @@ void CodeCompletionCallbacksImpl::doneParsing() {
 
   case CompletionKind::AttributeBegin: {
     Lookup.getAttributeDeclCompletions(IsInSil, AttTargetDK);
+
+    // Provide any type name for property delegate.
+    if (!AttTargetDK || *AttTargetDK == DeclKind::Var)
+      Lookup.getTypeCompletionsInDeclContext(
+          P.Context.SourceMgr.getCodeCompletionLoc());
     break;
   }
   case CompletionKind::AttributeDeclParen: {

lib/Parse/ParseDecl.cpp

@@ -2940,7 +2940,8 @@ Parser::parseDecl(ParseDeclOptions Flags,
       if (!CodeCompletion) {
         delayParseFromBeginningToHere(BeginParserPosition, Flags);
       } else {
-        CodeCompletion->completeDeclAttrKeyword(nullptr, isInSILMode(), false);
+        CodeCompletion->completeDeclAttrBeginning(nullptr, isInSILMode(),
+                                                  false);
       }
     }
 
@@ -3012,9 +3013,8 @@ Parser::parseDecl(ParseDeclOptions Flags,
 
   if (FoundCCTokenInAttr) {
     if (CodeCompletion) {
-      CodeCompletion->completeDeclAttrKeyword(DeclResult.getPtrOrNull(),
-                                              isInSILMode(),
-                                              false);
+      CodeCompletion->completeDeclAttrBeginning(DeclResult.getPtrOrNull(),
+                                                isInSILMode(), false);
     } else {
       delayParseFromBeginningToHere(BeginParserPosition, Flags);
       return makeParserError();

lib/Parse/ParsePattern.cpp

@@ -224,7 +224,7 @@ Parser::parseParameterClause(SourceLoc &leftParenLoc,
       parseDeclAttributeList(param.Attrs, FoundCCToken);
     if (FoundCCToken) {
       if (CodeCompletion) {
-        CodeCompletion->completeDeclAttrKeyword(nullptr, isInSILMode(), true);
+        CodeCompletion->completeDeclAttrBeginning(nullptr, isInSILMode(), true);
       } else {
         status |= makeParserCodeCompletionStatus();
       }