[Parser] refactor nested 'init' diagnostics. NFC

Add better comment. Replace a dyn_cast with cast per Jordan's observation.

Author: dduan

lib/Parse/ParseDecl.cpp

@@ -1733,12 +1733,14 @@ static bool isParenthesizedUnowned(Parser &P) {
 }
 
 
-
 bool Parser::isStartOfDecl() {
   // If this is obviously not the start of a decl, then we're done.
   if (!isKeywordPossibleDeclStart(Tok)) return false;
 
-  // 'init' invocation is not start of a declaration
+  // When 'init' appears inside another 'init', it's likely the user wants to
+  // invoke a initializer but forgets to prefix it with 'self.' or 'super.'
+  // Otherwise, expect 'init' to be the start of a declaration (and complain
+  // when the expectation is not fullfilled).
   if (Tok.is(tok::kw_init)) {
     return !isa<ConstructorDecl>(CurDeclContext);
   }

lib/Parse/ParseStmt.cpp

@@ -408,34 +408,31 @@ ParserStatus Parser::parseBraceItems(SmallVectorImpl<ASTNode> &Entries,
           }
         }
       }
-    } else {
+    } else if (Tok.is(tok::kw_init) && isa<ConstructorDecl>(CurDeclContext)) {
       SourceLoc StartLoc = Tok.getLoc();
-      if (Tok.is(tok::kw_init)) {
-        if (auto CD = dyn_cast<ConstructorDecl>(CurDeclContext)) {
-          // Hint at missing 'self.' or 'super.' then skip this statement.
-          bool isConvenient = CD->isConvenienceInit();
-          diagnose(StartLoc, diag::invalid_nested_init, isConvenient)
-            .fixItInsert(StartLoc, isConvenient ? "self." : "super.");
-          NeedParseErrorRecovery = true;
-        }
-      } else {
-        ParserStatus ExprOrStmtStatus = parseExprOrStmt(Result);
-        BraceItemsStatus |= ExprOrStmtStatus;
-        if (ExprOrStmtStatus.isError())
-          NeedParseErrorRecovery = true;
-        diagnoseDiscardedClosure(*this, Result);
-        if (ExprOrStmtStatus.isSuccess() && IsTopLevel) {
-          // If this is a normal library, you can't have expressions or
-          // statements outside at the top level.
-          diagnose(StartLoc,
-                   Result.is<Stmt*>() ? diag::illegal_top_level_stmt
-                                      : diag::illegal_top_level_expr);
-          Result = ASTNode();
-        }
-
-        if (!Result.isNull())
-          Entries.push_back(Result);
+      auto CD = cast<ConstructorDecl>(CurDeclContext);
+      // Hint at missing 'self.' or 'super.' then skip this statement.
+      bool isConvenient = CD->isConvenienceInit();
+      diagnose(StartLoc, diag::invalid_nested_init, isConvenient)
+        .fixItInsert(StartLoc, isConvenient ? "self." : "super.");
+      NeedParseErrorRecovery = true;
+    } else {
+      ParserStatus ExprOrStmtStatus = parseExprOrStmt(Result);
+      BraceItemsStatus |= ExprOrStmtStatus;
+      if (ExprOrStmtStatus.isError())
+        NeedParseErrorRecovery = true;
+      diagnoseDiscardedClosure(*this, Result);
+      if (ExprOrStmtStatus.isSuccess() && IsTopLevel) {
+        // If this is a normal library, you can't have expressions or
+        // statements outside at the top level.
+        diagnose(Tok.getLoc(),
+                 Result.is<Stmt*>() ? diag::illegal_top_level_stmt
+                                    : diag::illegal_top_level_expr);
+        Result = ASTNode();
       }
+
+      if (!Result.isNull())
+        Entries.push_back(Result);
     }
 
     if (!NeedParseErrorRecovery && !PreviousHadSemi && Tok.is(tok::semi)) {