Merge pull request swiftlang#23368 from akyrtzi/comment-parsestmt-isstartofstmt

[parser] Add comments that parseStmt() and isStartOfStmt() must be kept in sync, and assertion the parseStmt() makes progress

Author: akyrtzi

include/swift/Parse/Parser.h

@@ -230,7 +230,22 @@ class Parser {
     // Cut off parsing by acting as if we reached the end-of-file.
     Tok.setKind(tok::eof);
   }
-  
+
+  /// Use this to assert that the parser has advanced the lexing location, e.g.
+  /// before a specific parser function has returned.
+  class AssertParserMadeProgressBeforeLeavingScopeRAII {
+    Parser &P;
+    SourceLoc InitialLoc;
+  public:
+    AssertParserMadeProgressBeforeLeavingScopeRAII(Parser &parser) : P(parser) {
+      InitialLoc = P.Tok.getLoc();
+    }
+    ~AssertParserMadeProgressBeforeLeavingScopeRAII() {
+      assert(InitialLoc != P.Tok.getLoc() &&
+        "parser did not make progress, this can result in infinite loop");
+    }
+  };
+
   /// A RAII object for temporarily changing CurDeclContext.
   class ContextChange {
   protected:

lib/Parse/ParseStmt.cpp

@@ -36,6 +36,9 @@ using namespace swift::syntax;
 /// isStartOfStmt - Return true if the current token starts a statement.
 ///
 bool Parser::isStartOfStmt() {
+  // This needs to be kept in sync with `Parser::parseStmt()`. If a new token
+  // kind is accepted here as start of statement, it should also be handled in
+  // `Parser::parseStmt()`.
   switch (Tok.getKind()) {
   default: return false;
   case tok::kw_return:
@@ -565,6 +568,8 @@ static ParserResult<Stmt> recoverFromInvalidCase(Parser &P) {
 }
 
 ParserResult<Stmt> Parser::parseStmt() {
+  AssertParserMadeProgressBeforeLeavingScopeRAII apmp(*this);
+
   SyntaxParsingContext LocalContext(SyntaxContext, SyntaxContextKind::Stmt);
 
   // Note that we're parsing a statement.
@@ -588,7 +593,9 @@ ParserResult<Stmt> Parser::parseStmt() {
   if (isContextualYieldKeyword()) {
     Tok.setKind(tok::kw_yield);
   }
-  
+
+  // This needs to handle everything that `Parser::isStartOfStmt()` accepts as
+  // start of statement.
   switch (Tok.getKind()) {
   case tok::pound_line:
   case tok::pound_sourceLocation: