[Preprocessor] Fix incorrect token caching that occurs when lexing _Pragma

in macro argument pre-expansion mode when skipping a function body

This commit fixes a token caching problem that currently occurs when clang is
skipping a function body (e.g. when looking for a code completion token) and at
the same time caching the tokens for _Pragma when lexing it in macro argument
pre-expansion mode.

When _Pragma is being lexed in macro argument pre-expansion mode, it caches the
tokens so that it can avoid interpreting the pragma immediately (as the macro
argument may not be used in the macro body), and then either backtracks over or
commits these tokens. The problem is that, when we're backtracking/committing in
such a scenario, there's already a previous backtracking position stored in
BacktrackPositions (as we're skipping the function body), and this leads to a
situation where the cached tokens from the pragma (like '(' 'string_literal'
and ')') will remain in the cached tokens array incorrectly even after they're
consumed (in the case of backtracking) or just ignored (in the case when they're
committed). Furthermore, what makes it even worse, is that because of a previous
backtracking position, the logic that deals with when should we call
ExitCachingLexMode in CachingLex no longer works for us in this situation, and
more tokens in the macro argument get cached, to the point where the EOF token
that corresponds to the macro argument EOF is cached. This problem leads to all
sorts of issues in code completion mode, where incorrect errors get presented
and code completion completely fails to produce completion results.

rdar://28523863

Differential Revision: https://reviews.llvm.org/D28772


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@296140 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 0 deletions

diff --git a/include/clang/Lex/Preprocessor.h b/include/clang/Lex/Preprocessor.h
index 7ce1aad36d..26efa8b8a1 100644
--- a/include/clang/Lex/Preprocessor.h
+++ b/include/clang/Lex/Preprocessor.h
@@ -1077,6 +1077,24 @@ public:
   /// \brief Disable the last EnableBacktrackAtThisPos call.
   void CommitBacktrackedTokens();
 
+  struct CachedTokensRange {
+    CachedTokensTy::size_type Begin, End;
+  };
+
+private:
+  /// \brief A range of cached tokens that should be erased after lexing
+  /// when backtracking requires the erasure of such cached tokens.
+  Optional<CachedTokensRange> CachedTokenRangeToErase;
+
+public:
+  /// \brief Returns the range of cached tokens that were lexed since
+  /// EnableBacktrackAtThisPos() was previously called.
+  CachedTokensRange LastCachedTokenRange();
+
+  /// \brief Erase the range of cached tokens that were lexed since
+  /// EnableBacktrackAtThisPos() was previously called.
+  void EraseCachedTokens(CachedTokensRange TokenRange);
+
   /// \brief Make Preprocessor re-lex the tokens that were lexed since
   /// EnableBacktrackAtThisPos() was previously called.
   void Backtrack();