Turn -Wmax-tokens off by default

[hans]

Summary

On the one hand, one might interpret the use of the max-token pragmas or -fmax-tokens flag as an opt-in to the warning. However, in Chromium we've found it useful to only opt in selected build configurations, even though we have the pragmas in the code. For that reason, we think it makes sense to turn it off by default.

clang/include/clang/Basic/DiagnosticGroups.td

3. As a per-translation unit limit using the ``clang max_tokens_total`` pragma,
    which works like and overrides the ``-fmax-tokens=`` flag:
 
    .. code-block: c++
       #pragma clang max_tokens_total 1234
 
 These limits can be helpful in limiting code growth through included files.
 
 Setting a token limit of zero means no limit.
+
+Note that the warning is disabled by default, so -Wmax-tokens must be used
+in addition with the pragmas or -fmax-tokens flag to get any warnings.
 }];
 }

clang/include/clang/Basic/DiagnosticParseKinds.td

def ext_concept_legacy_bool_keyword : ExtWarn<
   "ISO C++20 does not permit the 'bool' keyword after 'concept'">,
   InGroup<DiagGroup<"concepts-ts-compat">>;
 def err_placeholder_expected_auto_or_decltype_auto : Error<
   "expected 'auto' or 'decltype(auto)' after concept name">;
 }
 
 def warn_max_tokens : Warning<
   "the number of preprocessor source tokens (%0) exceeds this token limit (%1)">,
-  InGroup<MaxTokens>;
+  InGroup<MaxTokens>, DefaultIgnore;
 
 def warn_max_tokens_total : Warning<
   "the total number of preprocessor source tokens (%0) exceeds the token limit (%1)">,
-  InGroup<MaxTokens>;
+  InGroup<MaxTokens>, DefaultIgnore;
 
 def note_max_tokens_total_override : Note<"total token limit set here">;
 
 } // end of Parser diagnostics

clang/test/Parser/max-tokens.cpp

-// RUN: %clang_cc1 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -fsyntax-only -verify %s -DMAX_TOKENS          -fmax-tokens=2
-// RUN: %clang_cc1 -fsyntax-only -verify %s -DMAX_TOKENS_OVERRIDE -fmax-tokens=9
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wmax-tokens
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wmax-tokens -DMAX_TOKENS          -fmax-tokens=2
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wmax-tokens -DMAX_TOKENS_OVERRIDE -fmax-tokens=9
 
 int x, y, z;
 
 #pragma clang max_tokens_here         // expected-error  {{missing argument to '#pragma clang max_tokens_here'; expected integer}}
 #pragma clang max_tokens_here foo     // expected-error  {{expected an integer argument in '#pragma clang max_tokens_here'}}
 #pragma clang max_tokens_here 123 456 // expected-warning{{extra tokens at end of '#pragma clang max_tokens_here' - ignored}}
 
 #pragma clang max_tokens_here 1 // expected-warning{{the number of preprocessor source tokens (7) exceeds this token limit (1)}}