[cfe-dev] [PATCH] Automatic detection of compatibility macros for non-portable diagnostics

[Alexander Kornienko]

This patch adds an automatic detection of a compatibility macros used in
specific projects to hide constructions based on non-portable features
(e.g. custom C++11 attributes). It helps to adapt diagnostic messages and
fix-it hints to use these compatibility macros instead of the actual
construct.

This feature is implemented in AnalysisBasedWarnings.cpp, as there's
currently only one diagnostic which gets profit from this - diagnostic of
unannotated fall-through between switch labels. But the code of the
CompatibilityMacroFinder class was intentionally made reasonably generic,
and doesn't contain any specific bindings to this diagnostic. The class is
a lightweight handler of PPCallbacks::MacroDefined calls. An instance of it
is registered via Preprocessor::addPPCallbacks for each token
sequence (specified in plain text) to find in macros (currently only
one). It keeps all macros with the specified expansion token sequence and
then can determine which one can be used instead of the actual construct in
a specific code location.

A motivating example for this feature:
There's the -Wimplicit-fallthrough warning, which detects
[[clang::fallthrough]]; construct as an annotation of an intended
fall-through. In projects which should be compiled both in C++11 mode and
in C++03 mode, this construct can not be used as is, so it should be
wrapped in a macro, e.g.:

#ifdef __clang__
#if __has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough")
#define LLVM_FALLTHROUGH [[clang::fallthrough]]
#endif
#endif

#ifndef LLVM_FALLTHROUGH
#define LLVM_FALLTHROUGH do { } while (0)
#endif


Prior to this feature diagnostic messages would say:
test.cpp:156:5: *warning: unannotated fall-through between switch labels*
    case 223:
    ^
test.cpp:156:5: *note: insert '[[clang::fallthrough]];' to silence this
warning*
    case 223:
    ^
    *[[clang::fallthrough]];*
test.cpp:156:5: *note: insert 'break;' to avoid fall-through*
    case 223:
    ^
    break;

Applying the first of these fix-it hints will lead to broken builds in
C++03 mode, which is usually not desired.

But with the automatic detection feature the diagnostic will suggest the
correct compatibility macro available in the corresponding source location:
...
test.cpp:156:5: *note: insert 'LLVM_FALLTHROUGH;' to silence this warning*
    case 223:
    ^
    *LLVM_FALLTHROUGH; *
...

Please, review this patch. Thank you!

-- 
Best regards,
Alexander Kornienko
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