[PATCH] D154880: [-Wunsafe-buffer-usage][WIP] Add a facility for debugging low fixit coverage.

[Artem Dergachev via Phabricator via cfe-commits]

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This patch adds extra notes to `-Wunsafe-buffer-usage` warnings, which explain why a fixit wasn't produced. When applied to a large body of real-world code, it'll help us gather statistics that will help us figure out which fixable gadgets (or other features of the fixit machine) to "invest" into.

This is a debugging facility intended for developer use only; it is activated by passing `-mllvm -debug-only=SafeBuffers` to clang, so it's carefully hidden and undiscoverable, and it's only available in builds with assertions.

Offline we've identified the following sources of false negatives which these notes can help us categorize:

1. unsafe operation not performed on a supported kind of variable (eg. member variable);
2. use site of the unsafe variable not claimed by any fixable gadgets (so we need to cover it with a new fixable);
3. one of the "implicated" variables has unclaimed uses (so we can't build the implication graph);
4. fixit generation for the declaration of the variable has failed (eg. declaration is in a macro);
5. fixit generation for one of the fixable gadgets has failed (eg. the use is in a macro).

Currently this patch covers #5; it probably makes sense to cover all of these except maybe #1 (this one's usually obvious).


Repository:
  rC Clang

https://reviews.llvm.org/D154880

Files:
  clang/include/clang/Analysis/Analyses/UnsafeBufferUsage.h
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/Analysis/UnsafeBufferUsage.cpp
  clang/lib/Sema/AnalysisBasedWarnings.cpp
  clang/test/SemaCXX/warn-unsafe-buffer-usage-debug.cpp

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