[clang] [clang] Implement __attribute__((format_matches)) (PR #116708)

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=?utf-8?q?Félix?= Cloutier <fcloutier at apple.com>,
=?utf-8?q?Félix?= Cloutier <fcloutier at apple.com>
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In-Reply-To: <llvm.org/llvm/llvm-project/pull/116708 at github.com>


apple-fcloutier wrote:

As an update, I've been adding more tests and I found that I can make improvements with follow-up work. Since these aren't stability problems or false positives, and we think the change is already big enough, I am currently planning to submit them as a separate PR once this lands. The things in question that are addressed:

* Passing a format string with the wrong type (for instance, a `NSString` format string to a function accepting a `printf` format string) terminates verification without emitting a diagnostic. This is an issue with the `format` attribute too (it emits a -Wformat-nonliteral warning for it, which is bad categorization and unspecific about what needs to happen).
* If you have a function with `format_matches` that calls another function with `format_matches` passing its own format argument, Clang doesn't verify that the two format strings are compatible.
* `format_matches` diagnostics always point at the format string: for instance, directly at the format string argument in `printf("%s", "hello")`, but at the `fmt` variable definition in `const char *const fmt = "%s"; printf(fmt, "hello")`. This can create situations where the diagnostic shows the problematic format strings, but not the format function call that references them. `Format` attribute checking uses `CheckFormatHandler::EmitFormatDiagnostic` to ensure that the format function call is consistently there (either directly where the warning points, or with a note if the call is elsewhere). Diagnostics for `format_matches` should do that too.
* Reusing the "data argument not used by format string" diagnostic in the context of `format_matches` is not a great experience, this should be "more specifiers in format string than expected".

Let me know if you would like me to fold anything from this list into this PR.

https://github.com/llvm/llvm-project/pull/116708