[-Wunsafe-buffer-usage] Hide fixits/suggestions behind an extra flag.

This patch implements a new clang driver flag -fsafe-buffer-usage-suggestions
which allows turning the smart suggestion machine

[-Wunsafe-buffer-usage] Hide fixits/suggestions behind an extra flag.

This patch implements a new clang driver flag -fsafe-buffer-usage-suggestions
which allows turning the smart suggestion machine on and off (defaults to off).
This is valuable for stability reasons, as the machine is being rapidly improved\
and we don't want accidental breakages to ruin the build for innocent users.
It is also arguably useful in general because it enables separation of concerns
between project contributors: some users will actively update the code to
conform to the programming model, while others simply want to make sure that
they aren't regressing it. Finally, there could be other valid reasons to
opt out of suggestions entirely on some codebases (while continuing to enforce
-Wunsafe-buffer-usage warnings), such as lack of access to hardened libc++
(or even to the C++ standard library in general) on the target platform.

When the flag is disabled, the unsafe buffer usage analysis is reduced to
an extremely minimal mode of operation that contains virtually no smarts:
not only it doesn't offer automatic fixits, but also textual suggestions
such as "change the type of this variable to std::span to preserve bounds
information" are not displayed, and in fact the machine doesn't even try
to blame specific variables in the first place, it simply warns on
the operations and leaves everything else to the user. So this flag turns off
a lot more of our complex machinery than what we already turn off in presence
of say -fno-diagnostic-fixit-info.

The flag is discoverable: when it's off, the warnings are accompanied by a note:
telling the user that there's a flag they can use.

Differential Revision: https://reviews.llvm.org/D146669

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[-Wunsafe-buffer-usage] Add unsafe buffer checking opt-out pragmas

Add a pair of clang pragmas:
- `#pragma clang unsafe_buffer_usage begin` and
- `#pragma clang unsafe_buffer_usage end`,
which speci

[-Wunsafe-buffer-usage] Add unsafe buffer checking opt-out pragmas

Add a pair of clang pragmas:
- `#pragma clang unsafe_buffer_usage begin` and
- `#pragma clang unsafe_buffer_usage end`,
which specify the start and end of an (unsafe buffer checking) opt-out
region, respectively.

Behaviors of opt-out regions conform to the following rules:

- No nested nor overlapped opt-out regions are allowed. One cannot
start an opt-out region with `... unsafe_buffer_usage begin` but never
close it with `... unsafe_buffer_usage end`. Mis-use of the pragmas
will be warned.
- Warnings raised from unsafe buffer operations inside such an opt-out
region will always be suppressed. This behavior CANNOT be changed by
`clang diagnostic` pragmas or command-line flags.
- Warnings raised from unsafe operations outside of such opt-out
regions may be reported on declarations inside opt-out
regions. These warnings are NOT suppressed.
- An un-suppressed unsafe operation warning may be attached with
notes. These notes are NOT suppressed as well regardless of whether
they are in opt-out regions.

The implementation maintains a separate sequence of location pairs
representing opt-out regions in `Preprocessor`. The `UnsafeBufferUsage`
analyzer reads the region sequence to check if an unsafe operation is
in an opt-out region. If it is, discard the warning raised from the
operation immediately.

This is a re-land after I reverting it at 9aa00c8a306561c4e3ddb09058e66bae322a0769.
The compilation error should be resolved.

Reviewed by: NoQ

Differential revision: https://reviews.llvm.org/D140179

show more ...

[-Wunsafe-buffer-usage] Add unsafe buffer checking opt-out pragmas

Add a pair of clang pragmas:
- `#pragma clang unsafe_buffer_usage begin` and
- `#pragma clang unsafe_buffer_usage end`,
which speci

[-Wunsafe-buffer-usage] Add unsafe buffer checking opt-out pragmas

Add a pair of clang pragmas:
- `#pragma clang unsafe_buffer_usage begin` and
- `#pragma clang unsafe_buffer_usage end`,
which specify the start and end of an (unsafe buffer checking) opt-out
region, respectively.

Behaviors of opt-out regions conform to the following rules:

- No nested nor overlapped opt-out regions are allowed. One cannot
start an opt-out region with `... unsafe_buffer_usage begin` but never
close it with `... unsafe_buffer_usage end`. Mis-use of the pragmas
will be warned.
- Warnings raised from unsafe buffer operations inside such an opt-out
region will always be suppressed. This behavior CANNOT be changed by
`clang diagnostic` pragmas or command-line flags.
- Warnings raised from unsafe operations outside of such opt-out
regions may be reported on declarations inside opt-out
regions. These warnings are NOT suppressed.
- An un-suppressed unsafe operation warning may be attached with
notes. These notes are NOT suppressed as well regardless of whether
they are in opt-out regions.

The implementation maintains a separate sequence of location pairs
representing opt-out regions in `Preprocessor`. The `UnsafeBufferUsage`
analyzer reads the region sequence to check if an unsafe operation is
in an opt-out region. If it is, discard the warning raised from the
operation immediately.

Reviewed by: NoQ

Differential revision: https://reviews.llvm.org/D140179

show more ...