[-Wunsafe-buffer-usage] Minimize fixit range for pointer variables (#81935)

Example:
int * const my_var = my_initializer;

Currently when transforming my_var to std::span the fixits:
- replace "

[-Wunsafe-buffer-usage] Minimize fixit range for pointer variables (#81935)

Example:
int * const my_var = my_initializer;

Currently when transforming my_var to std::span the fixits:
- replace "int * const my_var = " with "std::span<int> const my_var {"
- add ", SIZE}" after "my_initializer" where SIZE is either inferred or
a placeholder

This patch makes that behavior less intrusive by not modifying variable
cv-qualifiers and initialization syntax.
The new behavior is:
- replace "int *" with "std::span<int>"
- add "{" before "my_initializer"
- add ", SIZE}" after "my_initializer"

This is an improvement on its own - since we don't touch the identifier,
we automatically can handle macros in them.
It also simplifies future work on initializer fixits.

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[-Wunsafe-buffer-usage] Ignore constant safe indices in array subscripts (#80504)

[-Wunsafe-buffer-usage] Ignore safe array subscripts
Don't emit warnings for array subscripts on constant size arra

[-Wunsafe-buffer-usage] Ignore constant safe indices in array subscripts (#80504)

[-Wunsafe-buffer-usage] Ignore safe array subscripts
Don't emit warnings for array subscripts on constant size arrays where the index is constant and within bounds.

Example:
int arr[10];
arr[5] = 0; //safe, no warning

This patch recognizes only array indices that are integer literals - it doesn't understand more complex expressions (arithmetic on constants, etc.).

-Warray-bounds implemented in Sema::CheckArrayAccess() already solves a similar
(opposite) problem, handles complex expressions and is battle-tested.

Adding -Wunsafe-buffer-usage diagnostics to Sema is a non-starter as we need to emit
both the warnings and fixits and the performance impact of the fixit machine is
unacceptable for Sema.

CheckArrayAccess() as is doesn't distinguish between "safe" and "unknown" array
accesses. It also mixes the analysis that decides if an index is out of bounds
with crafting the diagnostics.

A refactor of CheckArrayAccess() might serve both the original purpose
and help us avoid false-positive with -Wunsafe-buffer-usage on constant
size arrrays.

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[-Wunsafe-buffer-usage] Emit fixits for array decayed to pointer (#80347)

Covers cases where DeclRefExpr referring to a const-size array decays to a
pointer and is used "as a pointer" (e. g. passed

[-Wunsafe-buffer-usage] Emit fixits for array decayed to pointer (#80347)

Covers cases where DeclRefExpr referring to a const-size array decays to a
pointer and is used "as a pointer" (e. g. passed to a pointer type
parameter).

Since std::array<T, N> doesn't implicitly convert to pointer to its element
type T* the cast needs to be done explicitly as part of the fixit
when we retrofit std::array to code that previously worked with constant
size array. std::array::data() method is used for the explicit
cast.

In terms of the fixit machine this covers the UPC(DRE) case for Array fixit strategy.
The emitted fixit inserts call to std::array::data() method similarly to
analogous fixit for Span strategy.

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[-Wunsafe-buffer-usage][NFC] Add testcase for non-unsafe pointer (#80076)

This adds a missing CHECK-NOT directive for an existing test case.

[-Wunsafe-buffer-usage] Refactor to let local variable fix-its and parameter fix-its share common code

Refactor the code for local variable fix-its so that it reuses the
code for parameter fix-its,

[-Wunsafe-buffer-usage] Refactor to let local variable fix-its and parameter fix-its share common code

Refactor the code for local variable fix-its so that it reuses the
code for parameter fix-its, which is in general better. For example,
cv-qualifiers are supported.

Reviewed by: NoQ (Artem Dergachev), t-rasmud (Rashmi Mudduluru)

Differential revision: https://reviews.llvm.org/D156189

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[WIP][-Wunsafe-buffer-usage] Handle lambda expressions within a method.

Differential Revision: https://reviews.llvm.org/D150386

[-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] FixableGadget for handling stand alone pointers under UPC

This patch introduces UPCStandalonePointerGadget, a FixableGadget that emits fixits to
handle cases where a pointer

[-Wunsafe-buffer-usage] FixableGadget for handling stand alone pointers under UPC

This patch introduces UPCStandalonePointerGadget, a FixableGadget that emits fixits to
handle cases where a pointer identified as unsafe is simply referenced. An example of
such a case is when the pointer is input as an argument to a method call, where we can
not change the type of the argument. For cases where the strategy for the unsafe pointer is
to use std::span, the idea is to extract the underlying pointer by invoking the "data()"
method on the span instance.

For example, the gadget emits a fixit for S3, where S1, S2 are handled by other gadgets:
S1: int *ptr = new int[10];
S2: int val1 = ptr[k]; // Unsafe operation on ptr
S3: foo(ptr); // Some method that accepts raw pointer => FIXIT: foo(ptr.data());

Reviewed by: NoQ, ziqingluo-90, jkorous

Differential revision: https://reviews.llvm.org/D143676

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