[clang] [BoundsSafety] Initial documentation for -fbounds-safety (PR #70749)

[Aaron Ballman via cfe-commits]

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+==================================================
+``-fbounds-safety``: Enforcing bounds safety for C
+==================================================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+``-fbounds-safety`` is a C extension to enforce bounds safety to prevent
+out-of-bounds (OOB) memory accesses, which remain a major source of security
+vulnerabilities in C. ``-fbounds-safety`` aims to eliminate this class of bugs
+by turning OOB accesses into deterministic traps.
+
+The ``-fbounds-safety`` extension offers bounds annotations that programmers can
+use to attach bounds to pointers. For example, programmers can add the
+``__counted_by(N)`` annotation to parameter ``ptr``, indicating that the pointer
+has ``N`` valid elements:
+
+.. code-block:: c
+
+   void foo(int *__counted_by(N) ptr, size_t N);
+
+Using this bounds information, the compiler inserts bounds checks on every
+pointer dereference, ensuring that the program does not access memory outside
+the specified bounds. The compiler requires programmers to provide enough bounds
+information so that the accesses can be checked at either run time or compile
+time — and it rejects code if it cannot.
+
+The most important contribution of ``-fbounds-safety`` is how it reduces the
+programmer’s annotation burden by reconciling bounds annotations at ABI
+boundaries with the use of implicit wide pointers (a.k.a. “fat” pointers) that
+carry bounds information on local variables without the need for annotations. We
+designed this model so that it preserves ABI compatibility with C while
+minimizing adoption effort.
+
+The ``-fbounds-safety`` extension has been adopted on millions of lines of
+production C code and proven to work in a consumer operating system setting. The
+extension was designed to enable incremental adoption — a key requirement in
+real-world settings where modifying an entire project and its dependencies all
+at once is often not possible. It also addresses multiple of other practical
+challenges that have made existing approaches to safer C dialects difficult to
+adopt, offering these properties that make it widely adoptable in practice:
+
+* It is designed to preserve the Application Binary Interface (ABI).
+* It interoperates well with plain C code.
+* It can be adopted partially and incrementally while still providing safety
+  benefits.
+* It is a conforming extension to C.
+* Consequently, source code that adopts the extension can continue to be
+  compiled by toolchains that do not support the extension (CAVEAT: this still
+  requires inclusion of a header file micro-defining bounds annotations to
----------------
AaronBallman wrote:

```suggestion
  requires inclusion of a header file macro-defining bounds annotations to
```

https://github.com/llvm/llvm-project/pull/70749