[clang] [Clang] Document the interaction of malloc_span and alloc_size (PR #216967)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Nikolas Klauser (philnik777)

<details>
<summary>Changes</summary>

The `malloc_span` attribute is intended to be used for `allocate_at_least`-like functions. Part of that interface is a guarantee that the number of bytes returned are actually dereferenceable. Document that, so that we can optimize on it in the future.


---
Full diff: https://github.com/llvm/llvm-project/pull/216967.diff


1 Files Affected:

- (modified) clang/include/clang/Basic/AttrDocs.td (+5) 


``````````diff
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 3052dd6c77ab1..22355bf47c2a8 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -5592,6 +5592,11 @@ members, one of which is a pointer to the start of the allocated memory and
 the other one is either an integer type containing the size of the actually
 allocated memory or a pointer to the end of the allocated region. Note, static
 data members do not impact whether a type is span-like or not.
+
+In combination with the `alloc_size` attribute, if the begin pointer is
+non-null, the size of the returned span-like object has to be greater or equal
+to the number of bytes guaranteed to be dereferenceable by `alloc_size`. It also
+guarantees that the number of dereferenceable bytes is at least size.
   }];
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/216967