Auto merge of rust-lang#97521 - SkiFire13:clarify-vec-as-ptr, r=Dylan-DPC

Clarify the guarantees of Vec::as_ptr and Vec::as_mut_ptr when there's no allocation

Currently the documentation says they return a pointer to the vector's buffer, which has the implied precondition that the vector allocated some memory. However `Vec`'s documentation also specifies that it won't always allocate, so it's unclear whether the pointer returned is valid in that case. Of course you won't be able to read/write actual bytes to/from it since the capacity is 0, but there's an exception: zero sized read/writes. They are still valid as long as the pointer is not null and the memory it points to wasn't deallocated, but `Vec::as_ptr` and `Vec::as_mut_ptr` don't specify that's not the case. This PR thus specifies they are actually valid for zero sized reads since `Vec` is implemented to hold a dangling pointer in those cases, which is neither null nor was deallocated.

Author: bors

library/alloc/src/vec/mod.rs

@@ -1107,7 +1107,8 @@ impl<T, A: Allocator> Vec<T, A> {
         self
     }
 
-    /// Returns a raw pointer to the vector's buffer.
+    /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
+    /// valid for zero sized reads if the vector didn't allocate.
     ///
     /// The caller must ensure that the vector outlives the pointer this
     /// function returns, or else it will end up pointing to garbage.
@@ -1144,7 +1145,8 @@ impl<T, A: Allocator> Vec<T, A> {
         ptr
     }
 
-    /// Returns an unsafe mutable pointer to the vector's buffer.
+    /// Returns an unsafe mutable pointer to the vector's buffer, or a dangling
+    /// raw pointer valid for zero sized reads if the vector didn't allocate.
     ///
     /// The caller must ensure that the vector outlives the pointer this
     /// function returns, or else it will end up pointing to garbage.