Adds a signed offset to a pointer.
count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.
If any of the following conditions are violated, the result is Undefined Behavior:
The offset in bytes, count * size_of::<T>(), computed on mathematical integers (without “wrapping around”), must fit in an isize.
If the computed offset is non-zero, then self must be [derived from][crate::ptr#provenance] a pointer to some allocation, and the entire memory range between self and the result must be in bounds of that allocation. In particular, this range must not “wrap around” the edge of the address space. Note that “range” here refers to a half-open range as usual in Rust, i.e., self..result for non-negative offsets and result..self for negative offsets.
Allocations can never be larger than isize::MAX bytes, so if the computed offset stays in bounds of the allocation, it is guaranteed to satisfy the first requirement. This implies, for instance, that vec.as_ptr().add(vec.len()) (for vec: Vec<T>) is always safe.
Consider using wrapping_offset instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.