| Subtracts an unsigned offset from a pointer. |
| |
| This can only move the pointer backward (or not move it). If you need to move forward or |
| backward depending on the value, then you might want [`offset`](#method.offset) instead |
| which takes a signed offset. |
| |
| `count` is in units of T; e.g., a `count` of 3 represents a pointer |
| offset of `3 * size_of::<T>()` bytes. |
| |
| # Safety |
| |
| 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`. |
| |
| * Let `result` be `self.addr() - count * size_of::<T>()`, computed on mathematical integers. |
| This must fit in a `usize`. |
| |
| * 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 `result` |
| (i.e., `result..self.addr()`) must be in bounds of that allocation. |
| |
| Allocations can never be larger than `isize::MAX` bytes and they can only contain addresses |
| representable by `usize`, so technically the last condition implies the first two. |
| |
| [allocation]: crate::ptr#allocation |
