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 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.
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.