[Coding Guideline] Don't use the isize type

[rcseacord]

Exceptions:

FFI is definitely one reason—C's ssize_t maps to isize, and you'll encounter it in POSIX APIs like read() that return -1 on error.
But there are a few other legitimate cases:

• Pointer arithmetic with negative offsets. If you're doing pointer::offset or working with slices where you need to express "go backwards," isize is the natural type. The offset method takes isize specifically for this reason.
• Difference between pointers or indices. If you subtract two pointers or two indices, the result can be negative. pointer::offset_from returns isize.
• Algorithms that use sentinel values. Some algorithms use -1 to represent "not found" or "invalid." While Rust idiomatically uses Option<usize> for this, when porting code or interfacing with such conventions, isize fits naturally.
• Relative offsets in data structures. If you're implementing something like a self-referential structure with relative pointers, or a compact representation where offsets can be negative, isize makes intent clear.

That said, you're right to reach for usize by default. Most indexing, sizes, and counts are non-negative, and usize makes that invariant explicit. isize is the exception for when you genuinely need signedness at pointer width.

[felix91gr]

Edit notes: I've added some formatting for type and type::method, as well as some bullet-list formatting for ease-of-reading. Hope that's alright, @rcseacord ^.^