There is a cost to having multiple language-level types that represent the exact same set of values, as C has (and is really noticeable in C++). Rust made an early, fairly explicit decision that a) usize is a distinct fundamental type from the other types, and not merely a target-specific typedef, and b) not to introduce more types for things like uindex or uaddr or uptr, which are the same as usize on nearly every platform.
Rust worded in its initial guarantee that usize was sufficient to roundtrip a pointer (making it effectively uptr), and there remains concern among several of the maintainers about breaking that guarantee, despite the fact that people on the only target that would be affected basically saying they'd rather see that guarantee broken. Sort of the more fundamental problem is that many crates are perfectly happy opting out of compiling for weirder platform--I've designed some stuff that relies on 64-bit system properties, and I'd rather like to have the ability to say "no compile for you on platform where usize-is-not-u64" and get impl From<usize> for u64 and impl From<u64> for usize. If you've got something like that, it also provides a neat way to say "I don't want to opt out of [or into] compiling for usize≠uptr" and keeping backwards compatibility.
> ...not to introduce more types for things like uindex or uaddr or uptr, which are the same as usize on nearly every platform. ... there remains concern among several of the maintainers about breaking that guarantee, despite the fact that people on the only target that would be affected basically saying they'd rather see that guarantee broken.
The proper approach to resolving this in an elegant way is to make the guarantee target-dependent. Require all depended-upon crates to acknowledge that usize might differ from uptr in order to unlock building for "exotic" architectures, much like how no-std works today. That way "nearly every platform" can still rely on the guarantee with no rise in complexity.
I brought up Go because it was designed around the same time and, while it gets a lot of flack for some of its other design decisions, this particular one seems prescient. However, I would be remiss if I gave the impression that the reasoning behind the decision was anticipation of some yet unseen future; the reality was that int and uint (which are not aliases for sized intN or uintN) were not initially the same as ptrdiff_t and size_t (respectively) on all platforms. Early versions of Go for 64-bit systems had 32-bit int and uint, so naturally uintptr had to be different (and it's also not an alias). It was only later that int and uint became machine-word-sized on all platforms and so made uintptr seem a bit redundant. However, this distinction is fortuitous for CHERI etc. support. Still, Go on CHERI with 128-bit uintptr might break some code, however such code is likely in violation of the unsafe pointer rules anyway: https://pkg.go.dev/unsafe#Pointer
Yet Rust is not Go and this solution is probably not the right one for Rust. As laid out in a link on a sibling comment, one possibility is to do away with pointer <=> integer conversions entirely, and use methods on pointers to access and mutate their addresses (which may be the only thing they represent on some platforms, but is just a part of their representation on others). The broader issue is really about evolving the language and ecosystem away from the mentality that "pointers are just integers with fancy sigil names".
I'd say, that even more than pointer sizes, the idea that a pointer is just a number really needs to die, and is in no way a forward looking decision expected of a modern language.
Pointers should at no point be converted into numbers and back as that trips up many assumptions (special runtimes, static/dynamic analysis tools, compiler optimizations).
Additionally, I would make it a priority that writing FFIs should be as easy as possible, and requires as little human deliberation as possible. Even if Rust is safe, its safety can only be assumed as long as the underlying external code upholds the invariants.
Which is a huge risk factor for Rust, especially in today's context of the Linux kernel. If I have an object created/handled by external native code, how do I make sure that it respects Rust's lifetime/aliasing rules?
What's the exact list of rules my C code must conform to?
Are there any static analysis/fuzzing tools that can verify that my code is indeed compliant?
C doesn't require convertibility to an integer and recognizes that pointers may have atypical representations. Casting to integer types has always been implementation defined. [u]intptr_t is optional specifically to allow such platforms to claim standard conformance.
> Which is a huge risk factor for Rust, especially in today's context of the Linux kernel. If I have an object created/handled by external native code, how do I make sure that it respects Rust's lifetime/aliasing rules?
Can you expand on this point? Like are you worried about whether the external code is going to free the memory out from under you? That is part of a guarantee, the compiler cannot guarantee what happens at runtime no matter what the author of a language wants, the CPU will do what it's told, it couldn't care about Rusts guarantees even if you built your code entirely with rust.
When you are interacting with the real world and real things you need to work with different assumptions, if you don't trust that the data will remain unmodified then copy it.
No matter how many abstractions you put on top of it there is still lighting in a rock messing with 1s and 0s.
Rust worded in its initial guarantee that usize was sufficient to roundtrip a pointer (making it effectively uptr), and there remains concern among several of the maintainers about breaking that guarantee, despite the fact that people on the only target that would be affected basically saying they'd rather see that guarantee broken. Sort of the more fundamental problem is that many crates are perfectly happy opting out of compiling for weirder platform--I've designed some stuff that relies on 64-bit system properties, and I'd rather like to have the ability to say "no compile for you on platform where usize-is-not-u64" and get impl From<usize> for u64 and impl From<u64> for usize. If you've got something like that, it also provides a neat way to say "I don't want to opt out of [or into] compiling for usize≠uptr" and keeping backwards compatibility.
If you want to see some long, gory debates on the topic, https://internals.rust-lang.org/t/pre-rfc-usize-is-not-size-... is a good starting point.