`cast_possible_wrap` for convertion from `u8` to `isize`

[quarthex]

Summary

The lint cast_possible_wrap is triggered by a conversion from u8 to isize.

The displayed message is:

casting `u8` to `isize` may wrap around the value on targets with 32-bit wide pointers

On 16/32/64-bits platforms, the message is wrong. An 8-bit unsigned value fits in a 16/32/64-bits signed register.

Reproducer

#![warn(clippy::cast_possible_wrap)]
const A: u8 = 0;
const B: isize = A as isize;

Version

rustc 1.63.0 (4b91a6ea7 2022-08-08)
binary: rustc
commit-hash: 4b91a6ea7258a947e59c6522cd5898e7c0a6a88f
commit-date: 2022-08-08
host: x86_64-unknown-linux-gnu
release: 1.63.0
LLVM version: 14.0.5

Additional Labels

@rustbot label +I-false-positive +L-pedantic

[quarthex]

The lint applies on 8-bits platforms, though.

[asquared31415]

Rust will never support platforms where u/isize is smaller than 16 bits and this lint is always incorrect. In fact isize::from(u8) exists, which means it definitely is a lossless conversion.

[quarthex]

@asquared31415 If it want to be considered as a language targeted for embedded development, Rust should be able to produce code for such platforms. In my company, we are trying to deal with shortages of microcontrollers such as STM32 (mainly reserved by the car industry) and we will use TinyAVR micrcontrollers instead. So, saying that Rust will never support 8-bit platforms is a little dramatic from my point of view.

[asquared31415]

Unfortunately Rust has already made guarantees that are incompatible with that. It's well documented that usize is at least 16 bits large in The Reference and changing that would also require breaking changes to the standard library, namely the removal of the isize::from(u8) impl.