Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc

[bend-n]

implements #136067

Rust has helper methods for many kinds of safe transmutes, for example integer<->bytes. This is a lint against using transmute for these cases.

fn bytes_at_home(x: [u8; 4]) -> u32 {
   transmute(x)
}

// other examples
transmute::<[u8; 2], u16>();
transmute::<[u8; 8], f64>();
transmute::<u32, [u8; 4]>();
transmute::<char, u32>();
transmute::<u32, char>();

It would be handy to suggest u32::from_ne_bytes(x).
This is implemented for [u8; _] -> {float int}

This also implements the cases:
char -> u32 via as
u32 -> char via from_u32_unchecked (note: notes from_u32().unwrap())
uXX -> iXX via as (note: not sure if this could be improved)

[rustbot]

r? @wesleywiser

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[bend-n]

how do i fix stdarch?

[rust-log-analyzer]

The job mingw-check-tidy failed! Check out the build log: (web) (plain)

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info: removing rustup binaries
info: rustup is uninstalled
##[group]Image checksum input
mingw-check-tidy
# We use the ghcr base image because ghcr doesn't have a rate limit
# and the mingw-check-tidy job doesn't cache docker images in CI.
FROM ghcr.io/rust-lang/ubuntu:22.04
ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install -y --no-install-recommends \
  g++ \
  make \
---

COPY host-x86_64/mingw-check/validate-toolstate.sh /scripts/
COPY host-x86_64/mingw-check/validate-error-codes.sh /scripts/

# NOTE: intentionally uses python2 for x.py so we can test it still works.
# validate-toolstate only runs in our CI, so it's ok for it to only support python3.
ENV SCRIPT TIDY_PRINT_DIFF=1 python2.7 ../x.py test \
           --stage 0 src/tools/tidy tidyselftest --extra-checks=py,cpp
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downloading https://static.rust-lang.org/dist/2025-01-08/rustfmt-nightly-x86_64-unknown-linux-gnu.tar.xz
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[juntyr]

I wouldn't suggest an as conversion here as as is so overloaded that I at least always forget what each conversion does. I remember there was an attempt in the past to create explicit methods for each as behaviour - if they've been merged, perhaps we could recommend them?

[bend-n]

I'm not sure what methods you're referring to... I don't think i should suggest i32::from_ne_bytes(x.to_ne_bytes())? And try_from has different behavior.

[juntyr]

There are the unstable cast_unsigned and cast_signed methods, and there's an ACP for additional methods rust-lang/libs-team#453.

Perhaps for now the code just gets a FIXME so that the recommendation is changed once #125882 is stabilized (feature integer_sign_cast)?

[Noratrieb]

Is there a reason you made it a MIR lint instead of a HIR lint? HIR lints will make it easier to do the spans.

[bend-n]

@Noratrieb i dont know the difference; i looked for transmute and copied that (ptr_to_integer_transmute_in_consts) lint.

[bend-n]

Also, how should i fix stdarch? Should i make a PR over there to remove the unnecessary trans?

[Noratrieb]

I would first recommend checking out if the lang team wants this lint.
Can you write out a clear PR description with the exact cases you're covering and some examples for the lang team?