Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc #136083
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r? @wesleywiser rustbot has assigned @wesleywiser. Use |
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how do i fix stdarch? |
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juntyr
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Jan 26, 2025
| Bool | Uint(..) | Int(..) => { | ||
| matches!(fn_sig.output().kind(), Int(..) | Uint(..)).then(|| { | ||
| errors::RedundantTransmute { | ||
| sugg: format!("({arg}) as {}", fn_sig.output()), |
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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?
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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.
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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)?
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The cast_(un)signed methods will be stable on nightly in another 2 days: #125882 (comment)
So maybe we just hold off on merging this PR until that stabilization PR lands? I'm also not a fan of as, so would like to avoid suggesting something that clippy will soon then start suggesting to change again.
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oh wow! i guess ill make it suggest those methods then
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what about bool as integer though?
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let b: bool = true;
let x = u8::from(b);
assert_eq!(x, 1);.into() would be nicer but depends on knowing u8 or i8 from type inference. u8::from and i8::from work in any context, though they don't work in postfix position. For the purposes of this lint, though, the only integer types you can transmute a bool to would be u8 or i8, and transmute already isn't postfix, so I'd suggest that transmutes from bool to u8/i8 should suggest u8::from and i8::from, respectively.
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i can do that with u32::from(char) i think, as well
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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. |
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@Noratrieb i dont know the difference; i looked for |
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Also, how should i fix |
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I would first recommend checking out if the lang team wants this lint. |
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We talked about this briefly in today's @rust-lang/lang meeting. This seems likely to always be desirable, and it helps people rewrite unsafe code as safe code. Let's give it a try in nightly and see how it goes! (In particular, that'll let us evaluate possible false positives.) |
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I'm slightly torn on this one. It feels more like a clippy lint to me, because to me what we're approving here as a lang team is "suggesting safe alternatives to unsafe transmutes", not "well there's a different unsafe version that would be better" -- it's the removing of unsafe that gets it to the "yes this is in rustc" bar, in my mind. (Other team members may disagree, however.) |
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I'm torn on it too. It is marginally safer, because then at least it goes through the pub(super) const unsafe fn from_u32_unchecked(i: u32) -> char {
// SAFETY: the caller must guarantee that `i` is a valid char value.
unsafe {
assert_unsafe_precondition!(
check_language_ub,
"invalid value for `char`",
(i: u32 = i) => char_try_from_u32(i).is_ok()
);
transmute(i)
}
}The assertion only fires on debug builds, though. |
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Some changes occurred to MIR optimizations cc @rust-lang/wg-mir-opt |
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@bors r+ rollup |
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lcnr
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Mar 26, 2025
| @@ -1,7 +1,7 @@ | |||
| #![no_std] | |||
| #![feature(lang_items)] | |||
| #![warn(clippy::transmute_int_to_char)] | |||
| #![allow(clippy::missing_transmute_annotations)] | |||
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we should likely remove these lints from clippy once this is merged, shouldn't we?
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Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc
implements rust-lang#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.
```rs
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:
`fXX` <-> `uXX` = `{from_bits, to_bits}`
`uXX` -> `iXX` via `cast_unsigned` and `cast_signed`
{`char` -> `u32`, `bool` -> `n8`} via `from`
`u32` -> `char` via `from_u32_unchecked` (note: notes `from_u32().unwrap()`) (contested)
`u8` -> `bool` via `==` (debatable)
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Rollup of 10 pull requests Successful merges: - rust-lang#136083 (Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc) - rust-lang#138344 (Enable `reliable_f16_math` on x86) - rust-lang#138624 (Add mipsel maintainer) - rust-lang#138935 (Update wg-prio triagebot config) - rust-lang#138946 (Un-bury chapters from the chapter list in rustc book) - rust-lang#138964 (Implement lint against using Interner and InferCtxtLike in random compiler crates) - rust-lang#138977 (Don't deaggregate InvocationParent just to reaggregate it again) - rust-lang#138980 (Collect items referenced from var_debug_info) - rust-lang#138985 (Use the correct binder scope for elided lifetimes in assoc consts) - rust-lang#138987 (Always emit `native-static-libs` note, even if it is empty) r? `@ghost` `@rustbot` modify labels: rollup
jhpratt
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Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc
implements rust-lang#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.
```rs
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:
`fXX` <-> `uXX` = `{from_bits, to_bits}`
`uXX` -> `iXX` via `cast_unsigned` and `cast_signed`
{`char` -> `u32`, `bool` -> `n8`} via `from`
`u32` -> `char` via `from_u32_unchecked` (note: notes `from_u32().unwrap()`) (contested)
`u8` -> `bool` via `==` (debatable)
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Rollup of 9 pull requests Successful merges: - rust-lang#136083 (Suggest {to,from}_ne_bytes for transmutations between arrays and integers, etc) - rust-lang#138624 (Add mipsel maintainer) - rust-lang#138935 (Update wg-prio triagebot config) - rust-lang#138946 (Un-bury chapters from the chapter list in rustc book) - rust-lang#138964 (Implement lint against using Interner and InferCtxtLike in random compiler crates) - rust-lang#138977 (Don't deaggregate InvocationParent just to reaggregate it again) - rust-lang#138980 (Collect items referenced from var_debug_info) - rust-lang#138985 (Use the correct binder scope for elided lifetimes in assoc consts) - rust-lang#138987 (Always emit `native-static-libs` note, even if it is empty) r? `@ghost` `@rustbot` modify labels: rollup
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Looks like this failed in rollup: #139009 (comment) @bors r- |
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☔ The latest upstream changes (presumably #119220) made this pull request unmergeable. Please resolve the merge conflicts. |
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The job Click to see the possible cause of the failure (guessed by this bot) |
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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.
It would be handy to suggest
u32::from_ne_bytes(x).This is implemented for
[u8; _]->{float int}This also implements the cases:
fXX<->uXX={from_bits, to_bits}uXX->iXXviacast_unsignedandcast_signed{
char->u32,bool->n8} viafromu32->charviafrom_u32_unchecked(note: notesfrom_u32().unwrap()) (contested)u8->boolvia==(debatable)