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matthiaskrgr opened this issue Jun 5, 2025 · 2 comments · May be fixed by #14996
Open

missing_transmute_annotations suggests impl Future which is not allowed #14984

matthiaskrgr opened this issue Jun 5, 2025 · 2 comments · May be fixed by #14996
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@samueltardieu
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C-bug Category: Clippy is not doing the correct thing I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied T-async-await Type: Issues related to async/await

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@matthiaskrgr
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matthiaskrgr commented Jun 5, 2025 •
edited by rustbot
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Using the following flags

--force-warn clippy::missing_transmute_annotations

this code:

async fn empty() {}

async fn meow(a: [u8; 100]) {
	empty().await;
	dbg!(a);
}

fn main() {
	let v = meow([0; 100]);
	let e = [0u8; 202];
	let _ = || unsafe {
		use std::{mem, ptr};
		let mut copy = ptr::read(&v);
		ptr::write(&mut copy, mem::transmute(ptr::read(&e)));
		mem::forget(copy);
	};
}

caused the following diagnostics:

    Checking _110151 v0.1.0 (/tmp/icemaker_global_tempdir.t8dYaH8PqaqW/icemaker_clippyfix_tempdir.QCq7tmv8PvKz/_110151)
warning: transmute used without annotations
  --> src/main.rs:14:30
   |
14 |         ptr::write(&mut copy, mem::transmute(ptr::read(&e)));
   |                                    ^^^^^^^^^ help: consider adding missing annotations: `transmute::<[u8; 202], impl std::future::Future<Output = ()>>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_transmute_annotations
   = note: requested on the command line with `--force-warn clippy::missing-transmute-annotations`

warning: `_110151` (bin "_110151") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s

However after applying these diagnostics, the resulting code:

async fn empty() {}

async fn meow(a: [u8; 100]) {
	empty().await;
	dbg!(a);
}

fn main() {
	let v = meow([0; 100]);
	let e = [0u8; 202];
	let _ = || unsafe {
		use std::{mem, ptr};
		let mut copy = ptr::read(&v);
		ptr::write(&mut copy, mem::transmute::<[u8; 202], impl std::future::Future<Output = ()>>(ptr::read(&e)));
		mem::forget(copy);
	};
}

no longer compiled:

    Checking _110151 v0.1.0 (/tmp/icemaker_global_tempdir.t8dYaH8PqaqW/icemaker_clippyfix_tempdir.QCq7tmv8PvKz/_110151)
error[E0562]: `impl Trait` is not allowed in paths
  --> src/main.rs:14:53
   |
14 |         ptr::write(&mut copy, mem::transmute::<[u8; 202], impl std::future::Future<Output = ()>>(ptr::read(&e)));
   |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

For more information about this error, try `rustc --explain E0562`.
error: could not compile `_110151` (bin "_110151") due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `_110151` (bin "_110151" test) due to 1 previous error

Version:

rustc 1.89.0-nightly (076ec59ff 2025-06-05)
binary: rustc
commit-hash: 076ec59ff1dcf538b9d3a0b8e0d7f4edd0559959
commit-date: 2025-06-05
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5
@matthiaskrgr matthiaskrgr added C-bug Category: Clippy is not doing the correct thing I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied T-async-await Type: Issues related to async/await labels Jun 6, 2025
@matthiaskrgr
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simple example I think:

Using the following flags

--force-warn clippy::missing_transmute_annotations

this code:

//! Check that typeck can observe the size of an opaque type.
//@ check-pass
use std::mem::transmute;
fn foo() -> impl Sized {
    0u8
}

fn main() {
    unsafe {
        transmute::<_, u8>(foo());
    }
}

caused the following diagnostics:

    Checking _outside-of-defining-scope v0.1.0 (/tmp/icemaker_global_tempdir.o4hDRjvshrmO/icemaker_clippyfix_tempdir.ekcifGNUFKDC/_outside-of-defining-scope)
warning: transmute used without annotations
  --> src/main.rs:10:9
   |
10 |         transmute::<_, u8>(foo());
   |         ^^^^^^^^^^^^^^^^^^ help: consider adding missing annotations: `transmute::<impl Sized, u8>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_transmute_annotations
   = note: requested on the command line with `--force-warn clippy::missing-transmute-annotations`

warning: `_outside-of-defining-scope` (bin "_outside-of-defining-scope") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s

However after applying these diagnostics, the resulting code:

//! Check that typeck can observe the size of an opaque type.
//@ check-pass
use std::mem::transmute;
fn foo() -> impl Sized {
    0u8
}

fn main() {
    unsafe {
        transmute::<impl Sized, u8>(foo());
    }
}

no longer compiled:

    Checking _outside-of-defining-scope v0.1.0 (/tmp/icemaker_global_tempdir.o4hDRjvshrmO/icemaker_clippyfix_tempdir.ekcifGNUFKDC/_outside-of-defining-scope)
error[E0562]: `impl Trait` is not allowed in paths
  --> src/main.rs:10:21
   |
10 |         transmute::<impl Sized, u8>(foo());
   |                     ^^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

For more information about this error, try `rustc --explain E0562`.
error: could not compile `_outside-of-defining-scope` (bin "_outside-of-defining-scope" test) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `_outside-of-defining-scope` (bin "_outside-of-defining-scope") due to 1 previous error

Version:

rustc 1.89.0-nightly (cf423712b 2025-06-05)
binary: rustc
commit-hash: cf423712b9e95e9f6ec84b1ecb3d125e55ac8d56
commit-date: 2025-06-05
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

@samueltardieu
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@rustbot claim

@samueltardieu samueltardieu linked a pull request Jun 6, 2025 that will close this issue
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Labels
C-bug Category: Clippy is not doing the correct thing I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied T-async-await Type: Issues related to async/await
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Suggest naming types before using explicit type names samueltardieu/rust-clippy
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