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Diagnostic improvement request: Enums that contain non-Send aliased types produce confusing error messages #146016
Description
Consider this code (also in this playground):
use std::sync::mpsc;
use std::thread;
mod other {
use std::ffi::c_void;
pub type Bar = *const c_void;
}
use other::Bar;
enum Foo {
Case(i32),
Case2(Bar),
}
fn main() {
let (tx, rx) = mpsc::channel();
let h = thread::spawn(move || match rx.recv().unwrap() {
Foo::Case(x) => println!("{:?}", x),
Foo::Case2(b) => println!("{:?}", b),
});
tx.send(Foo::Case2(std::ptr::null()));
h.join();
}When I compile this with the latest nightly (1.91.0-nightly (2025-08-28 f2824da98d44c4a4e17b), from rust playground), I see this error, which mentions neither Case2 nor Bar. In this case, it's easy enough to see what the problem is, but in a more complex example -- e.g. if Bar comes from a crate whose implementation the user is unfamiliar with -- it becomes difficult to understand what the cause of the problem is.
Compiling playground v0.0.1 (/playground)
error[E0277]: `*const c_void` cannot be sent between threads safely
--> src/main.rs:18:27
|
18 | let h = thread::spawn(move ||
| _____________-------------_^
| | |
| | required by a bound introduced by this call
19 | | match rx.recv().unwrap() {
20 | | Foo::Case(x) => println!("{:?}", x),
21 | | Foo::Case2(b) => println!("{:?}", b),
22 | | });
| |_____^ `*const c_void` cannot be sent between threads safely
|
= help: within `Foo`, the trait `Send` is not implemented for `*const c_void`
note: required because it appears within the type `Foo`
--> src/main.rs:12:6
|
12 | enum Foo {
| ^^^
= note: required for `std::sync::mpsc::Receiver<Foo>` to implement `Send`
note: required because it's used within this closure
--> src/main.rs:18:27
|
18 | let h = thread::spawn(move ||
| ^^^^^^^
note: required by a bound in `spawn`
--> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/mod.rs:726:8
|
723 | pub fn spawn<F, T>(f: F) -> JoinHandle<T>
| ----- required by a bound in this function
...
726 | F: Send + 'static,
| ^^^^ required by this bound in `spawn`
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error
It would be better if the error mentioned either Case2, Bar, or both. (Likely mentioning Case2 would suffice, if the type alias presents issues with providing more helpful diagnostics).
Activity
Rollup merge of rust-lang#163060 - chenyukang:yukang-fix-146016-non-s…
Rollup merge of rust-lang#163060 - chenyukang:yukang-fix-146016-non-s…
Unrolled build for #163060 in rollup 163071