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I/O safety breaks existing code #89955
Description
As reported here, the I/O safety feature introduces a breaking change.
Reduced standalone testcase (playground link):
use std::os::unix::io::FromRawFd;
use std::fs::File;
pub fn create_fd<F: FromRawFd>() -> F {
todo!()
}
fn main() {
let _f = File::from(create_fd());
}error[E0283]: type annotations needed
--> src/main.rs:9:14
|
9 | let _f = File::from(create_fd());
| ^^^^^^^^^^ cannot infer type for type parameter `T` declared on the trait `From`
|
= note: cannot satisfy `File: From<_>`
note: required by `from`
--> /.../lib/rustlib/src/rust/library/core/src/convert/mod.rs:371:5
|
371 | fn from(_: T) -> Self;
| ^^^^^^^^^^^^^^^^^^^^^^
It's possible to change the code to avoid the error, for example by changing the let _f = ... line to either of:
let _f = create_fd::<File>();
let _f: File = create_fd(); or, once I/O safety is stabilized, by changing create_fd to return OwnedFd instead of F: FromRawFd. The reporter also reported that they've already fixed their own code. Nevertheless, it is breakage observed in real-world code.
The problem is caused by I/O safety adding an impl From<OwnedFd> for File. We added these From impls as an alternative to adding a new FromFd trait. However, this means that File now has multiple From impls for types that implement AsRawFd, so one can no longer do File::from(create_fd()) where create_fd is defined like fn create_fd<F: FromRawFd>() -> F, because it's now ambiguous which type it should use for F.
So the questions here are:
- Is this breakage severe enough to block stabilization for I/O safety?
- If so, is there a way to fix it other than going back to adding a
FromFdtrait?
Activity
scottmcm commentedon Oct 16, 2021
I think this is officially a "minor" change, and thus allowed -- it's like all the breakage that can happen when
AsRefimplementations are added. Assuming that's true, it's a libs judgment call on how bad the impact is in practice.cuviper commentedon Oct 17, 2021
This is about to ship in 1.56 -- cc @rust-lang/release
But AFAIK it didn't show problems in crater, so we'll probably let it go as a minor change...
Mark-Simulacrum commentedon Oct 17, 2021
Certainly nothing significant, I think. Working with (raw) file descriptors is probably pretty rare, so I'm not expecting much breakage.
sunfishcode commentedon Oct 23, 2021
1.56 has shipped now, and I haven't seen any other reports of breakage. So following the comments above, I'll close this issue now. If anyone does see any other instances of breakage, please report it!