Auto merge of #85746 - m-ou-se:io-error-other, r=joshtriplett

Redefine `ErrorKind::Other` and stop using it in std.

This implements the idea I shared yesterday in the libs meeting when we were discussing how to handle adding new `ErrorKind`s to the standard library: This redefines `Other` to be for *user defined errors only*, and changes all uses of `Other` in the standard library to a `#[doc(hidden)]` and permanently `#[unstable]` `ErrorKind` that users can not match on. This ensures that adding `ErrorKind`s at a later point in time is not a breaking change, since the user couldn't match on these errors anyway. This way, we use the `#[non_exhaustive]` property of the enum in a more effective way.

Open questions:
- How do we check this change doesn't cause too much breakage? Will a crate run help and be enough?
- How do we ensure we don't accidentally start using `Other` again in the standard library? We don't have a `pub(not crate)` or `#[deprecated(in this crate only)]`.

cc https://github.com/rust-lang/rust/pull/79965

cc `@rust-lang/libs` `@ijackson`

r? `@dtolnay`

1 files changed, 3 insertions, 1 deletions

diff --git a/src/test/ui/write-fmt-errors.rs b/src/test/ui/write-fmt-errors.rs
index 7dd98564425..3fcaefaa63e 100644
--- a/src/test/ui/write-fmt-errors.rs
+++ b/src/test/ui/write-fmt-errors.rs
@@ -1,5 +1,7 @@
 // run-pass
 
+#![feature(io_error_uncategorized)]
+
 use std::fmt;
 use std::io::{self, Error, Write, sink};
 
@@ -13,7 +15,7 @@ impl fmt::Display for ErrorDisplay {
 
 struct ErrorWriter;
 
-const FORMAT_ERROR: io::ErrorKind = io::ErrorKind::Other;
+const FORMAT_ERROR: io::ErrorKind = io::ErrorKind::Uncategorized;
 const WRITER_ERROR: io::ErrorKind = io::ErrorKind::NotConnected;
 
 impl Write for ErrorWriter {