Add dedicated feature for core::io stability

Both `io_error_kind_in_core` and `core_io_borrowed_buf` require an unstable `core::io` module, but there is no way to make a module unstable and activated by `any(...)`

Author: bushrat011899

library/core/src/lib.rs

@@ -294,7 +294,7 @@ pub mod bstr;
 pub mod cell;
 pub mod char;
 pub mod ffi;
-#[unstable(feature = "core_io_borrowed_buf", issue = "117693")]
+#[unstable(feature = "core_io", issue = "154046")]
 pub mod io;
 pub mod iter;
 pub mod net;

library/coretests/tests/lib.rs

@@ -35,6 +35,7 @@
 #![feature(const_unsigned_bigint_helpers)]
 #![feature(core_intrinsics)]
 #![feature(core_intrinsics_fallbacks)]
+#![feature(core_io)]
 #![feature(core_io_borrowed_buf)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]

library/std/src/lib.rs

@@ -320,6 +320,7 @@
 #![feature(const_default)]
 #![feature(core_float_math)]
 #![feature(core_intrinsics)]
+#![feature(core_io)]
 #![feature(core_io_borrowed_buf)]
 #![feature(cstr_display)]
 #![feature(drop_guard)]

library/std/src/os/windows/fs.rs

@@ -61,6 +61,7 @@ pub trait FileExt {
     /// # Examples
     ///
     /// ```no_run
+    /// #![feature(core_io)]
     /// #![feature(core_io_borrowed_buf)]
     /// #![feature(read_buf_at)]
     ///

tests/ui/feature-gates/feature-gate-io_error_kind_in_core.rs

@@ -0,0 +1,12 @@
+// Ensure `ErrorKind` from `core` is gated behind `io_error_kind_in_core`
+//@ edition:2024
+
+use std::io::ErrorKind as ErrorKindFromStd;
+
+use core::io::ErrorKind as ErrorKindFromCore;
+//~^ ERROR use of unstable library feature `core_io`
+
+// Asserting both ErrorKinds are the same.
+const _: [ErrorKindFromCore; 1] = [ErrorKindFromStd::Other];
+
+fn main() {}

tests/ui/feature-gates/feature-gate-io_error_kind_in_core.stderr

@@ -0,0 +1,13 @@
+error[E0658]: use of unstable library feature `core_io`
+  --> $DIR/feature-gate-io_error_kind_in_core.rs:6:5
+   |
+LL | use core::io::ErrorKind as ErrorKindFromCore;
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #154046 <https://github.com/rust-lang/rust/issues/154046> for more information
+   = help: add `#![feature(core_io)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.