std: expose const_io_error! as const_error!

ACP: rust-lang/libs-team#205
Tracking issue: #133448

library/std/src/io/error.rs

@@ -151,27 +151,38 @@ pub type RawOsError = sys::RawOsError;
 // (For the sake of being explicit: the alignment requirement here only matters
 // if `error/repr_bitpacked.rs` is in use — for the unpacked repr it doesn't
 // matter at all)
+#[doc(hidden)]
+#[unstable(feature = "io_const_error_internals", issue = "none")]
 #[repr(align(4))]
 #[derive(Debug)]
-pub(crate) struct SimpleMessage {
-    kind: ErrorKind,
-    message: &'static str,
+pub struct SimpleMessage {
+    pub kind: ErrorKind,
+    pub message: &'static str,
 }
 
-impl SimpleMessage {
-    pub(crate) const fn new(kind: ErrorKind, message: &'static str) -> Self {
-        Self { kind, message }
-    }
-}
-
-/// Creates and returns an `io::Error` for a given `ErrorKind` and constant
-/// message. This doesn't allocate.
-pub(crate) macro const_io_error($kind:expr, $message:expr $(,)?) {
-    $crate::io::error::Error::from_static_message({
-        const MESSAGE_DATA: $crate::io::error::SimpleMessage =
-            $crate::io::error::SimpleMessage::new($kind, $message);
-        &MESSAGE_DATA
-    })
+/// Creates a new I/O error from a known kind of error and a string literal.
+///
+/// Contrary to [`Error::new`], this macro does not allocate and can be used in
+/// `const` contexts.
+///
+/// # Example
+/// ```
+/// #![feature(io_const_error)]
+/// use std::io::{const_error, Error, ErrorKind};
+///
+/// const FAIL: Error = const_error!(ErrorKind::Unsupported, "tried something that never works");
+///
+/// fn not_here() -> Result<(), Error> {
+///     Err(FAIL)
+/// }
+/// ```
+#[rustc_macro_transparency = "semitransparent"]
+#[unstable(feature = "io_const_error", issue = "133448")]
+#[allow_internal_unstable(hint_must_use, io_const_error_internals)]
+pub macro const_error($kind:expr, $message:expr $(,)?) {
+    $crate::hint::must_use($crate::io::Error::from_static_message(
+        const { &$crate::io::SimpleMessage { kind: $kind, message: $message } },
+    ))
 }
 
 // As with `SimpleMessage`: `#[repr(align(4))]` here is just because
@@ -598,7 +609,9 @@ impl Error {
     /// This function should maybe change to `from_static_message<const MSG: &'static
     /// str>(kind: ErrorKind)` in the future, when const generics allow that.
     #[inline]
-    pub(crate) const fn from_static_message(msg: &'static SimpleMessage) -> Error {
+    #[doc(hidden)]
+    #[unstable(feature = "io_const_error_internals", issue = "none")]
+    pub const fn from_static_message(msg: &'static SimpleMessage) -> Error {
         Self { repr: Repr::new_simple_message(msg) }
     }