demo of moving std::io::Error to core

library/alloc/src/io/error.rs

@@ -0,0 +1,203 @@
+use crate::{alloc::Global, boxed::Box, string::String};
+use core::io::{
+    const_io_error,
+    error_internals::{AllocVTable, Custom, ErrorBox, ErrorString},
+    Error, ErrorKind,
+};
+use core::{alloc::Allocator, error, ptr, result};
+
+unsafe fn set_alloc_vtable() {
+    static ALLOC_VTABLE: AllocVTable =
+        AllocVTable { deallocate: |ptr, layout| unsafe { Global.deallocate(ptr, layout) } };
+    unsafe {
+        ALLOC_VTABLE.install();
+    }
+}
+
+fn into_error_box<T: ?Sized>(value: Box<T>) -> ErrorBox<T> {
+    unsafe {
+        set_alloc_vtable();
+        ErrorBox::from_raw(Box::into_raw(value))
+    }
+}
+
+fn into_box<T: ?Sized>(v: ErrorBox<T>) -> Box<T> {
+    unsafe { Box::from_raw(v.into_raw()) }
+}
+
+impl From<String> for ErrorString {
+    fn from(value: String) -> Self {
+        unsafe {
+            set_alloc_vtable();
+            let (buf, length, capacity) = value.into_raw_parts();
+            ErrorString::from_raw_parts(
+                ErrorBox::from_raw(ptr::slice_from_raw_parts_mut(buf.cast(), capacity)).into(),
+                length,
+            )
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<crate::ffi::NulError> for Error {
+    /// Converts a [`crate::ffi::NulError`] into a [`Error`].
+    fn from(_: crate::ffi::NulError) -> Error {
+        const_io_error!(ErrorKind::InvalidInput, "data provided contains a nul byte")
+    }
+}
+
+impl Error {
+    /// Creates a new I/O error from a known kind of error as well as an
+    /// arbitrary error payload.
+    ///
+    /// This function is used to generically create I/O errors which do not
+    /// originate from the OS itself. The `error` argument is an arbitrary
+    /// payload which will be contained in this [`Error`].
+    ///
+    /// Note that this function allocates memory on the heap.
+    /// If no extra payload is required, use the `From` conversion from
+    /// `ErrorKind`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// // errors can be created from strings
+    /// let custom_error = Error::new(ErrorKind::Other, "oh no!");
+    ///
+    /// // errors can also be created from other errors
+    /// let custom_error2 = Error::new(ErrorKind::Interrupted, custom_error);
+    ///
+    /// // creating an error without payload (and without memory allocation)
+    /// let eof_error = Error::from(ErrorKind::UnexpectedEof);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_allow_incoherent_impl]
+    #[inline(never)]
+    pub fn new<E>(kind: ErrorKind, error: E) -> Error
+    where
+        E: Into<Box<dyn error::Error + Send + Sync>>,
+    {
+        Self::_new(kind, error.into())
+    }
+
+    /// Creates a new I/O error from an arbitrary error payload.
+    ///
+    /// This function is used to generically create I/O errors which do not
+    /// originate from the OS itself. It is a shortcut for [`Error::new`]
+    /// with [`ErrorKind::Other`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Error;
+    ///
+    /// // errors can be created from strings
+    /// let custom_error = Error::other("oh no!");
+    ///
+    /// // errors can also be created from other errors
+    /// let custom_error2 = Error::other(custom_error);
+    /// ```
+    ///
+    /// [`Error::new`]: https://doc.rust-lang.org/std/io/struct.Error.html#method.new
+    #[stable(feature = "io_error_other", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_allow_incoherent_impl]
+    pub fn other<E>(error: E) -> Error
+    where
+        E: Into<Box<dyn error::Error + Send + Sync>>,
+    {
+        Self::_new(ErrorKind::Other, error.into())
+    }
+
+    #[rustc_allow_incoherent_impl]
+    fn _new(kind: ErrorKind, error: Box<dyn error::Error + Send + Sync>) -> Error {
+        Error::_new_custom(into_error_box(Box::new(Custom { kind, error: into_error_box(error) })))
+    }
+
+    /// Consumes the `Error`, returning its inner error (if any).
+    ///
+    /// If this [`Error`] was constructed via [`new`] then this function will
+    /// return [`Some`], otherwise it will return [`None`].
+    ///
+    /// [`new`]: https://doc.rust-lang.org/std/io/struct.Error.html#method.new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// fn print_error(err: Error) {
+    ///     if let Some(inner_err) = err.into_inner() {
+    ///         println!("Inner error: {inner_err}");
+    ///     } else {
+    ///         println!("No inner error");
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     // Will print "No inner error".
+    ///     print_error(Error::last_os_error());
+    ///     // Will print "Inner error: ...".
+    ///     print_error(Error::new(ErrorKind::Other, "oh no!"));
+    /// }
+    /// ```
+    #[stable(feature = "io_error_inner", since = "1.3.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[inline]
+    #[rustc_allow_incoherent_impl]
+    pub fn into_inner(self) -> Option<Box<dyn error::Error + Send + Sync>> {
+        self.into_inner_impl().map(into_box)
+    }
+
+    /// Attempt to downgrade the inner error to `E` if any.
+    ///
+    /// If this [`Error`] was constructed via [`new`] then this function will
+    /// attempt to perform downgrade on it, otherwise it will return [`Err`].
+    ///
+    /// If downgrade succeeds, it will return [`Ok`], otherwise it will also
+    /// return [`Err`].
+    ///
+    /// [`new`]: https://doc.rust-lang.org/std/io/struct.Error.html#method.new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![allow(dead_code)]
+    /// #![feature(io_error_downcast)]
+    ///
+    /// use std::fmt;
+    /// use std::io;
+    /// use std::error::Error;
+    ///
+    /// #[derive(Debug)]
+    /// enum E {
+    ///     Io(io::Error),
+    ///     SomeOtherVariant,
+    /// }
+    ///
+    /// impl fmt::Display for E {
+    ///    // ...
+    /// #    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    /// #        todo!()
+    /// #    }
+    /// }
+    /// impl Error for E {}
+    ///
+    /// impl From<io::Error> for E {
+    ///     fn from(err: io::Error) -> E {
+    ///         err.downcast::<E>()
+    ///             .map(|b| *b)
+    ///             .unwrap_or_else(E::Io)
+    ///     }
+    /// }
+    /// ```
+    #[unstable(feature = "io_error_downcast", issue = "99262")]
+    #[rustc_allow_incoherent_impl]
+    pub fn downcast<E>(self) -> result::Result<Box<E>, Self>
+    where
+        E: error::Error + Send + Sync + 'static,
+    {
+        self.downcast_impl::<E>().map(|p| unsafe { Box::from_raw(p) })
+    }
+}

library/alloc/src/io/mod.rs

@@ -0,0 +1,8 @@
+//! Traits, helpers, and type definitions for core I/O functionality.
+
+#![unstable(feature = "alloc_io", issue = "none")]
+
+pub use core::io::*;
+
+#[cfg(target_has_atomic_load_store = "ptr")]
+mod error;

library/alloc/src/lib.rs

@@ -121,6 +121,8 @@
 #![feature(const_size_of_val)]
 #![feature(const_waker)]
 #![feature(core_intrinsics)]
+#![feature(core_io)]
+#![feature(core_io_error_internals)]
 #![feature(core_panic)]
 #![feature(deprecated_suggestion)]
 #![feature(dispatch_from_dyn)]
@@ -177,6 +179,7 @@
 #![feature(associated_type_bounds)]
 #![feature(c_unwind)]
 #![feature(cfg_sanitize)]
+#![feature(cfg_target_has_atomic)]
 #![feature(const_mut_refs)]
 #![feature(const_precise_live_drops)]
 #![feature(const_ptr_write)]
@@ -246,6 +249,7 @@ pub mod collections;
 #[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
 pub mod ffi;
 pub mod fmt;
+pub mod io;
 #[cfg(not(no_rc))]
 pub mod rc;
 pub mod slice;

library/alloc/tests/io.rs

@@ -0,0 +1 @@
+mod error;

library/alloc/tests/io/tests.rs

@@ -0,0 +1,26 @@
+use core::error;
+use core::fmt;
+use core::io::{Error, ErrorKind};
+
+#[test]
+fn test_downcasting() {
+    #[derive(Debug)]
+    struct TestError;
+
+    impl fmt::Display for TestError {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            f.write_str("asdf")
+        }
+    }
+
+    impl error::Error for TestError {}
+
+    // we have to call all of these UFCS style right now since method
+    // resolution won't implicitly drop the Send+Sync bounds
+    let mut err = Error::new(ErrorKind::Other, TestError);
+    assert!(err.get_ref().unwrap().is::<TestError>());
+    assert_eq!("asdf", err.get_ref().unwrap().to_string());
+    assert!(err.get_mut().unwrap().is::<TestError>());
+    let extracted = err.into_inner().unwrap();
+    extracted.downcast::<TestError>().unwrap();
+}