Auto merge of #73265 - mark-i-m:mv-std, r=<try>tmp-nightly

mv std libs to library/

This is the first step in refactoring the directory layout of this repository, with further followup steps planned (but not done yet).

Background: currently, all crates are under src/, without nested src directories and with the unconventional `lib*` prefixes (e.g., `src/libcore/lib.rs`). This directory structures is not idiomatic and makes the `src/` directory rather overwhelming. To improve contributor experience and make things a bit more approachable, we are reorganizing the repo a bit.

In this PR, we move the standard libs (basically anything that is "runtime", as opposed to part of the compiler, build system, or one of the tools, etc). The new layout moves these libraries to a new `library/` directory in the root of the repo. Additionally, we remove the `lib*` prefixes and add nested `src/` directories.  The other crates/tools in this repo are not touched. So in summary:

```
library/<crate>/src/*.rs
src/<all the rest>     // unchanged
```

where `<crate>` is:
- core
- alloc
- std
- test
- proc_macro
- panic_abort
- panic_unwind
- profiler_builtins
- term
- unwind
- rtstartup
- backtrace
- rustc-std-workspace-*

There was a lot of discussion about this and a few rounds of compiler team approvals, FCPs, MCPs, and nominations. The original MCP is https://github.com/rust-lang/compiler-team/issues/298. The final approval of the compiler team was given here: https://github.com/rust-lang/rust/pull/73265#issuecomment-659498446.

The name `library` was chosen to complement a later move of the compiler crates to a `compiler/` directory. There was a lot of discussion around adding the nested `src/` directories. Note that this does increase the nesting depth (plausibly important for manual traversal of the tree, e.g., through GitHub's UI or `cd`), but this is deemed to be better as it fits the standard layout of Rust crates throughout most of the ecosystem, though there is some debate about how much this should apply to multi-crate projects. Overall, there seem to be more people in favor of nested `src/` than against.

After this PR, there are no dependencies out of the `library/` directory except on the `build_helper` (or crates.io crates).

1 files changed, 0 insertions, 802 deletions

diff --git a/src/libstd/error.rs b/src/libstd/error.rs
deleted file mode 100644
index 3b4cb859dd4..00000000000
--- a/src/libstd/error.rs
+++ /dev/null
@@ -1,802 +0,0 @@
-//! Traits for working with Errors.
-
-#![stable(feature = "rust1", since = "1.0.0")]
-
-// A note about crates and the facade:
-//
-// Originally, the `Error` trait was defined in libcore, and the impls
-// were scattered about. However, coherence objected to this
-// arrangement, because to create the blanket impls for `Box` required
-// knowing that `&str: !Error`, and we have no means to deal with that
-// sort of conflict just now. Therefore, for the time being, we have
-// moved the `Error` trait into libstd. As we evolve a sol'n to the
-// coherence challenge (e.g., specialization, neg impls, etc) we can
-// reconsider what crate these items belong in.
-
-use core::array;
-use core::convert::Infallible;
-
-use crate::alloc::{AllocErr, LayoutErr};
-use crate::any::TypeId;
-use crate::backtrace::Backtrace;
-use crate::borrow::Cow;
-use crate::cell;
-use crate::char;
-use crate::fmt::{self, Debug, Display};
-use crate::mem::transmute;
-use crate::num;
-use crate::str;
-use crate::string;
-
-/// `Error` is a trait representing the basic expectations for error values,
-/// i.e., values of type `E` in [`Result<T, E>`]. Errors must describe
-/// themselves through the [`Display`] and [`Debug`] traits, and may provide
-/// cause chain information:
-///
-/// The [`source`] method is generally used when errors cross "abstraction
-/// boundaries". If one module must report an error that is caused by an error
-/// from a lower-level module, it can allow access to that error via the
-/// [`source`] method. This makes it possible for the high-level module to
-/// provide its own errors while also revealing some of the implementation for
-/// debugging via [`source`] chains.
-///
-/// [`Result<T, E>`]: ../result/enum.Result.html
-/// [`Display`]: ../fmt/trait.Display.html
-/// [`Debug`]: ../fmt/trait.Debug.html
-/// [`source`]: trait.Error.html#method.source
-#[stable(feature = "rust1", since = "1.0.0")]
-pub trait Error: Debug + Display {
-    /// The lower-level source of this error, if any.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::fmt;
-    ///
-    /// #[derive(Debug)]
-    /// struct SuperError {
-    ///     side: SuperErrorSideKick,
-    /// }
-    ///
-    /// impl fmt::Display for SuperError {
-    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f, "SuperError is here!")
-    ///     }
-    /// }
-    ///
-    /// impl Error for SuperError {
-    ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
-    ///         Some(&self.side)
-    ///     }
-    /// }
-    ///
-    /// #[derive(Debug)]
-    /// struct SuperErrorSideKick;
-    ///
-    /// impl fmt::Display for SuperErrorSideKick {
-    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f, "SuperErrorSideKick is here!")
-    ///     }
-    /// }
-    ///
-    /// impl Error for SuperErrorSideKick {}
-    ///
-    /// fn get_super_error() -> Result<(), SuperError> {
-    ///     Err(SuperError { side: SuperErrorSideKick })
-    /// }
-    ///
-    /// fn main() {
-    ///     match get_super_error() {
-    ///         Err(e) => {
-    ///             println!("Error: {}", e);
-    ///             println!("Caused by: {}", e.source().unwrap());
-    ///         }
-    ///         _ => println!("No error"),
-    ///     }
-    /// }
-    /// ```
-    #[stable(feature = "error_source", since = "1.30.0")]
-    fn source(&self) -> Option<&(dyn Error + 'static)> {
-        None
-    }
-
-    /// Gets the `TypeId` of `self`.
-    #[doc(hidden)]
-    #[unstable(
-        feature = "error_type_id",
-        reason = "this is memory-unsafe to override in user code",
-        issue = "60784"
-    )]
-    fn type_id(&self, _: private::Internal) -> TypeId
-    where
-        Self: 'static,
-    {
-        TypeId::of::<Self>()
-    }
-
-    /// Returns a stack backtrace, if available, of where this error occurred.
-    ///
-    /// This function allows inspecting the location, in code, of where an error
-    /// happened. The returned `Backtrace` contains information about the stack
-    /// trace of the OS thread of execution of where the error originated from.
-    ///
-    /// Note that not all errors contain a `Backtrace`. Also note that a
-    /// `Backtrace` may actually be empty. For more information consult the
-    /// `Backtrace` type itself.
-    #[unstable(feature = "backtrace", issue = "53487")]
-    fn backtrace(&self) -> Option<&Backtrace> {
-        None
-    }
-
-    /// ```
-    /// if let Err(e) = "xc".parse::<u32>() {
-    ///     // Print `e` itself, no need for description().
-    ///     eprintln!("Error: {}", e);
-    /// }
-    /// ```
-    #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_deprecated(since = "1.42.0", reason = "use the Display impl or to_string()")]
-    fn description(&self) -> &str {
-        "description() is deprecated; use Display"
-    }
-
-    #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_deprecated(
-        since = "1.33.0",
-        reason = "replaced by Error::source, which can support downcasting"
-    )]
-    #[allow(missing_docs)]
-    fn cause(&self) -> Option<&dyn Error> {
-        self.source()
-    }
-}
-
-mod private {
-    // This is a hack to prevent `type_id` from being overridden by `Error`
-    // implementations, since that can enable unsound downcasting.
-    #[unstable(feature = "error_type_id", issue = "60784")]
-    #[derive(Debug)]
-    pub struct Internal;
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
-    /// Converts a type of [`Error`] into a box of dyn [`Error`].
-    ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::fmt;
-    /// use std::mem;
-    ///
-    /// #[derive(Debug)]
-    /// struct AnError;
-    ///
-    /// impl fmt::Display for AnError {
-    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f , "An error")
-    ///     }
-    /// }
-    ///
-    /// impl Error for AnError {}
-    ///
-    /// let an_error = AnError;
-    /// assert!(0 == mem::size_of_val(&an_error));
-    /// let a_boxed_error = Box::<dyn Error>::from(an_error);
-    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    fn from(err: E) -> Box<dyn Error + 'a> {
-        Box::new(err)
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
-    /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
-    /// dyn [`Error`] + [`Send`] + [`Sync`].
-    ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::fmt;
-    /// use std::mem;
-    ///
-    /// #[derive(Debug)]
-    /// struct AnError;
-    ///
-    /// impl fmt::Display for AnError {
-    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f , "An error")
-    ///     }
-    /// }
-    ///
-    /// impl Error for AnError {}
-    ///
-    /// unsafe impl Send for AnError {}
-    ///
-    /// unsafe impl Sync for AnError {}
-    ///
-    /// let an_error = AnError;
-    /// assert!(0 == mem::size_of_val(&an_error));
-    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
-    /// assert!(
-    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
-        Box::new(err)
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl From<String> for Box<dyn Error + Send + Sync> {
-    /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
-    ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::mem;
-    ///
-    /// let a_string_error = "a string error".to_string();
-    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
-    /// assert!(
-    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    #[inline]
-    fn from(err: String) -> Box<dyn Error + Send + Sync> {
-        struct StringError(String);
-
-        impl Error for StringError {
-            #[allow(deprecated)]
-            fn description(&self) -> &str {
-                &self.0
-            }
-        }
-
-        impl Display for StringError {
-            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                Display::fmt(&self.0, f)
-            }
-        }
-
-        // Purposefully skip printing "StringError(..)"
-        impl Debug for StringError {
-            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                Debug::fmt(&self.0, f)
-            }
-        }
-
-        Box::new(StringError(err))
-    }
-}
-
-#[stable(feature = "string_box_error", since = "1.6.0")]
-impl From<String> for Box<dyn Error> {
-    /// Converts a [`String`] into a box of dyn [`Error`].
-    ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::mem;
-    ///
-    /// let a_string_error = "a string error".to_string();
-    /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
-    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    fn from(str_err: String) -> Box<dyn Error> {
-        let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
-        let err2: Box<dyn Error> = err1;
-        err2
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
-    /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
-    ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::mem;
-    ///
-    /// let a_str_error = "a str error";
-    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
-    /// assert!(
-    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    #[inline]
-    fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
-        From::from(String::from(err))
-    }
-}
-
-#[stable(feature = "string_box_error", since = "1.6.0")]
-impl From<&str> for Box<dyn Error> {
-    /// Converts a [`str`] into a box of dyn [`Error`].
-    ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::mem;
-    ///
-    /// let a_str_error = "a str error";
-    /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
-    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    fn from(err: &str) -> Box<dyn Error> {
-        From::from(String::from(err))
-    }
-}
-
-#[stable(feature = "cow_box_error", since = "1.22.0")]
-impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
-    /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
-    ///
-    /// [`Cow`]: ../borrow/enum.Cow.html
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::mem;
-    /// use std::borrow::Cow;
-    ///
-    /// let a_cow_str_error = Cow::from("a str error");
-    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
-    /// assert!(
-    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
-        From::from(String::from(err))
-    }
-}
-
-#[stable(feature = "cow_box_error", since = "1.22.0")]
-impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
-    /// Converts a [`Cow`] into a box of dyn [`Error`].
-    ///
-    /// [`Cow`]: ../borrow/enum.Cow.html
-    /// [`Error`]: ../error/trait.Error.html
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::error::Error;
-    /// use std::mem;
-    /// use std::borrow::Cow;
-    ///
-    /// let a_cow_str_error = Cow::from("a str error");
-    /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
-    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
-    /// ```
-    fn from(err: Cow<'a, str>) -> Box<dyn Error> {
-        From::from(String::from(err))
-    }
-}
-
-#[unstable(feature = "never_type", issue = "35121")]
-impl Error for ! {}
-
-#[unstable(
-    feature = "allocator_api",
-    reason = "the precise API and guarantees it provides may be tweaked.",
-    issue = "32838"
-)]
-impl Error for AllocErr {}
-
-#[unstable(
-    feature = "allocator_api",
-    reason = "the precise API and guarantees it provides may be tweaked.",
-    issue = "32838"
-)]
-impl Error for LayoutErr {}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Error for str::ParseBoolError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "failed to parse bool"
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Error for str::Utf8Error {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "invalid utf-8: corrupt contents"
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Error for num::ParseIntError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        self.__description()
-    }
-}
-
-#[stable(feature = "try_from", since = "1.34.0")]
-impl Error for num::TryFromIntError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        self.__description()
-    }
-}
-
-#[stable(feature = "try_from", since = "1.34.0")]
-impl Error for array::TryFromSliceError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        self.__description()
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Error for num::ParseFloatError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        self.__description()
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Error for string::FromUtf8Error {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "invalid utf-8"
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Error for string::FromUtf16Error {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "invalid utf-16"
-    }
-}
-
-#[stable(feature = "str_parse_error2", since = "1.8.0")]
-impl Error for Infallible {
-    fn description(&self) -> &str {
-        match *self {}
-    }
-}
-
-#[stable(feature = "decode_utf16", since = "1.9.0")]
-impl Error for char::DecodeUtf16Error {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "unpaired surrogate found"
-    }
-}
-
-#[stable(feature = "box_error", since = "1.8.0")]
-impl<T: Error> Error for Box<T> {
-    #[allow(deprecated, deprecated_in_future)]
-    fn description(&self) -> &str {
-        Error::description(&**self)
-    }
-
-    #[allow(deprecated)]
-    fn cause(&self) -> Option<&dyn Error> {
-        Error::cause(&**self)
-    }
-
-    fn source(&self) -> Option<&(dyn Error + 'static)> {
-        Error::source(&**self)
-    }
-}
-
-#[stable(feature = "fmt_error", since = "1.11.0")]
-impl Error for fmt::Error {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "an error occurred when formatting an argument"
-    }
-}
-
-#[stable(feature = "try_borrow", since = "1.13.0")]
-impl Error for cell::BorrowError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "already mutably borrowed"
-    }
-}
-
-#[stable(feature = "try_borrow", since = "1.13.0")]
-impl Error for cell::BorrowMutError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "already borrowed"
-    }
-}
-
-#[stable(feature = "try_from", since = "1.34.0")]
-impl Error for char::CharTryFromError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "converted integer out of range for `char`"
-    }
-}
-
-#[stable(feature = "char_from_str", since = "1.20.0")]
-impl Error for char::ParseCharError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        self.__description()
-    }
-}
-
-#[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
-impl Error for alloc::collections::TryReserveError {}
-
-// Copied from `any.rs`.
-impl dyn Error + 'static {
-    /// Returns `true` if the boxed type is the same as `T`
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn is<T: Error + 'static>(&self) -> bool {
-        // Get `TypeId` of the type this function is instantiated with.
-        let t = TypeId::of::<T>();
-
-        // Get `TypeId` of the type in the trait object.
-        let boxed = self.type_id(private::Internal);
-
-        // Compare both `TypeId`s on equality.
-        t == boxed
-    }
-
-    /// Returns some reference to the boxed value if it is of type `T`, or
-    /// `None` if it isn't.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
-        if self.is::<T>() {
-            unsafe { Some(&*(self as *const dyn Error as *const T)) }
-        } else {
-            None
-        }
-    }
-
-    /// Returns some mutable reference to the boxed value if it is of type `T`, or
-    /// `None` if it isn't.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
-        if self.is::<T>() {
-            unsafe { Some(&mut *(self as *mut dyn Error as *mut T)) }
-        } else {
-            None
-        }
-    }
-}
-
-impl dyn Error + 'static + Send {
-    /// Forwards to the method defined on the type `dyn Error`.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn is<T: Error + 'static>(&self) -> bool {
-        <dyn Error + 'static>::is::<T>(self)
-    }
-
-    /// Forwards to the method defined on the type `dyn Error`.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
-        <dyn Error + 'static>::downcast_ref::<T>(self)
-    }
-
-    /// Forwards to the method defined on the type `dyn Error`.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
-        <dyn Error + 'static>::downcast_mut::<T>(self)
-    }
-}
-
-impl dyn Error + 'static + Send + Sync {
-    /// Forwards to the method defined on the type `dyn Error`.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn is<T: Error + 'static>(&self) -> bool {
-        <dyn Error + 'static>::is::<T>(self)
-    }
-
-    /// Forwards to the method defined on the type `dyn Error`.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
-        <dyn Error + 'static>::downcast_ref::<T>(self)
-    }
-
-    /// Forwards to the method defined on the type `dyn Error`.
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    #[inline]
-    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
-        <dyn Error + 'static>::downcast_mut::<T>(self)
-    }
-}
-
-impl dyn Error {
-    #[inline]
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    /// Attempts to downcast the box to a concrete type.
-    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
-        if self.is::<T>() {
-            unsafe {
-                let raw: *mut dyn Error = Box::into_raw(self);
-                Ok(Box::from_raw(raw as *mut T))
-            }
-        } else {
-            Err(self)
-        }
-    }
-
-    /// Returns an iterator starting with the current error and continuing with
-    /// recursively calling [`source`].
-    ///
-    /// If you want to omit the current error and only use its sources,
-    /// use `skip(1)`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(error_iter)]
-    /// use std::error::Error;
-    /// use std::fmt;
-    ///
-    /// #[derive(Debug)]
-    /// struct A;
-    ///
-    /// #[derive(Debug)]
-    /// struct B(Option<Box<dyn Error + 'static>>);
-    ///
-    /// impl fmt::Display for A {
-    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f, "A")
-    ///     }
-    /// }
-    ///
-    /// impl fmt::Display for B {
-    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f, "B")
-    ///     }
-    /// }
-    ///
-    /// impl Error for A {}
-    ///
-    /// impl Error for B {
-    ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
-    ///         self.0.as_ref().map(|e| e.as_ref())
-    ///     }
-    /// }
-    ///
-    /// let b = B(Some(Box::new(A)));
-    ///
-    /// // let err : Box<Error> = b.into(); // or
-    /// let err = &b as &(dyn Error);
-    ///
-    /// let mut iter = err.chain();
-    ///
-    /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
-    /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
-    /// assert!(iter.next().is_none());
-    /// assert!(iter.next().is_none());
-    /// ```
-    ///
-    /// [`source`]: trait.Error.html#method.source
-    #[unstable(feature = "error_iter", issue = "58520")]
-    #[inline]
-    pub fn chain(&self) -> Chain<'_> {
-        Chain { current: Some(self) }
-    }
-}
-
-/// An iterator over an [`Error`] and its sources.
-///
-/// If you want to omit the initial error and only process
-/// its sources, use `skip(1)`.
-///
-/// [`Error`]: trait.Error.html
-#[unstable(feature = "error_iter", issue = "58520")]
-#[derive(Clone, Debug)]
-pub struct Chain<'a> {
-    current: Option<&'a (dyn Error + 'static)>,
-}
-
-#[unstable(feature = "error_iter", issue = "58520")]
-impl<'a> Iterator for Chain<'a> {
-    type Item = &'a (dyn Error + 'static);
-
-    fn next(&mut self) -> Option<Self::Item> {
-        let current = self.current;
-        self.current = self.current.and_then(Error::source);
-        current
-    }
-}
-
-impl dyn Error + Send {
-    #[inline]
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    /// Attempts to downcast the box to a concrete type.
-    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
-        let err: Box<dyn Error> = self;
-        <dyn Error>::downcast(err).map_err(|s| unsafe {
-            // Reapply the `Send` marker.
-            transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
-        })
-    }
-}
-
-impl dyn Error + Send + Sync {
-    #[inline]
-    #[stable(feature = "error_downcast", since = "1.3.0")]
-    /// Attempts to downcast the box to a concrete type.
-    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
-        let err: Box<dyn Error> = self;
-        <dyn Error>::downcast(err).map_err(|s| unsafe {
-            // Reapply the `Send + Sync` marker.
-            transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
-        })
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::Error;
-    use crate::fmt;
-
-    #[derive(Debug, PartialEq)]
-    struct A;
-    #[derive(Debug, PartialEq)]
-    struct B;
-
-    impl fmt::Display for A {
-        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            write!(f, "A")
-        }
-    }
-    impl fmt::Display for B {
-        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            write!(f, "B")
-        }
-    }
-
-    impl Error for A {}
-    impl Error for B {}
-
-    #[test]
-    fn downcasting() {
-        let mut a = A;
-        let a = &mut a as &mut (dyn Error + 'static);
-        assert_eq!(a.downcast_ref::<A>(), Some(&A));
-        assert_eq!(a.downcast_ref::<B>(), None);
-        assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
-        assert_eq!(a.downcast_mut::<B>(), None);
-
-        let a: Box<dyn Error> = Box::new(A);
-        match a.downcast::<B>() {
-            Ok(..) => panic!("expected error"),
-            Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
-        }
-    }
-}