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Add downcasting to std::error::Error
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This commit brings the `Error` trait in line with the [Error interoperation
RFC](rust-lang/rfcs#201) by adding downcasting,
which has long been intended. This change means that for any `Error`
trait objects that are `'static`, you can downcast to concrete error
types.

To make this work, it is necessary for `Error` to inherit from
`Reflect` (which is currently used to mark concrete types as "permitted
for reflection, aka downcasting"). This is a breaking change: it means
that impls like

```rust
impl<T> Error for MyErrorType<T> { ... }
```

must change to something like

```rust
impl<T: Reflect> Error for MyErrorType<T> { ... }
```

except that `Reflect` is currently unstable (and should remain so for
the time being). For now, code can instead bound by `Any`:

```rust
impl<T: Any> Error for MyErrorType<T> { ... }
```

which *is* stable and has `Reflect` as a super trait. The downside is
that this imposes a `'static` constraint, but that only
constrains *when* `Error` is implemented -- it does not actually
constrain the types that can implement `Error`.

[breaking-change]
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aturon committed Apr 30, 2015
1 parent 2214860 commit a010bf6
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Showing 7 changed files with 150 additions and 20 deletions.
9 changes: 6 additions & 3 deletions src/liballoc/boxed.rs
Original file line number Diff line number Diff line change
Expand Up @@ -257,11 +257,14 @@ impl Box<Any> {
}
}

impl Box<Any+Send> {
impl Box<Any + Send> {
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
<Box<Any>>::downcast(self)
pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
<Box<Any>>::downcast(self).map_err(|s| unsafe {
// reapply the Send marker
mem::transmute::<Box<Any>, Box<Any + Send>>(s)
})
}
}

Expand Down
6 changes: 2 additions & 4 deletions src/libcore/any.rs
Original file line number Diff line number Diff line change
Expand Up @@ -97,9 +97,7 @@ pub trait Any: Reflect + 'static {
fn get_type_id(&self) -> TypeId;
}

impl<T> Any for T
where T: Reflect + 'static
{
impl<T: Reflect + 'static> Any for T {
fn get_type_id(&self) -> TypeId { TypeId::of::<T>() }
}

Expand Down Expand Up @@ -222,7 +220,7 @@ impl TypeId {
/// Returns the `TypeId` of the type this generic function has been
/// instantiated with
#[stable(feature = "rust1", since = "1.0.0")]
pub fn of<T: ?Sized + Any>() -> TypeId {
pub fn of<T: ?Sized + Reflect + 'static>() -> TypeId {
TypeId {
t: unsafe { intrinsics::type_id::<T>() },
}
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3 changes: 2 additions & 1 deletion src/libcore/marker.rs
Original file line number Diff line number Diff line change
Expand Up @@ -462,6 +462,8 @@ mod impls {
#[unstable(feature = "core", reason = "requires RFC and more experience")]
#[allow(deprecated)]
#[cfg(not(stage0))]
#[rustc_on_unimplemented = "`{Self}` does not implement `Any`; \
ensure all type parameters are bounded by `Any`"]
pub trait Reflect {}

/// dox
Expand All @@ -471,4 +473,3 @@ pub trait Reflect {}
pub trait Reflect: MarkerTrait {}

impl Reflect for .. { }

136 changes: 131 additions & 5 deletions src/libstd/error.rs
Original file line number Diff line number Diff line change
Expand Up @@ -47,19 +47,22 @@
// coherence challenge (e.g., specialization, neg impls, etc) we can
// reconsider what crate these items belong in.

use boxed::Box;
use any::TypeId;
use boxed::{self, Box};
use convert::From;
use fmt::{self, Debug, Display};
use marker::{Send, Sync};
use marker::{Send, Sync, Reflect};
use mem::transmute;
use num;
use option::Option;
use option::Option::None;
use option::Option::{self, Some, None};
use result::Result::{self, Ok, Err};
use raw::TraitObject;
use str;
use string::{self, String};

/// Base functionality for all errors in Rust.
#[stable(feature = "rust1", since = "1.0.0")]
pub trait Error: Debug + Display {
pub trait Error: Debug + Display + Reflect {
/// A short description of the error.
///
/// The description should not contain newlines or sentence-ending
Expand All @@ -71,6 +74,14 @@ pub trait Error: Debug + Display {
/// The lower-level cause of this error, if any.
#[stable(feature = "rust1", since = "1.0.0")]
fn cause(&self) -> Option<&Error> { None }

/// Get the `TypeId` of `self`
#[doc(hidden)]
#[unstable(feature = "core",
reason = "unclear whether to commit to this public implementation detail")]
fn type_id(&self) -> TypeId where Self: 'static {
TypeId::of::<Self>()
}
}

#[stable(feature = "rust1", since = "1.0.0")]
Expand Down Expand Up @@ -154,3 +165,118 @@ impl Error for string::FromUtf16Error {
}
}

// copied from any.rs
impl Error + 'static {
/// Returns true if the boxed type is the same as `T`
#[unstable(feature = "error_downcast", reason = "recently added")]
#[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();

// Compare both TypeIds on equality
t == boxed
}

/// Returns some reference to the boxed value if it is of type `T`, or
/// `None` if it isn't.
#[unstable(feature = "error_downcast", reason = "recently added")]
#[inline]
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
if self.is::<T>() {
unsafe {
// Get the raw representation of the trait object
let to: TraitObject = transmute(self);

// Extract the data pointer
Some(transmute(to.data))
}
} else {
None
}
}

/// Returns some mutable reference to the boxed value if it is of type `T`, or
/// `None` if it isn't.
#[unstable(feature = "error_downcast", reason = "recently added")]
#[inline]
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
if self.is::<T>() {
unsafe {
// Get the raw representation of the trait object
let to: TraitObject = transmute(self);

// Extract the data pointer
Some(transmute(to.data))
}
} else {
None
}
}
}

impl Error + 'static + Send {
/// Forwards to the method defined on the type `Any`.
#[unstable(feature = "error_downcast", reason = "recently added")]
#[inline]
pub fn is<T: Error + 'static>(&self) -> bool {
<Error + 'static>::is::<T>(self)
}

/// Forwards to the method defined on the type `Any`.
#[unstable(feature = "error_downcast", reason = "recently added")]
#[inline]
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
<Error + 'static>::downcast_ref::<T>(self)
}

/// Forwards to the method defined on the type `Any`.
#[unstable(feature = "error_downcast", reason = "recently added")]
#[inline]
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
<Error + 'static>::downcast_mut::<T>(self)
}
}

#[unstable(feature = "error_downcast", reason = "recently added")]
/// An extension trait for downcasting from boxed Errors
pub trait BoxErrorExt {
#[unstable(feature = "error_downcast", reason = "recently added")]
/// Converts a boxed error to a concrete type if it is of type `T`, or
/// yields the original box if it isn't.
fn downcast<T: Error>(self) -> Result<Box<T>, Self>;
}

impl BoxErrorExt for Box<Error> {
#[inline]
#[unstable(feature = "error_downcast", reason = "recently added")]
fn downcast<T: Error + 'static>(self) -> Result<Box<T>, Box<Error>> {
if self.is::<T>() {
unsafe {
// Get the raw representation of the trait object
let raw = boxed::into_raw(self);
let to: TraitObject =
transmute::<*mut Error, TraitObject>(raw);

// Extract the data pointer
Ok(Box::from_raw(to.data as *mut T))
}
} else {
Err(self)
}
}
}

impl BoxErrorExt for Box<Error + Send> {
#[inline]
#[unstable(feature = "error_downcast", reason = "recently added")]
fn downcast<T: Error + 'static>(self) -> Result<Box<T>, Box<Error + Send>> {
<Box<Error>>::downcast(self).map_err(|s| unsafe {
// reapply the Send marker
transmute::<Box<Error>, Box<Error + Send>>(s)
})
}
}
3 changes: 2 additions & 1 deletion src/libstd/io/buffered.rs
Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@
use prelude::v1::*;
use io::prelude::*;

use marker::Reflect;
use cmp;
use error;
use fmt;
Expand Down Expand Up @@ -323,7 +324,7 @@ impl<W> From<IntoInnerError<W>> for Error {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<W: Send + fmt::Debug> error::Error for IntoInnerError<W> {
impl<W: Reflect + Send + fmt::Debug> error::Error for IntoInnerError<W> {
fn description(&self) -> &str {
error::Error::description(self.error())
}
Expand Down
6 changes: 3 additions & 3 deletions src/libstd/sync/mpsc/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -272,6 +272,7 @@ use error;
use fmt;
use mem;
use cell::UnsafeCell;
use marker::Reflect;

pub use self::select::{Select, Handle};
use self::select::StartResult;
Expand Down Expand Up @@ -958,8 +959,7 @@ impl<T> fmt::Display for SendError<T> {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Send> error::Error for SendError<T> {

impl<T: Send + Reflect> error::Error for SendError<T> {
fn description(&self) -> &str {
"sending on a closed channel"
}
Expand Down Expand Up @@ -994,7 +994,7 @@ impl<T> fmt::Display for TrySendError<T> {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Send> error::Error for TrySendError<T> {
impl<T: Send + Reflect> error::Error for TrySendError<T> {

fn description(&self) -> &str {
match *self {
Expand Down
7 changes: 4 additions & 3 deletions src/libstd/sys/common/poison.rs
Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@

use prelude::v1::*;

use marker::Reflect;
use cell::UnsafeCell;
use error::{Error};
use fmt;
Expand Down Expand Up @@ -109,7 +110,7 @@ impl<T> fmt::Display for PoisonError<T> {
}
}

impl<T: Send> Error for PoisonError<T> {
impl<T: Send + Reflect> Error for PoisonError<T> {
fn description(&self) -> &str {
"poisoned lock: another task failed inside"
}
Expand Down Expand Up @@ -155,13 +156,13 @@ impl<T> fmt::Debug for TryLockError<T> {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Send> fmt::Display for TryLockError<T> {
impl<T: Send + Reflect> fmt::Display for TryLockError<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.description().fmt(f)
}
}

impl<T: Send> Error for TryLockError<T> {
impl<T: Send + Reflect> Error for TryLockError<T> {
fn description(&self) -> &str {
match *self {
TryLockError::Poisoned(ref p) => p.description(),
Expand Down

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