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// Tagged dispatch mechanism for resolving the behavior of `anyhow!($expr)`.
//
// When anyhow! is given a single expr argument to turn into anyhow::Error, we
// want the resulting Error to pick up the input's implementation of source()
// and backtrace() if it has a std::error::Error impl, otherwise require nothing
// more than Display and Debug.
//
// Expressed in terms of specialization, we want something like:
//
// trait AnyhowNew {
// fn new(self) -> Error;
// }
//
// impl<T> AnyhowNew for T
// where
// T: Display + Debug + Send + Sync + 'static,
// {
// default fn new(self) -> Error {
// /* no std error impl */
// }
// }
//
// impl<T> AnyhowNew for T
// where
// T: std::error::Error + Send + Sync + 'static,
// {
// fn new(self) -> Error {
// /* use std error's source() and backtrace() */
// }
// }
//
// Since specialization is not stable yet, instead we rely on autoref behavior
// of method resolution to perform tagged dispatch. Here we have two traits
// AdhocKind and TraitKind that both have an anyhow_kind() method. AdhocKind is
// implemented whether or not the caller's type has a std error impl, while
// TraitKind is implemented only when a std error impl does exist. The ambiguity
// is resolved by AdhocKind requiring an extra autoref so that it has lower
// precedence.
//
// The anyhow! macro will set up the call in this form:
//
// #[allow(unused_imports)]
// use $crate::__private::{AdhocKind, TraitKind};
// let error = $msg;
// (&error).anyhow_kind().new(error)
use crate::Error;
use core::fmt::{Debug, Display};
#[cfg(feature = "std")]
use crate::StdError;
pub struct Adhoc;
#[doc(hidden)]
pub trait AdhocKind: Sized {
#[inline]
fn anyhow_kind(&self) -> Adhoc {
Adhoc
}
}
impl<T> AdhocKind for &T where T: ?Sized + Display + Debug + Send + Sync + 'static {}
impl Adhoc {
#[cold]
pub fn new<M>(self, message: M) -> Error
where
M: Display + Debug + Send + Sync + 'static,
{
Error::from_adhoc(message, backtrace!())
}
}
pub struct Trait;
#[doc(hidden)]
pub trait TraitKind: Sized {
#[inline]
fn anyhow_kind(&self) -> Trait {
Trait
}
}
impl<E> TraitKind for E where E: Into<Error> {}
impl Trait {
#[cold]
pub fn new<E>(self, error: E) -> Error
where
E: Into<Error>,
{
error.into()
}
}
#[cfg(feature = "std")]
pub struct Boxed;
#[cfg(feature = "std")]
#[doc(hidden)]
pub trait BoxedKind: Sized {
#[inline]
fn anyhow_kind(&self) -> Boxed {
Boxed
}
}
#[cfg(feature = "std")]
impl BoxedKind for Box<dyn StdError + Send + Sync> {}
#[cfg(feature = "std")]
impl Boxed {
#[cold]
pub fn new(self, error: Box<dyn StdError + Send + Sync>) -> Error {
let backtrace = backtrace_if_absent!(&*error);
Error::from_boxed(error, backtrace)
}
}
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