1// Tagged dispatch mechanism for resolving the behavior of `anyhow!($expr)`.
2//
3// When anyhow! is given a single expr argument to turn into anyhow::Error, we
4// want the resulting Error to pick up the input's implementation of source()
5// and backtrace() if it has a std::error::Error impl, otherwise require nothing
6// more than Display and Debug.
7//
8// Expressed in terms of specialization, we want something like:
9//
10// trait AnyhowNew {
11// fn new(self) -> Error;
12// }
13//
14// impl<T> AnyhowNew for T
15// where
16// T: Display + Debug + Send + Sync + 'static,
17// {
18// default fn new(self) -> Error {
19// /* no std error impl */
20// }
21// }
22//
23// impl<T> AnyhowNew for T
24// where
25// T: std::error::Error + Send + Sync + 'static,
26// {
27// fn new(self) -> Error {
28// /* use std error's source() and backtrace() */
29// }
30// }
31//
32// Since specialization is not stable yet, instead we rely on autoref behavior
33// of method resolution to perform tagged dispatch. Here we have two traits
34// AdhocKind and TraitKind that both have an anyhow_kind() method. AdhocKind is
35// implemented whether or not the caller's type has a std error impl, while
36// TraitKind is implemented only when a std error impl does exist. The ambiguity
37// is resolved by AdhocKind requiring an extra autoref so that it has lower
38// precedence.
39//
40// The anyhow! macro will set up the call in this form:
41//
42// #[allow(unused_imports)]
43// use $crate::__private::{AdhocKind, TraitKind};
44// let error = $msg;
45// (&error).anyhow_kind().new(error)
46
47use crate::Error;
48use core::fmt::{Debug, Display};
49
50#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
51use crate::StdError;
52#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
53use alloc::boxed::Box;
54
55pub struct Adhoc;
56
57#[doc(hidden)]
58pub trait AdhocKind: Sized {
59 #[inline]
60 fn anyhow_kind(&self) -> Adhoc {
61 Adhoc
62 }
63}
64
65impl<T> AdhocKind for &T where T: ?Sized + Display + Debug + Send + Sync + 'static {}
66
67impl Adhoc {
68 #[cold]
69 pub fn new<M>(self, message: M) -> Error
70 where
71 M: Display + Debug + Send + Sync + 'static,
72 {
73 Error::construct_from_adhoc(message, backtrace!())
74 }
75}
76
77pub struct Trait;
78
79#[doc(hidden)]
80pub trait TraitKind: Sized {
81 #[inline]
82 fn anyhow_kind(&self) -> Trait {
83 Trait
84 }
85}
86
87impl<E> TraitKind for E where E: Into<Error> {}
88
89impl Trait {
90 #[cold]
91 pub fn new<E>(self, error: E) -> Error
92 where
93 E: Into<Error>,
94 {
95 error.into()
96 }
97}
98
99#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
100pub struct Boxed;
101
102#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
103#[doc(hidden)]
104pub trait BoxedKind: Sized {
105 #[inline]
106 fn anyhow_kind(&self) -> Boxed {
107 Boxed
108 }
109}
110
111#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
112impl BoxedKind for Box<dyn StdError + Send + Sync> {}
113
114#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
115impl Boxed {
116 #[cold]
117 pub fn new(self, error: Box<dyn StdError + Send + Sync>) -> Error {
118 let backtrace: Option = backtrace_if_absent!(&*error);
119 Error::construct_from_boxed(error, backtrace)
120 }
121}
122