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 | |
47 | use crate::Error; |
48 | use core::fmt::{Debug, Display}; |
49 | |
50 | #[cfg (feature = "std" )] |
51 | use crate::StdError; |
52 | #[cfg (feature = "std" )] |
53 | use alloc::boxed::Box; |
54 | |
55 | pub struct Adhoc; |
56 | |
57 | #[doc (hidden)] |
58 | pub trait AdhocKind: Sized { |
59 | #[inline ] |
60 | fn anyhow_kind(&self) -> Adhoc { |
61 | Adhoc |
62 | } |
63 | } |
64 | |
65 | impl<T> AdhocKind for &T where T: ?Sized + Display + Debug + Send + Sync + 'static {} |
66 | |
67 | impl Adhoc { |
68 | #[cold ] |
69 | pub fn new<M>(self, message: M) -> Error |
70 | where |
71 | M: Display + Debug + Send + Sync + 'static, |
72 | { |
73 | Error::from_adhoc(message, backtrace!()) |
74 | } |
75 | } |
76 | |
77 | pub struct Trait; |
78 | |
79 | #[doc (hidden)] |
80 | pub trait TraitKind: Sized { |
81 | #[inline ] |
82 | fn anyhow_kind(&self) -> Trait { |
83 | Trait |
84 | } |
85 | } |
86 | |
87 | impl<E> TraitKind for E where E: Into<Error> {} |
88 | |
89 | impl 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 (feature = "std" )] |
100 | pub struct Boxed; |
101 | |
102 | #[cfg (feature = "std" )] |
103 | #[doc (hidden)] |
104 | pub trait BoxedKind: Sized { |
105 | #[inline ] |
106 | fn anyhow_kind(&self) -> Boxed { |
107 | Boxed |
108 | } |
109 | } |
110 | |
111 | #[cfg (feature = "std" )] |
112 | impl BoxedKind for Box<dyn StdError + Send + Sync> {} |
113 | |
114 | #[cfg (feature = "std" )] |
115 | impl 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::from_boxed(error, backtrace) |
120 | } |
121 | } |
122 | |