Repository: JelteF/derive_more (rust)

Issue:
Support trait objects for source in Error
Currently you can't just write
```rust
#[derive(Debug, Display, Error)]
pub enum MyError {
    #[display(fmt = "something failed")]
    Failed {
        #[error(source)]
        source: Box<dyn Error + Send + 'static>,
    },
   ...
}
```

This is what compiler says:
```
100 | #[derive(Debug, Display, Error)]
    |                          ^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `(dyn std::error::Error + std::marker::Send + 'static)`
```

BTW `thiserror` supports this pattern.

Pull request:
Support trait objects for source in Error
If you used boxed `dyn Error` traits as a source a compile error would
occur:
```
 --> tests/error/derives_for_enums_with_source.rs:5:17
  |
5 | #[derive(Debug, Error)]
  |                 ^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `(dyn std::error::Error + Send + 'static)`
  = help: the trait `std::error::Error` is implemented for `Box<T>`
  = note: required for `Box<(dyn std::error::Error + Send + 'static)>` to implement `std::error::Error`
  = note: required for the cast from `Box<(dyn std::error::Error + Send + 'static)>` to the object type `dyn std::error::Error`
  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)
```

This fixes that by shamelessly "borrowing" a fix from [`thiserror`].

[`thiserror`]: https://github.com/dtolnay/thiserror

Resolves #122

## Checklist

- ~[ ] Documentation is updated (if required)~
- [x] Tests are added/updated (if required)
- [x] [CHANGELOG entry][l:1] is added (if required)




[l:1]: /CHANGELOG.md


Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 554f3ed..1026715 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -61,6 +61,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   to work correctly.
 - Use `Provider` API for backtraces in `Error` derive.
 - Fix `Error` derive not working with `const` generics.
+- Support trait objects for source in Error, e.g.
+  `Box<dyn Error + Send + 'static>`
 
 ## 0.99.10 - 2020-09-11
 
diff --git a/Cargo.toml b/Cargo.toml
index e17fc54..c65cb13 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -50,7 +50,7 @@ debug = ["derive_more-impl/debug"]
 deref = ["derive_more-impl/deref"]
 deref_mut = ["derive_more-impl/deref_mut"]
 display = ["derive_more-impl/display"]
-error = ["derive_more-impl/error"]
+error = ["derive_more-impl/error", "std"]
 from = ["derive_more-impl/from"]
 from_str = ["derive_more-impl/from_str"]
 index = ["derive_more-impl/index"]
diff --git a/impl/Cargo.toml b/impl/Cargo.toml
index cf15027..b905452 100644
--- a/impl/Cargo.toml
+++ b/impl/Cargo.toml
@@ -33,7 +33,7 @@ unicode-xid = { version = "0.2.2", optional = true }
 rustc_version = { version = "0.4", optional = true }
 
 [dev-dependencies]
-derive_more = { path = "..", features = ["add", "debug", "from_str", "not", "try_into", "try_unwrap"] }
+derive_more = { path = "..", features = ["add", "debug", "error", "from_str", "not", "try_into", "try_unwrap"] }
 itertools = "0.11.0"
 
 [badges]
diff --git a/impl/src/error.rs b/impl/src/error.rs
index 3db73bb..3fd10ea 100644
--- a/impl/src/error.rs
+++ b/impl/src/error.rs
@@ -40,6 +40,7 @@ pub fn expand(
     let source = source.map(|source| {
         quote! {
             fn source(&self) -> Option<&(dyn ::std::error::Error + 'static)> {
+                use ::derive_more::__private::AsDynError;
                 #source
             }
         }
@@ -190,7 +191,7 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
     fn render_source_as_struct(&self) -> Option<TokenStream> {
         let source = self.source?;
         let ident = &self.data.members[source];
-        Some(render_some(quote! { &#ident }))
+        Some(render_some(quote! { #ident }))
     }
 
     fn render_source_as_enum_variant_match_arm(&self) -> Option<TokenStream> {
@@ -268,7 +269,7 @@ fn render_some<T>(expr: T) -> TokenStream
 where
     T: quote::ToTokens,
 {
-    quote! { Some(#expr as &(dyn ::std::error::Error + 'static)) }
+    quote! { Some(#expr.as_dyn_error()) }
 }
 
 fn parse_fields<'input, 'state>(
diff --git a/src/lib.rs b/src/lib.rs
index 7e10f6b..911b544 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -70,6 +70,16 @@ mod r#str;
 #[cfg(feature = "from_str")]
 pub use self::r#str::FromStrError;
 
+#[cfg(feature = "error")]
+mod vendor;
+
+// Not public API.
+#[doc(hidden)]
+#[cfg(feature = "error")]
+pub mod __private {
+    pub use crate::vendor::thiserror::aserror::AsDynError;
+}
+
 #[cfg(not(any(
     feature = "full",
     feature = "add_assign",
diff --git a/src/vendor/mod.rs b/src/vendor/mod.rs
new file mode 100644
index 0000000..a174757
--- /dev/null
+++ b/src/vendor/mod.rs
@@ -0,0 +1,2 @@
+#[cfg(feature = "error")]
+pub mod thiserror;
diff --git a/src/vendor/thiserror/LICENSE-APACHE b/src/vendor/thiserror/LICENSE-APACHE
new file mode 100644
index 0000000..1b5ec8b
--- /dev/null
+++ b/src/vendor/thiserror/LICENSE-APACHE
@@ -0,0 +1,176 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
+
+   "License" shall mean the terms and conditions for use, reproduction,
+   and distribution as defined by Sections 1 through 9 of this document.
+
+   "Licensor" shall mean the copyright owner or entity authorized by
+   the copyright owner that is granting the License.
+
+   "Legal Entity" shall mean the union of the acting entity and all
+   other entities that control, are controlled by, or are under common
+   control with that entity. For the purposes of this definition,
+   "control" means (i) the power, direct or indirect, to cause the
+   direction or management of such entity, whether by contract or
+   otherwise, or (ii) ownership of fifty percent (50%) or more of the
+   outstanding shares, or (iii) beneficial ownership of such entity.
+
+   "You" (or "Your") shall mean an individual or Legal Entity
+   exercising permissions granted by this License.
+
+   "Source" form shall mean the preferred form for making modifications,
+   including but not limited to software source code, documentation
+   source, and configuration files.
+
+   "Object" form shall mean any form resulting from mechanical
+   transformation or translation of a Source form, including but
+   not limited to compiled object code, generated documentation,
+   and conversions to other media types.
+
+   "Work" shall mean the work of authorship, whether in Source or
+   Object form, made available under the License, as indicated by a
+   copyright notice that is included in or attached to the work
+   (an example is provided in the Appendix below).
+
+   "Derivative Works" shall mean any work, whether in Source or Object
+   form, that is based on (or derived from) the Work and for which the
+   editorial revisions, annotations, elaborations, or other modifications
+   represent, as a whole, an original work of authorship. For the purposes
+   of this License, Derivative Works shall not include works that remain
+   separable from, or merely link (or bind by name) to the interfaces of,
+   the Work and Derivative Works thereof.
+
+   "Contribution" shall mean any work of authorship, including
+   the original version of the Work and any modifications or additions
+   to that Work or Derivative Works thereof, that is intentionally
+   submitted to Licensor for inclusion in the Work by the copyright owner
+   or by an individual or Legal Entity authorized to submit on behalf of
+   the copyright owner. For the purposes of this definition, "submitted"
+   means any form of electronic, verbal, or written communication sent
+   to the Licensor or its representatives, including but not limited to
+   communication on electronic mailing lists, source code control systems,
+   and issue tracking systems that are managed by, or on behalf of, the
+   Licensor for the purpose of discussing and improving the Work, but
+   excluding communication that is conspicuously marked or otherwise
+   designated in writing by the copyright owner as "Not a Contribution."
+
+   "Contributor" shall mean Licensor and any individual or Legal Entity
+   on behalf of whom a Contribution has been received by Licensor and
+   subsequently incorporated within the Work.
+
+2. Grant of Copyright License. Subject to the terms and conditions of
+   this License, each Contributor hereby grants to You a perpetual,
+   worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+   copyright license to reproduce, prepare Derivative Works of,
+   publicly display, publicly perform, sublicense, and distribute the
+   Work and such Derivative Works in Source or Object form.
+
+3. Grant of Patent License. Subject to the terms and conditions of
+   this License, each Contributor hereby grants to You a perpetual,
+   worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+   (except as stated in this section) patent license to make, have made,
+   use, offer to sell, sell, import, and otherwise transfer the Work,
+   where such license applies only to those patent claims licensable
+   by such Contributor that are necessarily infringed by their
+   Contribution(s) alone or by combination of their Contribution(s)
+   with the Work to which such Contribution(s) was submitted. If You
+   institute patent litigation against any entity (including a
+   cross-claim or counterclaim in a lawsuit) alleging that the Work
+   or a Contribution incorporated within the Work constitutes direct
+   or contributory patent infringement, then any patent licenses
+   granted to You under this License for that Work shall terminate
+   as of the date such litigation is filed.
+
+4. Redistribution. You may reproduce and distribute copies of the
+   Work or Derivative Works thereof in any medium, with or without
+   modifications, and in Source or Object form, provided that You
+   meet the following conditions:
+
+   (a) You must give any other recipients of the Work or
+       Derivative Works a copy of this License; and
+
+   (b) You must cause any modified files to carry prominent notices
+       stating that You changed the files; and
+
+   (c) You must retain, in the Source form of any Derivative Works
+       that You distribute, all copyright, patent, trademark, and
+       attribution notices from the Source form of the Work,
+       excluding those notices that do not pertain to any part of
+       the Derivative Works; and
+
+   (d) If the Work includes a "NOTICE" text file as part of its
+       distribution, then any Derivative Works that You distribute must
+       include a readable copy of the attribution notices contained
+       within such NOTICE file, excluding those notices that do not
+       pertain to any part of the Derivative Works, in at least one
+       of the following places: within a NOTICE text file distributed
+       as part of the Derivative Works; within the Source form or
+       documentation, if provided along with the Derivative Works; or,
+       within a display generated by the Derivative Works, if and
+       wherever such third-party notices normally appear. The contents
+       of the NOTICE file are for informational purposes only and
+       do not modify the License. You may add Your own attribution
+       notices within Derivative Works that You distribute, alongside
+       or as an addendum to the NOTICE text from the Work, provided
+       that such additional attribution notices cannot be construed
+       as modifying the License.
+
+   You may add Your own copyright statement to Your modifications and
+   may provide additional or different license terms and conditions
+   for use, reproduction, or distribution of Your modifications, or
+   for any such Derivative Works as a whole, provided Your use,
+   reproduction, and distribution of the Work otherwise complies with
+   the conditions stated in this License.
+
+5. Submission of Contributions. Unless You explicitly state otherwise,
+   any Contribution intentionally submitted for inclusion in the Work
+   by You to the Licensor shall be under the terms and conditions of
+   this License, without any additional terms or conditions.
+   Notwithstanding the above, nothing herein shall supersede or modify
+   the terms of any separate license agreement you may have executed
+   with Licensor regarding such Contributions.
+
+6. Trademarks. This License does not grant permission to use the trade
+   names, trademarks, service marks, or product names of the Licensor,
+   except as required for reasonable and customary use in describing the
+   origin of the Work and reproducing the content of the NOTICE file.
+
+7. Disclaimer of Warranty. Unless required by applicable law or
+   agreed to in writing, Licensor provides the Work (and each
+   Contributor provides its Contributions) on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+   implied, including, without limitation, any warranties or conditions
+   of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+   PARTICULAR PURPOSE. You are solely responsible for determining the
+   appropriateness of using or redistributing the Work and assume any
+   risks associated with Your exercise of permissions under this License.
+
+8. Limitation of Liability. In no event and under no legal theory,
+   whether in tort (including negligence), contract, or otherwise,
+   unless required by applicable law (such as deliberate and grossly
+   negligent acts) or agreed to in writing, shall any Contributor be
+   liable to You for damages, including any direct, indirect, special,
+   incidental, or consequential damages of any character arising as a
+   result of this License or out of the use or inability to use the
+   Work (including but not limited to damages for loss of goodwill,
+   work stoppage, computer failure or malfunction, or any and all
+   other commercial damages or losses), even if such Contributor
+   has been advised of the possibility of such damages.
+
+9. Accepting Warranty or Additional Liability. While redistributing
+   the Work or Derivative Works thereof, You may choose to offer,
+   and charge a fee for, acceptance of support, warranty, indemnity,
+   or other liability obligations and/or rights consistent with this
+   License. However, in accepting such obligations, You may act only
+   on Your own behalf and on Your sole responsibility, not on behalf
+   of any other Contributor, and only if You agree to indemnify,
+   defend, and hold each Contributor harmless for any liability
+   incurred by, or claims asserted against, such Contributor by reason
+   of your accepting any such warranty or additional liability.
+
+END OF TERMS AND CONDITIONS
diff --git a/src/vendor/thiserror/LICENSE-MIT b/src/vendor/thiserror/LICENSE-MIT
new file mode 100644
index 0000000..31aa793
--- /dev/null
+++ b/src/vendor/thiserror/LICENSE-MIT
@@ -0,0 +1,23 @@
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/src/vendor/thiserror/README.md b/src/vendor/thiserror/README.md
new file mode 100644
index 0000000..0c7f124
--- /dev/null
+++ b/src/vendor/thiserror/README.md
@@ -0,0 +1,6 @@
+# Vendored files from `thiserror`
+
+These are vendored files from the [`thiserror`] crate. The license files in this
+directory only apply to the files in this subdirectory of `derive_more`.
+
+[`thiserror`]: https://github.com/dtolnay/thiserror
diff --git a/src/vendor/thiserror/aserror.rs b/src/vendor/thiserror/aserror.rs
new file mode 100644
index 0000000..5fea84e
--- /dev/null
+++ b/src/vendor/thiserror/aserror.rs
@@ -0,0 +1,48 @@
+use std::error::Error;
+use std::panic::UnwindSafe;
+
+pub trait AsDynError<'a>: Sealed {
+    fn as_dyn_error(&self) -> &(dyn Error + 'a);
+}
+
+impl<'a, T: Error + 'a> AsDynError<'a> for T {
+    #[inline]
+    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
+        self
+    }
+}
+
+impl<'a> AsDynError<'a> for dyn Error + 'a {
+    #[inline]
+    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
+        self
+    }
+}
+
+impl<'a> AsDynError<'a> for dyn Error + Send + 'a {
+    #[inline]
+    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
+        self
+    }
+}
+
+impl<'a> AsDynError<'a> for dyn Error + Send + Sync + 'a {
+    #[inline]
+    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
+        self
+    }
+}
+
+impl<'a> AsDynError<'a> for dyn Error + Send + Sync + UnwindSafe + 'a {
+    #[inline]
+    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
+        self
+    }
+}
+
+pub trait Sealed {}
+impl<'a, T: Error + 'a> Sealed for T {}
+impl<'a> Sealed for dyn Error + 'a {}
+impl<'a> Sealed for dyn Error + Send + 'a {}
+impl<'a> Sealed for dyn Error + Send + Sync + 'a {}
+impl<'a> Sealed for dyn Error + Send + Sync + UnwindSafe + 'a {}
diff --git a/src/vendor/thiserror/mod.rs b/src/vendor/thiserror/mod.rs
new file mode 100644
index 0000000..a6add8b
--- /dev/null
+++ b/src/vendor/thiserror/mod.rs
@@ -0,0 +1,1 @@
+pub mod aserror;


Test patch:
diff --git a/tests/error/derives_for_enums_with_source.rs b/tests/error/derives_for_enums_with_source.rs
index 9a732fa..f1867fa 100644
--- a/tests/error/derives_for_enums_with_source.rs
+++ b/tests/error/derives_for_enums_with_source.rs
@@ -12,6 +12,10 @@ enum TestErr {
         source: SimpleErr,
         field: i32,
     },
+    NamedImplicitBoxedSource {
+        source: Box<dyn Error + Send + 'static>,
+        field: i32,
+    },
     NamedExplicitNoSource {
         #[error(not(source))]
         source: SimpleErr,
@@ -94,6 +98,17 @@ fn named_implicit_source() {
     assert!(err.source().unwrap().is::<SimpleErr>());
 }
 
+#[test]
+fn named_implicit_boxed_source() {
+    let err = TestErr::NamedImplicitBoxedSource {
+        source: Box::new(SimpleErr),
+        field: 0,
+    };
+
+    assert!(err.source().is_some());
+    assert!(err.source().unwrap().is::<SimpleErr>());
+}
+
 #[test]
 fn named_explicit_no_source() {
     let err = TestErr::NamedExplicitNoSource {