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 description:
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
+
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+   any Contribution intentionally submitted for inclusion in the Work
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+   the terms of any separate license agreement you may have executed
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+6. Trademarks. This License does not grant permission to use the trade
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+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
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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 {