Repository: shepmaster/snafu (rust)

Issue:
Breaking changing 0.7.2
I had an error roughly defined as such:

```
#[derive(Debug, Snafu)]
enum Error {
    Variant1 {
        #[snafu(source(from(String, Into::into)))]
        source: Box<dyn std::error::Error + Send + Sync>,
    },

    // ...
}

fn return_error() -> Result<(), String> {
    Err("nope!".to_string())
}

return_error().context(Variant1Snafu)?;
```

In 0.7.1 this was acceptable. In 0.7.2, it is now saying that `String` does not implement `std::error::Error` which is a requirement for `AsErrorSource`. I believe the derive macro is no longer performing the type conversion before attempting to construct the context.

Pull request description:
Uniformly use the transformed error source in the error constructor
Previously, we only converted the source in the source field initializer. However, `GenerateImplicitData::generate_with_source` also needs access to the post-converted value.

Fixes #364


Patch:
diff --git a/snafu-derive/src/lib.rs b/snafu-derive/src/lib.rs
index d12733a..f89e965 100644
--- a/snafu-derive/src/lib.rs
+++ b/snafu-derive/src/lib.rs
@@ -194,15 +194,28 @@ impl SourceField {
 }
 
 enum Transformation {
-    None { ty: syn::Type },
-    Transform { ty: syn::Type, expr: syn::Expr },
+    None {
+        ty: syn::Type,
+    },
+    Transform {
+        source_ty: syn::Type,
+        target_ty: syn::Type,
+        expr: syn::Expr,
+    },
 }
 
 impl Transformation {
-    fn ty(&self) -> &syn::Type {
+    fn source_ty(&self) -> &syn::Type {
+        match self {
+            Transformation::None { ty } => ty,
+            Transformation::Transform { source_ty, .. } => source_ty,
+        }
+    }
+
+    fn target_ty(&self) -> &syn::Type {
         match self {
             Transformation::None { ty } => ty,
-            Transformation::Transform { ty, .. } => ty,
+            Transformation::Transform { target_ty, .. } => target_ty,
         }
     }
 
@@ -936,7 +949,11 @@ fn field_container(
                 name, ty, provide, ..
             } = field;
             let transformation = maybe_transformation
-                .map(|(ty, expr)| Transformation::Transform { ty, expr })
+                .map(|(source_ty, expr)| Transformation::Transform {
+                    source_ty,
+                    target_ty: ty.clone(),
+                    expr,
+                })
                 .unwrap_or_else(|| Transformation::None { ty });
 
             source_fields.add(
@@ -1237,12 +1254,15 @@ fn parse_snafu_tuple_struct(
         return Err(vec![one_field_error(span)]);
     }
 
+    let ty = inner.into_value().ty;
     let (maybe_transformation, errs) = transformations.finish();
     let transformation = maybe_transformation
-        .map(|(ty, expr)| Transformation::Transform { ty, expr })
-        .unwrap_or_else(|| Transformation::None {
-            ty: inner.into_value().ty,
-        });
+        .map(|(source_ty, expr)| Transformation::Transform {
+            source_ty,
+            target_ty: ty.clone(),
+            expr,
+        })
+        .unwrap_or_else(|| Transformation::None { ty });
     errors.extend(errs);
 
     let (maybe_crate_root, errs) = crate_roots.finish();
@@ -1878,7 +1898,7 @@ impl TupleStructInfo {
             provides,
         } = self;
 
-        let inner_type = transformation.ty();
+        let inner_type = transformation.source_ty();
         let transformation = transformation.transformation();
 
         let where_clauses: Vec<_> = generics
diff --git a/snafu-derive/src/shared.rs b/snafu-derive/src/shared.rs
index 8580ea6..7d7c342 100644
--- a/snafu-derive/src/shared.rs
+++ b/snafu-derive/src/shared.rs
@@ -314,12 +314,20 @@ pub mod context_selector {
                 self.construct_implicit_fields()
             };
 
-            let (source_ty, transfer_source_field) = match source_field {
+            let (source_ty, transform_source, transfer_source_field) = match source_field {
                 Some(source_field) => {
-                    let (ty, transfer) = build_source_info(source_field);
-                    (quote! { #ty }, transfer)
+                    let SourceInfo {
+                        source_field_type,
+                        transform_source,
+                        transfer_source_field,
+                    } = build_source_info(source_field);
+                    (
+                        quote! { #source_field_type },
+                        Some(transform_source),
+                        Some(transfer_source_field),
+                    )
                 }
-                None => (quote! { #crate_root::NoneError }, quote! {}),
+                None => (quote! { #crate_root::NoneError }, None, None),
             };
 
             let track_caller = track_caller();
@@ -334,6 +342,7 @@ pub mod context_selector {
 
                     #track_caller
                     fn into_error(self, error: Self::Source) -> #parameterized_error_name {
+                        #transform_source;
                         #error_constructor_name {
                             #construct_implicit_fields
                             #transfer_source_field
@@ -360,7 +369,7 @@ pub mod context_selector {
 
             let (source_ty, transfer_source_field, empty_source_field) = match source_field {
                 Some(f) => {
-                    let source_field_type = f.transformation.ty();
+                    let source_field_type = f.transformation.source_ty();
                     let source_field_name = &f.name;
                     let source_transformation = f.transformation.transformation();
 
@@ -411,7 +420,11 @@ pub mod context_selector {
             let user_field_generics = self.user_field_generics();
             let where_clauses = self.where_clauses;
 
-            let (source_field_type, transfer_source_field) = build_source_info(source_field);
+            let SourceInfo {
+                source_field_type,
+                transform_source,
+                transfer_source_field,
+            } = build_source_info(source_field);
 
             let track_caller = track_caller();
 
@@ -422,6 +435,7 @@ pub mod context_selector {
                 {
                     #track_caller
                     fn from(error: #source_field_type) -> Self {
+                        #transform_source;
                         #error_constructor_name {
                             #construct_implicit_fields_with_source
                             #transfer_source_field
@@ -432,16 +446,28 @@ pub mod context_selector {
         }
     }
 
+    struct SourceInfo<'a> {
+        source_field_type: &'a syn::Type,
+        transform_source: TokenStream,
+        transfer_source_field: TokenStream,
+    }
+
     // Assumes that the error is in a variable called "error"
-    fn build_source_info(source_field: &crate::SourceField) -> (&syn::Type, TokenStream) {
+    fn build_source_info(source_field: &crate::SourceField) -> SourceInfo<'_> {
         let source_field_name = source_field.name();
-        let source_field_type = source_field.transformation.ty();
+        let source_field_type = source_field.transformation.source_ty();
+        let target_field_type = source_field.transformation.target_ty();
         let source_transformation = source_field.transformation.transformation();
 
-        (
+        let transform_source =
+            quote! { let error: #target_field_type = (#source_transformation)(error) };
+        let transfer_source_field = quote! { #source_field_name: error, };
+
+        SourceInfo {
             source_field_type,
-            quote! { #source_field_name: (#source_transformation)(error), },
-        )
+            transform_source,
+            transfer_source_field,
+        }
     }
 
     fn track_caller() -> proc_macro2::TokenStream {
@@ -750,7 +776,8 @@ pub mod error {
                 .source_field()
                 .filter(|f| f.provide);
 
-            let source_provide_ref = provided_source.map(|f| (f.transformation.ty(), f.name()));
+            let source_provide_ref =
+                provided_source.map(|f| (f.transformation.source_ty(), f.name()));
 
             let provide_refs = provide_refs.chain(source_provide_ref);
 


Test patch:
diff --git a/tests/implicit.rs b/tests/implicit.rs
index 41ed519..4c7737b 100644
--- a/tests/implicit.rs
+++ b/tests/implicit.rs
@@ -157,3 +157,34 @@ mod with_and_without_source {
         assert_eq!(e.data.0, ItWas::GenerateWithSource);
     }
 }
+
+mod converted_sources {
+    use snafu::{prelude::*, IntoError};
+
+    #[derive(Debug)]
+    struct ImplicitData;
+
+    impl snafu::GenerateImplicitData for ImplicitData {
+        fn generate() -> Self {
+            Self
+        }
+    }
+
+    #[derive(Debug, Snafu)]
+    struct HasSource {
+        backtrace: snafu::Backtrace,
+
+        #[snafu(implicit)]
+        data: ImplicitData,
+
+        #[snafu(source(from(String, Into::into)))]
+        source: Box<dyn std::error::Error>,
+    }
+
+    #[test]
+    fn receives_the_error_after_conversion() {
+        let e = HasSourceSnafu.into_error(String::from("bad"));
+        // Mostly testing that this compiles; assertion is bonus
+        assert_eq!(e.source.to_string(), "bad");
+    }
+}