Fix up PR 137 for merge

README.md

@@ -137,6 +137,20 @@ pub enum DataStoreError {
   }
   ```
 
+- If a field is both a source (named `source`, or has `#[source]` or `#[from]`
+  attribute) *and* is marked `#[backtrace]`, then the Error trait's
+  `backtrace()` method is forwarded to the source's backtrace.
+
+  ```rust
+  #[derive(Error, Debug)]
+  pub enum MyError {
+      Io {
+          #[backtrace]
+          source: io::Error,
+      },
+  }
+  ```
+
 - Errors may use `error(transparent)` to forward the source and Display methods
   straight through to an underlying error without adding an additional message.
   This would be appropriate for enums that need an "anything else" variant.

impl/src/expand.rs

@@ -58,7 +58,9 @@ fn impl_struct(input: Struct) -> TokenStream {
                     self.#source.as_dyn_error().backtrace()
                 }
             };
-            let combinator = if type_is_option(backtrace_field.ty) {
+            let combinator = if source == backtrace {
+                source_backtrace
+            } else if type_is_option(backtrace_field.ty) {
                 quote! {
                     #source_backtrace.or(self.#backtrace.as_ref())
                 }
@@ -128,7 +130,7 @@ fn impl_struct(input: Struct) -> TokenStream {
     });
 
     let from_impl = input.from_field().map(|from_field| {
-        let backtrace_field = input.backtrace_field();
+        let backtrace_field = input.distinct_backtrace_field();
         let from = from_field.ty;
         let body = from_initializer(from_field, backtrace_field);
         quote! {
@@ -238,6 +240,27 @@ fn impl_enum(input: Enum) -> TokenStream {
                         }
                     }
                 }
+                (Some(backtrace_field), Some(source_field))
+                    if backtrace_field.member == source_field.member =>
+                {
+                    let backtrace = &backtrace_field.member;
+                    let varsource = quote!(source);
+                    let source_backtrace = if type_is_option(source_field.ty) {
+                        quote_spanned! {backtrace.span()=>
+                            #varsource.as_ref().and_then(|source| source.as_dyn_error().backtrace())
+                        }
+                    } else {
+                        quote_spanned! {backtrace.span()=>
+                            #varsource.as_dyn_error().backtrace()
+                        }
+                    };
+                    quote! {
+                        #ty::#ident {#backtrace: #varsource, ..} => {
+                            use thiserror::private::AsDynError;
+                            #source_backtrace
+                        }
+                    }
+                }
                 (Some(backtrace_field), _) => {
                     let backtrace = &backtrace_field.member;
                     let body = if type_is_option(backtrace_field.ty) {
@@ -326,7 +349,7 @@ fn impl_enum(input: Enum) -> TokenStream {
 
     let from_impls = input.variants.iter().filter_map(|variant| {
         let from_field = variant.from_field()?;
-        let backtrace_field = variant.backtrace_field();
+        let backtrace_field = variant.distinct_backtrace_field();
         let variant = &variant.ident;
         let from = from_field.ty;
         let body = from_initializer(from_field, backtrace_field);

impl/src/prop.rs

@@ -13,6 +13,11 @@ impl Struct<'_> {
     pub(crate) fn backtrace_field(&self) -> Option<&Field> {
         backtrace_field(&self.fields)
     }
+
+    pub(crate) fn distinct_backtrace_field(&self) -> Option<&Field> {
+        let backtrace_field = self.backtrace_field()?;
+        distinct_backtrace_field(backtrace_field, self.from_field())
+    }
 }
 
 impl Enum<'_> {
@@ -54,6 +59,11 @@ impl Variant<'_> {
     pub(crate) fn backtrace_field(&self) -> Option<&Field> {
         backtrace_field(&self.fields)
     }
+
+    pub(crate) fn distinct_backtrace_field(&self) -> Option<&Field> {
+        let backtrace_field = self.backtrace_field()?;
+        distinct_backtrace_field(backtrace_field, self.from_field())
+    }
 }
 
 impl Field<'_> {
@@ -100,6 +110,20 @@ fn backtrace_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
     None
 }
 
+// The #[backtrace] field, if it is not the same as the #[from] field.
+fn distinct_backtrace_field<'a, 'b>(
+    backtrace_field: &'a Field<'b>,
+    from_field: Option<&Field>,
+) -> Option<&'a Field<'b>> {
+    if from_field.map_or(false, |from_field| {
+        from_field.member == backtrace_field.member
+    }) {
+        None
+    } else {
+        Some(backtrace_field)
+    }
+}
+
 fn type_is_backtrace(ty: &Type) -> bool {
     let path = match ty {
         Type::Path(ty) => &ty.path,

src/lib.rs

@@ -161,6 +161,22 @@
 //!   # };
 //!   ```
 //!
+//! - If a field is both a source (named `source`, or has `#[source]` or
+//!   `#[from]` attribute) *and* is marked `#[backtrace]`, then the Error
+//!   trait's `backtrace()` method is forwarded to the source's backtrace.
+//!
+//!   ```rust
+//!   # const IGNORE: &str = stringify! {
+//!   #[derive(Error, Debug)]
+//!   pub enum MyError {
+//!       Io {
+//!           #[backtrace]
+//!           source: io::Error,
+//!       },
+//!   }
+//!   # };
+//!   ```
+//!
 //! - Errors may use `error(transparent)` to forward the source and Display
 //!   methods straight through to an underlying error without adding an
 //!   additional message. This would be appropriate for enums that need an

tests/test_backtrace.rs

@@ -10,9 +10,16 @@ use thiserror::Error;
 #[error("...")]
 pub struct Inner;
 
+#[cfg(thiserror_nightly_testing)]
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct InnerBacktrace {
+    backtrace: std::backtrace::Backtrace,
+}
+
 #[cfg(thiserror_nightly_testing)]
 pub mod structs {
-    use super::Inner;
+    use super::{Inner, InnerBacktrace};
     use std::backtrace::Backtrace;
     use std::error::Error;
     use std::sync::Arc;
@@ -54,6 +61,14 @@ pub mod structs {
         backtrace: Backtrace,
     }
 
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct CombinedBacktraceFrom {
+        #[from]
+        #[backtrace]
+        source: InnerBacktrace,
+    }
+
     #[derive(Error, Debug)]
     #[error("...")]
     pub struct OptBacktraceFrom {
@@ -97,6 +112,11 @@ pub mod structs {
         let error = BacktraceFrom::from(Inner);
         assert!(error.backtrace().is_some());
 
+        let error = CombinedBacktraceFrom::from(InnerBacktrace {
+            backtrace: Backtrace::capture(),
+        });
+        assert!(error.backtrace().is_some());
+
         let error = OptBacktraceFrom::from(Inner);
         assert!(error.backtrace().is_some());
 
@@ -107,7 +127,7 @@ pub mod structs {
 
 #[cfg(thiserror_nightly_testing)]
 pub mod enums {
-    use super::Inner;
+    use super::{Inner, InnerBacktrace};
     use std::backtrace::Backtrace;
     use std::error::Error;
     use std::sync::Arc;
@@ -157,6 +177,16 @@ pub mod enums {
         },
     }
 
+    #[derive(Error, Debug)]
+    pub enum CombinedBacktraceFrom {
+        #[error("...")]
+        Test {
+            #[from]
+            #[backtrace]
+            source: InnerBacktrace,
+        },
+    }
+
     #[derive(Error, Debug)]
     pub enum OptBacktraceFrom {
         #[error("...")]
@@ -204,6 +234,11 @@ pub mod enums {
         let error = BacktraceFrom::from(Inner);
         assert!(error.backtrace().is_some());
 
+        let error = CombinedBacktraceFrom::from(InnerBacktrace {
+            backtrace: Backtrace::capture(),
+        });
+        assert!(error.backtrace().is_some());
+
         let error = OptBacktraceFrom::from(Inner);
         assert!(error.backtrace().is_some());