Support backtrace in From impl

impl/src/expand.rs

@@ -90,12 +90,13 @@ fn impl_struct(input: Struct) -> TokenStream {
     });
 
     let from_impl = input.from_field().map(|from_field| {
-        let member = &from_field.member;
+        let backtrace_field = input.backtrace_field();
         let from = from_field.ty;
+        let body = from_initializer(from_field, backtrace_field);
         quote! {
             impl #impl_generics std::convert::From<#from> for #ty #ty_generics #where_clause {
                 fn from(source: #from) -> Self {
-                    #ty { #member: source }
+                    #ty #body
                 }
             }
         }
@@ -243,13 +244,14 @@ 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 variant = &variant.ident;
-        let member = &from_field.member;
         let from = from_field.ty;
+        let body = from_initializer(from_field, backtrace_field);
         Some(quote! {
             impl #impl_generics std::convert::From<#from> for #ty #ty_generics #where_clause {
                 fn from(source: #from) -> Self {
-                    #ty::#variant { #member: source }
+                    #ty::#variant #body
                 }
             }
         })
@@ -280,6 +282,26 @@ fn fields_pat(fields: &[Field]) -> TokenStream {
     }
 }
 
+fn from_initializer(from_field: &Field, backtrace_field: Option<&Field>) -> TokenStream {
+    let from_member = &from_field.member;
+    let backtrace = backtrace_field.map(|backtrace_field| {
+        let backtrace_member = &backtrace_field.member;
+        if type_is_option(backtrace_field.ty) {
+            quote! {
+                #backtrace_member: std::option::Option::Some(std::backtrace::Backtrace::capture()),
+            }
+        } else {
+            quote! {
+                #backtrace_member: std::backtrace::Backtrace::capture(),
+            }
+        }
+    });
+    quote!({
+        #from_member: source,
+        #backtrace
+    })
+}
+
 fn type_is_option(ty: &Type) -> bool {
     let path = match ty {
         Type::Path(ty) => &ty.path,

impl/src/prop.rs

@@ -51,6 +51,12 @@ impl Variant<'_> {
     }
 }
 
+impl Field<'_> {
+    pub(crate) fn is_backtrace(&self) -> bool {
+        type_is_backtrace(self.ty)
+    }
+}
+
 fn from_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
     for field in fields {
         if field.attrs.from.is_some() {
@@ -82,7 +88,7 @@ fn backtrace_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
         }
     }
     for field in fields {
-        if type_is_backtrace(field.ty) {
+        if field.is_backtrace() {
             return Some(&field);
         }
     }

impl/src/valid.rs

@@ -104,6 +104,7 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
     let mut from_field = None;
     let mut source_field = None;
     let mut backtrace_field = None;
+    let mut has_backtrace = false;
     for field in fields {
         if let Some(from) = field.attrs.from {
             if from_field.is_some() {
@@ -125,7 +126,9 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
                 ));
             }
             backtrace_field = Some(field);
+            has_backtrace = true;
         }
+        has_backtrace |= field.is_backtrace();
     }
     if let (Some(from_field), Some(source_field)) = (from_field, source_field) {
         if !same_member(from_field, source_field) {
@@ -136,10 +139,10 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
         }
     }
     if let Some(from_field) = from_field {
-        if fields.len() > 1 {
+        if fields.len() > 1 + has_backtrace as usize {
             return Err(Error::new_spanned(
                 from_field.attrs.from,
-                "deriving From requires no fields other than source",
+                "deriving From requires no fields other than source and backtrace",
             ));
         }
     }