Support Option<$source>

impl/src/expand.rs

@@ -3,7 +3,7 @@ use crate::valid;
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, quote_spanned};
 use syn::spanned::Spanned;
-use syn::{DeriveInput, Member, Result};
+use syn::{DeriveInput, Member, PathArguments, Result, Type};
 
 pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
     let input = Input::from_syn(node)?;
@@ -18,8 +18,14 @@ fn impl_struct(input: Struct) -> TokenStream {
     let ty = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
 
-    let source_method = input.source_member().map(|source| {
+    let source_method = input.source_field().map(|source_field| {
-        let dyn_error = quote_spanned!(source.span()=> self.#source.as_dyn_error());
+        let source = &source_field.member;
+        let asref = if type_is_option(source_field.ty) {
+            Some(quote_spanned!(source.span()=> .as_ref()?))
+        } else {
+            None
+        };
+        let dyn_error = quote_spanned!(source.span()=> self.#source #asref.as_dyn_error());
         quote! {
             fn source(&self) -> std::option::Option<&(dyn std::error::Error + 'static)> {
                 use thiserror::private::AsDynError;
@@ -30,11 +36,20 @@ fn impl_struct(input: Struct) -> TokenStream {
 
     let backtrace_method = input.backtrace_field().map(|backtrace| {
         let backtrace = &backtrace.member;
-        let body = if let Some(source) = input.source_member() {
+        let body = if let Some(source_field) = input.source_field() {
-            let dyn_error = quote_spanned!(source.span()=> self.#source.as_dyn_error());
+            let source = &source_field.member;
+            let source_backtrace = if type_is_option(source_field.ty) {
+                quote_spanned! {source.span()=>
+                    self.#source.as_ref().and_then(|source| source.as_dyn_error().backtrace())
+                }
+            } else {
+                quote_spanned! {source.span()=>
+                    self.#source.as_dyn_error().backtrace()
+                }
+            };
             quote!({
                 use thiserror::private::AsDynError;
-                #dyn_error.backtrace().unwrap_or(&self.#backtrace)
+                #source_backtrace.unwrap_or(&self.#backtrace)
             })
         } else {
             quote! {
@@ -77,9 +92,15 @@ fn impl_enum(input: Enum) -> TokenStream {
     let source_method = if input.has_source() {
         let arms = input.variants.iter().map(|variant| {
             let ident = &variant.ident;
-            match variant.source_member() {
+            match variant.source_field() {
-                Some(source) => {
+                Some(source_field) => {
-                    let dyn_error = quote_spanned!(source.span()=> source.as_dyn_error());
+                    let source = &source_field.member;
+                    let asref = if type_is_option(source_field.ty) {
+                        Some(quote_spanned!(source.span()=> .as_ref()?))
+                    } else {
+                        None
+                    };
+                    let dyn_error = quote_spanned!(source.span()=> source #asref.as_dyn_error());
                     quote! {
                         #ty::#ident {#source: source, ..} => std::option::Option::Some(#dyn_error),
                     }
@@ -104,10 +125,19 @@ fn impl_enum(input: Enum) -> TokenStream {
     let backtrace_method = if input.has_backtrace() {
         let arms = input.variants.iter().map(|variant| {
             let ident = &variant.ident;
-            match (variant.backtrace_field(), variant.source_member()) {
+            match (variant.backtrace_field(), variant.source_field()) {
-                (Some(backtrace), Some(source)) if backtrace.attrs.backtrace.is_none() => {
+                (Some(backtrace), Some(source_field)) if backtrace.attrs.backtrace.is_none() => {
                     let backtrace = &backtrace.member;
-                    let dyn_error = quote_spanned!(source.span()=> source.as_dyn_error());
+                    let source = &source_field.member;
+                    let source_backtrace = if type_is_option(source_field.ty) {
+                        quote_spanned! {source.span()=>
+                            source.as_ref().and_then(|source| source.as_dyn_error().backtrace())
+                        }
+                    } else {
+                        quote_spanned! {source.span()=>
+                            source.as_dyn_error().backtrace()
+                        }
+                    };
                     quote! {
                         #ty::#ident {
                             #backtrace: backtrace,
@@ -115,7 +145,7 @@ fn impl_enum(input: Enum) -> TokenStream {
                             ..
                         } => std::option::Option::Some({
                             use thiserror::private::AsDynError;
-                            #dyn_error.backtrace().unwrap_or(backtrace)
+                            #source_backtrace.unwrap_or(backtrace)
                         }),
                     }
                 }
@@ -192,3 +222,20 @@ fn fields_pat(fields: &[Field]) -> TokenStream {
         None => quote!({}),
     }
 }
+
+fn type_is_option(ty: &Type) -> bool {
+    let path = match ty {
+        Type::Path(ty) => &ty.path,
+        _ => return false,
+    };
+
+    let last = path.segments.last().unwrap();
+    if last.ident != "Option" {
+        return false;
+    }
+
+    match &last.arguments {
+        PathArguments::AngleBracketed(bracketed) => bracketed.args.len() == 1,
+        _ => false,
+    }
+}

impl/src/prop.rs

@@ -2,8 +2,8 @@ use crate::ast::{Enum, Field, Struct, Variant};
 use syn::{Member, Type};
 
 impl Struct<'_> {
-    pub(crate) fn source_member(&self) -> Option<&Member> {
+    pub(crate) fn source_field(&self) -> Option<&Field> {
-        source_member(&self.fields)
+        source_field(&self.fields)
     }
 
     pub(crate) fn backtrace_field(&self) -> Option<&Field> {
@@ -15,7 +15,7 @@ impl Enum<'_> {
     pub(crate) fn has_source(&self) -> bool {
         self.variants
             .iter()
-            .any(|variant| variant.source_member().is_some())
+            .any(|variant| variant.source_field().is_some())
     }
 
     pub(crate) fn has_backtrace(&self) -> bool {
@@ -34,8 +34,8 @@ impl Enum<'_> {
 }
 
 impl Variant<'_> {
-    pub(crate) fn source_member(&self) -> Option<&Member> {
+    pub(crate) fn source_field(&self) -> Option<&Field> {
-        source_member(&self.fields)
+        source_field(&self.fields)
     }
 
     pub(crate) fn backtrace_field(&self) -> Option<&Field> {
@@ -43,15 +43,15 @@ impl Variant<'_> {
     }
 }
 
-fn source_member<'a>(fields: &'a [Field]) -> Option<&'a Member> {
+fn source_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
     for field in fields {
         if field.attrs.source.is_some() {
-            return Some(&field.member);
+            return Some(&field);
         }
     }
     for field in fields {
         match &field.member {
-            Member::Named(ident) if ident == "source" => return Some(&field.member),
+            Member::Named(ident) if ident == "source" => return Some(&field),
             _ => {}
         }
     }