Merge pull request #28 from dtolnay/backtrace

Prefer the source's backtrace if it has one

impl/src/attr.rs

@@ -10,6 +10,7 @@ use syn::{
 pub struct Attrs<'a> {
     pub display: Option<Display<'a>>,
     pub source: Option<Source<'a>>,
+    pub backtrace: Option<Backtrace<'a>>,
 }
 
 #[derive(Clone)]
@@ -24,10 +25,15 @@ pub struct Source<'a> {
     pub original: &'a Attribute,
 }
 
+pub struct Backtrace<'a> {
+    pub original: &'a Attribute,
+}
+
 pub fn get(input: &[Attribute]) -> Result<Attrs> {
     let mut attrs = Attrs {
         display: None,
         source: None,
+        backtrace: None,
     };
 
     for attr in input {
@@ -46,6 +52,12 @@ pub fn get(input: &[Attribute]) -> Result<Attrs> {
                 return Err(Error::new_spanned(attr, "duplicate #[source] attribute"));
             }
             attrs.source = Some(source);
+        } else if attr.path.is_ident("backtrace") {
+            let backtrace = parse_backtrace(attr)?;
+            if attrs.backtrace.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[backtrace] attribute"));
+            }
+            attrs.backtrace = Some(backtrace);
         }
     }
 
@@ -118,6 +130,11 @@ fn parse_source(attr: &Attribute) -> Result<Source> {
     Ok(Source { original: attr })
 }
 
+fn parse_backtrace(attr: &Attribute) -> Result<Backtrace> {
+    syn::parse2::<Nothing>(attr.tokens.clone())?;
+    Ok(Backtrace { original: attr })
+}
+
 impl ToTokens for Display<'_> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let fmt = &self.fmt;

impl/src/expand.rs

@@ -28,10 +28,22 @@ fn impl_struct(input: Struct) -> TokenStream {
         }
     });
 
-    let backtrace_method = input.backtrace_member().map(|backtrace| {
+    let backtrace_method = input.backtrace_field().map(|backtrace| {
+        let backtrace = &backtrace.member;
+        let body = if let Some(source) = input.source_member() {
+            let dyn_error = quote_spanned!(source.span()=> self.#source.as_dyn_error());
+            quote!({
+                use thiserror::private::AsDynError;
+                #dyn_error.backtrace().unwrap_or(&self.#backtrace)
+            })
+        } else {
+            quote! {
+                &self.#backtrace
+            }
+        };
         quote! {
             fn backtrace(&self) -> std::option::Option<&std::backtrace::Backtrace> {
-                std::option::Option::Some(&self.#backtrace)
+                std::option::Option::Some(#body)
             }
         }
     });
@@ -67,10 +79,9 @@ fn impl_enum(input: Enum) -> TokenStream {
             let ident = &variant.ident;
             match variant.source_member() {
                 Some(source) => {
-                    let var = quote_spanned!(source.span()=> source);
-                    let dyn_error = quote_spanned!(source.span()=> #var.as_dyn_error());
+                    let dyn_error = quote_spanned!(source.span()=> source.as_dyn_error());
                     quote! {
-                        #ty::#ident {#source: #var, ..} => std::option::Option::Some(#dyn_error),
+                        #ty::#ident {#source: source, ..} => std::option::Option::Some(#dyn_error),
                     }
                 }
                 None => quote! {
@@ -93,11 +104,28 @@ 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_member() {
-                Some(backtrace) => quote! {
-                    #ty::#ident {#backtrace: backtrace, ..} => std::option::Option::Some(backtrace),
-                },
-                None => quote! {
+            match (variant.backtrace_field(), variant.source_member()) {
+                (Some(backtrace), Some(source)) if backtrace.attrs.backtrace.is_none() => {
+                    let backtrace = &backtrace.member;
+                    let dyn_error = quote_spanned!(source.span()=> source.as_dyn_error());
+                    quote! {
+                        #ty::#ident {
+                            #backtrace: backtrace,
+                            #source: source,
+                            ..
+                        } => std::option::Option::Some({
+                            use thiserror::private::AsDynError;
+                            #dyn_error.backtrace().unwrap_or(backtrace)
+                        }),
+                    }
+                }
+                (Some(backtrace), _) => {
+                    let backtrace = &backtrace.member;
+                    quote! {
+                        #ty::#ident {#backtrace: backtrace, ..} => std::option::Option::Some(backtrace),
+                    }
+                }
+                (None, _) => quote! {
                     #ty::#ident {..} => std::option::Option::None,
                 },
             }

impl/src/lib.rs

@@ -10,7 +10,7 @@ mod valid;
 use proc_macro::TokenStream;
 use syn::{parse_macro_input, DeriveInput};
 
-#[proc_macro_derive(Error, attributes(error, source))]
+#[proc_macro_derive(Error, attributes(backtrace, error, source))]
 pub fn derive_error(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
     expand::derive(&input)

impl/src/prop.rs

@@ -6,8 +6,8 @@ impl Struct<'_> {
         source_member(&self.fields)
     }
 
-    pub(crate) fn backtrace_member(&self) -> Option<&Member> {
-        backtrace_member(&self.fields)
+    pub(crate) fn backtrace_field(&self) -> Option<&Field> {
+        backtrace_field(&self.fields)
     }
 }
 
@@ -21,7 +21,7 @@ impl Enum<'_> {
     pub(crate) fn has_backtrace(&self) -> bool {
         self.variants
             .iter()
-            .any(|variant| variant.backtrace_member().is_some())
+            .any(|variant| variant.backtrace_field().is_some())
     }
 
     pub(crate) fn has_display(&self) -> bool {
@@ -38,14 +38,8 @@ impl Variant<'_> {
         source_member(&self.fields)
     }
 
-    pub(crate) fn backtrace_member(&self) -> Option<&Member> {
-        backtrace_member(&self.fields)
-    }
-}
-
-impl Field<'_> {
-    fn is_backtrace(&self) -> bool {
-        type_is_backtrace(self.ty)
+    pub(crate) fn backtrace_field(&self) -> Option<&Field> {
+        backtrace_field(&self.fields)
     }
 }
 
@@ -64,11 +58,18 @@ fn source_member<'a>(fields: &'a [Field]) -> Option<&'a Member> {
     None
 }
 
-fn backtrace_member<'a>(fields: &'a [Field]) -> Option<&'a Member> {
-    fields
-        .iter()
-        .find(|field| field.is_backtrace())
-        .map(|field| &field.member)
+fn backtrace_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.backtrace.is_some() {
+            return Some(&field);
+        }
+    }
+    for field in fields {
+        if type_is_backtrace(field.ty) {
+            return Some(&field);
+        }
+    }
+    None
 }
 
 fn type_is_backtrace(ty: &Type) -> bool {

impl/src/valid.rs

@@ -15,8 +15,8 @@ impl Input<'_> {
 
 impl Struct<'_> {
     fn validate(&self) -> Result<()> {
-        check_no_source(&self.attrs)?;
-        find_duplicate_source(&self.fields)?;
+        check_no_source_or_backtrace(&self.attrs)?;
+        check_no_duplicate_source_or_backtrace(&self.fields)?;
         for field in &self.fields {
             field.validate()?;
         }
@@ -26,7 +26,7 @@ impl Struct<'_> {
 
 impl Enum<'_> {
     fn validate(&self) -> Result<()> {
-        check_no_source(&self.attrs)?;
+        check_no_source_or_backtrace(&self.attrs)?;
         let has_display = self.has_display();
         for variant in &self.variants {
             variant.validate()?;
@@ -43,8 +43,8 @@ impl Enum<'_> {
 
 impl Variant<'_> {
     fn validate(&self) -> Result<()> {
-        check_no_source(&self.attrs)?;
-        find_duplicate_source(&self.fields)?;
+        check_no_source_or_backtrace(&self.attrs)?;
+        check_no_duplicate_source_or_backtrace(&self.fields)?;
         for field in &self.fields {
             field.validate()?;
         }
@@ -64,18 +64,25 @@ impl Field<'_> {
     }
 }
 
-fn check_no_source(attrs: &Attrs) -> Result<()> {
+fn check_no_source_or_backtrace(attrs: &Attrs) -> Result<()> {
     if let Some(source) = &attrs.source {
         return Err(Error::new_spanned(
             source.original,
             "not expected here; the #[source] attribute belongs on a specific field",
         ));
     }
+    if let Some(backtrace) = &attrs.backtrace {
+        return Err(Error::new_spanned(
+            backtrace.original,
+            "not expected here; the #[backtrace] attribute belongs on a specific field",
+        ));
+    }
     Ok(())
 }
 
-fn find_duplicate_source(fields: &[Field]) -> Result<()> {
+fn check_no_duplicate_source_or_backtrace(fields: &[Field]) -> Result<()> {
     let mut has_source = false;
+    let mut has_backtrace = false;
     for field in fields {
         if let Some(source) = &field.attrs.source {
             if has_source {
@@ -86,6 +93,15 @@ fn find_duplicate_source(fields: &[Field]) -> Result<()> {
             }
             has_source = true;
         }
+        if let Some(backtrace) = &field.attrs.backtrace {
+            if has_backtrace {
+                return Err(Error::new_spanned(
+                    backtrace.original,
+                    "duplicate #[backtrace] attribute",
+                ));
+            }
+            has_backtrace = true;
+        }
     }
     Ok(())
 }