#[backtrace] attribute for explicitly selecting the backtrace field

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/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

@@ -43,12 +43,6 @@ impl Variant<'_> {
     }
 }
 
-impl Field<'_> {
-    fn is_backtrace(&self) -> bool {
-        type_is_backtrace(self.ty)
-    }
-}
-
 fn source_member<'a>(fields: &'a [Field]) -> Option<&'a Member> {
     for field in fields {
         if field.attrs.source.is_some() {
@@ -65,10 +59,17 @@ fn source_member<'a>(fields: &'a [Field]) -> Option<&'a Member> {
 }
 
 fn backtrace_member<'a>(fields: &'a [Field]) -> Option<&'a Member> {
-    fields
-        .iter()
-        .find(|field| field.is_backtrace())
-        .map(|field| &field.member)
+    for field in fields {
+        if field.attrs.backtrace.is_some() {
+            return Some(&field.member);
+        }
+    }
+    for field in fields {
+        if type_is_backtrace(field.ty) {
+            return Some(&field.member);
+        }
+    }
+    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(())
 }