Infer default source/backtrace fields

CHANGELOG.md

@@ -6,6 +6,17 @@ All notable changes to this project will be documented in this file.
 ### Added
 - _Nothing yet._
 
+## [0.5.8] - 2025-09-30
+
+### Changed
+- `masterror::Error` now infers sources named `source` and backtrace fields of
+  type `std::backtrace::Backtrace`/`Option<std::backtrace::Backtrace>` even
+  without explicit attributes, matching `thiserror`'s ergonomics.
+
+### Tests
+- Expanded derive tests to cover implicit `source`/`backtrace` detection across
+  structs and enums.
+
 ## [0.5.7] - 2025-09-29
 
 ### Added

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "masterror"
-version = "0.5.7"
+version = "0.5.8"
 rust-version = "1.90"
 edition = "2024"
 license = "MIT OR Apache-2.0"
@@ -49,7 +49,7 @@ turnkey = []
 openapi = ["dep:utoipa"]
 
 [workspace.dependencies]
-masterror-derive = { version = "0.1.3", path = "masterror-derive" }
+masterror-derive = { version = "0.1.4", path = "masterror-derive" }
 masterror-template = { version = "0.1.2", path = "masterror-template" }
 
 [dependencies]

README.md

@@ -29,9 +29,9 @@ Stable categories, conservative HTTP mapping, no `unsafe`.
 
 ~~~toml
 [dependencies]
-masterror = { version = "0.5.7", default-features = false }
+masterror = { version = "0.5.8", default-features = false }
 # or with features:
-# masterror = { version = "0.5.7", features = [
+# masterror = { version = "0.5.8", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -66,10 +66,10 @@ masterror = { version = "0.5.7", default-features = false }
 ~~~toml
 [dependencies]
 # lean core
-masterror = { version = "0.5.7", default-features = false }
+masterror = { version = "0.5.8", default-features = false }
 
 # with Axum/Actix + JSON + integrations
-# masterror = { version = "0.5.7", features = [
+# masterror = { version = "0.5.8", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -342,13 +342,13 @@ assert_eq!(resp.status, 401);
 Minimal core:
 
 ~~~toml
-masterror = { version = "0.5.7", default-features = false }
+masterror = { version = "0.5.8", default-features = false }
 ~~~
 
 API (Axum + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.5.7", features = [
+masterror = { version = "0.5.8", features = [
   "axum", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }
@@ -357,7 +357,7 @@ masterror = { version = "0.5.7", features = [
 API (Actix + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.5.7", features = [
+masterror = { version = "0.5.8", features = [
   "actix", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }

masterror-derive/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "masterror-derive"
 rust-version = "1.90"
-version = "0.1.3"
+version = "0.1.4"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/RAprogramm/masterror"

masterror-derive/src/error_trait.rs

@@ -82,7 +82,7 @@ fn struct_source_body(fields: &Fields, display: &DisplaySpec) -> TokenStream {
             }
         }
         DisplaySpec::Template(_) => {
-            if let Some(field) = fields.iter().find(|field| field.attrs.source.is_some()) {
+            if let Some(field) = fields.iter().find(|field| field.attrs.has_source()) {
                 let member = &field.member;
                 field_source_expr(quote!(self.#member), quote!(&self.#member), &field.ty)
             } else {
@@ -135,10 +135,7 @@ fn variant_transparent_source(variant: &VariantData) -> TokenStream {
 
 fn variant_template_source(variant: &VariantData) -> TokenStream {
     let variant_ident = &variant.ident;
-    let source_field = variant
-        .fields
-        .iter()
-        .find(|field| field.attrs.source.is_some());
+    let source_field = variant.fields.iter().find(|field| field.attrs.has_source());
 
     match (&variant.fields, source_field) {
         (Fields::Unit, _) => quote! { Self::#variant_ident => None },

masterror-derive/src/from_impl.rs

@@ -127,11 +127,11 @@ fn field_value_expr(field: &Field, from_field: &Field) -> Result<TokenStream, Er
         return Ok(quote! { value });
     }
 
-    if field.attrs.backtrace.is_some() {
+    if field.attrs.has_backtrace() {
         return Ok(backtrace_initializer(field));
     }
 
-    if field.attrs.source.is_some() && field.attrs.from.is_none() {
+    if field.attrs.has_source() && field.attrs.from.is_none() {
         return source_initializer(field);
     }
 

masterror-derive/src/input.rs

@@ -96,7 +96,7 @@ impl Fields {
     }
 
     pub fn backtrace_field(&self) -> Option<&Field> {
-        self.iter().find(|field| field.attrs.backtrace.is_some())
+        self.iter().find(|field| field.attrs.has_backtrace())
     }
 }
 
@@ -134,7 +134,7 @@ impl Field {
             None => syn::Member::Unnamed(syn::Index::from(index))
         };
 
-        let attrs = FieldAttrs::from_attrs(&field.attrs, errors);
+        let attrs = FieldAttrs::from_attrs(&field.attrs, ident.as_ref(), &field.ty, errors);
 
         Self {
             ident,
@@ -149,13 +149,20 @@ impl Field {
 
 #[derive(Debug, Default)]
 pub struct FieldAttrs {
-    pub from:      Option<Attribute>,
-    pub source:    Option<Attribute>,
-    pub backtrace: Option<Attribute>
+    pub from:           Option<Attribute>,
+    pub source:         Option<Attribute>,
+    pub backtrace:      Option<Attribute>,
+    inferred_source:    bool,
+    inferred_backtrace: bool
 }
 
 impl FieldAttrs {
-    fn from_attrs(attrs: &[Attribute], errors: &mut Vec<Error>) -> Self {
+    fn from_attrs(
+        attrs: &[Attribute],
+        ident: Option<&Ident>,
+        ty: &syn::Type,
+        errors: &mut Vec<Error>
+    ) -> Self {
         let mut result = FieldAttrs::default();
 
         for attr in attrs {
@@ -198,8 +205,42 @@ impl FieldAttrs {
             result.source = Some(attr.clone());
         }
 
+        if result.source.is_none() && ident.is_some_and(|ident| ident == "source") {
+            result.inferred_source = true;
+        }
+
+        if result.backtrace.is_none() {
+            if is_option_type(ty) {
+                if option_inner_type(ty).is_some_and(is_backtrace_type) {
+                    result.inferred_backtrace = true;
+                }
+            } else if is_backtrace_type(ty) {
+                result.inferred_backtrace = true;
+            }
+        }
+
         result
     }
+
+    pub fn has_source(&self) -> bool {
+        self.source.is_some() || self.inferred_source
+    }
+
+    pub fn has_backtrace(&self) -> bool {
+        self.backtrace.is_some() || self.inferred_backtrace
+    }
+
+    pub fn is_backtrace_inferred(&self) -> bool {
+        self.inferred_backtrace
+    }
+
+    pub fn source_attribute(&self) -> Option<&Attribute> {
+        self.source.as_ref()
+    }
+
+    pub fn backtrace_attribute(&self) -> Option<&Attribute> {
+        self.backtrace.as_ref()
+    }
 }
 
 #[derive(Debug)]
@@ -403,16 +444,23 @@ fn validate_from_usage(fields: &Fields, display: &DisplaySpec, errors: &mut Vec<
                 continue;
             }
 
-            if companion.attrs.backtrace.is_some() {
+            if companion.attrs.has_backtrace() {
                 continue;
             }
 
-            if let Some(attr) = &companion.attrs.source {
+            if companion.attrs.has_source() {
                 if companion.attrs.from.is_none() && !is_option_type(&companion.ty) {
-                    errors.push(Error::new_spanned(
-                        attr,
-                        "additional #[source] fields used with #[from] must be Option<_>"
-                    ));
+                    if let Some(attr) = companion.attrs.source_attribute() {
+                        errors.push(Error::new_spanned(
+                            attr,
+                            "additional #[source] fields used with #[from] must be Option<_>"
+                        ));
+                    } else {
+                        errors.push(Error::new(
+                            companion.span,
+                            "additional #[source] fields used with #[from] must be Option<_>"
+                        ));
+                    }
                 }
                 continue;
             }
@@ -442,7 +490,7 @@ fn validate_from_usage(fields: &Fields, display: &DisplaySpec, errors: &mut Vec<
 fn validate_backtrace_usage(fields: &Fields, errors: &mut Vec<Error>) {
     let backtrace_fields: Vec<_> = fields
         .iter()
-        .filter(|field| field.attrs.backtrace.is_some())
+        .filter(|field| field.attrs.has_backtrace())
         .collect();
 
     for field in &backtrace_fields {
@@ -454,17 +502,25 @@ fn validate_backtrace_usage(fields: &Fields, errors: &mut Vec<Error>) {
     }
 
     for field in backtrace_fields.iter().skip(1) {
-        if let Some(attr) = &field.attrs.backtrace {
+        if let Some(attr) = field.attrs.backtrace_attribute() {
             errors.push(Error::new_spanned(
                 attr,
                 "multiple #[backtrace] fields are not supported"
             ));
+        } else {
+            errors.push(Error::new(
+                field.span,
+                "multiple #[backtrace] fields are not supported"
+            ));
         }
     }
 }
 
 fn validate_backtrace_field_type(field: &Field, errors: &mut Vec<Error>) {
-    let Some(attr) = &field.attrs.backtrace else {
+    let Some(attr) = field.attrs.backtrace_attribute() else {
+        if field.attrs.is_backtrace_inferred() {
+            return;
+        }
         return;
     };