Support fmt handlers and update diagnostics

CHANGELOG.md

@@ -3,6 +3,20 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+## [0.6.3] - 2025-10-10
+
+### Added
+- Invoke custom `#[error(fmt = <path>)]` handlers for structs and enum variants,
+  borrowing fields and forwarding the formatter reference just like `thiserror`.
+
+### Changed
+- Ensure duplicate `fmt` attributes report a single diagnostic without
+  suppressing the derived display implementation.
+
+### Tests
+- Extend the formatter trybuild suite with success cases covering struct and
+  enum formatter paths.
+
 ## [0.6.2] - 2025-10-09
 
 ### Added

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "masterror"
-version = "0.6.2"
+version = "0.6.3"
 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 = "0.2.2", path = "masterror-derive" }
+masterror-derive = { version = "0.2.3", path = "masterror-derive" }
 masterror-template = { version = "0.2.0", path = "masterror-template" }
 
 [dependencies]

README.md

@@ -29,9 +29,9 @@ Stable categories, conservative HTTP mapping, no `unsafe`.
 
 ~~~toml
 [dependencies]
-masterror = { version = "0.6.2", default-features = false }
+masterror = { version = "0.6.3", default-features = false }
 # or with features:
-# masterror = { version = "0.6.2", features = [
+# masterror = { version = "0.6.3", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -66,10 +66,10 @@ masterror = { version = "0.6.2", default-features = false }
 ~~~toml
 [dependencies]
 # lean core
-masterror = { version = "0.6.2", default-features = false }
+masterror = { version = "0.6.3", default-features = false }
 
 # with Axum/Actix + JSON + integrations
-# masterror = { version = "0.6.2", features = [
+# masterror = { version = "0.6.3", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -383,13 +383,13 @@ assert_eq!(resp.status, 401);
 Minimal core:
 
 ~~~toml
-masterror = { version = "0.6.2", default-features = false }
+masterror = { version = "0.6.3", default-features = false }
 ~~~
 
 API (Axum + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.6.2", features = [
+masterror = { version = "0.6.3", features = [
   "axum", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }
@@ -398,7 +398,7 @@ masterror = { version = "0.6.2", features = [
 API (Actix + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.6.2", features = [
+masterror = { version = "0.6.3", 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 = "0.2.2"
+version = "0.2.3"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/RAprogramm/masterror"

masterror-derive/src/display.rs

@@ -3,7 +3,7 @@ use std::collections::HashMap;
 use masterror_template::template::{TemplateFormatter, TemplateFormatterKind};
 use proc_macro2::{Ident, Literal, Span, TokenStream};
 use quote::{format_ident, quote, quote_spanned};
-use syn::{Error, spanned::Spanned};
+use syn::Error;
 
 use crate::{
     input::{
@@ -42,9 +42,7 @@ fn expand_struct(input: &ErrorInput, data: &StructData) -> Result<TokenStream, E
         }
         DisplaySpec::FormatterPath {
             path, ..
-        } => {
-            return Err(Error::new(path.span(), "`fmt = ...` is not supported yet"));
-        }
+        } => render_struct_formatter_path(&data.fields, path)
     };
 
     let ident = &input.ident;
@@ -93,6 +91,26 @@ fn render_struct_transparent(fields: &Fields) -> TokenStream {
     }
 }
 
+fn struct_formatter_arguments(fields: &Fields) -> Vec<TokenStream> {
+    match fields {
+        Fields::Unit => Vec::new(),
+        Fields::Named(fields) | Fields::Unnamed(fields) => fields
+            .iter()
+            .map(|field| {
+                let member = &field.member;
+                quote!(&self.#member)
+            })
+            .collect()
+    }
+}
+
+fn formatter_path_call(path: &syn::ExprPath, mut args: Vec<TokenStream>) -> TokenStream {
+    args.push(quote!(f));
+    quote! {
+        #path(#(#args),*)
+    }
+}
+
 fn render_variant(variant: &VariantData) -> Result<TokenStream, Error> {
     match &variant.display {
         DisplaySpec::Transparent {
@@ -105,10 +123,15 @@ fn render_variant(variant: &VariantData) -> Result<TokenStream, Error> {
         } => render_variant_template(variant, template, Some(args)),
         DisplaySpec::FormatterPath {
             path, ..
-        } => Err(Error::new(path.span(), "`fmt = ...` is not supported yet"))
+        } => render_variant_formatter_path(variant, path)
     }
 }
 
+fn render_struct_formatter_path(fields: &Fields, path: &syn::ExprPath) -> TokenStream {
+    let args = struct_formatter_arguments(fields);
+    formatter_path_call(path, args)
+}
+
 #[derive(Debug)]
 struct ResolvedPlaceholderExpr {
     expr:          TokenStream,
@@ -279,6 +302,47 @@ fn render_variant_transparent(variant: &VariantData) -> Result<TokenStream, Erro
     }
 }
 
+fn render_variant_formatter_path(
+    variant: &VariantData,
+    path: &syn::ExprPath
+) -> Result<TokenStream, Error> {
+    let variant_ident = &variant.ident;
+    match &variant.fields {
+        Fields::Unit => {
+            let call = formatter_path_call(path, Vec::new());
+            Ok(quote! {
+                Self::#variant_ident => {
+                    #call
+                }
+            })
+        }
+        Fields::Unnamed(fields) => {
+            let bindings: Vec<_> = fields.iter().map(binding_ident).collect();
+            let pattern = quote!(Self::#variant_ident(#(#bindings),*));
+            let call = formatter_path_call(path, variant_formatter_arguments(&bindings));
+            Ok(quote! {
+                #pattern => {
+                    #call
+                }
+            })
+        }
+        Fields::Named(fields) => {
+            let bindings: Vec<_> = fields.iter().map(binding_ident).collect();
+            let pattern = quote!(Self::#variant_ident { #(#bindings),* });
+            let call = formatter_path_call(path, variant_formatter_arguments(&bindings));
+            Ok(quote! {
+                #pattern => {
+                    #call
+                }
+            })
+        }
+    }
+}
+
+fn variant_formatter_arguments(bindings: &[Ident]) -> Vec<TokenStream> {
+    bindings.iter().map(|binding| quote!(#binding)).collect()
+}
+
 fn render_variant_template(
     variant: &VariantData,
     template: &DisplayTemplate,

masterror-derive/src/input.rs

@@ -404,12 +404,15 @@ fn extract_display_spec(
     errors: &mut Vec<Error>
 ) -> Result<DisplaySpec, ()> {
     let mut display = None;
+    let mut saw_error_attribute = false;
 
     for attr in attrs {
         if !path_is(attr, "error") {
             continue;
         }
 
+        saw_error_attribute = true;
+
         if display.is_some() {
             errors.push(Error::new_spanned(attr, "duplicate #[error] attribute"));
             continue;
@@ -424,7 +427,9 @@ fn extract_display_spec(
     match display {
         Some(spec) => Ok(spec),
         None => {
-            errors.push(Error::new(missing_span, "missing #[error(...)] attribute"));
+            if !saw_error_attribute {
+                errors.push(Error::new(missing_span, "missing #[error(...)] attribute"));
+            }
             Err(())
         }
     }

tests/ui/formatter/fail/duplicate_fmt.stderr

@@ -3,9 +3,3 @@ error: duplicate `fmt` handler specified
   |
 4 | #[error(fmt = crate::format_error, fmt = crate::format_error)]
   |                                    ^^^
-
-error: missing #[error(...)] attribute
- --> tests/ui/formatter/fail/duplicate_fmt.rs:5:8
-  |
-5 | struct DuplicateFmt;
-  |        ^^^^^^^^^^^^

tests/ui/formatter/fail/unsupported_flag.stderr

@@ -3,9 +3,3 @@ error: placeholder spanning bytes 0..11 uses an unsupported formatter
   |
 4 | #[error("{value:##x}")]
   |         ^^^^^^^^^^^^^
-
-error: missing #[error(...)] attribute
- --> tests/ui/formatter/fail/unsupported_flag.rs:5:8
-  |
-5 | struct UnsupportedFlag {
-  |        ^^^^^^^^^^^^^^^

tests/ui/formatter/fail/unsupported_formatter.stderr

@@ -3,9 +3,3 @@ error: placeholder spanning bytes 0..9 uses an unsupported formatter
   |
 4 | #[error("{value:y}")]
   |         ^^^^^^^^^^^
-
-error: missing #[error(...)] attribute
- --> tests/ui/formatter/fail/unsupported_formatter.rs:5:8
-  |
-5 | struct UnsupportedFormatter {
-  |        ^^^^^^^^^^^^^^^^^^^^

tests/ui/formatter/fail/uppercase_binary.stderr

@@ -3,9 +3,3 @@ error: placeholder spanning bytes 0..9 uses an unsupported formatter
   |
 4 | #[error("{value:B}")]
   |         ^^^^^^^^^^^
-
-error: missing #[error(...)] attribute
- --> tests/ui/formatter/fail/uppercase_binary.rs:5:8
-  |
-5 | struct UppercaseBinary {
-  |        ^^^^^^^^^^^^^^^

tests/ui/formatter/fail/uppercase_pointer.stderr

@@ -3,9 +3,3 @@ error: placeholder spanning bytes 0..9 uses an unsupported formatter
   |
 4 | #[error("{value:P}")]
   |         ^^^^^^^^^^^
-
-error: missing #[error(...)] attribute
- --> tests/ui/formatter/fail/uppercase_pointer.rs:5:8
-  |
-5 | struct UppercasePointer {
-  |        ^^^^^^^^^^^^^^^^

tests/ui/formatter/pass/fmt_path.rs

@@ -0,0 +1,53 @@
+use masterror::Error;
+
+fn format_unit(f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+    f.write_str("unit")
+}
+
+fn format_pair(left: &i32, right: &i32, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+    write!(f, "pair={left}:{right}")
+}
+
+fn format_struct_fields(
+    count: &usize,
+    label: &&'static str,
+    f: &mut core::fmt::Formatter<'_>
+) -> core::fmt::Result {
+    write!(f, "struct={count}:{label}")
+}
+
+#[derive(Debug, Error)]
+#[error(fmt = crate::format_struct_fields)]
+struct StructFormatter {
+    count: usize,
+    label: &'static str,
+}
+
+#[derive(Debug, Error)]
+enum EnumFormatter {
+    #[error(fmt = crate::format_unit)]
+    Unit,
+    #[error(fmt = crate::format_pair)]
+    Tuple(i32, i32),
+    #[error(fmt = crate::format_pair)]
+    Named { left: i32, right: i32 },
+    #[error(fmt = crate::format_struct_fields)]
+    Struct { count: usize, label: &'static str }
+}
+
+fn main() {
+    let _ = StructFormatter {
+        count: 1,
+        label: "alpha"
+    }
+    .to_string();
+
+    let _ = EnumFormatter::Unit.to_string();
+    let _ = EnumFormatter::Tuple(10, 20).to_string();
+    let _ = EnumFormatter::Named { left: 5, right: 15 }.to_string();
+    let _ = EnumFormatter::Struct {
+        count: 2,
+        label: "beta"
+    }
+    .to_string();
+}

tests/ui/transparent/arguments_not_supported.stderr

@@ -3,9 +3,3 @@ error: format arguments are not supported with #[error(transparent)]
   |
 4 | #[error(transparent, code = 42)]
   |                    ^
-
-error: missing #[error(...)] attribute
- --> tests/ui/transparent/arguments_not_supported.rs:5:8
-  |
-5 | struct TransparentWithArgs(#[from] std::io::Error);
-  |        ^^^^^^^^^^^^^^^^^^^