Add `pack_err!` macro for error structs with automatic name field

2 files changed, 213 insertions, 0 deletions

diff --git a/mingling_macros/src/lib.rs b/mingling_macros/src/lib.rs
index 97dd824..408450a 100644
--- a/mingling_macros/src/lib.rs
+++ b/mingling_macros/src/lib.rs
@@ -158,6 +158,8 @@ mod help;
 mod node;
 mod pack;
 #[cfg(feature = "extra_macros")]
+mod pack_err;
+#[cfg(feature = "extra_macros")]
 mod program_setup;
 mod render;
 mod renderer;
@@ -319,6 +321,74 @@ pub fn pack(input: TokenStream) -> TokenStream {
     pack::pack(input)
 }
 
+/// Creates an error struct with a `name: String` field and optional `info: Type` field.
+///
+/// This macro provides a concise way to define error types that implement `Groupped`
+/// and are registered for inclusion in the program enum.
+///
+/// The `name` field is automatically set to the snake_case version of the struct name
+/// at compile time.
+///
+/// # Syntax
+///
+/// Two forms are supported:
+///
+/// ```rust,ignore
+/// // Simple form — generates a struct with only `name: String` and a `Default` impl:
+/// pack_err!(ErrorNotFound);
+///
+/// // Typed form — generates a struct with `name: String` + `info: Type` and a `new(info)` constructor:
+/// pack_err!(ErrorNotDir = PathBuf);
+/// ```
+///
+/// # Generated code
+///
+/// For `pack_err!(ErrorNotFound)`:
+///
+/// ```rust,ignore
+/// #[derive(::mingling::Groupped)]
+/// pub struct ErrorNotFound {
+///     name: String,
+/// }
+///
+/// impl Default for ErrorNotFound {
+///     fn default() -> Self {
+///         Self {
+///             name: "error_not_found".into(),
+///         }
+///     }
+/// }
+/// ```
+///
+/// For `pack_err!(ErrorNotDir = PathBuf)`:
+///
+/// ```rust,ignore
+/// #[derive(::mingling::Groupped)]
+/// pub struct ErrorNotDir {
+///     name: String,
+///     info: PathBuf,
+/// }
+///
+/// impl ErrorNotDir {
+///     pub fn new(info: PathBuf) -> Self {
+///         Self {
+///             name: "error_not_dir".into(),
+///             info,
+///         }
+///     }
+/// }
+/// ```
+///
+/// When the `general_renderer` feature is enabled, the struct also gets
+/// `#[derive(serde::Serialize)]`.
+///
+/// This macro is only available with the `extra_macros` feature.
+#[cfg(feature = "extra_macros")]
+#[proc_macro]
+pub fn pack_err(input: TokenStream) -> TokenStream {
+    pack_err::pack_err(input)
+}
+
 /// Early-returns an error from a `Result`, converting the `Ok` branch to a
 /// `ChainProcess`.
 ///
diff --git a/mingling_macros/src/pack_err.rs b/mingling_macros/src/pack_err.rs
new file mode 100644
index 0000000..dd7b083
--- /dev/null
+++ b/mingling_macros/src/pack_err.rs
@@ -0,0 +1,143 @@
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{Ident, Token, Type, parse_macro_input};
+
+/// Converts a PascalCase/UpperCamelCase identifier string to snake_case.
+///
+/// Examples:
+/// - `ErrorNotFound` → `"error_not_found"`
+/// - `ErrorNotDir` → `"error_not_dir"`
+/// - `FileIO` → `"file_io"`
+/// - `XMLParser` → `"xml_parser"`
+fn to_snake_case(ident: &str) -> String {
+    let mut result = String::new();
+    let mut prev_is_upper = false;
+
+    for (i, c) in ident.chars().enumerate() {
+        if c.is_uppercase() {
+            if i > 0 && !prev_is_upper {
+                result.push('_');
+            }
+            for lower_c in c.to_lowercase() {
+                result.push(lower_c);
+            }
+            prev_is_upper = true;
+        } else {
+            result.push(c);
+            prev_is_upper = false;
+        }
+    }
+
+    result
+}
+
+enum PackErrInput {
+    /// pack_err!(ErrorNotFound)
+    Simple { type_name: Ident },
+    /// pack_err!(ErrorNotDir = PathBuf)
+    Typed {
+        type_name: Ident,
+        inner_type: Box<Type>,
+    },
+}
+
+impl syn::parse::Parse for PackErrInput {
+    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
+        let type_name: Ident = input.parse()?;
+
+        if input.peek(Token![=]) {
+            input.parse::<Token![=]>()?;
+            let inner_type: Type = input.parse()?;
+            Ok(PackErrInput::Typed {
+                type_name,
+                inner_type: Box::new(inner_type),
+            })
+        } else {
+            Ok(PackErrInput::Simple { type_name })
+        }
+    }
+}
+
+#[allow(clippy::too_many_lines)]
+pub fn pack_err(input: TokenStream) -> TokenStream {
+    let parsed = parse_macro_input!(input as PackErrInput);
+
+    match parsed {
+        PackErrInput::Simple { type_name } => {
+            let name_str = type_name.to_string();
+            let snake_name = to_snake_case(&name_str);
+
+            #[cfg(not(feature = "general_renderer"))]
+            let derive = quote! {
+                #[derive(::mingling::Groupped)]
+            };
+
+            #[cfg(feature = "general_renderer")]
+            let derive = quote! {
+                #[derive(::mingling::Groupped, ::serde::Serialize)]
+            };
+
+            let expanded = quote! {
+                #derive
+                pub struct #type_name {
+                    /// The snake_case name of this error, automatically set at compile time.
+                    name: String,
+                }
+
+                impl ::std::default::Default for #type_name {
+                    fn default() -> Self {
+                        Self {
+                            name: #snake_name.into(),
+                        }
+                    }
+                }
+
+                ::mingling::macros::register_type!(#type_name);
+            };
+
+            expanded.into()
+        }
+        PackErrInput::Typed {
+            type_name,
+            inner_type,
+        } => {
+            let name_str = type_name.to_string();
+            let snake_name = to_snake_case(&name_str);
+
+            #[cfg(not(feature = "general_renderer"))]
+            let derive = quote! {
+                #[derive(::mingling::Groupped)]
+            };
+
+            #[cfg(feature = "general_renderer")]
+            let derive = quote! {
+                #[derive(::mingling::Groupped, ::serde::Serialize)]
+            };
+
+            let expanded = quote! {
+                #derive
+                pub struct #type_name {
+                    /// The snake_case name of this error, automatically set at compile time.
+                    name: String,
+                    /// Additional context info for this error.
+                    info: #inner_type,
+                }
+
+                impl #type_name {
+                    /// Creates a new error with the given info.
+                    /// The `name` field is automatically set to the snake_case of the struct name.
+                    pub fn new(info: #inner_type) -> Self {
+                        Self {
+                            name: #snake_name.into(),
+                            info,
+                        }
+                    }
+                }
+
+                ::mingling::macros::register_type!(#type_name);
+            };
+
+            expanded.into()
+        }
+    }
+}