Add internal template parser for custom error derive

CHANGELOG.md

@@ -3,7 +3,9 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
-_No changes yet._
+### Added
+- `masterror::error::template` module providing a parsed representation of
+  `#[error("...")]` strings and a formatter hook for future custom derives.
 
 ## [0.5.0] - 2025-09-23
 

README.md

@@ -147,6 +147,22 @@ assert_eq!(wrapped.to_string(), "I/O failed: disk offline");
   valid.
 - `#[error(transparent)]` enforces single-field wrappers that forward
   `Display`/`source` to the inner error.
+- `masterror::error::template::ErrorTemplate` parses `#[error("...")]`
+  strings, exposing literal and placeholder segments so custom derives can be
+  implemented without relying on `thiserror`.
+
+```rust
+use masterror::error::template::{ErrorTemplate, TemplateIdentifier};
+
+let template = ErrorTemplate::parse("{code}: {message}").expect("parse");
+let display = template.display_with(|placeholder, f| match placeholder.identifier() {
+    TemplateIdentifier::Named("code") => write!(f, "{}", 404),
+    TemplateIdentifier::Named("message") => f.write_str("Not Found"),
+    _ => Ok(()),
+});
+
+assert_eq!(display.to_string(), "404: Not Found");
+```
 
 </details>
 
@@ -329,4 +345,3 @@ MSRV = 1.89 (may raise in minor, never in patch).
 Apache-2.0 OR MIT, at your option.
 
 </details>
-

README.template.md

@@ -141,6 +141,22 @@ assert_eq!(wrapped.to_string(), "I/O failed: disk offline");
   valid.
 - `#[error(transparent)]` enforces single-field wrappers that forward
   `Display`/`source` to the inner error.
+- `masterror::error::template::ErrorTemplate` parses `#[error("...")]`
+  strings, exposing literal and placeholder segments so custom derives can be
+  implemented without relying on `thiserror`.
+
+```rust
+use masterror::error::template::{ErrorTemplate, TemplateIdentifier};
+
+let template = ErrorTemplate::parse("{code}: {message}").expect("parse");
+let display = template.display_with(|placeholder, f| match placeholder.identifier() {
+    TemplateIdentifier::Named("code") => write!(f, "{}", 404),
+    TemplateIdentifier::Named("message") => f.write_str("Not Found"),
+    _ => Ok(()),
+});
+
+assert_eq!(display.to_string(), "404: Not Found");
+```
 
 </details>
 

src/error.rs

@@ -0,0 +1,14 @@
+//! Utilities for building custom error derive infrastructure.
+//!
+//! This module exposes lower-level building blocks that will eventually power
+//! a native replacement for the `thiserror` derive. The initial goal is to
+//! parse and validate display templates (`#[error("...")]`) in a reusable
+//! and well-tested manner so that future procedural macros can focus on
+//! generating code.
+//!
+//! The API is intentionally low level. It makes no assumptions about how the
+//! parsed data is going to be used and instead provides precise spans and
+//! formatting metadata that higher-level code can rely on.
+
+/// Parser and formatter helpers for `#[error("...")]` templates.
+pub mod template;