Document the feature flag (perfect-derive) inside the Readme and on the main docs page

Signed-off-by: Justus Flügel <justusfluegel@gmail.com>

README.md

@@ -53,6 +53,7 @@ diagnostic error code: ruget::api::bad_json
   - [... syntax highlighting](#-syntax-highlighting)
   - [... primary label](#-primary-label)
   - [... collection of labels](#-collection-of-labels)
+  - [... with generic errors](#-with-generic-errors)
 - [Acknowledgements](#acknowledgements)
 - [License](#license)
 
@@ -782,6 +783,58 @@ let report: miette::Report = MyError {
 
 println!("{:?}", report.with_source_code("About something or another or yet another ...".to_string()));
 ```
+#### ... with generic errors
+
+When tring to build more complex error types, it can often be useful to use generics.
+
+```rust
+#[derive(Debug, Diagnostic, Error)]
+enum MyError<T> {
+    #[error(transparent)]
+    #[diagnostic(transparent)]
+    Base(T),
+    #[error("Some other error occured")]
+    #[diagnostic(help = "See the manual.")]
+    OtherError
+}
+```
+To enable this pattern, you can enable **the `perfect-derive` feature** on miette.
+This will add trait bounds on generics in the `Diagnostic` implementation, depending on how
+they are used inside the struct/enum.
+
+This should work for all other attributes as well, like `#[label]` or `#[diagnostic_source]`.
+
+<details>
+<summary>
+
+##### ⚠ Warning: (Small) Gotcha with the `#[related]` attribute
+
+</summary>
+
+Because of current lifetime constraints, only generic collection elements but not generic
+collections are currently supported, meaning the following works:
+
+```rust
+#[derive(Debug, Diagnostic, Error)]
+#[error("Some example error")]
+struct MyError<T> {
+    #[related]
+    related_errors: Vec<T>
+}
+```
+but the following does not:
+```rust
+#[derive(Debug, Diagnostic, Error)]
+#[error("Some example error")]
+struct MyError<T> {
+    // See the difference here?
+    // Note that the collection is general, and not
+    // the elements inside the vec. This is **not** supported.
+    #[related]
+    related_errors: T // <- here
+}
+```
+</details>
 
 ### MSRV
 

src/lib.rs

@@ -53,6 +53,7 @@
 //!   - [... syntax highlighting](#-syntax-highlighting)
 //!   - [... primary label](#-primary-label)
 //!   - [... collection of labels](#-collection-of-labels)
+//!   - [... with generic errors](#-with-generic-errors)
 //! - [Acknowledgements](#acknowledgements)
 //! - [License](#license)
 //!
@@ -783,6 +784,59 @@
 //!
 //! println!("{:?}", report.with_source_code("About something or another or yet another ...".to_string()));
 //! ```
+//! ### ... with generic errors
+//!
+//! When tring to build more complex error types, it can often be useful to use generics.
+//!
+//! ```rust,ignore
+//! #[derive(Debug, Diagnostic, Error)]
+//! enum MyError<T> {
+//!     #[error(transparent)]
+//!     #[diagnostic(transparent)]
+//!     Base(T),
+//!     #[error("Some other error occured")]
+//!     #[diagnostic(help = "See the manual.")]
+//!     OtherError
+//! }
+//! ```
+//! To enable this pattern, you can enable **the `perfect-derive` feature** on miette.
+//! This will add trait bounds on generics in the `Diagnostic` implementation, depending on how
+//! they are used inside the struct/enum.
+//!
+//! This should work for all other attributes as well, like `#[label]` or `#[diagnostic_source]`.
+//!
+//! <details>
+//! <summary>
+//!
+//! #### ⚠ Warning: (Small) Gotcha with the `#[related]` attribute
+//!
+//! </summary>
+//!
+//! Because of current lifetime constraints, only generic collection elements but not generic
+//! collections are currently supported, meaning the following works:
+//!
+//! ```rust,ignore
+//! #[derive(Debug, Diagnostic, Error)]
+//! #[error("Some example error")]
+//! struct MyError<T> {
+//!     #[related]   
+//!     related_errors: Vec<T>
+//! }
+//! ```
+//! but the following does not:
+//! ```rust,ignore
+//! #[derive(Debug, Diagnostic, Error)]
+//! #[error("Some example error")]
+//! struct MyError<T> {
+//!     // See the difference here?
+//!     // Note that the collection is general, and not
+//!     // the elements inside the vec. This is **not** supported.
+//!     #[related]   
+//!     related_errors: T // <- here
+//! }
+//! ```
+
+//! </details>
 //!
 //! ## MSRV
 //!