Documenting Generic APIs

In this lesson, you will learn how to write clear, actionable documentation for generic Rust code by explicitly calling out the "contracts" imposed by your trait bounds.

When you write a generic function like fn process<T: Serialize>(item: T), you aren't just writing code; you are establishing a contract. If you don't document that contract, your users will only discover what T requires by reading compiler error messages—which is a frustrating experience.

Communicating Constraints

Effective documentation for generics should prioritize the Trait Bounds. Instead of just saying "this function works with anything," explain why it requires a specific trait.

Consider this example:

/// Serializes a data structure and sends it over the network.
///
/// # Panics
/// This function will fail if the provided `T` cannot be converted 
/// to JSON via `serde::Serialize`.
pub fn send_over_wire<T>(data: T) 
where 
    T: serde::Serialize + Send 
{
    // Implementation details...
}

Why this matters

Notice that the documentation explicitly mentions serde::Serialize and Send. By documenting these bounds, you tell the user two vital things:

1. Capabilities: The user knows they can pass any struct they’ve implemented Serialize for.
2. Safety/Concurrency: The Send bound warns the user that this function is designed to work in multi-threaded contexts.

Writing "User-First" Documentation

When writing documentation for your generic APIs, follow these three rules:

• Be explicit about the bounds: Don't assume the reader can infer why a type needs Clone or Default. Briefly state: "T must implement Clone to allow for internal buffering."
• Mention trait requirements in the preamble: Use the # Errors or # Panics sections if the generic constraints impact runtime behavior.
• Show a concrete example: Always provide a code block showing the function being called with a standard type (like String or i32) so the user can copy-paste and verify the implementation immediately.

If you omit these details, your library becomes a "black box" where developers have to guess which types are compatible. By documenting the trait bounds, you turn a confusing generic signature into a clear, predictable interface.

Key Takeaway: Always document your trait bounds explicitly in the function's doc comments so users understand both the capabilities and the requirements of your generic API.