Default Method Implementations

// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

pub trait CollectLeaves {
    type Leaf;

    // Required Method
    fn collect_leaves_buffered(&self, buf: &mut Vec<Self::Leaf>);

    // Default implementation
    fn collect_leaves(&self) -> Vec<Self::Leaf> {
        let mut buf = vec![];
        self.collect_leaves_buffered(&mut buf);
        buf
    }
}

Traits often have methods that are implemented for you already, once you implement the required methods.
A trait method has a default implementation if the function body is present. This implementation can be written in terms of other methods available, such as other methods in the trait or methods of a supertrait.
Often you’ll see methods that provide the broad functionality that is necessary to implement (like Ord’s compare) with default implementations for functions that can be implemented in terms of those methods (like Ord’s max/min/clamp).
Default methods can be overridden by derive macros, as derive macros produce arbitrary ASTs in the implementation.

ref:

https://doc.rust-lang.org/reference/items/traits.html#r-items.traits.associated-item-decls

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Default Method Implementations