rust - 为什么 Fn 派生自 FnMut(派生自 FnOnce)？

Question

如果您查看 official Rust doc ，您会看到特征 Fn 派生自 FnMut，或者，要实现 Fn，您必须实现 FnMut(之后是 FnOnce，因为 FnMut 也派生自它)。

为什么会这样？我简直无法理解。是因为您可以将每个 Fn 调用为 FnOnce 或 FnMut 吗？

Answer 1

对此最好的引用是优秀的 Finding Closure in Rust博客文章。我将引用重要部分:

There’s three traits, and so seven non-empty sets of traits that could possibly be implemented… but there’s actually only three interesting configurations:

• Fn, FnMut and FnOnce,
• FnMut and FnOnce,
• only FnOnce.

Why? Well, the three closure traits are actually three nested sets: every closure that implements Fn can also implement FnMut (if &self works, &mut self also works; proof: &*self), and similarly every closure implementing FnMut can also implement FnOnce. This hierarchy is enforced at the type level