A Rust trick: use method-call resolution order to implement trait fallback

Before explaining what the trick is and how it works, I'll take a general example: let's say in a Rust codebase or library you find a better way of doing something, but you still want to keep compatibility with the rest of the codebase or ecosystem. Say you have a new trait Awesome that does what Debug does, but better (for some definition of better). You may want to express code that can consume a value that it either Awesome or Debug like this: fn do_it(value: impl Awesome or Debug) { if impls!(value, Awesome) { // use Awesome's logic } else { // use Debug's logic } } This syntax is invented, but the idea is: if the caller gives you a type that implements Awesome use its implementation, otherwise fallback to Debug. A concrete example of such a need is the crate maud, that declares the Render trait, which is a better Display because it handles html-escaping more efficiently. A simplifier version of it is: trait Render { /// Append the data to the buffer, /// escaping it as necessary…

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