I don’t think you have to do any of this trickery.
Parse it as let <pat> = (<expr> || <expr>);. Have a transformation that depending on context makes the || expand to { let temp = <lhs>; if temp { temp } else { <rhs> } or let <pat> = match <lhs> { <pat> => <pat>, _ => <rhs>.
Or if we’re ok with || only working with Try types rather than with refutable patterns, add impl Try for bool and desugar || to (approx) match <lhs>.as_result() { Ok(t) => typeof::<lhs>::from_ok(t), => <rhs> }.
We have a lot of power to (ab)use || (and &&) since they’re only magic for bool-typed expressions currently. I’m not sure when they’re “desugared”, but it’s definitely “possible” to intertwine it with type resolution, if not ideal. And the Try version doesn’t require it.
In any case, I personally prefer magic semantics for || over magic semantics for let ||.
Edit bad thought:
What keeps us from “just” introducing a new statement of form let <pat> = <expr - ||> $(|| <expr - ||>)* ; that “just” works for bools?