Borrowed matching

match &value, or matching an existing &E, inspects an enum without consuming it. Named payload bindings have type &T, even for scalar or unit payloads. match &mut value, or matching an existing &mut E, binds &mut T payloads. _ creates no binding and moves no borrowed payload. _ as name binds &E or &mut E: a reborrow of the whole scrutinee, whose loan protects all of it. The scrutinee evaluates once; patterns, coverage, and arm scope follow ordinary matching. A binding inside a nested pattern is a reference to that inner payload, through boxes as well, and a name bound by p | q refers to the payload of whichever alternative matched.

Mutable matching supports standard sums and user enums, both copyable and affine. Mutating a copyable enum’s inline payload leaves independent copies unchanged. Matching and its payload borrows allocate nothing.

Payload loans descend from the scrutinee loan and protect the active variant against conflicting access, replacement, movement, or destruction. A nested binding’s loan also protects each enclosing variant. Disjoint payload fields can be borrowed mutably together; precise class-field origins remain precise, while collection and imprecise returned origins protect their whole target. Reading a discriminant conflicts with an existing exclusive loan into the enum.

Bindings exist only in their arm. Last-use checking permits replacement after the final payload use, even within that arm. No live payload loan may cross yield. Payload references can be returned under the single reference-parameter rule; the returned payload conservatively protects the caller’s whole argument. Borrowing a temporary, returning a local owner’s payload, and storing payload references in aggregates are rejected. A mut reference binding can be reassigned to a payload borrowed from itself; the arm’s payload bindings are immutable.

A returned Option[&T] or Result[&T, E] already holds a reference: match it by value, not with match &result. Its payload bindings are the carried reference and extend the call’s origin loan; see optional and fallible references.