ADR-242: A `match` Over a Place Lends What an Arm Only Reads

Status: Superseded by ADR-291 — its D1-D3 are D22-D24 there Date: September 27, 2026 Target Version: Nikaia 0.0.229 Answers: #168, a match that binds a field of a name moves the name Related: Part I 6.5 (a reader takes a view), ADR-094 D2 (what keeps a value is given it, and a copy is the program’s to write), ADR-214 D2 (what is handed over, and the part of a whole), ADR-137 (patterns)

1. Context

match s { Shape::Named(n) => println(f"{n}") } lowered to a Rust match that binds n by value, which takes the String out of s. A second read of s — another match, a print, a hand-over — was rustc’s use of moved value, and in a catch, match error { … } followed by {error} was borrow of partially moved value: a correct program refused in a file nobody wrote.

The checker could not have said otherwise: a pattern’s bindings were all typed unknown, so it knew neither that n is text nor what the arm does with it.

2. Decisions

D1 — Over a place the function owns, an arm’s bindings are typed from the variant.

A name, or a field of one, that is not itself a view: a positional variant’s parts in order, a variant with named fields by name, a library’s variants from its ledger entry. A temporary is gone after the match anyway, and a view’s parts are views already, so neither changes. A binding the variant does not type — a nested pattern, an or-pattern — stays as it was.

D2 — A part that does not copy and that the arm only reads is lent.

It is written ref n below, which leaves the scrutinee whole. The arm keeps a part — and it is taken, as before — where it:

hands it to something that keeps it: a list’s push, a field, a rename, a callee whose keeps says so (what ADR-214 already records);
calls a method on it that changes it or takes it: mutates, or a receiver by value, such as drain;
assigns to it;
returns it, or hands it back as the arm’s value.

A number, a bool and a char are copied as they were, so an arm that compares one with a number is unchanged.

D3 — The typing is the checker’s everywhere it reaches.

A method on a bound part now resolves, so what it pauses, fails with and touches is counted where it used to be unknown. One std test probed a grammar action’s pause through exactly that gap; it now pauses where it says it does.

3. Rejected alternatives

match &s — every binding a reference, numbers included, so r > 0 compares a &i64 with an i64 and fails; the arms would need a * the source does not write.
A copy of the scrutinee — match s.clone(): a copy the program did not write (ADR-107 D3).

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ADR-242: A match Over a Place Lends What an Arm Only Reads