A record rest pattern cannot borrow the remainder through a reference

GAP-EXPRESSIONS-001

No form binds the remainder of a record through a reference: a borrowed view of "everything but these fields" has no spelling.

Lifecycle

known

Owner

language

Scope

reference/spec/expressions.md#struct-patterns

Origin

RFC-0178 review: borrowed rest patterns need the &var splitting rules of RFC-0122

Affects · linked records

spec.expressions.struct-patterns.legality-1language spec rule

gapKnown gaps ↗

Gap

RFC-0178's rest pattern (let { x, ..rest } := r) consumes an owned record. Applied to a reference, let { x, ..rest } := &r could bind x: &T and rest: &{ ..Rest }, a borrowed view of the remainder, and with &var a split mutable borrow. That needs the shared/exclusive rules that borrow checking provides, so it is reserved: the form is rejected with a message pointing at the owned form. Until then a function that must not consume its argument cannot take a remainder out of it generically.

Impact

Visible to Metel programmers: typestate steps that take self by value are unaffected. Code that wants "this field, and a view of everything else" from a borrowed row has no spelling; read the named fields through the reference instead.

Affects

• spec.expressions.struct-patterns.legality-1

Resolution

Not scheduled. Revisit once RFC-0122 (borrow checking) is implemented; RFC-0175's drain_field sketch has the same shape.