FA-43124 / Borrow checking / Member archive

A mutable output is justified by mutability of an unrelated input · case 04

A mutable output is justified by mutability of an unrelated input.

Member previewVariant 4 · 3 implementations · 21 checks per implementation

Case contract

Check borrowed function signature well-formedness: lifetime parameters unique; all occurrences declared or static; output elision needs exactly one input lifetime; receiver elision uses receiver lifetime; every output source is an input; by-reference return cannot originate in a local; returned mutable borrow requires mutable input; output extent must fit source extent; bound lifetime cannot escape binder depth; unused lifetime parameters rejected in this toy language. Input is a fully explicit descriptor of the stated toy IR. Return rule identifiers in declaration order; absent optional obligations use the provided neutral defaults. No rule is an assertion about a production language.

Why this case matters

A finite offline static-analysis model of ownership and borrowing; it does not execute the analyzed program.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
mutable-source regression 0	`[]`	`["mutable-source"]`	Failed

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