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FA-43981 / Borrow checking / Open access

An empty iterator summary invents a yielded borrow · case 01

An empty iterator summary invents a yielded borrow.

Verified by executionVariant 1 · 21 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The static analyzer mishandles empty yield: an empty iterator summary invents a yielded borrow.

THE FAILURE

The static analyzer mishandles empty yield: an empty iterator summary invents a yielded borrow.

Unsuccessful approach: The partial repair uses if d['empty'] and len(d['yielded_loans'])>1: errors.append('empty-yield'), which still violates the stipulated analysis contract.

Case contract

Check iterator lifetime summaries. Yielded shared items borrow backing storage; lending items borrow the iterator until next call; mutable yields must have disjoint element footprints; next consumes the prior lending-item loan; double-ended iteration cannot yield a crossed element twice; chunked iteration retains remainder ownership; adapter closures cannot extend item lifetime; collection of lending references cannot outlive iteration; empty iterator yields no loan; iterator destruction releases its backing borrow but not independently owned items. 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.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(d):
    errors=[]
    if not set(d['yield_origins'])<=set(d['backing_origins']): errors.append('yield-origin')
    if d['lending'] and d['item_end']>d['next_point']: errors.append('lending-call-bound')
    if d['mutable'] and bool(set(d['yield_a'])&set(d['yield_b'])): errors.append('mutable-yield-disjoint')
    if d['next_called'] and d['prior_lending_live']: errors.append('next-reborrow-kill')
    if bool(set(d['front_indices'])&set(d['back_indices'])): errors.append('double-ended-crossing')
    if not set(d['remainder'])<=set(d['accounted']): errors.append('chunk-remainder')
    if d['adapter_bound']>d['item_bound']: errors.append('adapter-lifetime')
    if d['collected'] and d['collection_end']>d['iteration_end']: errors.append('lending-collection')
    if False: errors.append('empty-yield')
    if d['destroyed'] and bool(set(d['owned_items'])&set(d['invalidated_items'])): errors.append('iterator-drop-owned-items')
    return errors
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
base={'yield_origins': [], 'backing_origins': [], 'lending': False, 'item_end': 0, 'next_point': 0, 'yield_a': [], 'yield_b': [], 'mutable': False, 'prior_lending_live': False, 'next_called': False, 'front_indices': [], 'back_indices': [], 'remainder': [], 'accounted': [], 'adapter_bound': 0, 'item_bound': 0, 'collected': False, 'collection_end': 0, 'iteration_end': 0, 'empty': False, 'yielded_loans': [], 'destroyed': False, 'backing_live': False, 'owned_items': [], 'invalidated_items': []}
check('well formed empty obligations',solve(base),[])
check('yield-origin regression 0', solve(dict(base, **({'yield_origins':['a','b'],'backing_origins':['a']}))), ['yield-origin'])
check('yield-origin regression 1', solve(dict(base, **({'yield_origins':[N,N+1],'backing_origins':[N]}))), ['yield-origin'])
check('lending-call-bound regression 0', solve(dict(base, **({'lending':True,'item_end':N+1,'next_point':N}))), ['lending-call-bound'])
check('lending-call-bound regression 1', solve(dict(base, **({'lending':True,'item_end':N+2,'next_point':N+1}))), ['lending-call-bound'])
check('mutable-yield-disjoint regression 0', solve(dict(base, **({'mutable':True,'yield_a':[N,N+1],'yield_b':[N]}))), ['mutable-yield-disjoint'])
check('mutable-yield-disjoint regression 1', solve(dict(base, **({'mutable':True,'yield_a':[N],'yield_b':[N,N+1]}))), ['mutable-yield-disjoint'])
check('next-reborrow-kill regression 0', solve(dict(base, **({'next_called':True,'prior_lending_live':True}))), ['next-reborrow-kill'])
check('next-reborrow-kill regression 1', solve(dict(base, **({'next_called':True,'prior_lending_live':True,'next_point':N}))), ['next-reborrow-kill'])
check('double-ended-crossing regression 0', solve(dict(base, **({'front_indices':[N,N+1],'back_indices':[N+1]}))), ['double-ended-crossing'])
check('double-ended-crossing regression 1', solve(dict(base, **({'front_indices':[N],'back_indices':[N,N+1]}))), ['double-ended-crossing'])
check('chunk-remainder regression 0', solve(dict(base, **({'remainder':[N,N+1],'accounted':[N]}))), ['chunk-remainder'])
check('chunk-remainder regression 1', solve(dict(base, **({'remainder':list(range(N+1)),'accounted':[0]}))), ['chunk-remainder'])
check('adapter-lifetime regression 0', solve(dict(base, **({'adapter_bound':N+1,'item_bound':N}))), ['adapter-lifetime'])
check('adapter-lifetime regression 1', solve(dict(base, **({'adapter_bound':N+2,'item_bound':N+1}))), ['adapter-lifetime'])
check('lending-collection regression 0', solve(dict(base, **({'collected':True,'collection_end':N+1,'iteration_end':N}))), ['lending-collection'])
check('lending-collection regression 1', solve(dict(base, **({'collected':True,'collection_end':N+2,'iteration_end':N+1}))), ['lending-collection'])
check('empty-yield regression 0', solve(dict(base, **({'empty':True,'yielded_loans':['r']}))), ['empty-yield'])
check('empty-yield regression 1', solve(dict(base, **({'empty':True,'yielded_loans':[N]}))), ['empty-yield'])
check('iterator-drop-owned-items regression 0', solve(dict(base, **({'destroyed':True,'owned_items':['x'],'invalidated_items':['x']}))), ['iterator-drop-owned-items'])
check('iterator-drop-owned-items regression 1', solve(dict(base, **({'destroyed':True,'owned_items':[N],'invalidated_items':[N]}))), ['iterator-drop-owned-items'])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
well formed empty obligations[][]Passed
yield-origin regression 0['yield-origin']['yield-origin']Passed
yield-origin regression 1['yield-origin']['yield-origin']Passed
lending-call-bound regression 0['lending-call-bound']['lending-call-bound']Passed
lending-call-bound regression 1['lending-call-bound']['lending-call-bound']Passed
mutable-yield-disjoint regression 0['mutable-yield-disjoint']['mutable-yield-disjoint']Passed
mutable-yield-disjoint regression 1['mutable-yield-disjoint']['mutable-yield-disjoint']Passed
next-reborrow-kill regression 0['next-reborrow-kill']['next-reborrow-kill']Passed
next-reborrow-kill regression 1['next-reborrow-kill']['next-reborrow-kill']Passed
double-ended-crossing regression 0['double-ended-crossing']['double-ended-crossing']Passed
double-ended-crossing regression 1['double-ended-crossing']['double-ended-crossing']Passed
chunk-remainder regression 0['chunk-remainder']['chunk-remainder']Passed
chunk-remainder regression 1['chunk-remainder']['chunk-remainder']Passed
adapter-lifetime regression 0['adapter-lifetime']['adapter-lifetime']Passed
adapter-lifetime regression 1['adapter-lifetime']['adapter-lifetime']Passed
lending-collection regression 0['lending-collection']['lending-collection']Passed
lending-collection regression 1['lending-collection']['lending-collection']Passed
empty-yield regression 0[]['empty-yield']Failed
empty-yield regression 1[]['empty-yield']Failed
iterator-drop-owned-items regression 0['iterator-drop-owned-items']['iterator-drop-owned-items']Passed
iterator-drop-owned-items regression 1['iterator-drop-owned-items']['iterator-drop-owned-items']Passed

SHA-256 / ebb93f0a9d70a20954d12b2455e353236e2e7d09df8e44cdea79ce598e40e276

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(d):
    errors=[]
    if not set(d['yield_origins'])<=set(d['backing_origins']): errors.append('yield-origin')
    if d['lending'] and d['item_end']>d['next_point']: errors.append('lending-call-bound')
    if d['mutable'] and bool(set(d['yield_a'])&set(d['yield_b'])): errors.append('mutable-yield-disjoint')
    if d['next_called'] and d['prior_lending_live']: errors.append('next-reborrow-kill')
    if bool(set(d['front_indices'])&set(d['back_indices'])): errors.append('double-ended-crossing')
    if not set(d['remainder'])<=set(d['accounted']): errors.append('chunk-remainder')
    if d['adapter_bound']>d['item_bound']: errors.append('adapter-lifetime')
    if d['collected'] and d['collection_end']>d['iteration_end']: errors.append('lending-collection')
    if d['empty'] and len(d['yielded_loans'])>1: errors.append('empty-yield')
    if d['destroyed'] and bool(set(d['owned_items'])&set(d['invalidated_items'])): errors.append('iterator-drop-owned-items')
    return errors
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
base={'yield_origins': [], 'backing_origins': [], 'lending': False, 'item_end': 0, 'next_point': 0, 'yield_a': [], 'yield_b': [], 'mutable': False, 'prior_lending_live': False, 'next_called': False, 'front_indices': [], 'back_indices': [], 'remainder': [], 'accounted': [], 'adapter_bound': 0, 'item_bound': 0, 'collected': False, 'collection_end': 0, 'iteration_end': 0, 'empty': False, 'yielded_loans': [], 'destroyed': False, 'backing_live': False, 'owned_items': [], 'invalidated_items': []}
check('well formed empty obligations',solve(base),[])
check('yield-origin regression 0', solve(dict(base, **({'yield_origins':['a','b'],'backing_origins':['a']}))), ['yield-origin'])
check('yield-origin regression 1', solve(dict(base, **({'yield_origins':[N,N+1],'backing_origins':[N]}))), ['yield-origin'])
check('lending-call-bound regression 0', solve(dict(base, **({'lending':True,'item_end':N+1,'next_point':N}))), ['lending-call-bound'])
check('lending-call-bound regression 1', solve(dict(base, **({'lending':True,'item_end':N+2,'next_point':N+1}))), ['lending-call-bound'])
check('mutable-yield-disjoint regression 0', solve(dict(base, **({'mutable':True,'yield_a':[N,N+1],'yield_b':[N]}))), ['mutable-yield-disjoint'])
check('mutable-yield-disjoint regression 1', solve(dict(base, **({'mutable':True,'yield_a':[N],'yield_b':[N,N+1]}))), ['mutable-yield-disjoint'])
check('next-reborrow-kill regression 0', solve(dict(base, **({'next_called':True,'prior_lending_live':True}))), ['next-reborrow-kill'])
check('next-reborrow-kill regression 1', solve(dict(base, **({'next_called':True,'prior_lending_live':True,'next_point':N}))), ['next-reborrow-kill'])
check('double-ended-crossing regression 0', solve(dict(base, **({'front_indices':[N,N+1],'back_indices':[N+1]}))), ['double-ended-crossing'])
check('double-ended-crossing regression 1', solve(dict(base, **({'front_indices':[N],'back_indices':[N,N+1]}))), ['double-ended-crossing'])
check('chunk-remainder regression 0', solve(dict(base, **({'remainder':[N,N+1],'accounted':[N]}))), ['chunk-remainder'])
check('chunk-remainder regression 1', solve(dict(base, **({'remainder':list(range(N+1)),'accounted':[0]}))), ['chunk-remainder'])
check('adapter-lifetime regression 0', solve(dict(base, **({'adapter_bound':N+1,'item_bound':N}))), ['adapter-lifetime'])
check('adapter-lifetime regression 1', solve(dict(base, **({'adapter_bound':N+2,'item_bound':N+1}))), ['adapter-lifetime'])
check('lending-collection regression 0', solve(dict(base, **({'collected':True,'collection_end':N+1,'iteration_end':N}))), ['lending-collection'])
check('lending-collection regression 1', solve(dict(base, **({'collected':True,'collection_end':N+2,'iteration_end':N+1}))), ['lending-collection'])
check('empty-yield regression 0', solve(dict(base, **({'empty':True,'yielded_loans':['r']}))), ['empty-yield'])
check('empty-yield regression 1', solve(dict(base, **({'empty':True,'yielded_loans':[N]}))), ['empty-yield'])
check('iterator-drop-owned-items regression 0', solve(dict(base, **({'destroyed':True,'owned_items':['x'],'invalidated_items':['x']}))), ['iterator-drop-owned-items'])
check('iterator-drop-owned-items regression 1', solve(dict(base, **({'destroyed':True,'owned_items':[N],'invalidated_items':[N]}))), ['iterator-drop-owned-items'])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
well formed empty obligations[][]Passed
yield-origin regression 0['yield-origin']['yield-origin']Passed
yield-origin regression 1['yield-origin']['yield-origin']Passed
lending-call-bound regression 0['lending-call-bound']['lending-call-bound']Passed
lending-call-bound regression 1['lending-call-bound']['lending-call-bound']Passed
mutable-yield-disjoint regression 0['mutable-yield-disjoint']['mutable-yield-disjoint']Passed
mutable-yield-disjoint regression 1['mutable-yield-disjoint']['mutable-yield-disjoint']Passed
next-reborrow-kill regression 0['next-reborrow-kill']['next-reborrow-kill']Passed
next-reborrow-kill regression 1['next-reborrow-kill']['next-reborrow-kill']Passed
double-ended-crossing regression 0['double-ended-crossing']['double-ended-crossing']Passed
double-ended-crossing regression 1['double-ended-crossing']['double-ended-crossing']Passed
chunk-remainder regression 0['chunk-remainder']['chunk-remainder']Passed
chunk-remainder regression 1['chunk-remainder']['chunk-remainder']Passed
adapter-lifetime regression 0['adapter-lifetime']['adapter-lifetime']Passed
adapter-lifetime regression 1['adapter-lifetime']['adapter-lifetime']Passed
lending-collection regression 0['lending-collection']['lending-collection']Passed
lending-collection regression 1['lending-collection']['lending-collection']Passed
empty-yield regression 0[]['empty-yield']Failed
empty-yield regression 1[]['empty-yield']Failed
iterator-drop-owned-items regression 0['iterator-drop-owned-items']['iterator-drop-owned-items']Passed
iterator-drop-owned-items regression 1['iterator-drop-owned-items']['iterator-drop-owned-items']Passed

SHA-256 / c97aa8eee6e5568c77be5401dacaf9fd4eda95766906958b967df096435d513a

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 21 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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Verification & scope

The explicitly stated toy language is the complete scope; this is not a production compiler or a claim about Rust semantics. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:44:07.503704+00:00.

Case digest / 6a5a454f5dd2576959bc86feaec2113f5f69321ecdd7b0b0652c89a0f17e614b