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FA-43846 / Borrow checking / Open access

A guard consumes a noncopy place needed by later pattern alternatives · case 01

A guard consumes a noncopy place needed by later pattern alternatives.

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

ROOT CAUSE

The static analyzer mishandles guard move: a guard consumes a noncopy place needed by later pattern alternatives.

THE FAILURE

The static analyzer mishandles guard move: a guard consumes a noncopy place needed by later pattern alternatives.

Unsuccessful approach: The partial repair uses if not d['copyable'] and bool(d['guard_moves']): errors.append('guard-move'), which still violates the stipulated analysis contract.

Case contract

Check borrow state across guarded matches. Guard failure restores prearm ownership; guard evaluation cannot move a noncopy scrutinee; arm-local loans end before another arm; guard mutation cannot change a discriminant supporting a projected borrow; arm move commits only after guard success; pattern tests are shared observations; exhaustive join includes every reachable arm; nonexhaustive matches retain fallback state; guard temporaries die on failed guard; a borrowed binding used by guard remains valid through guard expression. 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 d['guard_failed'] and set(d['pre_owned'])!=set(d['after_owned']): errors.append('failed-guard-restore')
    if False: errors.append('guard-move')
    if bool(set(d['arm_local'])&set(d['next_arm_live'])): errors.append('arm-local-end')
    if d['discriminant_mutated'] and d['projected_loan']: errors.append('discriminant-stability')
    if not d['guard_succeeded'] and bool(d['committed_moves']): errors.append('move-commit-point')
    if d['test_unique']: errors.append('pattern-test-capability')
    if not set(d['arms'])<=set(d['joined']): errors.append('reachable-arm-join')
    if not d['exhaustive'] and not d['fallback_included']: errors.append('fallback-state')
    if bool(set(d['guard_temps'])&set(d['failed_guard_live'])): errors.append('failed-guard-temporaries')
    if d['binding_end']<d['guard_end']: errors.append('binding-guard-duration')
    return errors
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
base={'guard_failed': False, 'pre_owned': [], 'after_owned': [], 'guard_moves': [], 'copyable': [], 'arm_local': [], 'next_arm_live': [], 'discriminant_mutated': False, 'projected_loan': False, 'guard_succeeded': True, 'committed_moves': [], 'test_unique': False, 'arms': [], 'joined': [], 'exhaustive': True, 'fallback_included': True, 'guard_temps': [], 'failed_guard_live': [], 'guard_end': 0, 'binding_end': 0}
check('well formed empty obligations',solve(base),[])
check('failed-guard-restore regression 0', solve(dict(base, **({'guard_failed':True,'pre_owned':['x']}))), ['failed-guard-restore'])
check('failed-guard-restore regression 1', solve(dict(base, **({'guard_failed':True,'pre_owned':[N],'after_owned':[]}))), ['failed-guard-restore'])
check('guard-move regression 0', solve(dict(base, **({'guard_moves':['a','b'],'copyable':['a']}))), ['guard-move'])
check('guard-move regression 1', solve(dict(base, **({'guard_moves':[N,N+1],'copyable':[N]}))), ['guard-move'])
check('arm-local-end regression 0', solve(dict(base, **({'arm_local':['r'],'next_arm_live':['r']}))), ['arm-local-end'])
check('arm-local-end regression 1', solve(dict(base, **({'arm_local':[N],'next_arm_live':[N]}))), ['arm-local-end'])
check('discriminant-stability regression 0', solve(dict(base, **({'discriminant_mutated':True,'projected_loan':True}))), ['discriminant-stability'])
check('discriminant-stability regression 1', solve(dict(base, **({'discriminant_mutated':True,'projected_loan':True,'guard_end':N,'binding_end':N}))), ['discriminant-stability'])
check('move-commit-point regression 0', solve(dict(base, **({'guard_succeeded':False,'committed_moves':['x']}))), ['move-commit-point'])
check('move-commit-point regression 1', solve(dict(base, **({'guard_succeeded':False,'committed_moves':[N]}))), ['move-commit-point'])
check('pattern-test-capability regression 0', solve(dict(base, **({'test_unique':True}))), ['pattern-test-capability'])
check('pattern-test-capability regression 1', solve(dict(base, **({'test_unique':True,'guard_end':N,'binding_end':N}))), ['pattern-test-capability'])
check('reachable-arm-join regression 0', solve(dict(base, **({'arms':['a','b'],'joined':['a']}))), ['reachable-arm-join'])
check('reachable-arm-join regression 1', solve(dict(base, **({'arms':list(range(N+1)),'joined':[0]}))), ['reachable-arm-join'])
check('fallback-state regression 0', solve(dict(base, **({'exhaustive':False,'fallback_included':False,'arms':['a'],'joined':['a']}))), ['fallback-state'])
check('fallback-state regression 1', solve(dict(base, **({'exhaustive':False,'fallback_included':False,'arms':[N],'joined':[N]}))), ['fallback-state'])
check('failed-guard-temporaries regression 0', solve(dict(base, **({'guard_temps':['a','b'],'failed_guard_live':['b']}))), ['failed-guard-temporaries'])
check('failed-guard-temporaries regression 1', solve(dict(base, **({'guard_temps':[N,N+1],'failed_guard_live':[N]}))), ['failed-guard-temporaries'])
check('binding-guard-duration regression 0', solve(dict(base, **({'guard_end':N+1,'binding_end':N}))), ['binding-guard-duration'])
check('binding-guard-duration regression 1', solve(dict(base, **({'guard_end':N+2,'binding_end':N+1}))), ['binding-guard-duration'])
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
failed-guard-restore regression 0['failed-guard-restore']['failed-guard-restore']Passed
failed-guard-restore regression 1['failed-guard-restore']['failed-guard-restore']Passed
guard-move regression 0[]['guard-move']Failed
guard-move regression 1[]['guard-move']Failed
arm-local-end regression 0['arm-local-end']['arm-local-end']Passed
arm-local-end regression 1['arm-local-end']['arm-local-end']Passed
discriminant-stability regression 0['discriminant-stability']['discriminant-stability']Passed
discriminant-stability regression 1['discriminant-stability']['discriminant-stability']Passed
move-commit-point regression 0['move-commit-point']['move-commit-point']Passed
move-commit-point regression 1['move-commit-point']['move-commit-point']Passed
pattern-test-capability regression 0['pattern-test-capability']['pattern-test-capability']Passed
pattern-test-capability regression 1['pattern-test-capability']['pattern-test-capability']Passed
reachable-arm-join regression 0['reachable-arm-join']['reachable-arm-join']Passed
reachable-arm-join regression 1['reachable-arm-join']['reachable-arm-join']Passed
fallback-state regression 0['fallback-state']['fallback-state']Passed
fallback-state regression 1['fallback-state']['fallback-state']Passed
failed-guard-temporaries regression 0['failed-guard-temporaries']['failed-guard-temporaries']Passed
failed-guard-temporaries regression 1['failed-guard-temporaries']['failed-guard-temporaries']Passed
binding-guard-duration regression 0['binding-guard-duration']['binding-guard-duration']Passed
binding-guard-duration regression 1['binding-guard-duration']['binding-guard-duration']Passed

SHA-256 / 2171f8687eb82126a9c467c7db64d6f89ac80cbdbe7bdf82de0262ea492909e9

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(d):
    errors=[]
    if d['guard_failed'] and set(d['pre_owned'])!=set(d['after_owned']): errors.append('failed-guard-restore')
    if not d['copyable'] and bool(d['guard_moves']): errors.append('guard-move')
    if bool(set(d['arm_local'])&set(d['next_arm_live'])): errors.append('arm-local-end')
    if d['discriminant_mutated'] and d['projected_loan']: errors.append('discriminant-stability')
    if not d['guard_succeeded'] and bool(d['committed_moves']): errors.append('move-commit-point')
    if d['test_unique']: errors.append('pattern-test-capability')
    if not set(d['arms'])<=set(d['joined']): errors.append('reachable-arm-join')
    if not d['exhaustive'] and not d['fallback_included']: errors.append('fallback-state')
    if bool(set(d['guard_temps'])&set(d['failed_guard_live'])): errors.append('failed-guard-temporaries')
    if d['binding_end']<d['guard_end']: errors.append('binding-guard-duration')
    return errors
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
base={'guard_failed': False, 'pre_owned': [], 'after_owned': [], 'guard_moves': [], 'copyable': [], 'arm_local': [], 'next_arm_live': [], 'discriminant_mutated': False, 'projected_loan': False, 'guard_succeeded': True, 'committed_moves': [], 'test_unique': False, 'arms': [], 'joined': [], 'exhaustive': True, 'fallback_included': True, 'guard_temps': [], 'failed_guard_live': [], 'guard_end': 0, 'binding_end': 0}
check('well formed empty obligations',solve(base),[])
check('failed-guard-restore regression 0', solve(dict(base, **({'guard_failed':True,'pre_owned':['x']}))), ['failed-guard-restore'])
check('failed-guard-restore regression 1', solve(dict(base, **({'guard_failed':True,'pre_owned':[N],'after_owned':[]}))), ['failed-guard-restore'])
check('guard-move regression 0', solve(dict(base, **({'guard_moves':['a','b'],'copyable':['a']}))), ['guard-move'])
check('guard-move regression 1', solve(dict(base, **({'guard_moves':[N,N+1],'copyable':[N]}))), ['guard-move'])
check('arm-local-end regression 0', solve(dict(base, **({'arm_local':['r'],'next_arm_live':['r']}))), ['arm-local-end'])
check('arm-local-end regression 1', solve(dict(base, **({'arm_local':[N],'next_arm_live':[N]}))), ['arm-local-end'])
check('discriminant-stability regression 0', solve(dict(base, **({'discriminant_mutated':True,'projected_loan':True}))), ['discriminant-stability'])
check('discriminant-stability regression 1', solve(dict(base, **({'discriminant_mutated':True,'projected_loan':True,'guard_end':N,'binding_end':N}))), ['discriminant-stability'])
check('move-commit-point regression 0', solve(dict(base, **({'guard_succeeded':False,'committed_moves':['x']}))), ['move-commit-point'])
check('move-commit-point regression 1', solve(dict(base, **({'guard_succeeded':False,'committed_moves':[N]}))), ['move-commit-point'])
check('pattern-test-capability regression 0', solve(dict(base, **({'test_unique':True}))), ['pattern-test-capability'])
check('pattern-test-capability regression 1', solve(dict(base, **({'test_unique':True,'guard_end':N,'binding_end':N}))), ['pattern-test-capability'])
check('reachable-arm-join regression 0', solve(dict(base, **({'arms':['a','b'],'joined':['a']}))), ['reachable-arm-join'])
check('reachable-arm-join regression 1', solve(dict(base, **({'arms':list(range(N+1)),'joined':[0]}))), ['reachable-arm-join'])
check('fallback-state regression 0', solve(dict(base, **({'exhaustive':False,'fallback_included':False,'arms':['a'],'joined':['a']}))), ['fallback-state'])
check('fallback-state regression 1', solve(dict(base, **({'exhaustive':False,'fallback_included':False,'arms':[N],'joined':[N]}))), ['fallback-state'])
check('failed-guard-temporaries regression 0', solve(dict(base, **({'guard_temps':['a','b'],'failed_guard_live':['b']}))), ['failed-guard-temporaries'])
check('failed-guard-temporaries regression 1', solve(dict(base, **({'guard_temps':[N,N+1],'failed_guard_live':[N]}))), ['failed-guard-temporaries'])
check('binding-guard-duration regression 0', solve(dict(base, **({'guard_end':N+1,'binding_end':N}))), ['binding-guard-duration'])
check('binding-guard-duration regression 1', solve(dict(base, **({'guard_end':N+2,'binding_end':N+1}))), ['binding-guard-duration'])
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
failed-guard-restore regression 0['failed-guard-restore']['failed-guard-restore']Passed
failed-guard-restore regression 1['failed-guard-restore']['failed-guard-restore']Passed
guard-move regression 0[]['guard-move']Failed
guard-move regression 1[]['guard-move']Failed
arm-local-end regression 0['arm-local-end']['arm-local-end']Passed
arm-local-end regression 1['arm-local-end']['arm-local-end']Passed
discriminant-stability regression 0['discriminant-stability']['discriminant-stability']Passed
discriminant-stability regression 1['discriminant-stability']['discriminant-stability']Passed
move-commit-point regression 0['move-commit-point']['move-commit-point']Passed
move-commit-point regression 1['move-commit-point']['move-commit-point']Passed
pattern-test-capability regression 0['pattern-test-capability']['pattern-test-capability']Passed
pattern-test-capability regression 1['pattern-test-capability']['pattern-test-capability']Passed
reachable-arm-join regression 0['reachable-arm-join']['reachable-arm-join']Passed
reachable-arm-join regression 1['reachable-arm-join']['reachable-arm-join']Passed
fallback-state regression 0['fallback-state']['fallback-state']Passed
fallback-state regression 1['fallback-state']['fallback-state']Passed
failed-guard-temporaries regression 0['failed-guard-temporaries']['failed-guard-temporaries']Passed
failed-guard-temporaries regression 1['failed-guard-temporaries']['failed-guard-temporaries']Passed
binding-guard-duration regression 0['binding-guard-duration']['binding-guard-duration']Passed
binding-guard-duration regression 1['binding-guard-duration']['binding-guard-duration']Passed

SHA-256 / 43f7034921d6cd0dde17c51dc38c1e6262cfcd3c9ffbe1da7825c9c81af41a99

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 2026-09-29T14:44:06.686141+00:00.

Case digest / fedee1d341033fdfc7208785328df04d40cd47586492e1a74583c03900399512