Reject top-level single-value decoding

[dduan]

Summary

• Reject single-value decoding requests when the decoder is wrapping a top-level TOML table.
• Return a DecodingError.typeMismatch instead of attempting to unpack an uninitialized token.
• Add a Swift Testing regression for decoding a top-level table as Int.

Root cause

A top-level TOMLDecoder wraps a keyed TOMLTable and leaves its scalar token at the empty sentinel. When a scalar Decodable type requested a single-value container, the decoder returned itself and integer decoding indexed that empty token range, producing a SIGTRAP during decoder-fuzzer corpus initialization.

Impact

Wrong-shape top-level decoding now fails normally with DecodingError instead of terminating the process. This allows the decoder fuzz target to load valid table-shaped corpus entries and continue fuzzing.

Validation

• swift test — 750 tests passed.
• swift test --filter TOMLDecoderTests.topLevelTableCannotDecodeAsSingleValue
• Decoder libFuzzer startup corpus replay — 217 inputs passed without trapping.
• Multi-worker decoder fuzzing completed without crash artifacts.

[github-actions]

�[33mWarning: running with the Swift runtime interposer on Linux to avoid the Swift 6.3 runtime hook crash. See https://github.com/ordo-one/benchmark/issues/349�[0m

Comparing results between 'main' and 'pull_request'

Host 'runnervm5mmn9' with 4 'x86_64' processors with 15 GB memory, running:
#18~24.04.1-Ubuntu SMP Thu May 28 16:39:11 UTC 2026

TOMLDecoderBenchmarks

Decode toml.io example metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (μs) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	64	64	65	67	72	101	171	7150
pull_request	62	64	64	65	70	96	191	7240
Δ	-2	0	-1	-2	-2	-5	20	90
Improvement %	3	0	2	3	3	5	-12	90

Retains: results within specified thresholds, fold down for details.

Retains *	p0	p25	p50	p75	p90	p99	p100	Samples
main	765	765	765	765	765	765	765	7150
pull_request	765	765	765	765	765	765	765	7240
Δ	0	0	0	0	0	0	0	90
Improvement %	0	0	0	0	0	0	0	90

Parse toml.io example metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (ns) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	5100	5243	5283	5323	5451	7435	34925	10000
pull_request	5029	5143	5171	5211	5271	7287	37250	10000
Δ	-71	-100	-112	-112	-180	-148	2325	0
Improvement %	1	2	2	2	3	2	-7	0

Retains: results within specified thresholds, fold down for details.

Retains *	p0	p25	p50	p75	p90	p99	p100	Samples
main	31	31	31	31	31	31	31	10000
pull_request	31	31	31	31	31	31	31	10000
Δ	0	0	0	0	0	0	0	0
Improvement %	0	0	0	0	0	0	0	0

decode canada.toml metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (ms) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	256	256	257	261	261	261	261	4
pull_request	256	256	257	261	263	263	263	4
Δ	0	0	0	0	2	2	2	0
Improvement %	0	0	0	0	-1	-1	-1	0

Retains: results within specified thresholds, fold down for details.

Retains (K) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	1286	1286	1286	1286	1286	1286	1286	4
pull_request	1286	1286	1286	1286	1286	1286	1286	4
Δ	0	0	0	0	0	0	0	0
Improvement %	0	0	0	0	0	0	0	0

decode twitter.toml metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (μs) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	9093	9921	10002	10060	10150	10546	10546	99
pull_request	8688	9404	9429	9478	9544	9789	10106	106
Δ	-405	-517	-573	-582	-606	-757	-440	7
Improvement %	4	5	6	6	6	7	4	7

Retains: results within specified thresholds, fold down for details.

Retains (K) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	113	113	113	113	113	113	113	99
pull_request	113	113	113	113	113	113	113	106
Δ	0	0	0	0	0	0	0	7
Improvement %	0	0	0	0	0	0	0	7

parse GitHub events archive metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (ms) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	129	130	131	134	141	141	141	8
pull_request	120	121	121	125	133	133	133	9
Δ	-9	-9	-10	-9	-8	-8	-8	1
Improvement %	7	7	8	7	6	6	6	1

Retains: results within specified thresholds, fold down for details.

Retains (K) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	394	394	394	394	394	394	394	8
pull_request	394	394	394	394	394	394	394	9
Δ	0	0	0	0	0	0	0	1
Improvement %	0	0	0	0	0	0	0	1

parse canada.toml metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (ms) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	16	16	16	16	16	23	23	62
pull_request	15	15	15	15	15	23	23	66
Δ	-1	-1	-1	-1	-1	0	0	4
Improvement %	6	6	6	6	6	0	0	4

Retains: results within specified thresholds, fold down for details.

Retains *	p0	p25	p50	p75	p90	p99	p100	Samples
main	47	47	47	47	47	47	47	62
pull_request	47	47	47	47	47	47	47	66
Δ	0	0	0	0	0	0	0	4
Improvement %	0	0	0	0	0	0	0	4

parse twitter.toml metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (μs) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	2339	2367	2386	2402	2427	2591	3547	406
pull_request	2196	2222	2230	2243	2271	2929	3510	434
Δ	-143	-145	-156	-159	-156	338	-37	28
Improvement %	6	6	7	7	6	-13	1	28

Retains: results within specified thresholds, fold down for details.

Retains *	p0	p25	p50	p75	p90	p99	p100	Samples
main	7736	7736	7736	7736	7736	7736	7736	406
pull_request	7736	7736	7736	7736	7736	7736	7736	434
Δ	0	0	0	0	0	0	0	28
Improvement %	0	0	0	0	0	0	0	28