Improve performance of JSONDecoder and JSONEncoder for large apps

[ChrisBenua]

In short: T.self is _JSONStringDictionaryDecodableMarker.Type, value as? _JSONStringDictionaryEncodableMarker and value as? _JSONDirectArrayEncodable are really slow operations. And the bigger binary gets the slower this check works for the first time for each pair of class/struct/enum and protocol.

But checking whether current type conforms to _JSONStringDictionaryDecodableMarker and _JSONStringDictionaryEncodableMarker is needed only when we use custom keyDecoding/EncodingStrategy. Comments in code confirm this:

/// A marker protocol used to determine whether a value is a `String`-keyed `Dictionary`
/// containing `Decodable` values (in which case it should be exempt from key conversion strategies).
///
/// The marker protocol also provides access to the type of the `Decodable` values,
/// which is needed for the implementation of the key conversion strategy exemption.
private protocol _JSONStringDictionaryDecodableMarker {
    static var elementType: Decodable.Type { get }
}

/// A marker protocol used to determine whether a value is a `String`-keyed `Dictionary`
/// containing `Encodable` values (in which case it should be exempt from key conversion strategies).
private protocol _JSONStringDictionaryEncodableMarker { }

So we can easily skip this checks when default keyDecoding/EncodingStrategy is used.

For details: see this issue: #1480

[ChrisBenua]

Benchmarking results

Collected using this command swift package --allow-writing-to-package-directory benchmark baseline compare new_coders --target JSONBenchmarks --format markdown

Comparing results between 'old_coders' and 'Current_run'

Host 'Christians-MacBook-Pro.local' with 8 'arm64' processors with 16 GB memory, running:
Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:34 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T8103

JSONBenchmarks

Canada-decodeFromJSON metrics

Time (total CPU): results within specified thresholds, fold down for details.

Time (total CPU) (μs) *	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	27	27	27	27	27	27	28	112
Current_run	27	27	28	28	28	28	28	109
Δ	0	0	1	1	1	1	0	-3
Improvement %	0	0	-4	-4	-4	-4	0	-3

Throughput (# / s): results within specified thresholds, fold down for details.

Throughput (# / s) (K)	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	38	38	37	37	37	37	36	112
Current_run	37	36	36	36	36	36	36	109
Δ	-1	-2	-1	-1	-1	-1	0	-3
Improvement %	-3	-5	-3	-3	-3	-3	0	-3

Canada-encodeToJSON metrics

Time (total CPU): results within specified thresholds, fold down for details.

Time (total CPU) (μs) *	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	54	54	54	54	55	55	55	56
Current_run	54	54	54	54	55	55	55	56
Δ	0	0	0	0	0	0	0	0
Improvement %	0	0	0	0	0	0	0	0

Throughput (# / s): results within specified thresholds, fold down for details.

Throughput (# / s) (K)	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	19	18	18	18	18	18	18	56
Current_run	19	18	18	18	18	18	18	56
Δ	0	0	0	0	0	0	0	0
Improvement %	0	0	0	0	0	0	0	0

Twitter-decodeFromJSON metrics

Time (total CPU): results within specified thresholds, fold down for details.

Time (total CPU) (ns) *	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	2150	2163	2169	2198	2224	2292	2448	1374
Current_run	2152	2165	2167	2175	2204	2247	2477	1379
Δ	2	2	-2	-23	-20	-45	29	5
Improvement %	0	0	0	1	1	2	-1	5

Throughput (# / s): results within specified thresholds, fold down for details.

Throughput (# / s) (K)	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	465	463	461	456	450	435	389	1374
Current_run	465	463	462	460	454	446	404	1379
Δ	0	0	1	4	4	11	15	5
Improvement %	0	0	0	1	1	3	4	5

Twitter-encodeToJSON metrics

Time (total CPU): results within specified thresholds, fold down for details.

Time (total CPU) (ns) *	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	1078	1083	1085	1089	1094	1155	1314	2752
Current_run	1085	1091	1093	1095	1100	1135	1280	2738
Δ	7	8	8	6	6	-20	-34	-14
Improvement %	-1	-1	-1	-1	-1	2	3	-14

Throughput (# / s): results within specified thresholds, fold down for details.

Throughput (# / s) (K)	p0	p25	p50	p75	p90	p99	p100	Samples
old_coders	928	924	922	920	915	864	629	2752
Current_run	922	918	917	914	910	882	782	2738
Δ	-6	-6	-5	-6	-5	18	153	-14
Improvement %	-1	-1	-1	-1	-1	2	24	-14