fix: resolve IEnumerable<T> of an unregistered element type as an empty sequence

[vbreuss]

MS.DI guarantees that IEnumerable<T> resolves for every T, empty when nothing is registered, so consumers enumerate one without a null check and frameworks use a collection as their extension point. The provider-replacement path answered null instead, because the generator emits collection cases only for element types the graph mentions.

The objection recorded against this gap was that matching MS.DI means building a collection for a type unknown at compile time. That holds for a non-empty collection; an empty one constructs no elements, only an array of the element type the caller already handed over, so it needs no compile-time expansion.

An element type the container does have a registration for is excluded, so a collection with an async-tainted member still throws the container's guidance rather than quietly reporting nothing. Only IEnumerable<T> is covered, matching MS.DI, which answers null for T[], IList<T> and IReadOnlyList<T> of an unregistered element type as well. The keyed surface is untouched: MS.DI has no equivalent guarantee there.

A value-typed element stays unreported, which is the one remaining divergence and an AOT constraint rather than a semantic one: manifesting the array needs the T[] type, which native AOT generates on demand for every reference element kind but not for a value one, where Array.CreateInstance throws NotSupportedException.

[github-actions]

🚀 Benchmark Results

Details

BenchmarkDotNet v0.15.8, Linux Ubuntu 24.04.4 LTS (Noble Numbat)

AMD EPYC 7763 2.45GHz, 1 CPU, 4 logical and 2 physical cores

.NET SDK 10.0.302

[Host] : .NET 10.0.10 (10.0.10, 10.0.1026.32716), X64 RyuJIT x86-64-v3
Job=InProcess  Toolchain=InProcessEmitToolchain  IterationCount=15

LaunchCount=1  WarmupCount=10

Resolve	Size	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
baseline*	8	6.991 ns	0.0144 ns	0.0128 ns	1.00	-	NA
Awaiten	8	6.985 ns	0.0216 ns	0.0192 ns	1.00	-	NA
MsDI	8	7.387 ns	0.0111 ns	0.0098 ns	1.06	-	NA
Autofac	8	110.260 ns	2.2097 ns	2.0670 ns	15.79	656 B	NA
Jab	8	2.646 ns	0.0050 ns	0.0042 ns	0.38	-	NA
PureDI	8	5.429 ns	0.0128 ns	0.0114 ns	0.78	-	NA
DryIoc	8	8.450 ns	0.0093 ns	0.0072 ns	1.21	-	NA
SimpleInjector	8	11.202 ns	0.0103 ns	0.0086 ns	1.60	-	NA
baseline*	256	7.847 ns	0.0424 ns	0.0354 ns	1.00	-	NA
Awaiten	256	7.815 ns	0.0040 ns	0.0034 ns	1.00	-	NA
MsDI	256	7.245 ns	0.0126 ns	0.0106 ns	0.93	-	NA
Autofac	256	109.133 ns	3.0507 ns	2.8536 ns	13.96	656 B	NA
Jab	256	42.688 ns	0.0386 ns	0.0323 ns	5.46	-	NA
PureDI	256	7.651 ns	0.0034 ns	0.0029 ns	0.98	-	NA
DryIoc	256	8.436 ns	0.0046 ns	0.0036 ns	1.08	-	NA
SimpleInjector	256	14.858 ns	0.1155 ns	0.1024 ns	1.90	-	NA

Details

BenchmarkDotNet v0.15.8, Linux Ubuntu 24.04.4 LTS (Noble Numbat)

AMD EPYC 7763 2.61GHz, 1 CPU, 4 logical and 2 physical cores

.NET SDK 10.0.302

[Host] : .NET 10.0.10 (10.0.10, 10.0.1026.32716), X64 RyuJIT x86-64-v3
Job=InProcess  Toolchain=InProcessEmitToolchain  IterationCount=15

LaunchCount=1  WarmupCount=10

Realistic	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
baseline*	242.8 ns	1.51 ns	1.41 ns	0.96	568 B	1.00
Awaiten	251.8 ns	1.63 ns	1.45 ns	1.00	568 B	1.00
MsDI	617.2 ns	4.13 ns	3.66 ns	2.45	1104 B	1.94
Autofac	7,072.7 ns	44.30 ns	41.44 ns	28.09	10784 B	18.99
Jab	184.9 ns	1.19 ns	1.11 ns	0.73	432 B	0.76
DryIoc	418.6 ns	1.81 ns	1.61 ns	1.66	944 B	1.66
SimpleInjector	759.9 ns	3.73 ns	3.49 ns	3.02	1096 B	1.93
PureDI	199.2 ns	2.83 ns	2.65 ns	0.79	632 B	1.11

Details

BenchmarkDotNet v0.15.8, Linux Ubuntu 24.04.4 LTS (Noble Numbat)

AMD EPYC 7763 3.01GHz, 1 CPU, 4 logical and 2 physical cores

.NET SDK 10.0.302

[Host] : .NET 10.0.10 (10.0.10, 10.0.1026.32716), X64 RyuJIT x86-64-v3
Job=InProcess  Toolchain=InProcessEmitToolchain  IterationCount=15

LaunchCount=1  WarmupCount=10

Build	Size	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
baseline*	8	13.700 ns	0.1232 ns	0.1153 ns	0.64	144 B	1.00
Awaiten	8	21.57 ns	0.381 ns	0.318 ns	1.00	144 B	1.00
MsDI	8	1,735.12 ns	29.513 ns	27.606 ns	80.46	5688 B	39.50
Autofac	8	31,943.61 ns	126.550 ns	112.183 ns	1,481.29	33094 B	229.82
Jab	8	12.85 ns	0.298 ns	0.279 ns	0.60	96 B	0.67
PureDI	8	18.80 ns	0.114 ns	0.096 ns	0.87	128 B	0.89
DryIoc	8	913.35 ns	6.418 ns	5.690 ns	42.35	1528 B	10.61
SimpleInjector	8	13,795.51 ns	64.512 ns	60.345 ns	639.73	24760 B	171.94
baseline*	256	91.176 ns	0.5961 ns	0.5284 ns	0.59	2128 B	1.00
Awaiten	256	154.50 ns	2.945 ns	2.459 ns	1.00	2128 B	1.00
MsDI	256	16,803.40 ns	126.511 ns	98.772 ns	108.78	61016 B	28.67
Autofac	256	772,547.28 ns	4,288.717 ns	4,011.668 ns	5,001.36	720914 B	338.78
Jab	256	144.19 ns	4.009 ns	3.130 ns	0.93	2080 B	0.98
PureDI	256	157.48 ns	4.150 ns	3.679 ns	1.02	2112 B	0.99
DryIoc	256	51,437.41 ns	329.639 ns	292.216 ns	333.00	80098 B	37.64
SimpleInjector	256	411,296.90 ns	3,472.973 ns	3,078.700 ns	2,662.68	572947 B	269.24

baseline* rows show the corresponding Awaiten benchmark from the most recent successful main branch build with results, for regression comparison.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

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Test Results

   18 files  ± 0     18 suites  ±0   9m 1s ⏱️ +25s
1 299 tests + 2  1 298 ✅ + 2  1 💤 ±0  0 ❌ ±0 
6 294 runs  +14  6 293 ✅ +14  1 💤 ±0  0 ❌ ±0 

Results for commit 9a0d9ef. ± Comparison against base commit ebd1ab0.

This pull request removes 2 and adds 4 tests. Note that renamed tests count towards both.

Awaiten.Extensions.DependencyInjection.Tests.FeatureDetectionTests+AcceptedDivergences ‑ AnEnumerableOfAnUnmentionedElementTypeIsUnderReported
Awaiten.Extensions.DependencyInjection.Tests.MsDiConformanceTests+AbsentServices ‑ AnEnumerableOfAnUnmentionedTypeReturnsNull_KnownGap

Awaiten.Extensions.DependencyInjection.Tests.FeatureDetectionTests+AcceptedDivergences ‑ AnEnumerableOfAnUnmentionedValueElementTypeIsUnderReported
Awaiten.Extensions.DependencyInjection.Tests.MsDiConformanceTests+AbsentServices ‑ AnArrayOfAnUnmentionedTypeReturnsNull
Awaiten.Extensions.DependencyInjection.Tests.MsDiConformanceTests+AbsentServices ‑ AnEnumerableOfAnUnmentionedTypeIsEmpty
Awaiten.Extensions.DependencyInjection.Tests.MsDiConformanceTests+AbsentServices ‑ AnEnumerableOfAnUnmentionedValueTypeReturnsNull_AotConstraint