Avoid redundant boxing in ValueTuple<T1,...,T7,TRest>

[prozolic]

This PR use pattern variable capture (is IValueTupleInternal rest) instead of a separate type-check followed by a cast to IValueTupleInternal. This avoids a redundant boxing of TRest when it is a ValueTuple<TRest> containing reference-type fields, reducing heap allocations in GetHashCode, ToString, ITuple.Length, and the ITuple indexer.

Benchmark

Method	Job	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
GetHashCode_Int8	Job-QIDXWP	\main\corerun.exe	5.260 ns	0.1494 ns	0.1720 ns	5.318 ns	4.939 ns	5.557 ns	1.00	0.00	-	-	NA
GetHashCode_Int8	Job-TLPBEL	\pr\corerun.exe	4.964 ns	0.1389 ns	0.1599 ns	4.911 ns	4.764 ns	5.344 ns	0.94	0.04	-	-	NA
GetHashCode_Str8	Job-QIDXWP	\main\corerun.exe	13.155 ns	0.5284 ns	0.5873 ns	13.028 ns	12.442 ns	14.690 ns	1.00	0.00	0.0028	24 B	1.00
GetHashCode_Str8	Job-TLPBEL	\pr\corerun.exe	8.563 ns	0.1916 ns	0.2206 ns	8.447 ns	8.311 ns	8.969 ns	0.65	0.03	-	-	0.00
ToString_Int8	Job-QIDXWP	\main\corerun.exe	72.434 ns	2.1456 ns	2.4709 ns	71.774 ns	69.573 ns	76.946 ns	1.00	0.00	0.0304	256 B	1.00
ToString_Int8	Job-TLPBEL	\pr\corerun.exe	70.706 ns	1.8968 ns	2.1844 ns	70.259 ns	67.266 ns	75.064 ns	0.98	0.04	0.0305	256 B	1.00
ToString_Str8	Job-QIDXWP	\main\corerun.exe	76.118 ns	2.1219 ns	2.4436 ns	76.160 ns	71.786 ns	81.114 ns	1.00	0.00	0.0334	280 B	1.00
ToString_Str8	Job-TLPBEL	\pr\corerun.exe	71.267 ns	1.5937 ns	1.8353 ns	70.953 ns	68.286 ns	74.699 ns	0.94	0.04	0.0305	256 B	0.91
Length_Int8	Job-QIDXWP	\main\corerun.exe	4.052 ns	0.1157 ns	0.1287 ns	4.069 ns	3.882 ns	4.256 ns	1.00	0.00	0.0057	48 B	1.00
Length_Int8	Job-TLPBEL	\pr\corerun.exe	4.284 ns	0.1248 ns	0.1438 ns	4.287 ns	4.080 ns	4.595 ns	1.06	0.05	0.0057	48 B	1.00
Length_Str8	Job-QIDXWP	\main\corerun.exe	10.711 ns	0.2865 ns	0.3184 ns	10.782 ns	10.178 ns	11.242 ns	1.00	0.00	0.0095	80 B	1.00
Length_Str8	Job-TLPBEL	\pr\corerun.exe	6.129 ns	0.1166 ns	0.1248 ns	6.155 ns	5.816 ns	6.290 ns	0.57	0.02	0.0067	56 B	0.70
Indexer_Int8	Job-QIDXWP	\main\corerun.exe	6.039 ns	0.2171 ns	0.2500 ns	6.036 ns	5.719 ns	6.515 ns	1.00	0.00	0.0086	72 B	1.00
Indexer_Int8	Job-TLPBEL	\pr\corerun.exe	6.479 ns	0.2155 ns	0.2395 ns	6.487 ns	6.112 ns	7.022 ns	1.07	0.06	0.0086	72 B	1.00
Indexer_Str8	Job-QIDXWP	\main\corerun.exe	11.339 ns	0.2880 ns	0.3201 ns	11.413 ns	10.752 ns	11.848 ns	1.00	0.00	0.0095	80 B	1.00
Indexer_Str8	Job-TLPBEL	\pr\corerun.exe	6.356 ns	0.1778 ns	0.1976 ns	6.367 ns	6.058 ns	6.811 ns	0.56	0.02	0.0067	56 B	0.70

    public class Perf_ValueTuple
    {
        private ValueTuple<int, int, int, int, int, int, int, ValueTuple<int>> _tupleInt8;
        private ValueTuple<int, int, int, int, int, int, int, ValueTuple<string>> _tupleStr8;

        [GlobalSetup]
        public void Setup()
        {
            _tupleInt8 = new(1, 2, 3, 4, 5, 6, 7, new ValueTuple<int>(8));
            _tupleStr8 = new(1, 2, 3, 4, 5, 6, 7, new ValueTuple<string>("8"));
        }

        [Benchmark]
        public int GetHashCode_Int8() => _tupleInt8.GetHashCode();

        [Benchmark]
        public int GetHashCode_Str8() => _tupleStr8.GetHashCode();

        [Benchmark]
        public string ToString_Int8() => _tupleInt8.ToString();

        [Benchmark]
        public string ToString_Str8() => _tupleStr8.ToString();

        [Benchmark]
        public int Length_Int8() => ((ITuple)_tupleInt8).Length;

        [Benchmark]
        public int Length_Str8() => ((ITuple)_tupleStr8).Length;

        [Benchmark]
        public object Indexer_Int8() => ((ITuple)_tupleInt8)[7];

        [Benchmark]
        public object Indexer_Str8() => ((ITuple)_tupleStr8)[7];
    }

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Refactors ValueTuple.cs to use pattern matching with type-checked variable declarations, replacing redundant cast expressions after is checks.

Changes:

• Replaces is IValueTupleInternal checks followed by ((IValueTupleInternal)Rest) casts with is IValueTupleInternal rest pattern matching.
• Applies the change in GetHashCode, ToString, IValueTupleInternal.ToStringEnd, ITuple.Length, and the indexer.

[EgorBo]

There is a known issue somewhere against JIT for this pattern around shared generics, but I think it's fine to accept this change anyway as it simplifies code.

[prozolic]

Thank you for the review!
Just to add some context, ValueTuple<T1,...,T7,TRest> .GetHashCodeCore was already using pattern variable capture (is IValueTupleInternal rest). So I applied this partly to keep the pattern usage consistent.

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