Clean up Uri.UnescapeDataString

[stephentoub]

• Use string.IndexOf rather than an open-coded, unsafe loop.
• Avoid an unnecessary SequenceEquals at the end: we're only at this point if a % was found highlighting that something escaped was found.
• Use stack memory for smaller inputs if possible, to avoid unnecessary ArrayPool interaction
• Remove an unnecessary argument to a helper function.
• Fix ValueStringBuilder.Grow to only copy the contained data.

This was really meant as a minor cleanup, e.g. to remove unsafe code that needn't be. It does however have a perf benefit for longer inputs when nothing needs to be unescaped (or when the thing that needs to be unescaped is later in the input), as then the use of IndexOf provides vectorization "for free".

Method	Toolchain	Repeats	InputMode	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
Unescape	\new\corerun.exe	4	ASCII	13.73 ns	0.096 ns	0.085 ns	0.85	0.02	-	-	-	-
Unescape	\old\corerun.exe	4	ASCII	16.10 ns	0.265 ns	0.248 ns	1.00	0.00	-	-	-	-
Unescape	\new\corerun.exe	4	Emoji	250.76 ns	2.311 ns	1.929 ns	0.92	0.02	0.0367	-	-	232 B
Unescape	\old\corerun.exe	4	Emoji	273.92 ns	3.279 ns	3.068 ns	1.00	0.00	0.0367	-	-	232 B
Unescape	\new\corerun.exe	4	Escaped	58.47 ns	0.480 ns	0.425 ns	0.80	0.01	0.0050	-	-	32 B
Unescape	\old\corerun.exe	4	Escaped	72.96 ns	0.974 ns	0.864 ns	1.00	0.00	0.0050	-	-	32 B
Unescape	\new\corerun.exe	4	Mixed	84.71 ns	1.392 ns	1.302 ns	0.71	0.02	0.0101	-	-	64 B
Unescape	\old\corerun.exe	4	Mixed	118.71 ns	1.265 ns	1.183 ns	1.00	0.00	0.0100	-	-	64 B
Unescape	\new\corerun.exe	40	ASCII	16.75 ns	0.061 ns	0.054 ns	0.19	0.00	-	-	-	-
Unescape	\old\corerun.exe	40	ASCII	86.24 ns	0.791 ns	0.740 ns	1.00	0.00	-	-	-	-
Unescape	\new\corerun.exe	40	Emoji	1,719.34 ns	4.388 ns	4.105 ns	1.01	0.01	0.2651	-	-	1672 B
Unescape	\old\corerun.exe	40	Emoji	1,702.36 ns	15.955 ns	14.144 ns	1.00	0.00	0.2651	-	-	1672 B
Unescape	\new\corerun.exe	40	Escaped	257.49 ns	5.161 ns	5.944 ns	0.97	0.02	0.0162	-	-	104 B
Unescape	\old\corerun.exe	40	Escaped	265.17 ns	3.782 ns	3.538 ns	1.00	0.00	0.0162	-	-	104 B
Unescape	\new\corerun.exe	40	Mixed	670.76 ns	6.920 ns	6.473 ns	1.13	0.02	0.0668	-	-	424 B
Unescape	\old\corerun.exe	40	Mixed	595.95 ns	7.990 ns	7.473 ns	1.00	0.00	0.0668	-	-	424 B
Unescape	\new\corerun.exe	400	ASCII	63.74 ns	0.579 ns	0.541 ns	0.09	0.00	-	-	-	-
Unescape	\old\corerun.exe	400	ASCII	743.76 ns	9.243 ns	8.646 ns	1.00	0.00	-	-	-	-
Unescape	\new\corerun.exe	400	Emoji	15,545.16 ns	123.640 ns	115.653 ns	0.96	0.01	2.5330	-	-	16072 B
Unescape	\old\corerun.exe	400	Emoji	16,178.76 ns	84.439 ns	78.984 ns	1.00	0.00	2.5330	-	-	16072 B
Unescape	\new\corerun.exe	400	Escaped	2,126.10 ns	40.104 ns	46.184 ns	1.00	0.03	0.1297	-	-	824 B
Unescape	\old\corerun.exe	400	Escaped	2,123.31 ns	16.610 ns	15.537 ns	1.00	0.00	0.1297	-	-	824 B
Unescape	\new\corerun.exe	400	Mixed	5,248.00 ns	43.814 ns	38.840 ns	0.97	0.01	0.6409	0.0076	-	4024 B
Unescape	\old\corerun.exe	400	Mixed	5,391.88 ns	62.970 ns	58.902 ns	1.00	0.00	0.6409	0.0076	-	4024 B

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Running;
using System;
using System.Linq;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    [Params(4, 40, 400)] public int Repeats { get; set; }

    [Params("ASCII", "Mixed", "Escaped", "Emoji")] public string InputMode { get; set; }

    private string _value;

    [GlobalSetup]
    public void Setup() =>
        _value = InputMode switch
        {
            "ASCII" => string.Concat(Enumerable.Repeat("abcd", Repeats)),
            "Mixed" => string.Concat(Enumerable.Repeat("abcd%2B", Repeats)),
            "Escaped" => string.Concat(Enumerable.Repeat("%2B", Repeats)),
            "Emoji" => string.Concat(Enumerable.Repeat("%F0%9F%98%80", Repeats)),
            _ => throw new Exception()
        };

    [Benchmark]
    public string Unescape() => Uri.UnescapeDataString(_value);
}

[benaadams]

Much cleaner