Split non-ASCII Uri normalization out of ParseRemaining and avoid temporary allocations

[MihaZupan]

Review without whitespace changes

When we rebuild the _string for inputs with non-ASCII chars, we do so in two places:

• PrivateParseMinimal recreates the string up to the path (scheme, user info, host, port)
• ParseRemaining recreates the path, query and fragment

ParseRemaining is currently written along the lines of

int idx;
int origIdx;
int length;

if (HasUnicode)
{
    _string += Normalize(_originalString, origIdx, ...);
    GetLengthWithoutTrailingSpaces(_string, ref length, idx);
}

CheckCanonical(_string, idx, ...);

if (HasUnicode)
{
    _string += Normalize(_originalString, origIdx, ...);
    GetLengthWithoutTrailingSpaces(_string, ref length, idx);
}

CheckCanonical(_string, idx, ...);

if (HasUnicode)
{
    _string += Normalize(_originalString, origIdx, ...);
    GetLengthWithoutTrailingSpaces(_string, ref length, idx);
}

CheckCanonical(_string, idx, ...);

This makes it much harder to reason about because we're juggling two offsets into two different strings, while also changing one of them.

This PR splits the rebuilding logic into a separate helper we call first that normalizes the path/query/fragment.

if (HasUnicode)
{
    Rebuild();
}

CheckCanonical(_string, idx, ...);
CheckCanonical(_string, idx, ...);
CheckCanonical(_string, idx, ...);

void Rebuild()
{
    var builder = ...;

    Normalize(ref builder, span, ...);
    Normalize(ref builder, span, ...);
    Normalize(ref builder, span, ...);

    _string = builder.ToString();
}

Since all the transformations now happen in a single place, we can also easily avoid the ~5 temporary string allocations.
We're still allocating the host separately, but that can be improved later.

Benchmark

[MemoryDiagnoser(false)]
public class UriInit
{
    private string _uriString;

    [Params("Simple", "LongPath", "ShortSections")]
    public string Input;

    [GlobalSetup]
    public void Setup() => _uriString = Input switch
    {
        "Simple" => "https://mihubot.xyz/api/RuntimeUtils/CoreRoot/All?arch=x64&os=linux&type=release#hello",
        "LongPath" => $"https://contoso.com/{new string('a', 400)}?b#c",
        "ShortSections" => $"https://a@host/b?c#d"
    } + "🍉";

    [Benchmark]
    public Uri Ctor() => new Uri(_uriString);
}

Method	Toolchain	Input	Mean	Error	Ratio	Allocated	Alloc Ratio
Ctor	main	LongPath	666.8 ns	15.24 ns	1.00	3752 B	1.00
Ctor	pr	LongPath	552.8 ns	3.87 ns	0.83	1128 B	0.30
Ctor	main	ShortSections	158.4 ns	1.52 ns	1.00	608 B	1.00
Ctor	pr	ShortSections	122.4 ns	0.69 ns	0.77	360 B	0.59
Ctor	main	Simple	225.1 ns	1.82 ns	1.00	968 B	1.00
Ctor	pr	Simple	195.3 ns	3.08 ns	0.87	448 B	0.46

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[Copilot]

Pull request overview

This PR refactors the URI normalization logic to improve code maintainability and reduce memory allocations for URIs containing non-ASCII characters. The key change splits the string rebuilding logic out of ParseRemaining into a dedicated helper method ParseRemaining_RecreateNormalizedString, eliminating approximately 5 temporary string allocations per URI construction.

Key changes:

• Refactored ParseRemaining to separate Unicode normalization logic into ParseRemaining_RecreateNormalizedString
• Changed IriHelper.EscapeUnescapeIri from returning a string to accepting a ref ValueStringBuilder parameter, enabling allocation reuse
• Simplified UriSyntax helper methods by inlining IsFullMatch

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Private.Uri/tests/UnitTests/IriEscapeUnescapeTest.cs	Updated test helper to use new EscapeUnescapeIri API with ValueStringBuilder
src/libraries/System.Private.Uri/src/System/UriSyntax.cs	Simplified flag checking methods by inlining logic
src/libraries/System.Private.Uri/src/System/UriExt.cs	Removed obsolete EscapeUnescapeIri wrapper method
src/libraries/System.Private.Uri/src/System/Uri.cs	Split normalization logic into separate helper; updated to use new IriHelper API; minor BOM addition
src/libraries/System.Private.Uri/src/System/IriHelper.cs	Changed API from returning string to void with ref ValueStringBuilder parameter; removed unnecessary parentheses

[liveans]

LGTM with a question and nits