Avoid Substring() call in ConstructFunction

[Erarndt]

Fixes #
Helps with
#10961

Context

The existing implementation of ConstructFunction() called Substring() and passed that value to a handful of helper functions. It turns out that we can wrap the string in ReadOnlyMemory<char> and pass that to the helper functions instead to avoid the additional allocation. Some of the functions needed to be updated to accept the new type.

Before:

After:

Changes Made

Testing

Notes

[Copilot]

Pull Request Overview

This PR refactors ConstructFunction and related helpers in Expander.cs to eliminate intermediate Substring allocations by leveraging ReadOnlySpan<char> and ReadOnlyMemory<char>.

• Changed ScanForClosingParenthesis and ScanForClosingQuote to operate on spans.
• Updated ExtractFunctionArguments to accept a ReadOnlyMemory<char> and use span-based slicing.
• Replaced most Substring calls with span/memory slicing, except for a remaining Substring in ConstructIndexerFunction.

Comments suppressed due to low confidence (3)

src/Build/Evaluation/Expander.cs:708

• The XML doc comment still refers to string expression, but the parameter is now ReadOnlySpan<char>. Update the summary and <param> tags to match the new type.

private static int ScanForClosingParenthesis(ReadOnlySpan<char> expression, int index, out bool potentialPropertyFunction, out bool potentialRegistryFunction)

src/Build/Evaluation/Expander.cs:3925

• [nitpick] Mixing string and span/memory APIs can be confusing. Consider changing this method to accept ReadOnlyMemory<char> or ReadOnlySpan<char> for expressionFunction, so slicing logic is uniform and allocations are minimized.

private static void ConstructIndexerFunction(string expressionFunction, IElementLocation elementLocation, object propertyValue, int methodStartIndex, int indexerEndIndex, ref FunctionBuilder<T> functionBuilder)

src/Build/Evaluation/Expander.cs:828

• The new span-based parsing logic introduces multiple edge cases (nested properties, quotes, commas). Add unit tests covering mismatched parentheses/quotes and empty or complex argument lists to ensure correctness.

private static string[] ExtractFunctionArguments(IElementLocation elementLocation, string expressionFunction, ReadOnlyMemory<char> argumentsMemory)

[SimaTian]

This directly helps with
#10961
I like it.