Switch ItemExpressionCapture to a struct

[Erarndt]

Fixes #

Context

We can avoid several allocations by changing the usage of ItemExpressionCapture. Switching from a class to a struct avoids a large number of allocations, and we can further reduce the allocations using a struct-based enumerator rather than constructing a list.

Before:

After:

Changes Made

Testing

Notes

[Copilot]

Pull Request Overview

This PR changes ItemExpressionCapture from a class to a struct to reduce heap allocations, replaces the list-based item-expression capture with a struct-based enumerator, and updates all callers and tests to use the new pattern.

• Convert ItemExpressionCapture to a struct and make capture methods return nullable structs
• Introduce ReferencedItemExpressionsEnumerator in place of List<ItemExpressionCapture>
• Update callers and unit tests to use MoveNext() and Current

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/Build/Evaluation/LazyItemEvaluator.cs	Unwrap nullable struct before passing to helper
src/Build/Evaluation/ItemSpec.cs	Unwrap nullable struct when constructing fragments
src/Build/Evaluation/ExpressionShredder.cs	Define ReferencedItemExpressionsEnumerator and refactor enumeration code
src/Build/Evaluation/Expander.cs	Adapt callers to use the new struct enumerator
src/Build.UnitTests/Evaluation/ExpressionShredder_Tests.cs	Update tests to use MoveNext() and Current

Comments suppressed due to low confidence (3)

src/Build/Evaluation/ExpressionShredder.cs:103

• [nitpick] Consider adding XML doc-comments for MoveNext and Current to clarify usage, and indicate that reset behavior is not supported.

        internal struct ReferencedItemExpressionsEnumerator

src/Build/Evaluation/ExpressionShredder.cs:107

• [nitpick] The field currentIndex is a bit generic; consider renaming to something like scanPosition or cursor to better convey its role in the enumerator.

            private int currentIndex;

src/Build/Evaluation/ExpressionShredder.cs:103

• [nitpick] Rather than a standalone struct, consider implementing IEnumerator<ItemExpressionCapture> to align with standard .NET enumeration patterns and enable use in foreach.

        internal struct ReferencedItemExpressionsEnumerator