Change MS.Internal.Span (ItemSpan.h) to value struct to reduce text related internal allocations

[nietras]

For years I have observed MS.Internal.Span turn up as top 10 object being allocated in WPF app. Here I suggest to finally address this by turning the very small and simple Span(object element, int length) into a value type instead of reference type to reduce GC pressure and total memory use. I do not know how WPF does benchmarks or what metric is used to verify this change. I understand it changes a long standing internal type and hope the work is not seen as futile or irrelevant.

Example object allocation count for a WPF app that continuously updates simple TextBlocks with counts or similar. Span is almost always number one in any of our WPF apps. With other WPF allocs top too. Perhaps if this PR is accepted those can be addressed too.

Description

Changes internal Span type from reference type to value to reduce text related managed heap allocations and GC pressure. To ensure correctness type also changed to readonly/initonly and minor necessary related code changed related to this.

Customer Impact

Reduced GC pressure.

Risk

Minor breaking internal change which may cause unexpected usage or similar.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR reduces GC pressure in WPF text-related code paths by changing the internal MS.Internal.Span (defined in ItemSpan.h) from a reference type to a value type, and updating surrounding APIs/call sites to preserve correctness under value-type copy semantics.

Changes:

• Converted Span in the DirectWrite forwarder layer from ref struct to value struct (with initonly fields) and updated related IList<Span>-based APIs.
• Added/used “set-in-place” APIs (SetAt) so callers can replace spans in backing vectors/lists when lengths change (instead of mutating a referenced object).
• Updated text formatting code (TextStore, SpanVector) to rebuild-and-store updated spans rather than mutating fields on a shared reference.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Microsoft.DotNet.Wpf/src/Shared/MS/Utility/FrugalList.cs	Adds SetAt to support replacing items in frugal storage (needed for value-type spans).
src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/TextFormatting/TextStore.cs	Replaces span mutation with “recreate + SetAt” to handle value-type span updates.
src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/Span.cs	Updates span vector logic for value-type semantics and exposes SetAt on SpanVector.
src/Microsoft.DotNet.Wpf/src/DirectWriteForwarder/CPP/DWriteWrapper/TextItemizer.h	Updates itemizer API to return IList<Span> instead of IList<Span^>.
src/Microsoft.DotNet.Wpf/src/DirectWriteForwarder/CPP/DWriteWrapper/TextItemizer.cpp	Updates list element creation to add Span values (not heap-allocated Span^).
src/Microsoft.DotNet.Wpf/src/DirectWriteForwarder/CPP/DWriteWrapper/TextAnalyzer.h	Updates analyzer APIs to return IList<Span> instead of IList<Span^>.
src/Microsoft.DotNet.Wpf/src/DirectWriteForwarder/CPP/DWriteWrapper/TextAnalyzer.cpp	Updates analyzer implementation and exception strings for the new Span value type.
src/Microsoft.DotNet.Wpf/src/DirectWriteForwarder/CPP/DWriteWrapper/ItemSpan.h	Converts Span to a value struct and makes fields initonly to enforce immutability.

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