[iOS] IL2026 trim errors from Managed Registrar referencing HybridWebViewHandler nested classes

[PureWeen]

Description

When building a MAUI template app with TrimMode=full and TreatWarningsAsErrors=true on iOS, ILLink produces IL2026 errors from <Module>..cctor() referencing HybridWebViewHandler nested classes that have [RequiresUnreferencedCode].

This causes the RunOniOS_MauiReleaseTrimFull and RunOniOS_MauiReleaseTrimFull_CoreCLR integration tests to fail on the release/11.0.1xx-preview4 branch.

Error

ILLink : Trim analysis error IL2026: <Module>..cctor(): Using member
  'Microsoft.Maui.Handlers.HybridWebViewHandler.SchemeHandler.Handler.get'
  which has 'RequiresUnreferencedCodeAttribute' can break functionality
  when trimming application code. HybridWebView uses dynamic System.Text.Json
  serialization features.

Affected Members (all from <Module>..cctor() caller)

• HybridWebViewHandler.SchemeHandler..cctor()
• HybridWebViewHandler.SchemeHandler.GetResponseBytesAsync(String, NSUrlRequest, ILogger)
• HybridWebViewHandler.SchemeHandler.Handler.get
• HybridWebViewHandler.SchemeHandler.SchemeHandler(HybridWebViewHandler)
• HybridWebViewHandler.SchemeHandler.StartUrlSchemeTask(WKWebView, IWKUrlSchemeTask)
• HybridWebViewHandler.SchemeHandler.StopUrlSchemeTask(WKWebView, IWKUrlSchemeTask)
• HybridWebViewHandler.WebViewScriptMessageHandler..cctor()
• HybridWebViewHandler.WebViewScriptMessageHandler.DidReceiveScriptMessage(WKUserContentController, WKScriptMessage)
• HybridWebViewHandler.WebViewScriptMessageHandler.Handler.get

Root Cause

The iOS Managed Registrar generates a <Module>..cctor() that registers all NSObject subclasses with ObjC-exported interfaces. Both SchemeHandler (implements IWKUrlSchemeHandler) and WebViewScriptMessageHandler (implements IWKScriptMessageHandler) are NSObject subclasses marked with [RequiresUnreferencedCode].

The registrar-generated code references these types without propagating the trim suppression, causing IL2026 when TrimMode=full.

Context

• PRs Fix HybridWebView NativeAOT warning path maui#34958 and Move HybridWebView AOT helper from local function to private static method maui#34986 fixed the IL3050 (NativeAOT) warnings for HybridWebView — those are merged and working.
• This is IL2026 (trim analysis) from a different code path — the macios managed registrar.
• The [RequiresUnreferencedCode] attributes on the nested classes are correct and intentional.
• The fix likely needs to be in the managed registrar (xamarin-macios) to suppress IL2026 for types that already carry [RequiresUnreferencedCode].

Reproduction

Build a default MAUI template on iOS with:

dotnet build -c Release -f:net11.0-ios -p:TrimMode=full -p:TreatWarningsAsErrors=true -p:TrimmerSingleWarn=false

Build Reference

PR dotnet/maui#35171, Build 1402884 — RunOniOS_MauiReleaseTrimFull ARM64 job

[PureWeen]

@sbomer thoughts?