[dotnet-linker] Use [DynamicDependency] attributes instead of manual marking when preserving block code.

[rolfbjarne]

This makes it easier to move this code out of a custom linker step in the future.

Contributes towards #17693.

[Copilot]

Pull request overview

This PR updates the dotnet-linker workflow to preserve block-related code using [DynamicDependency] attributes (via assembly rewriting) instead of manual marking, aligning with the effort to remove custom linker-step dependencies (issue #17693).

Changes:

• Introduces a new PreserveBlockCodeStep that injects [DynamicDependency] attributes to preserve the Handler field and Invoke method for block trampolines.
• Refactors assembly-rewriting steps by adding an AssemblyModifierStep base and an IsActiveFor hook to ConfigurationAwareStep.
• Adds MSBuild toggles to choose between the new dynamic-dependency-based preservation and the legacy mark handler.

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
tools/dotnet-linker/Steps/PreserveBlockCodeStep.cs	New step that rewrites assemblies to preserve block code via dynamic dependencies.
tools/dotnet-linker/Steps/PreserveBlockCodeHandler.cs	Reuses the new member-detection helper; remains as fallback when not using dynamic dependencies.
tools/dotnet-linker/Steps/ConfigurationAwareStep.cs	Adds IsActiveFor(AssemblyDefinition) gating before processing assemblies.
tools/dotnet-linker/Steps/AssemblyModifierStep.cs	New base step to centralize AppBundleRewriter usage and nested-type traversal.
tools/dotnet-linker/PreserveSmartEnumConversionsStep.cs	Migrates to AssemblyModifierStep and uses the new activation hook.
tools/dotnet-linker/PreserveProtocolsStep.cs	Migrates to AssemblyModifierStep and uses the new activation hook.
tools/dotnet-linker/AppBundleRewriter.cs	Adds overload to create dynamic dependency attributes for methods + fixes param doc for fields.
dotnet/targets/Xamarin.Shared.Sdk.targets	Adds an MSBuild switch to enable dynamic-dependency-based block code preservation and conditionally disables the old handler.

Comments suppressed due to low confidence (2)

tools/dotnet-linker/PreserveSmartEnumConversionsStep.cs:70

• AssemblyModifierStep already recurses into nested types, but ProcessType still does its own nested-type recursion. This results in nested types being processed twice (and potentially adding the same dynamic dependencies multiple times, even if later deduped). Remove the recursion in ProcessType and rely on the base class traversal.

		protected override bool ProcessType (TypeDefinition type)
		{
			var modified = false;

			if (type.HasNestedTypes) {
				foreach (var nested in type.NestedTypes)
					modified |= ProcessType (nested);
			}

tools/dotnet-linker/PreserveProtocolsStep.cs:65

• AssemblyModifierStep already recursively walks nested types via ProcessTypeImpl, but this override also manually recurses into type.NestedTypes. That will process nested types twice (extra Resolve()/attribute work). Remove the manual recursion here and let AssemblyModifierStep handle nested types.

		protected override bool ProcessType (TypeDefinition type)
		{
			var modified = false;

			if (type.HasNestedTypes) {
				foreach (var nested in type.NestedTypes)
					modified |= ProcessType (nested);
			}

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