feat: cover ReactiveUI.SourceGenerators' ObservableAsProperty and IViewFor behaviour

[glennawatson]

Summary

[ObservableAsProperty] and view registration cover what ReactiveUI.SourceGenerators did, so it can drop its copies.

• [ObservableAsProperty] takes InitialValue, ReadOnly and UseProtected. The property returns its initial value until its helper is assigned. A non-string initial value is an expression, evaluated once, bound with the declaring file's usings. A non-nullable string property starts as string.Empty.
• RXUIBIND018 and a code fix move ReactiveUI.SourceGenerators' other forms onto partial properties. A field, a method returning an observable, and an observable property marked [ObservableAsProperty] get nothing generated. The fix rewrites a field as a property, carrying its initializer and Inheritance across. It gives a method or observable property a property of its own, assigned in a written InitializeOAPH method. The attribute is valid on those members so older code compiles, and PropertyName and Inheritance exist for the fix.
• RXUIBIND019 reports a marked method that takes parameters.
• Views declared with ReactiveUI.SourceGenerators' [IViewFor<T>] or [IViewFor("TypeName")] register. That generator adds IViewFor<T> in output other generators cannot see, so the attribute is read directly and the lookup checks for the interface when it resolves the view. RegistrationType LazySingleton and Constant keep one instance.
• Views register in a ReactiveUI.Binding.Reactive consumer. Only the lean IViewFor<T> was matched, so a System.Reactive consumer got no generated view lookup.
• Every DebuggerDisplay starts with the name of its type, and all of them are ASCII.

Why

ReactiveUI.SourceGenerators is removing [ObservableAsProperty] and [IViewFor] view registration because this library provides them (reactiveui/ReactiveUI.SourceGenerators#501). The options, forms and attributes above were the gaps. A debugger showing a bare value gave no hint of what was being looked at.

Breaking changes

• A non-nullable string property marked [ObservableAsProperty] now returns string.Empty rather than null before its helper is assigned.
• [ObservableAsProperty] on a field, a method or a non-partial property now reports RXUIBIND018 rather than doing nothing silently.
• Not taken from ReactiveUI.SourceGenerators: ReadOnly defaults to false, so existing code that assigns a helper outside a constructor keeps compiling. The [Obsolete] RegisterViewsForViewModelsSourceGenerated shim is not generated here, because it would declare the same type in every consumer assembly.

How this was verified

• Tests cover each option at run time in both runtime flavours, every field shape from ReactiveUI.SourceGenerators' own tests rewritten by the fix and then run, and the method and observable-property forms observed through WhenAnyValue.
• Views declared by attribute are resolved end to end, with a stand-in generator adding the interface the way ReactiveUI.SourceGenerators does. A System.Reactive consumer's views resolve too.
• Ran the Linux examples suite and the Windows examples on a real Windows machine.

Notes for the reviewer

• Start with ObservableAsPropertyGenerator.cs, ViewRegistrationExtractor.cs, and ObservableAsPropertyCodeFixProvider.cs with ObservableAsPropertyRewriter.cs.
• Mechanical: the DebuggerDisplay edits (a Roslyn rewrite adding the type name) and the regenerated PublicAPI.txt baselines, which change with them.
• The generator test projects reference the analyzer under the analyzer extern alias, because the analyzer compiles its own copies of the generator's shared internal types.
• Out of scope: attribute forwarding and the CS0657/CA1822 suppressions from the request.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.18%. Comparing base (1c622f6) to head (70caf4c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   99.15%   99.18%   +0.02%     
==========================================
  Files         370      374       +4     
  Lines       13416    13794     +378     
  Branches     1768     1880     +112     
==========================================
+ Hits        13303    13681     +378     
  Misses        113      113              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.