build(deps): bump ReactiveUI.Binding to 8.3.0 (#4520)

## Summary

**ReactiveUI now brings ReactiveUI.Binding 8.3.0, which reads members
ReactiveUI.SourceGenerators writes.**

- Bump `BindingVersion` to 8.3.0 in `src/Directory.Packages.props`,
covering `ReactiveUI.Binding` and `ReactiveUI.Binding.Reactive`.
- README: drop the advice to use a `[Reactive]` partial property for
anything passed to `WhenAnyValue` (Binding 8.3.0 fixes that gap), and
mention the new diagnostic RXUIBIND021 where the README covers Binding's
analyzers.
- Fix a real RXUIBIND021 hit the new analyzer found in
`ReactiveUI.Samples.Maui`: `Username`, `Password`, `LoginButton` and
`CancelButton` are fields the MAUI XAML compiler writes into
`LoginPage`, not members ReactiveUI.SourceGenerators writes, so
`Bind`/`BindCommand` had no generated binding for them. Switch those
four calls to `BindUnsafe`/`BindCommandUnsafe`, and convert
`LoginViewModel`'s hand-rolled `UserName`/`Password` properties to
`[Reactive]` partial properties.

## Why

Picks up ReactiveUI.Binding 8.3.0, whose source generator now writes
bindings for `[Reactive]` fields and partial properties,
`[ReactiveCollection]`, `[BindableDerivedList]`, `[ReactiveCommand]` and
`[IReactiveObject]` members that ReactiveUI.SourceGenerators adds.
Building the repo against it with `-warnaserror` surfaced one real
RXUIBIND021 case, fixed above.

## Breaking changes

None.

## How this was verified

Full solution build with `-warnaserror` is clean on every target
framework, including the Windows-only ones cross-compiled on Linux.
Built and ran every test project and the Windows-only examples (WPF,
WinForms, WinUI, MAUI) on a real Windows host, including the fixed
`ReactiveUI.Samples.Maui`. Ran the documentation example output checks
against every example project that runs on Linux; all match their
expected output. `ReactiveUI.Device.Tests` was not run; it needs an
emulator, per this repo's own CI split.

## Notes for the reviewer

The `Directory.Packages.props` change only touches `BindingVersion`, to
avoid a merge conflict with the concurrent `ReactiveUI.SourceGenerators`
version bump on another branch. The `ReactiveUI.Samples.Maui` fix is the
only behavior change outside dependency and doc updates; the rest is
mechanical.

## Checklist
- [x] I have read the [Contribute
guide](https://www.reactiveui.net/contribute/index.html)
- [x] The PR title follows [Conventional
Commits](https://www.conventionalcommits.org/)
- [x] Tests cover this change, or the summary says why they do not
- [ ] New or changed public API has XML documentation

Author: glennawatson

README.md

@@ -147,15 +147,14 @@ your project references and write code for that flavour.
 
 - **Remove your own reference to ReactiveUI.SourceGenerators**, or set it to the version ReactiveUI brings or later.
   An older version fails to restore with error NU1605.
-- **Use a partial property for any property you pass to `WhenAnyValue`.** `[Reactive]` also works on a field, but
-  ReactiveUI.Binding cannot see a property the generators write from a field. A `WhenAnyValue` call on that property
-  throws at run time.
 
 ### Analyzers
 
 An **analyzer** checks your code as you type and reports problems as warnings or errors. ReactiveUI.Binding and
 ReactiveUI.SourceGenerators run their analyzers in your project. They report binding calls and attributes that the
-generators cannot handle.
+generators cannot handle. Diagnostic RXUIBIND021 flags a binding call with no generated binding behind it, such as
+one that targets a member another source generator adds or a stored lambda; that call throws at run time, so treat
+the warning as a bug to fix rather than suppress.
 
 The analyzers inside ReactiveUI.Primitives stay out of your project. ReactiveUI references ReactiveUI.Primitives with
 `ExcludeAssets="analyzers"`. To use them, reference ReactiveUI.Primitives directly:

src/Directory.Packages.props

@@ -7,7 +7,7 @@
   <PropertyGroup Label="Shared Version Variables">
     <SplatVersion>21.0.0SplatVersion>
     <PrimitivesVersion>8.2.0PrimitivesVersion>
-    <BindingVersion>8.2.0BindingVersion>
+    <BindingVersion>8.3.0BindingVersion>
     <TUnitVersion>1.69.16TUnitVersion>
     

src/examples/ReactiveUI.Samples.Maui/LoginPage.xaml.cs

@@ -26,16 +26,20 @@ public LoginPage()
 
         _ = this.WhenActivated(d =>
         {
-            _ = this.Bind(ViewModel, vm => vm.UserName, v => v.Username.Text)
+            // Username, Password, LoginButton and CancelButton are fields the MAUI XAML compiler
+            // writes into this partial class from LoginPage.xaml, not members ReactiveUI.SourceGenerators
+            // adds, so ReactiveUI.Binding has no generated binding for them. The Unsafe calls are the
+            // documented fallback (RXUIBIND021) for a destination member another source generator owns.
+            _ = this.BindUnsafe(ViewModel, static vm => vm.UserName, static v => v.Username.Text)
                 .DisposeWith(d);
 
-            _ = this.Bind(ViewModel, vm => vm.Password, v => v.Password.Text)
+            _ = this.BindUnsafe(ViewModel, static vm => vm.Password, static v => v.Password.Text)
                 .DisposeWith(d);
 
-            _ = this.BindCommand(ViewModel, vm => vm.Login, v => v.LoginButton)
+            _ = this.BindCommandUnsafe(ViewModel, static vm => vm.Login, static v => v.LoginButton, toEvent: null)
                 .DisposeWith(d);
 
-            _ = this.BindCommand(ViewModel, vm => vm.Cancel, v => v.CancelButton)
+            _ = this.BindCommandUnsafe(ViewModel, static vm => vm.Cancel, static v => v.CancelButton, toEvent: null)
                 .DisposeWith(d);
 
             _ = ViewModel.Login

src/examples/ReactiveUI.Samples.Maui/LoginViewModel.cs

@@ -4,12 +4,13 @@
 // See the LICENSE file in the project root for full license information.
 
 using System.Diagnostics;
+using ReactiveUI.SourceGenerators;
 
 namespace ReactiveUI.Samples.Maui;
 
 /// A view model that handles user login with reactive validation and async execution.
 [DebuggerDisplay("LoginViewModel UserName={UserName}")]
-public class LoginViewModel : ReactiveObject, IDisposable
+public partial class LoginViewModel : ReactiveObject, IDisposable
 {
     /// Cancellation source for the in-flight login operation, signalled by the  command.
     private CancellationTokenSource? _loginCancellation;
@@ -52,18 +53,12 @@ public LoginViewModel(ISequencer scheduler)
     }
 
     /// Gets or sets the user name.
-    public string? UserName
-    {
-        get;
-        set => this.RaiseAndSetIfChanged(ref field, value);
-    }
+    [Reactive]
+    public partial string? UserName { get; set; }
 
     /// Gets or sets the password.
-    public string? Password
-    {
-        get;
-        set => this.RaiseAndSetIfChanged(ref field, value);
-    }
+    [Reactive]
+    public partial string? Password { get; set; }
 
     /// Gets the login command. Returns true on success, false on failure.
     public ReactiveCommand<RxVoid, bool> Login { get; }