XamlC should optimize OnPlatform

[eerhardt]

Description

In the .NET Podcasts app, it has the following XAML:

https://github.com/microsoft/dotnet-podcasts/blob/00be1bb25b6940fee77f0e7b711a04dea2358ced/src/Mobile/Controls/HeaderControl.xaml#L9-L32

            <OnPlatform x:TypeArguments="View">
                <On Platform="UWP, macOS">
                    <Grid RowDefinitions="*, auto">
                        <SearchBar x:Name="searchBar"
                                   HorizontalOptions="Start"
                                   WidthRequest="460"
                                   Text="{Binding TextToSearch, Source={x:Reference selfMediaElementView}}"
                                   SearchCommand="{Binding SearchCommand, Source={x:Reference selfMediaElementView}}" />
                        <Label Text="See All Categories"
                               Style="{StaticResource BodyLLabelStyle}"
                               TextColor="{StaticResource Primary}"
                               Grid.Row="1"
                               HorizontalOptions="End"
                               Margin="0,0,30,0"
                               IsVisible="{Binding ShowSearchCategories, Source={x:Reference selfMediaElementView}}"
                               FontSize="16">
                            <Label.GestureRecognizers>
                                <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped" />
                            </Label.GestureRecognizers>
                        </Label>
                    </Grid>
                </On>
                <On Platform="Android,iOS">
                    <Grid ColumnDefinitions="*,*"
                          Margin="16,12">
                        <Image Source="{AppThemeBinding Light=logo_header_horizontal.png,Dark=logo_color_horizontal_darkmode.png}"
                               HeightRequest="34"
                               WidthRequest="125"
                               VerticalOptions="Center"
                               HorizontalOptions="Start" />
                        <Image Source="search.png"
                               Grid.Column="1"
                               VerticalOptions="Center"
                               HorizontalOptions="End"
                               HeightRequest="20"
                               WidthRequest="20">
                            <Image.GestureRecognizers>
                                <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped" />
                            </Image.GestureRecognizers>
                        </Image>
                    </Grid>
                </On>
            </OnPlatform>

When building the app for Android, I'm still seeing the controls inside the <On Platform="UWP, macOS"> clause get created and properties set on them. For example, the SearchBar is only used in UWP, macOS, but in my compiled Android app I see:

private void InitializeComponent()
{
	ReferenceExtension referenceExtension = new ReferenceExtension();
	BindingExtension bindingExtension = new BindingExtension();
	ReferenceExtension referenceExtension2 = new ReferenceExtension();
	BindingExtension bindingExtension2 = new BindingExtension();
	SearchBar searchBar = new SearchBar();

This pattern is used in 3 places in the .NET Podcasts app:

1. The above
2. The Player control
3. The ShowDetailPage.

We should optimize this pattern so when building for Android, the other platforms' controls are not created.

cc @StephaneDelcroix @jonathanpeppers

Steps to Reproduce

1. git clone https://github.com/microsoft/dotnet-podcasts.git
2. cd dotnet-podcasts/src/Mobile
3. dotnet build Microsoft.NetConf2021.Maui.csproj -t:Run -c Release -f net6.0-android -r android-arm64
4. Inspect the IL of HeaderControl.InitializeComponent()

Version with bug

Preview 14 (current)

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android, Windows, macOS

Affected platform versions

All

Did you find any workaround?

No response

Relevant log output

No response

[StephaneDelcroix]

#4829 was about the {OnPlatform} markup extension, not the <OnPlatform\> generic construct. Looking if we can apply the same treatment over here...