[Rgen] Use a roslyn code generator to help generate the flag parsing …

…of all attributes.

The old Xamarin API used A LOT of attributes as flags to indicate
different generating methods. All this attributes do not have any data
and can be used as a feature flag.

For example, if we wanted to generated a static class we can do:
```csharp

[Static]
interface MyStaticClass {}
```
In this commit we are relaying in a custom roslyn code generator that
parses an attribute and generates the flag properties for our data
model. All the code is very repetitive and specially error prone. The
generator allows use just to focus on the name of the attribute and the
targets to which the attribute can be applied.

When you do the review you'll notice that the targets used in the
AttributeNames in the transformer do not match the ones in the
bgen/Attributes.cs file. The mismatch is because people back in the day
were careless and used the default flag, which is All. That is a
mistake, so I went throgh the trouble of using the correct target and
add a document string explaining the use of the flag and therefore the
reasoning of the updated target.

A sample of the generated code can be found
[here](https://gist.github.com/mandel-macaque/ed4277457a3afa6606af0f78ee9bf07e)

src/rgen/.gitignore

@@ -0,0 +1 @@
+Generated

src/rgen/Microsoft.Macios.Generator/DataModel/Parameter.cs

@@ -3,6 +3,7 @@
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.InteropServices;
 using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -13,7 +14,8 @@ namespace Microsoft.Macios.Generator.DataModel;
 /// <summary>
 /// Readonly structure that represents a change in a parameter.
 /// </summary>
-readonly struct Parameter : IEquatable<Parameter> {
+[StructLayout (LayoutKind.Auto)]
+readonly partial struct Parameter : IEquatable<Parameter> {
 	/// <summary>
 	/// Parameter position in the method.
 	/// </summary>

src/rgen/Microsoft.Macios.Generator/DataModel/TypeInfo.cs

@@ -3,6 +3,7 @@
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.InteropServices;
 using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.Macios.Generator.Attributes;
@@ -13,6 +14,7 @@ namespace Microsoft.Macios.Generator.DataModel;
 /// <summary>
 /// Readonly structure that represents a change in a method return type.
 /// </summary>
+[StructLayout (LayoutKind.Auto)]
 readonly partial struct TypeInfo : IEquatable<TypeInfo> {
 
 	public static TypeInfo Void = new ("void", SpecialType.System_Void) { Parents = ["System.ValueType", "object"], };

src/rgen/Microsoft.Macios.Generator/TabbedStringBuilder.Availability.cs

@@ -0,0 +1,56 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using Microsoft.Macios.Generator.Availability;
+using Xamarin.Utils;
+
+namespace Microsoft.Macios.Generator;
+
+partial class TabbedStringBuilder {
+
+	public TabbedStringBuilder AppendMemberAvailability (in SymbolAvailability allPlatformsAvailability)
+	{
+		foreach (var availability in allPlatformsAvailability.PlatformAvailabilities) {
+			var platformName = availability.Platform.AsString ().ToLower ();
+			if (availability.SupportedVersion is not null) {
+				var versionStr = (PlatformAvailability.IsDefaultVersion (availability.SupportedVersion))
+					? string.Empty
+					: availability.SupportedVersion.ToString ();
+				AppendLine ($"[SupportedOSPlatform (\"{platformName}{versionStr}\")]");
+			}
+
+			// loop over the unsupported versions of the platform 
+			foreach (var (version, message) in availability.UnsupportedVersions) {
+				var versionStr = (PlatformAvailability.IsDefaultVersion (version)) ? string.Empty : version.ToString ();
+				if (message is null) {
+					AppendLine ($"[UnsupportedOSPlatform (\"{platformName}{versionStr}\")]");
+				} else {
+					AppendLine ($"[UnsupportedOSPlatform (\"{platformName}{versionStr}\", \"{message}\")]");
+				}
+			}
+
+			// loop over the obsolete versions of the platform 
+			foreach (var (version, obsoleteInfo) in availability.ObsoletedVersions) {
+				var versionStr = (PlatformAvailability.IsDefaultVersion (version)) ? string.Empty : version.ToString ();
+
+				switch (obsoleteInfo) {
+				case (null, null):
+					AppendLine ($"[ObsoletedOSPlatform (\"{platformName}{versionStr}\")]");
+					break;
+				case (not null, null):
+					AppendLine ($"[ObsoletedOSPlatform (\"{platformName}{versionStr}\", \"{obsoleteInfo.Message}\")]");
+					break;
+				case (null, not null):
+					AppendLine ($"[ObsoletedOSPlatform (\"{platformName}{versionStr}\", Url=\"{obsoleteInfo.Url}\")]");
+					break;
+				case (not null, not null):
+					AppendLine (
+						$"[ObsoletedOSPlatform (\"{platformName}{versionStr}\", \"{obsoleteInfo.Message}\", Url=\"{obsoleteInfo.Url}\")]");
+					break;
+				}
+			}
+		}
+
+		return this;
+	}
+}

src/rgen/Microsoft.Macios.Generator/TabbedStringBuilder.cs

@@ -4,8 +4,6 @@
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Text;
-using Microsoft.Macios.Generator.Availability;
-using Xamarin.Utils;
 
 namespace Microsoft.Macios.Generator;
 
@@ -123,7 +121,7 @@ public TabbedStringBuilder AppendLine (ReadOnlySpan<char> span)
 
 		return this;
 	}
-
+	
 	public TabbedStringBuilder Append (ref DefaultInterpolatedStringHandler handler)
 	{
 		WriteTabs ().Append (handler.ToStringAndClear ());
@@ -135,6 +133,7 @@ public TabbedStringBuilder AppendLine (ref DefaultInterpolatedStringHandler hand
 		WriteTabs ().Append (handler.ToStringAndClear ()).AppendLine ();
 		return this;
 	}
+
 
 	/// <summary>
 	/// Append a new raw literal by prepending the correct indentation.
@@ -157,7 +156,7 @@ public TabbedStringBuilder AppendRaw (string rawString)
 
 		return this;
 	}
-
+	
 	/// <summary>
 	/// Append the generated code attribute to the current string builder. Added for convenience.
 	/// </summary>
@@ -176,52 +175,6 @@ public TabbedStringBuilder AppendGeneratedCodeAttribute (bool optimizable = true
 		return this;
 	}
 
-	public TabbedStringBuilder AppendMemberAvailability (in SymbolAvailability allPlatformsAvailability)
-	{
-		foreach (var availability in allPlatformsAvailability.PlatformAvailabilities) {
-			var platformName = availability.Platform.AsString ().ToLower ();
-			if (availability.SupportedVersion is not null) {
-				var versionStr = (PlatformAvailability.IsDefaultVersion (availability.SupportedVersion))
-					? string.Empty
-					: availability.SupportedVersion.ToString ();
-				AppendLine ($"[SupportedOSPlatform (\"{platformName}{versionStr}\")]");
-			}
-
-			// loop over the unsupported versions of the platform 
-			foreach (var (version, message) in availability.UnsupportedVersions) {
-				var versionStr = (PlatformAvailability.IsDefaultVersion (version)) ? string.Empty : version.ToString ();
-				if (message is null) {
-					AppendLine ($"[UnsupportedOSPlatform (\"{platformName}{versionStr}\")]");
-				} else {
-					AppendLine ($"[UnsupportedOSPlatform (\"{platformName}{versionStr}\", \"{message}\")]");
-				}
-			}
-
-			// loop over the obsolete versions of the platform 
-			foreach (var (version, obsoleteInfo) in availability.ObsoletedVersions) {
-				var versionStr = (PlatformAvailability.IsDefaultVersion (version)) ? string.Empty : version.ToString ();
-
-				switch (obsoleteInfo) {
-				case (null, null):
-					AppendLine ($"[ObsoletedOSPlatform (\"{platformName}{versionStr}\")]");
-					break;
-				case (not null, null):
-					AppendLine ($"[ObsoletedOSPlatform (\"{platformName}{versionStr}\", \"{obsoleteInfo.Message}\")]");
-					break;
-				case (null, not null):
-					AppendLine ($"[ObsoletedOSPlatform (\"{platformName}{versionStr}\", Url=\"{obsoleteInfo.Url}\")]");
-					break;
-				case (not null, not null):
-					AppendLine (
-						$"[ObsoletedOSPlatform (\"{platformName}{versionStr}\", \"{obsoleteInfo.Message}\", Url=\"{obsoleteInfo.Url}\")]");
-					break;
-				}
-			}
-		}
-
-		return this;
-	}
-
 	public TabbedStringBuilder AppendNotificationAdvice (in string className, in string notification)
 	{
 		string attr =

src/rgen/Microsoft.Macios.Transformer.Generator/Microsoft.Macios.Transformer.Generator/Microsoft.Macios.Transformer.Generator.csproj

@@ -0,0 +1,35 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net$(BundledNETCoreAppTargetFrameworkVersion)</TargetFramework>
+        <IsPackable>false</IsPackable>
+        <Nullable>enable</Nullable>
+        <LangVersion>latest</LangVersion>
+
+        <EnforceExtendedAnalyzerRules>true</EnforceExtendedAnalyzerRules>
+        <IsRoslynComponent>true</IsRoslynComponent>
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+
+        <RootNamespace>Microsoft.Macios.Transformer.Generator</RootNamespace>
+        <PackageId>Microsoft.Macios.Transformer.Generator</PackageId>
+        <!-- There is a bug in the roslyn analyzer for roslyn analyzers.... -->
+        <NoWarn>RS2007;RS1041;APL0003</NoWarn>
+
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.11">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.9.2"/>
+    </ItemGroup>
+
+    <ItemGroup>
+      <Compile Include="..\..\Microsoft.Macios.Generator\TabbedStringBuilder.cs">
+        <Link>TabbedStringBuilder.cs</Link>
+      </Compile>
+    </ItemGroup>
+
+
+</Project>

src/rgen/Microsoft.Macios.Transformer.Generator/Microsoft.Macios.Transformer.Generator/Properties/launchSettings.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://json.schemastore.org/launchsettings.json",
+  "profiles": {
+    "DebugRoslynSourceGenerator": {
+      "commandName": "DebugRoslynComponent",
+      "targetProject": "../Microsoft.Macios.Transformer.Generator.Sample/Microsoft.Macios.Transformer.Generator.Sample.csproj"
+    }
+  }
+}