Add breaking change for RID warning (#26203)

Author: gewarren

docs/core/compatibility/6.0.md

@@ -100,6 +100,7 @@ If you're migrating an app to .NET 6, the breaking changes listed here might aff
 | [Implicit global using directives in C# projects](sdk/6.0/implicit-namespaces.md) | Preview 7 |
 | [Implicit global using directives disabled](sdk/6.0/implicit-namespaces-rc1.md) | RC 1|
 | [OutputType not automatically set to WinExe](sdk/6.0/outputtype-not-set-automatically.md) | RC 1 |
+| [RuntimeIdentifier warning if self-contained is unspecified](sdk/6.0/runtimeidentifier-self-contained.md) | RC 1 |
 
 ## Serialization
 

docs/core/compatibility/sdk/6.0/runtimeidentifier-self-contained.md

@@ -0,0 +1,49 @@
+---
+title: "Breaking change: `RuntimeIdentifier` warning if self-contained is unspecified"
+description: Learn about the breaking change in .NET 6 where specifying a `RuntimeIdentifier` without specifying whether an app is self-contained results in a warning.
+ms.date: 09/20/2021
+---
+# RuntimeIdentifier warning if self-contained is unspecified
+
+If you specify a `RuntimeIdentifier` in your project file or use the `-r` option with `dotnet`, the .NET SDK defaults the build, publish, and run outputs to be self-contained applications. The default without specifying a `RuntimeIdentifier` is to have a framework-dependent application. This change introduces a new warning (NETSDK1179) if you specify a `RuntimeIdentifier` without specifying whether the application is self-contained.
+
+## Version introduced
+
+.NET 6 RC 1
+
+## Previous behavior
+
+In previous versions, specifying a `RuntimeIdentifier` would silently change the application from a framework-dependent application to a self-contained application.
+
+## New behavior
+
+In .NET 6, if you specify a `RuntimeIdentifier` without specifying whether the application is self-contained, you'll get the following warning:
+
+**warning NETSDK1179: One of '--self-contained' or '--no-self-contained' options are required when '--runtime' is used.**
+
+For example, the following command will generate the warning:
+
+```dotnetcli
+dotnet publish -r win-x86
+```
+
+## Change category
+
+This change may affect [*source compatibility*](../../categories.md#source-compatibility).
+
+## Reason for change
+
+The default without specifying a `RuntimeIdentifier` is to generate a framework-dependent application. This default caused confusion for many customers. The purpose of adding the warning is to:
+
+- Warn customers of the behavior change to default to a framework-dependent app.
+- Encourage customers to specifically choose the type of application they want to build.
+- Prepare customers for possibly changing the behavior in .NET 7 to default to framework-dependent.
+
+## Recommended action
+
+- Specify a boolean value in your project file for `SelfContained`.
+- Or, add `--sc` with a value to your build or publish command.
+
+## Affected APIs
+
+N/A

docs/core/compatibility/toc.yml

@@ -137,6 +137,8 @@ items:
           href: sdk/6.0/implicit-namespaces-rc1.md
         - name: OutputType not automatically set to WinExe
           href: sdk/6.0/outputtype-not-set-automatically.md
+        - name: RuntimeIdentifier warning if self-contained is unspecified
+          href: sdk/6.0/runtimeidentifier-self-contained.md
       - name: Windows Forms
         items:
         - name: APIs throw ArgumentNullException
@@ -765,6 +767,8 @@ items:
           href: sdk/6.0/implicit-namespaces-rc1.md
         - name: OutputType not automatically set to WinExe
           href: sdk/6.0/outputtype-not-set-automatically.md
+        - name: RuntimeIdentifier warning if self-contained is unspecified
+          href: sdk/6.0/runtimeidentifier-self-contained.md
       - name: .NET 5
         items:
         - name: Directory.Packages.props files imported by default