Add documentation for incompatible change process. (#7098)

* Add documentation for incompatible change process.

site/_layouts/documentation.html

@@ -44,7 +44,7 @@ <h3>Home</h3>
               <li><a href="/versions/{{ current_version }}/bazel-overview.html">Bazel Overview</a></li>
               <li><a href="/versions/{{ current_version }}/bazel-vision.html">Bazel Vision</a></li>
               <li><a href="/versions/{{ current_version }}/getting-started.html">Getting Started</a></li>
-              <li><a href="/versions/{{ current_version }}/skylark/backward-compatibility.html">Backward Compatibility</a></li>
+              <li><a href="/versions/{{ current_version }}/backward-compatibility.html">Backward Compatibility</a></li>
             </ul>
 
             <h3>Using Bazel</h3>

site/docs/backward-compatibility.md

@@ -0,0 +1,88 @@
+---
+layout: documentation
+title: Backward Compatibility
+---
+
+# Backward Compatibility
+
+Bazel is evolving, and we will make changes to Bazel that at times will be
+incompatible and require some changes from Bazel users.
+
+## GitHub labels
+
+* All incompatible changes: label [**incompatible-change**](https://github.com/bazelbuild/bazel/issues?q=label%3Aincompatible-change)
+* Expected breaking change in release X.Y: label **breaking-change-X.Y** (e.g. [**breaking-change-0.21**](https://github.com/bazelbuild/bazel/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3Abreaking-change-0.21))
+* Release X.Y is in a migration window: label **migration-X.Y** (e.g. [**migration-0.21**](https://github.com/bazelbuild/bazel/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3Amigration-0.21))
+
+## At a glance
+
+1. Every breaking change is guarded with an `--incompatible_*` flag.
+1. Newly introduced incompatible flags default to off.
+1. For every `--incompatible_*` flag we have a GitHub issue explaining
+   the change in behavior and giving a migration recipe.
+1. The migration window is at least one release long and is set by the author of the incompatible change.
+1. We announce what set of flags we indend to flip with the next release one release in advance.
+1. APIs and behavior guarded by an `--experimental_*` flag can change at any time.
+1. Users should never run their production builds with `--experimental_*`  or `--incompatible_*` flags.
+
+## What is stable functionality?
+
+In general, if an API or a behavior is available in Bazel without
+`--experimental_...` flag, we consider it a stable, supported feature.
+This includes:
+
+* Starlark language and APIs
+* Rules bundled with Bazel
+* Bazel APIs such as Remote Execution APIs or Build Event Protocol
+* Flags and their semantics
+
+## Incompatible changes and migration recipes
+
+When we introduce an incompatible change, we try to make it easier for Bazel
+users to update their code. We do this by means of _migration windows_ and
+_migration recipes_.
+
+Migration window is one or more release of Bazel during which a migration from
+old functionality to new functionality is possible, according to a migration
+recipe.
+
+During the migration window, both the old functionality and the new functionality
+are available in the Bazel release. For every incompatible change, we provide
+a _migration recipe_ that allows updating the user code (`BUILD` and `.bzl` files,
+as well as any Bazel usage in scripts, usage of Bazel API and so on) in such a
+way that **it works simultaneously without any flags with old and new
+functionality**.
+
+In other words, during a migration window for an incompatible change `foo`:
+
+1. `--incompatible_foo` flag is available in Bazel release and defaults to off.
+1. User code can be updated in such a way that it works simultaneously with
+   that flag being on or off.
+1. After the code is migrated, the users can check whether they migrated
+   successfully by building with `--incompatible_foo=true`. However, their
+   code will continue to work in the same release in default state (where
+   `--incompatible_foo` is of), as well after the migration window is over
+   (at which point the flag will be effectively on).
+
+## Communicating incompatible changes
+
+The primary source of information about incompatible changes are GitHub issues
+marked with an ["incompatible-change" label](https://github.com/bazelbuild/bazel/issues?q=label%3Aincompatible-change).
+
+For every incompatible change, the issue specifies:
+* the name of the flag controlling the incompatible change
+* the description of the changed functionality
+* the migration recipe
+
+The incompatible change issue is closed when the incompatible flag is flipped at
+HEAD.
+
+All the incompatible changes for which a Bazel releaze X.Y is part of a
+migration window are marked with a label "migration-X.Y" label (for example
+[migration-0.21](https://github.com/bazelbuild/bazel/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3Amigration-0.21)).
+
+All the incompatible changes that are expected to happen in release X.Y
+are marked with a label "breaking-change-X.Y" (for example
+[breaking-change-0.21](https://github.com/bazelbuild/bazel/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3Abreaking-change-0.21)).
+
+

site/docs/skylark/backward-compatibility.md

@@ -1,34 +1,16 @@
 ---
 layout: documentation
-title: Backward Compatibility
+title: Legacy Incompatible Changes List
 ---
 
-# Backward Compatibility
 
-Bazel is still in Beta and new releases may include backward incompatible
-changes. As we make changes and polish the extension mechanism, old features
-may be removed and new features that are not backward compatible may be added.
+# Legacy Incompatible Changes List
 
-Backward incompatible changes are introduced gradually:
+Legacy, partial list of [backward-incompatible changes](../backward-compatibility.md).
 
-1.  The backward incompatible change is introduced behind a flag with its
-    default value set to `false`.
-2.  In a later release, the flag's default value will be set to `true`. You
-    can still use the flag to disable the change.
-3.  Then in a later release, the flag will be removed and you will no longer be
-    able to disable the change.
+Full, authorative list of incompatible changes is [GitHub issues with
+"incompatible-change" label](https://github.com/bazelbuild/bazel/issues?q=is%3Aissue+label%3Aincompatible-change)
 
-To check if your code will be compatible with future releases you can:
-
-*   Build your code with the flag `--all_incompatible_changes`. This flag
-    enables all backward incompatible changes, and so you can ensure your code
-    is compatible with upcoming changes.
-*   Use boolean flags to enable/disable specific backward incompatible changes.
-
-## Current backward incompatible changes
-
-The following are the backward incompatible changes that are implemented and
-guarded behind flags in the current release:
 
 General Starlark