diff --git a/android/transition.go b/android/transition.go
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+// Copyright 2024 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package android
+
+import "github.com/google/blueprint"
+
+// TransitionMutator implements a top-down mechanism where a module tells its
+// direct dependencies what variation they should be built in but the dependency
+// has the final say.
+//
+// When implementing a transition mutator, one needs to implement four methods:
+//   - Split() that tells what variations a module has by itself
+//   - OutgoingTransition() where a module tells what it wants from its
+//     dependency
+//   - IncomingTransition() where a module has the final say about its own
+//     variation
+//   - Mutate() that changes the state of a module depending on its variation
+//
+// That the effective variation of module B when depended on by module A is the
+// composition the outgoing transition of module A and the incoming transition
+// of module B.
+//
+// the outgoing transition should not take the properties of the dependency into
+// account, only those of the module that depends on it. For this reason, the
+// dependency is not even passed into it as an argument. Likewise, the incoming
+// transition should not take the properties of the depending module into
+// account and is thus not informed about it. This makes for a nice
+// decomposition of the decision logic.
+//
+// A given transition mutator only affects its own variation; other variations
+// stay unchanged along the dependency edges.
+//
+// Soong makes sure that all modules are created in the desired variations and
+// that dependency edges are set up correctly. This ensures that "missing
+// variation" errors do not happen and allows for more flexible changes in the
+// value of the variation among dependency edges (as oppposed to bottom-up
+// mutators where if module A in variation X depends on module B and module B
+// has that variation X, A must depend on variation X of B)
+//
+// The limited power of the context objects passed to individual mutators
+// methods also makes it more difficult to shoot oneself in the foot. Complete
+// safety is not guaranteed because no one prevents individual transition
+// mutators from mutating modules in illegal ways and for e.g. Split() or
+// Mutate() to run their own visitations of the transitive dependency of the
+// module and both of these are bad ideas, but it's better than no guardrails at
+// all.
+//
+// This model is pretty close to Bazel's configuration transitions. The mapping
+// between concepts in Soong and Bazel is as follows:
+//   - Module == configured target
+//   - Variant == configuration
+//   - Variation name == configuration flag
+//   - Variation == configuration flag value
+//   - Outgoing transition == attribute transition
+//   - Incoming transition == rule transition
+//
+// The Split() method does not have a Bazel equivalent and Bazel split
+// transitions do not have a Soong equivalent.
+//
+// Mutate() does not make sense in Bazel due to the different models of the
+// two systems: when creating new variations, Soong clones the old module and
+// thus some way is needed to change it state whereas Bazel creates each
+// configuration of a given configured target anew.
+type TransitionMutator interface {
+	// Split returns the set of variations that should be created for a module no
+	// matter who depends on it. Used when Make depends on a particular variation
+	// or when the module knows its variations just based on information given to
+	// it in the Blueprint file. This method should not mutate the module it is
+	// called on.
+	Split(ctx BaseModuleContext) []string
+
+	// OutgoingTransition is called on a module to determine which variation it wants
+	// from its direct dependencies. The dependency itself can override this decision.
+	// This method should not mutate the module itself.
+	OutgoingTransition(ctx OutgoingTransitionContext, sourceVariation string) string
+
+	// IncomingTransition is called on a module to determine which variation it should
+	// be in based on the variation modules that depend on it want. This gives the module
+	// a final say about its own variations. This method should not mutate the module
+	// itself.
+	IncomingTransition(ctx IncomingTransitionContext, incomingVariation string) string
+
+	// Mutate is called after a module was split into multiple variations on each variation.
+	// It should not split the module any further but adding new dependencies is
+	// fine. Unlike all the other methods on TransitionMutator, this method is
+	// allowed to mutate the module.
+	Mutate(ctx BottomUpMutatorContext, variation string)
+}
+
+type IncomingTransitionContext interface {
+	ArchModuleContext
+	ModuleProviderContext
+	ModuleErrorContext
+
+	// Module returns the target of the dependency edge for which the transition
+	// is being computed
+	Module() Module
+
+	// Config returns the configuration for the build.
+	Config() Config
+
+	DeviceConfig() DeviceConfig
+
+	// IsAddingDependency returns true if the transition is being called while adding a dependency
+	// after the transition mutator has already run, or false if it is being called when the transition
+	// mutator is running.  This should be used sparingly, all uses will have to be removed in order
+	// to support creating variants on demand.
+	IsAddingDependency() bool
+}
+
+type OutgoingTransitionContext interface {
+	ArchModuleContext
+	ModuleProviderContext
+
+	// Module returns the target of the dependency edge for which the transition
+	// is being computed
+	Module() Module
+
+	// DepTag() Returns the dependency tag through which this dependency is
+	// reached
+	DepTag() blueprint.DependencyTag
+
+	// Config returns the configuration for the build.
+	Config() Config
+
+	DeviceConfig() DeviceConfig
+}
+
+type androidTransitionMutator struct {
+	finalPhase bool
+	mutator    TransitionMutator
+	name       string
+}
+
+func (a *androidTransitionMutator) Split(ctx blueprint.BaseModuleContext) []string {
+	if a.finalPhase {
+		panic("TransitionMutator not allowed in FinalDepsMutators")
+	}
+	if m, ok := ctx.Module().(Module); ok {
+		moduleContext := m.base().baseModuleContextFactory(ctx)
+		return a.mutator.Split(&moduleContext)
+	} else {
+		return []string{""}
+	}
+}
+
+func (a *androidTransitionMutator) OutgoingTransition(bpctx blueprint.OutgoingTransitionContext, sourceVariation string) string {
+	if m, ok := bpctx.Module().(Module); ok {
+		ctx := outgoingTransitionContextPool.Get().(*outgoingTransitionContextImpl)
+		defer outgoingTransitionContextPool.Put(ctx)
+		*ctx = outgoingTransitionContextImpl{
+			archModuleContext: m.base().archModuleContextFactory(bpctx),
+			bp:                bpctx,
+		}
+		return a.mutator.OutgoingTransition(ctx, sourceVariation)
+	} else {
+		return ""
+	}
+}
+
+func (a *androidTransitionMutator) IncomingTransition(bpctx blueprint.IncomingTransitionContext, incomingVariation string) string {
+	if m, ok := bpctx.Module().(Module); ok {
+		ctx := incomingTransitionContextPool.Get().(*incomingTransitionContextImpl)
+		defer incomingTransitionContextPool.Put(ctx)
+		*ctx = incomingTransitionContextImpl{
+			archModuleContext: m.base().archModuleContextFactory(bpctx),
+			bp:                bpctx,
+		}
+		return a.mutator.IncomingTransition(ctx, incomingVariation)
+	} else {
+		return ""
+	}
+}
+
+func (a *androidTransitionMutator) Mutate(ctx blueprint.BottomUpMutatorContext, variation string) {
+	if am, ok := ctx.Module().(Module); ok {
+		if variation != "" {
+			// TODO: this should really be checking whether the TransitionMutator affected this module, not
+			//  the empty variant, but TransitionMutator has no concept of skipping a module.
+			base := am.base()
+			base.commonProperties.DebugMutators = append(base.commonProperties.DebugMutators, a.name)
+			base.commonProperties.DebugVariations = append(base.commonProperties.DebugVariations, variation)
+		}
+
+		mctx := bottomUpMutatorContextFactory(ctx, am, a.finalPhase)
+		defer bottomUpMutatorContextPool.Put(mctx)
+		a.mutator.Mutate(mctx, variation)
+	}
+}
+
+type incomingTransitionContextImpl struct {
+	archModuleContext
+	bp blueprint.IncomingTransitionContext
+}
+
+func (c *incomingTransitionContextImpl) Module() Module {
+	return c.bp.Module().(Module)
+}
+
+func (c *incomingTransitionContextImpl) Config() Config {
+	return c.bp.Config().(Config)
+}
+
+func (c *incomingTransitionContextImpl) DeviceConfig() DeviceConfig {
+	return DeviceConfig{c.bp.Config().(Config).deviceConfig}
+}
+
+func (c *incomingTransitionContextImpl) IsAddingDependency() bool {
+	return c.bp.IsAddingDependency()
+}
+
+func (c *incomingTransitionContextImpl) provider(provider blueprint.AnyProviderKey) (any, bool) {
+	return c.bp.Provider(provider)
+}
+
+func (c *incomingTransitionContextImpl) ModuleErrorf(fmt string, args ...interface{}) {
+	c.bp.ModuleErrorf(fmt, args)
+}
+
+func (c *incomingTransitionContextImpl) PropertyErrorf(property, fmt string, args ...interface{}) {
+	c.bp.PropertyErrorf(property, fmt, args)
+}
+
+type outgoingTransitionContextImpl struct {
+	archModuleContext
+	bp blueprint.OutgoingTransitionContext
+}
+
+func (c *outgoingTransitionContextImpl) Module() Module {
+	return c.bp.Module().(Module)
+}
+
+func (c *outgoingTransitionContextImpl) DepTag() blueprint.DependencyTag {
+	return c.bp.DepTag()
+}
+
+func (c *outgoingTransitionContextImpl) Config() Config {
+	return c.bp.Config().(Config)
+}
+
+func (c *outgoingTransitionContextImpl) DeviceConfig() DeviceConfig {
+	return DeviceConfig{c.bp.Config().(Config).deviceConfig}
+}
+
+func (c *outgoingTransitionContextImpl) provider(provider blueprint.AnyProviderKey) (any, bool) {
+	return c.bp.Provider(provider)
+}