Add Rust support to Soong.

Adds support to Soong for building rust modules. This currently only
supports x86_64 device and x86 linux host targets. The functionality
is sufficient to build crosvm.

Bug: 136189233
Test: Test module builds.
Test: crosvm builds.
Change-Id: I6ea04615834a6d673578ab10ea1a2eb04259fe09

rust/config/toolchain.go

@@ -0,0 +1,124 @@
+// Copyright 2019 The Android Open Source Project
+//
+// 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 config
+
+import (
+	"android/soong/android"
+)
+
+type Toolchain interface {
+	RustTriple() string
+	ToolchainRustFlags() string
+	ToolchainLinkFlags() string
+
+	SharedLibSuffix() string
+	StaticLibSuffix() string
+	RlibSuffix() string
+	DylibSuffix() string
+	ProcMacroSuffix() string
+	ExecutableSuffix() string
+
+	Is64Bit() bool
+	Supported() bool
+}
+
+type toolchainBase struct {
+}
+
+func (toolchainBase) RustTriple() string {
+	panic("toolchainBase does not define a triple.")
+}
+
+func (toolchainBase) ToolchainRustFlags() string {
+	panic("toolchainBase does not provide rust flags.")
+}
+
+func (toolchainBase) ToolchainLinkFlags() string {
+	panic("toolchainBase does not provide link flags.")
+}
+
+func (toolchainBase) Is64Bit() bool {
+	panic("toolchainBase cannot determine datapath width.")
+}
+
+type toolchain64Bit struct {
+	toolchainBase
+}
+
+func (toolchain64Bit) Is64Bit() bool {
+	return true
+}
+
+type toolchain32Bit struct {
+	toolchainBase
+}
+
+func (toolchain32Bit) Is64Bit() bool {
+	return false
+}
+
+func (toolchain32Bit) Bionic() bool {
+	return true
+}
+
+func (toolchainBase) ExecutableSuffix() string {
+	return ""
+}
+
+func (toolchainBase) SharedLibSuffix() string {
+	return ".so"
+}
+
+func (toolchainBase) StaticLibSuffix() string {
+	return ".a"
+}
+
+func (toolchainBase) RlibSuffix() string {
+	return ".rlib"
+}
+func (toolchainBase) DylibSuffix() string {
+	return ".so"
+}
+
+func (toolchainBase) ProcMacroSuffix() string {
+	return ".so"
+}
+
+func (toolchainBase) Supported() bool {
+	return false
+}
+
+func toolchainBaseFactory() Toolchain {
+	return &toolchainBase{}
+}
+
+type toolchainFactory func(arch android.Arch) Toolchain
+
+var toolchainFactories = make(map[android.OsType]map[android.ArchType]toolchainFactory)
+
+func registerToolchainFactory(os android.OsType, arch android.ArchType, factory toolchainFactory) {
+	if toolchainFactories[os] == nil {
+		toolchainFactories[os] = make(map[android.ArchType]toolchainFactory)
+	}
+	toolchainFactories[os][arch] = factory
+}
+
+func FindToolchain(os android.OsType, arch android.Arch) Toolchain {
+	factory := toolchainFactories[os][arch.ArchType]
+	if factory == nil {
+		return toolchainBaseFactory()
+	}
+	return factory(arch)
+}