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/global.go

@@ -0,0 +1,65 @@
+// 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"
+	_ "android/soong/cc/config"
+)
+
+var pctx = android.NewPackageContext("android/soong/rust/config")
+
+var (
+	RustDefaultVersion = "1.35.0"
+	RustDefaultBase    = "prebuilts/rust/"
+	DefaultEdition     = "2018"
+	Stdlibs            = []string{
+		"libarena",
+		"libfmt_macros",
+		"libgraphviz",
+		"libserialize",
+		"libstd",
+		"libsyntax",
+		"libsyntax_ext",
+		"libsyntax_pos",
+		"libterm",
+	}
+)
+
+func init() {
+	pctx.SourcePathVariable("RustDefaultBase", RustDefaultBase)
+	pctx.VariableConfigMethod("HostPrebuiltTag", android.Config.PrebuiltOS)
+
+	pctx.VariableFunc("RustBase", func(ctx android.PackageVarContext) string {
+		if override := ctx.Config().Getenv("RUST_PREBUILTS_BASE"); override != "" {
+			return override
+		}
+		return "${RustDefaultBase}"
+	})
+
+	pctx.VariableFunc("RustVersion", func(ctx android.PackageVarContext) string {
+		if override := ctx.Config().Getenv("RUST_PREBUILTS_VERSION"); override != "" {
+			return override
+		}
+		return RustDefaultVersion
+	})
+
+	pctx.StaticVariable("RustPath", "${RustBase}/${HostPrebuiltTag}/${RustVersion}")
+	pctx.StaticVariable("RustBin", "${RustPath}/bin")
+
+	pctx.ImportAs("ccConfig", "android/soong/cc/config")
+	pctx.StaticVariable("RustLinker", "${ccConfig.ClangBin}/clang++")
+	pctx.StaticVariable("RustLinkerArgs", "-B ${ccConfig.ClangBin} -fuse-ld=lld")
+}