Allow a custom env for more xcrun invocations

Basically a continuation of #1766

Author: neonichu

Sources/Build/llbuild.swift

@@ -87,7 +87,7 @@ public struct LLBuildManifestGenerator {
     }
 
     /// Generate manifest at the given path.
-    public func generateManifest(at path: AbsolutePath) throws {
+    public func generateManifest(at path: AbsolutePath, environment: [String:String] = Process.env) throws {
         var targets = Targets()
 
         // Create commands for all target description in the plan.
@@ -99,7 +99,7 @@ public struct LLBuildManifestGenerator {
                     buildByDefault: plan.graph.reachableTargets.contains(target),
                     isTest: description.isTestTarget)
             case .clang(let description):
-                targets.append(try createClangCompileTarget(description),
+                targets.append(try createClangCompileTarget(description, environment: environment),
                     buildByDefault: plan.graph.reachableTargets.contains(target),
                     isTest: description.isTestTarget)
             }
@@ -215,7 +215,7 @@ public struct LLBuildManifestGenerator {
     }
 
     /// Create a llbuild target for a Clang target description.
-    private func createClangCompileTarget(_ target: ClangTargetBuildDescription) throws -> Target {
+    private func createClangCompileTarget(_ target: ClangTargetBuildDescription, environment: [String:String] = Process.env) throws -> Target {
 
         let standards = [
             (target.clangTarget.cxxLanguageStandard, SupportedLanguageExtension.cppExtensions),

Sources/Workspace/UserToolchain.swift

@@ -129,6 +129,9 @@ public final class UserToolchain: Toolchain {
         return lookupExecutablePath(filename: getenv(variable), searchPaths: searchPaths)
     }
 
+    /// Environment to use when looking up tools.
+    private let processEnvironment: [String: String]
+
     public func getClangCompiler() throws -> AbsolutePath {
 
         if let clangCompiler = _clangCompiler {
@@ -142,7 +145,7 @@ public final class UserToolchain: Toolchain {
             clangCompiler = value
         } else {
             // No value in env, so search for `clang`.
-            let foundPath = try Process.checkNonZeroExit(arguments: whichClangArgs).chomp()
+            let foundPath = try Process.checkNonZeroExit(arguments: whichClangArgs, environment: processEnvironment).chomp()
             guard !foundPath.isEmpty else {
                 throw InvalidToolchainDiagnostic("could not find `clang`")
             }
@@ -157,8 +160,9 @@ public final class UserToolchain: Toolchain {
         return clangCompiler
     }
 
-    public init(destination: Destination, environment: [String:String] = Process.env) throws {
+    public init(destination: Destination, environment: [String: String] = Process.env) throws {
         self.destination = destination
+        self.processEnvironment = environment
 
         // Get the search paths from PATH.
         let searchPaths = getEnvSearchPaths(