[GH:#9202] [Parity] error: unable to spawn process 'metal' (No such file or directory)

Fixtures/Metal/SimpleLibrary/Package.swift

@@ -0,0 +1,19 @@
+// swift-tools-version: 6.2
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+import PackageDescription
+
+let package = Package(
+    name: "MyRenderer",
+    products: [
+        .library(
+            name: "MyRenderer",
+            targets: ["MyRenderer"]),
+    ],
+    targets: [
+        .target(
+            name: "MyRenderer",
+            dependencies: ["MySharedTypes"]),
+
+        .target(name: "MySharedTypes")
+    ]
+)

Fixtures/Metal/SimpleLibrary/Sources/MyRenderer/Renderer.swift

@@ -0,0 +1,4 @@
+import MySharedTypes
+
+
+let vertex = AAPLVertex(position: .init(250, -250), color: .init(1, 0, 0, 1))

Fixtures/Metal/SimpleLibrary/Sources/MyRenderer/Shaders.metal

@@ -0,0 +1,12 @@
+// A relative path to SharedTypes.h.
+#import "../MySharedTypes/include/SharedTypes.h"
+
+#include <metal_stdlib>
+using namespace metal;
+
+vertex float4 simpleVertexShader(const device AAPLVertex *vertices [[buffer(0)]],
+                                  uint vertexID [[vertex_id]]) {
+    AAPLVertex in = vertices[vertexID];
+    return float4(in.position.x, in.position.y, 0.0, 1.0);
+}
+

Fixtures/Metal/SimpleLibrary/Sources/MySharedTypes/include/SharedTypes.h

@@ -0,0 +1,14 @@
+#ifndef SharedTypes_h
+#define SharedTypes_h
+
+
+#import <simd/simd.h>
+
+
+typedef struct {
+    vector_float2 position;
+    vector_float4 color;
+} AAPLVertex;
+
+
+#endif /* SharedTypes_h */

Fixtures/Metal/SimpleLibrary/Tests/MyRendererTests/MyRendererTests.swift

@@ -0,0 +1,6 @@
+import Testing
+@testable import MyRenderer
+
+@Test func example() async throws {
+    // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+}

Package.swift

@@ -991,6 +991,13 @@ let package = Package(
             name: "SwiftBuildSupportTests",
             dependencies: ["SwiftBuildSupport", "_InternalTestSupport", "_InternalBuildTestSupport"]
         ),
+        .testTarget(
+            name: "BuildMetalTests",
+            dependencies: [
+                "_InternalTestSupport",
+                "Basics"
+            ]
+        ),
         // Examples (These are built to ensure they stay up to date with the API.)
         .executableTarget(
             name: "package-info",

Sources/SwiftBuildSupport/PackagePIFBuilder.swift

@@ -19,6 +19,8 @@ import struct Basics.SourceControlURL
 import struct Basics.Diagnostic
 import struct Basics.ObservabilityMetadata
 import class Basics.ObservabilityScope
+import class Basics.AsyncProcess
+import struct Basics.AsyncProcessResult
 
 import class PackageModel.Manifest
 import class PackageModel.Package
@@ -179,6 +181,9 @@ public final class PackagePIFBuilder {
     /// The file system to read from.
     let fileSystem: FileSystem
 
+    /// Path to the Metal compiler, resolved via `xcrun --find metal` or 'metal' if not found.
+    let metalCompilerPath: String
+
     /// Whether to suppress warnings from compilers, linkers, and other build tools for package dependencies.
     private var suppressWarningsForPackageDependencies: Bool {
         UserDefaults.standard.bool(forKey: "SuppressWarningsForPackageDependencies", defaultValue: true)
@@ -215,6 +220,7 @@ public final class PackagePIFBuilder {
         self.fileSystem = fileSystem
         self.observabilityScope = observabilityScope
         self.addLocalRpaths = addLocalRpaths
+        self.metalCompilerPath = Self.findMetalCompilerPath(observabilityScope: observabilityScope)
     }
 
     public init(
@@ -239,6 +245,7 @@ public final class PackagePIFBuilder {
         self.packageDisplayVersion = packageDisplayVersion
         self.fileSystem = fileSystem
         self.observabilityScope = observabilityScope
+        self.metalCompilerPath = Self.findMetalCompilerPath(observabilityScope: observabilityScope)
     }
 
     /// Build an empty PIF project.
@@ -660,6 +667,36 @@ public final class PackagePIFBuilder {
             self.rule = resource.rule
         }
     }
+
+    private static func findMetalCompilerPath(observabilityScope: ObservabilityScope) -> String {
+        do {
+            let result = try AsyncProcess.popen(
+                arguments: ["xcrun", "--find", "metal"]
+            )
+            guard result.exitStatus == .terminated(code: 0) else {
+                observabilityScope.emit(
+                    debug: "Failed to find Metal compiler using xcrun, exited with \(result.exitStatus).  Defaulting to 'metal'"
+                )
+                return "metal"
+            }
+
+            let output = try result.utf8Output()
+
+            let path = output.trimmingCharacters(in: .whitespacesAndNewlines)
+
+            guard !path.isEmpty else {
+                observabilityScope.emit(debug: "Metal compiler path is empty, defaulting to 'metal'")
+                return "metal"
+            }
+
+            return path
+        } catch {
+            observabilityScope.emit(
+                debug: "Error \(error) finding Metal compiler, defaulting to 'metal'"
+            )
+            return "metal"
+        }
+    }
 }
 
 // MARK: - Helpers

Sources/SwiftBuildSupport/PackagePIFProjectBuilder.swift

@@ -220,6 +220,8 @@ struct PackagePIFProjectBuilder {
         settings[.COREML_COMPILER_CONTAINER] = "swift-package"
         settings[.COREML_CODEGEN_LANGUAGE] = "None"
 
+        settings[.MTL_COMPILER_PATH] = self.pifBuilder.metalCompilerPath
+
         self.project[keyPath: resourcesTargetKeyPath].common.addBuildConfig { id in
             BuildConfig(id: id, name: "Debug", settings: settings)
         }

Sources/XCBuildSupport/PIF.swift

@@ -925,6 +925,7 @@ public enum PIF {
             case CLANG_COVERAGE_MAPPING_LINKER_ARGS
             case MACH_O_TYPE
             case MACOSX_DEPLOYMENT_TARGET
+            case MTL_COMPILER_PATH
             case MODULEMAP_FILE
             case MODULEMAP_FILE_CONTENTS
             case MODULEMAP_PATH

Tests/BuildMetalTests/BuildMetalTests.swift

@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import _InternalTestSupport
+import Testing
+import Basics
+
+@Suite
+struct BuildMetalTests {
+
+    @Test(
+        .requireHostOS(.macOS),
+        arguments: getBuildData(for: [.swiftbuild]),
+    )
+    func simpleLibrary(data: BuildData) async throws {
+        let buildSystem = data.buildSystem
+        let configuration = data.config
+
+        try await fixture(name: "Metal/SimpleLibrary") { fixturePath in
+
+            // Build the package
+            let (_, _) = try await executeSwiftBuild(
+                fixturePath,
+                configuration: configuration,
+                buildSystem: buildSystem,
+                throwIfCommandFails: true
+            )
+
+            // Get the bin path
+            let (binPathOutput, _) = try await executeSwiftBuild(
+                fixturePath,
+                configuration: configuration,
+                extraArgs: ["--show-bin-path"],
+                buildSystem: buildSystem,
+                throwIfCommandFails: true
+            )
+
+            let binPath = try AbsolutePath(validating: binPathOutput.trimmingCharacters(in: .whitespacesAndNewlines))
+
+            // Check that default.metallib exists
+            let metallibPath = binPath.appending(components:["MyRenderer_MyRenderer.bundle", "Contents", "Resources", "default.metallib"])
+            #expect(
+                localFileSystem.exists(metallibPath),
+                "Expected default.metallib to exist at \(metallibPath)"
+            )
+        }
+    }
+
+}