Improved error messages for when swiftc, clang, or sysroot can't be found.

No fix-its yet, but they will come.
https://bugs.swift.org/browse/SR-2271

Author: abertelrud

Sources/Commands/Error.swift

@@ -19,7 +19,7 @@ import var Utility.stderr
 
 public enum Error: Swift.Error {
     case noManifestFound
-    case invalidToolchain
+    case invalidToolchain(problem: String)
     case buildYAMLNotFound(String)
     case repositoryHasChanges(String)
 }
@@ -29,8 +29,8 @@ extension Error: FixableError {
         switch self {
         case .noManifestFound:
             return "no \(Manifest.filename) file found"
-        case .invalidToolchain:
-            return "invalid inferred toolchain"
+        case .invalidToolchain(let problem):
+            return "invalid inferred toolchain: \(problem)"
         case .buildYAMLNotFound(let value):
             return "no build YAML found: \(value)"
         case .repositoryHasChanges(let value):

Sources/Commands/UserToolchain.swift

@@ -15,6 +15,7 @@ import protocol Build.Toolchain
 
 #if os(macOS)
     private let whichClangArgs = ["xcrun", "--find", "clang"]
+    private let whichDefaultSDKArgs = ["xcrun", "--sdk", "macosx", "--show-sdk-path"]
 #else
     private let whichClangArgs = ["which", "clang"]
 #endif
@@ -38,24 +39,75 @@ struct UserToolchain: Toolchain {
 #endif
 
     init() throws {
-        do {
-            SWIFT_EXEC = getenv("SWIFT_EXEC")
-                // use the swiftc installed alongside ourselves
-                ?? AbsolutePath(CommandLine.arguments[0], relativeTo: currentWorkingDirectory).parentDirectory.appending(component: "swiftc").asString
-
-            clang = try getenv("CC") ?? POSIX.popen(whichClangArgs).chomp()
-
-            #if os(macOS)
-                sysroot = try getenv("SYSROOT") ?? POSIX.popen(["xcrun", "--sdk", "macosx", "--show-sdk-path"]).chomp()
-            #else
-                sysroot = nil
-            #endif
-
-            guard !SWIFT_EXEC.isEmpty && !clang.isEmpty && (sysroot == nil || !sysroot!.isEmpty) else {
-                throw Error.invalidToolchain
+        let swiftCompiler: AbsolutePath
+        let clangCompiler: AbsolutePath
+        let defaultSDK: AbsolutePath?
+        
+        // Find the Swift compiler, looking first in the environment.
+        if let value = getenv("SWIFT_EXEC"), !value.isEmpty {
+            // We have a value, but it could be an absolute or a relative path.
+            swiftCompiler = AbsolutePath(value, relativeTo: currentWorkingDirectory)
+        }
+        else {
+            // No value in env, so look for `swiftc` alongside our own binary.
+            swiftCompiler = AbsolutePath(CommandLine.arguments[0], relativeTo: currentWorkingDirectory).parentDirectory.appending(component: "swiftc")
+        }
+        
+        // Check that it's valid in the file system.
+        // FIXME: We should also check that it resolves to an executable file
+        //        (it could be a symlink to such as file).
+        guard localFileSystem.exists(swiftCompiler) else {
+            throw Error.invalidToolchain(problem: "could not find `swiftc` at expected path \(swiftCompiler.asString)")
+        }
+        
+        // Find the Clang compiler, looking first in the environment.
+        if let value = getenv("CC"), !value.isEmpty {
+            // We have a value, but it could be an absolute or a relative path.
+            clangCompiler = AbsolutePath(value, relativeTo: currentWorkingDirectory)
+        }
+        else {
+            // No value in env, so search for `clang`.
+            guard let foundPath = try? POSIX.popen(whichClangArgs).chomp(), !foundPath.isEmpty else {
+                throw Error.invalidToolchain(problem: "could not find `clang`")
+            }
+            clangCompiler = AbsolutePath(foundPath, relativeTo: currentWorkingDirectory)
+        }
+        
+        // Check that it's valid in the file system.
+        // FIXME: We should also check that it resolves to an executable file
+        //        (it could be a symlink to such as file).
+        guard localFileSystem.exists(clangCompiler) else {
+            throw Error.invalidToolchain(problem: "could not find `clang` at expected path \(clangCompiler.asString)")
+        }
+        
+        // Find the default SDK (on macOS only).
+      #if os(macOS)
+        if let value = getenv("SYSROOT"), !value.isEmpty {
+            // We have a value, but it could be an absolute or a relative path.
+            defaultSDK = AbsolutePath(value, relativeTo: currentWorkingDirectory)
+        }
+        else {
+            // No value in env, so search for it.
+            guard let foundPath = try? POSIX.popen(whichDefaultSDKArgs).chomp(), !foundPath.isEmpty else {
+                throw Error.invalidToolchain(problem: "could not find default SDK")
+            }
+            defaultSDK = AbsolutePath(foundPath, relativeTo: currentWorkingDirectory)
+        }
+        
+        // If we have an SDK, we check that it's valid in the file system.
+        if let sdk = defaultSDK {
+            // FIXME: We should probably also check that it is a directory, etc.
+            guard localFileSystem.exists(sdk) else {
+                throw Error.invalidToolchain(problem: "could not find default SDK at expected path \(sdk.asString)")
             }
-        } catch POSIX.Error.exitStatus {
-            throw Error.invalidToolchain
         }
+      #else
+        defaultSDK = nil
+      #endif
+        
+        // Finally set the properties (we will refactor these to paths in the next diff).
+        SWIFT_EXEC = swiftCompiler.asString
+        clang = clangCompiler.asString
+        sysroot = defaultSDK?.asString
     }
 }