Do not inherit host destination's extra flags for target destination

Host environment specific flags, such as XCTest module directory, were
implicitly inherited by target destination. It caused unexpected
reference to host module file, so drop them explicitly

Author: kateinoigakukun

Sources/Commands/SwiftTool.swift

@@ -838,6 +838,9 @@ public class SwiftTool {
             // Create custom toolchain if present.
             if let customDestination = self.options.customCompileDestination {
                 destination = try Destination(fromFile: customDestination)
+            } else if let target = self.options.customCompileTriple,
+                      let targetDestination = Destination.defaultDestination(for: target, host: hostDestination) {
+                destination = targetDestination
             } else {
                 // Otherwise use the host toolchain.
                 destination = hostDestination
@@ -854,18 +857,6 @@ public class SwiftTool {
         }
         if let sdk = self.options.customCompileSDK {
             destination.sdk = sdk
-        } else if let target = destination.target, target.isWASI() {
-            // Set default SDK path when target is WASI whose SDK is embeded
-            // in Swift toolchain
-            do {
-                let compilers = try UserToolchain.determineSwiftCompilers(binDir: destination.binDir)
-                destination.sdk = compilers.compile
-                    .parentDirectory // bin
-                    .parentDirectory // usr
-                    .appending(components: "share", "wasi-sysroot")
-            } catch {
-                return .failure(error)
-            }
         }
         destination.archs = options.archs
 

Sources/Workspace/Destination.swift

@@ -178,6 +178,24 @@ public struct Destination: Encodable, Equatable {
     }
     /// Cache storage for sdk platform path.
     private static var _sdkPlatformFrameworkPath: (fwk: AbsolutePath, lib: AbsolutePath)? = nil
+
+    /// Returns a default destination of a given target environment
+    public static func defaultDestination(for triple: Triple, host: Destination) -> Destination? {
+        if triple.isWASI() {
+            let wasiSysroot = host.binDir
+                .parentDirectory // usr
+                .appending(components: "share", "wasi-sysroot")
+            return Destination(
+                target: triple,
+                sdk: wasiSysroot,
+                binDir: host.binDir,
+                extraCCFlags: [],
+                extraSwiftCFlags: [],
+                extraCPPFlags: []
+            )
+        }
+        return nil
+    }
 }
 
 extension Destination {