Throw error for mixed targets when tool version is insufficient

Author: ncooke3

Sources/PackageLoading/PackageBuilder.swift

@@ -153,6 +153,9 @@ extension Target {
             case emptyName
         }
 
+        /// The target contains an invalid mix of languages (e.g. both Swift and C).
+        case mixedSources(AbsolutePath)
+
     }
 }
 
@@ -161,6 +164,10 @@ extension Target.Error: CustomStringConvertible {
         switch self {
         case .invalidName(let path, let problem):
             return "invalid target name at '\(path)'; \(problem)"
+        case .mixedSources(let path):
+            // TODO(ncooke3): Update error message with support version.
+            return "target at '\(path)' contains mixed language source " +
+                    "files; feature not supported until tools version XX"
         }
     }
 }
@@ -1418,6 +1425,10 @@ extension Sources {
         }
     }
 
+    var containsMixedLanguage: Bool {
+        return hasSwiftSources && hasClangSources
+    }
+
     /// Determine target type based on the sources.
     fileprivate func computeTargetType() -> Target.Kind {
         let isLibrary = !relativePaths.contains { path in

Sources/PackageLoading/TargetSourcesBuilder.swift

@@ -180,6 +180,12 @@ public struct TargetSourcesBuilder {
         diagnoseInfoPlistConflicts(in: resources)
         diagnoseInvalidResource(in: target.resources)
 
+        // It's an error to contain mixed language source files.
+        // TODO(ncooke3): Update `toolsVersion` when the PR merges.
+        if sources.containsMixedLanguage, toolsVersion < .v4  {
+            throw Target.Error.mixedSources(targetPath)
+        }
+
         return (sources, resources, headers, ignored, others)
     }
 

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -40,6 +40,28 @@ class PackageBuilderTests: XCTestCase {
         }
     }
 
+    func testMixedSourcesWhenUnsupportedToolsVersion() throws {
+        let foo: AbsolutePath = AbsolutePath(path: "/Sources/foo")
+
+        let fs = InMemoryFileSystem(emptyFiles:
+            foo.appending(components: "Foo.swift").pathString,
+            foo.appending(components: "bar.c").pathString
+        )
+
+        let manifest = Manifest.createRootManifest(
+            name: "pkg",
+            path: .root,
+            toolsVersion: .v3,
+            targets: [
+                try TargetDescription(name: "foo"),
+            ]
+        )
+        PackageBuilderTester(manifest, in: fs) { _, diagnostics in
+            // TODO(ncooke3): Update error message with support version.
+            diagnostics.check(diagnostic: "target at '\(foo)' contains mixed language source files; feature not supported until tools version XX", severity: .error)
+        }
+    }
+
     func testMixedSources() throws {
         let foo: AbsolutePath = AbsolutePath(path: "/Sources/foo")