Validation of package products should check that a library product doesn't try to include executable targets

Currently a library product that includes an executable target in its `targets` array only results in a warning.  This was the best that could be done before `executableTarget` was introduced, since further checking was needed to see if a `target` was a library or executable target.  Since we have explicit types now, we should promote this to an error since people are running into it and it's not clear what the problem is.

This check is limited to tools version 5.7 or later.  This change includes a new test fixture and unit test.

rdar://95782540

Author: abertelrud

Fixtures/Miscellaneous/PackageWithMalformedLibraryProduct/Package.swift

@@ -0,0 +1,15 @@
+// swift-tools-version: 5.7
+import PackageDescription
+
+let package = Package(
+    name: "PackageWithMalformedLibraryProduct",
+    products: [
+        .library(
+            name: "PackageWithMalformedLibraryProduct",
+            targets: ["PackageWithMalformedLibraryProduct"]),
+    ],
+    targets: [
+        .executableTarget(
+            name: "PackageWithMalformedLibraryProduct")
+    ]
+)

Fixtures/Miscellaneous/PackageWithMalformedLibraryProduct/Sources/PackageWithMalformedLibraryProduct/main.swift

@@ -0,0 +1 @@
+print("Hello World")

Sources/PackageLoading/Diagnostics.swift

@@ -56,6 +56,10 @@ extension Basics.Diagnostic {
         .error("system library product \(product) shouldn't have a type and contain only one target")
     }
 
+    static func libraryProductWithExectuableTarget(product: String) -> Self {
+        .error("library product '\(product)' should not contain an executable target")
+    }
+
     static func nonPluginProductWithPluginTargets(product: String, type: ProductType, pluginTargets: [String]) -> Self {
         .error("\(type.description) product '\(product)' should not contain plugin targets (it has \(pluginTargets.map{ "'\($0)'" }.joined(separator: ", ")))")
     }

Sources/PackageLoading/PackageBuilder.swift

@@ -1222,6 +1222,13 @@ public final class PackageBuilder {
             self.observabilityScope.emit(.nonPluginProductWithPluginTargets(product: product.name, type: product.type, pluginTargets: pluginTargets.map{ $0.name }))
             return false
         }
+        if manifest.toolsVersion >= .v5_7 {
+            let executableTargets = targets.filter { $0.type == .executable }
+            guard executableTargets.isEmpty else {
+                self.observabilityScope.emit(.libraryProductWithExectuableTarget(product: product.name))
+                return false
+            }
+        }
         return true
     }
 

Tests/FunctionalTests/MiscellaneousTests.swift

@@ -690,6 +690,18 @@ class MiscellaneousTestCase: XCTestCase {
         }
     }
 
+    func testLibraryTriesToIncludeExecutableTarget() throws {
+        try fixture(name: "Miscellaneous/PackageWithMalformedLibraryProduct") { path in
+            XCTAssertThrowsCommandExecutionError(try executeSwiftBuild(path)) { error in
+                // if our code crashes we'll get an exit code of 256
+                guard error.result.exitStatus == .terminated(code: 1) else {
+                    return XCTFail("failed in an unexpected manner: \(error)")
+                }
+                XCTAssertMatch(error.stdout + error.stderr, .contains("library product 'PackageWithMalformedLibraryProduct' should not contain an executable target"))
+            }
+        }
+    }
+
     func testEditModeEndToEnd() throws {
         try fixture(name: "Miscellaneous/Edit") { fixturePath in
             let prefix = resolveSymlinks(fixturePath)