Be more strict about invalid -module-name arguments

In the case this argument is passed explicitly and invalid, it was
previous ignored and fell back to 'main' even in cases where you might
not want that like compiling object files. It doesn't seem necessary to
allow invalid cases like this through regardless of what object type is
being built.

Author: keith

Sources/SwiftDriver/Driver/Driver.swift

@@ -2321,32 +2321,6 @@ extension Driver {
     return ""
   }
 
-  /// Whether we are going to be building an executable.
-  ///
-  /// FIXME: Why "maybe"? Why isn't this all known in advance as captured in
-  /// linkerOutputType?
-  private static func maybeBuildingExecutable(
-    _ parsedOptions: inout ParsedOptions,
-    linkerOutputType: LinkOutputType?
-  ) -> Bool {
-    switch linkerOutputType {
-    case .executable:
-      return true
-
-    case .dynamicLibrary, .staticLibrary:
-      return false
-
-    default:
-      break
-    }
-
-    if parsedOptions.hasArgument(.parseAsLibrary, .parseStdlib) {
-      return false
-    }
-
-    return parsedOptions.allInputs.count == 1
-  }
-
   /// Determine how the module will be emitted and the name of the module.
   private static func computeModuleInfo(
     _ parsedOptions: inout ParsedOptions,
@@ -2419,10 +2393,9 @@ extension Driver {
 
     func fallbackOrDiagnose(_ error: Diagnostic.Message) {
       moduleNameIsFallback = true
-      if compilerOutputType == nil || maybeBuildingExecutable(&parsedOptions, linkerOutputType: linkerOutputType) {
+      if compilerOutputType == nil || !parsedOptions.hasArgument(.moduleName) {
         moduleName = "main"
-      }
-      else {
+      } else {
         diagnosticsEngine.emit(error)
         moduleName = "__bad__"
       }

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -757,6 +757,18 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertEqual(try Driver(args: ["swiftc", "foo.swift", "-o", "+++.out"]).moduleOutputInfo.name, "main")
     XCTAssertEqual(try Driver(args: ["swift"]).moduleOutputInfo.name, "REPL")
     XCTAssertEqual(try Driver(args: ["swiftc", "foo.swift", "-emit-library", "-o", "libBaz.dylib"]).moduleOutputInfo.name, "Baz")
+
+    try assertDriverDiagnostics(
+      args: ["swiftc", "foo.swift", "-module-name", "", "file.foo.swift"]
+    ) {
+      $1.expect(.error("module name \"\" is not a valid identifier"))
+    }
+
+    try assertDriverDiagnostics(
+      args: ["swiftc", "foo.swift", "-module-name", "123", "file.foo.swift"]
+    ) {
+      $1.expect(.error("module name \"123\" is not a valid identifier"))
+    }
   }
 
   func testEmitModuleSeparatelyDiagnosticPath() throws {