Exposes implementationOnlyImports option in SPM plugin

Plugins/SwiftProtobufPlugin/plugin.swift

@@ -62,6 +62,8 @@ struct SwiftProtobufPlugin: BuildToolPlugin {
             var visibility: Visibility?
             /// The file naming strategy to use.
             var fileNaming: FileNaming?
+            /// Whether internal imports should be annotated as `@_implementationOnly`.
+            var implementationOnlyImports: Bool?
         }
 
         /// The path to the `protoc` binary.
@@ -156,6 +158,10 @@ struct SwiftProtobufPlugin: BuildToolPlugin {
         var inputFiles = [Path]()
         var outputFiles = [Path]()
 
+        if let implementationOnlyImports = invocation.implementationOnlyImports {
+            protocArgs.append("--swift_opt=ImplementationOnlyImports=\(implementationOnlyImports)")
+        }
+
         for var file in invocation.protoFiles {
             // Append the file to the protoc args so that it is used for generating
             protocArgs.append("\(file)")

Sources/protoc-gen-swift/Docs.docc/index.md

@@ -164,6 +164,30 @@ mapping {
 The `proto_file_path` values here should match the paths used in the proto file
 `import` statements.
 
+
+##### Generation Option: `ImplementationOnlyImports` - `@_implementationOnly`-annotated imports
+
+By default, the code generator does not annotate any imports with `@_implementationOnly`.
+However, in some scenarios, such as when distributing an `XCFramework`, imports 
+for types used only internally should be annotated as `@_implementationOnly` to 
+avoid exposing internal symbols to clients.
+You can change this with the `ImplementationOnlyImports` option:
+
+```
+$ protoc --swift_opt=ImplementationOnlyImports=[value] --swift_out=. foo/bar/*.proto mumble/*.proto
+```
+
+The possible values for `ImplementationOnlyImports` are:
+
+* `false` (default): The `@_implementationOnly` annotation will never be used.
+* `true`: Imports of internal dependencies and any modules defined in the module
+mappings will be annotated as `@_implementationOnly`. 
+
+**Important:** Modules cannot be imported as implementation-only if they're 
+exposed via public API, so even if `ImplementationOnlyImports` is set to `true`,
+this will only work if the `Visibility` is set to `internal`. 
+
+
 ### Building your project
 
 After copying the `.pb.swift` files into your project, you will need

Sources/protoc-gen-swift/Docs.docc/spm-plugin.md

@@ -74,7 +74,8 @@ to the root of your target's source folder. An example configuration file looks
             "protoFiles": [
                 "Foo.proto",
             ],
-            "visibility": "internal"
+            "visibility": "internal",
+            "implementationOnlyImports": true
         },
         {
             "protoFiles": [