Add more documentation about code generation

Sources/GRPCProtobuf/Documentation.docc/Articles/Code-generation-with-protoc.md

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+# Code generation with protoc
+
+This article describes how to use the Swift Package Manager build plugin to
+generate gRPC Swift and Swift Protobuf code from your Protocol Buffers `.proto` files.
+
+## Overview
+
+If you've used Protocol Buffers before then generating gRPC Swift stubs should be simple. If you're
+unfamiliar with Protocol Buffers then you should get comfortable with the concepts before
+continuing; the [Protocol Buffers website](https://protobuf.dev/) is a great place to start.
+
+If you haven't installed `protoc` yet refer to <doc:Installing-protoc> for
+instructions.
+
+## Using protoc
+
+The [`grpc-swift-protobuf`](https://github.com/grpc/grpc-swift-protobuf) package provides
+`protoc-gen-grpc-swift-2`, a program which is a plugin for the Protocol Buffers compiler, `protoc`.
+To generate gRPC stubs for your `.proto` files directly you must run the `protoc` command with
+the `--grpc-swift-2_out=<DIRECTORY>` option:
+
+```console
+protoc --grpc-swift-2_out=. my-service.proto
+```
+
+> `protoc-gen-grpc-swift-2` only generates gRPC stubs, it doesn't generate messages. You must use
+> `protoc-gen-swift` to generate messages in addition to gRPC Stubs.
+
+The presence of `--grpc-swift-2_out` tells `protoc` to use the `protoc-gen-grpc-swift-2` plugin. By
+default it'll look for the plugin in your `PATH`. You can also specify the path to the plugin
+explicitly:
+
+```console
+protoc --plugin=/path/to/protoc-gen-grpc-swift-2 --grpc-swift-2_out=. my-service.proto
+```
+
+You can also specify various option the `protoc-gen-grpc-swift-2` via `protoc` using
+the `--grpc-swift-2_opt` argument:
+
+```console
+protoc --grpc-swift-2_opt=<OPTION_NAME>=<OPTION_VALUE> --grpc-swift-2_out=.
+```
+
+You can specify multiple options by passing the `--grpc-swift-2_opt` argument multiple times:
+
+```console
+protoc \
+  --grpc-swift-2_opt=<OPTION_NAME1>=<OPTION_VALUE1> \
+  --grpc-swift-2_opt=<OPTION_NAME2>=<OPTION_VALUE2> \
+  --grpc-swift-2_out=.
+```
+
+### Generator options
+
+| Name                      | Possible Values                             | Default         | Description                                              |
+|---------------------------|---------------------------------------------|-----------------|----------------------------------------------------------|
+| `Visibility`              | `Public`, `Package`, `Internal`             | `Internal`      | Access level for generated stubs                         |
+| `Server`                  | `True`, `False`                             | `True`          | Generate server stubs                                    |
+| `Client`                  | `True`, `False`                             | `True`          | Generate client stubs                                    |
+| `FileNaming`              | `FullPath`, `PathToUnderscores`, `DropPath` | `FullPath`      | How generated source files should be named. †            |
+| `ProtoPathModuleMappings` |                                             |                 | Path to module map `.asciipb` file. ‡                    |
+| `UseAccessLevelOnImports` | `True`, `False`                             | `False`         | Whether imports should have explicit access levels.      |
+| `GRPCModuleName`          |                                             | `GRPCCore`      | The name of the `GRPCCore` module.                       |
+| `GRPCProtobufModuleName`  |                                             | `GRPCProtobuf`  | The name of the `GRPCProtobuf` module.                   |
+| `SwiftProtobufModuleName` |                                             | `SwiftProtobuf` | The name of the `SwiftProtobuf` module.                  |
+| `Availability`            | String, in the form `OS Version`            |                 | Platform availability to use in generated code. §        |
+
+† The `FileNaming` option has three possible values, for an input of `foo/bar/baz.proto` the following
+output file will be generated:
+- `FullPath`: `foo/bar/baz.grpc.swift`.
+- `PathToUnderscores`: `foo_bar_baz.grpc.swift`
+- `DropPath`: `baz.grpc.swift`
+
+‡ The code generator assumes all inputs are generated into the same module, `ProtoPathModuleMappings`
+allows you to specify a mapping from `.proto` files to the Swift module they are generated in. This
+allows the code generator to add appropriate imports to your generated stubs. This is described in
+more detail in the [SwiftProtobuf documentation](https://github.com/apple/swift-protobuf/blob/main/Documentation/PLUGIN.md).
+
+§ If unspecified the following availability is used: macOS 15, iOS 18, tvOS 18,
+watchOS 11, visionOS 2. The `Availability` option may be specified multiple
+times, where each value is a space delimited pair of platform and version, e.g.
+`Availability=macOS 15.0`.
+
+### Building the protoc plugin
+
+> The version of `protoc-gen-grpc-swift-2` you use mustn't be newer than the version of
+> the `grpc-swift-protobuf` you're using.
+
+If your package depends on `grpc-swift-protobuf` then you can get a copy of `protoc-gen-grpc-swift-2`
+by building it directly:
+
+```console
+swift build --product protoc-gen-grpc-swift-2
+```
+
+This command will build the plugin into `.build/debug` directory. You can get the full path using
+`swift build --show-bin-path`.

Sources/GRPCProtobuf/Documentation.docc/Articles/Code-generation-with-the-build-plugin.md

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+# Code generation with the build plugin
+
+This article describes how to use `protoc` to generate stubs for gRPC Swift and
+Swift Protobuf from your Protocol Buffers `.proto` files.
+
+## Overview
+
+The build plugin (`GRPCProtobufGenerator`) is a great choice for convenient
+dynamic code generation, however it does come with some limitations. Because it
+generates the gRPC Swift stubs as part of the build it has the requirement that
+`protoc` must be available at compile time. This requirement means it is not a
+good fit for library authors who do not have direct control over this.
+
+To learn more about other options for code generation see <doc:Generating-stubs>.
+
+The build plugin works by detecting `.proto` files in the source tree and
+invokes `protoc` once for each file (caching results and performing the
+generation as necessary).
+
+If you haven't installed `protoc` yet refer to <doc:Installing-protoc> for
+instructions.
+
+### Adoption
+
+You must adopt Swift Package Manager build plugins on a per-target basis by
+modifying your package manifest (`Package.swift` file). To do this, declare the
+`grpc-swift-protobuf` package as a dependency and add the plugin to your desired
+targets.
+
+For example, to make use of the plugin for generating gRPC Swift stubs as part
+of the `echo-server` target:
+
+```swift
+targets: [
+   .executableTarget(
+     name: "echo-server",
+     dependencies: [
+       // ...
+     ],
+     plugins: [
+       .plugin(
+         name: "GRPCProtobufGenerator",
+         package: "grpc-swift-protobuf"
+       )
+     ]
+   )
+ ]
+```
+
+Once this is done you need to ensure that the `.proto` files to be used for
+generation are included in the target's source directory and that you have
+defined a configuration file.
+
+## Configuration
+
+You must provide a configuration file named
+`grpc-swift-proto-generator-config.json` in the directory which encloses all
+`.proto` files (in the same directory as the files or a parent directory). The
+configuration file tells the build plugin about the options used for `protoc`
+invocations.
+
+> Warning:
+> The name of the config file is important and must match exactly, the
+> plugin won't be applied if it can't find the config file.
+
+You can use the following as a starting point for your configuration:
+
+```json
+{
+  "generate": {
+    "clients": true,
+    "servers": true,
+    "messages": true
+  }
+}
+```
+
+By default clients, servers, and messages will be generated with the `internal`
+access level.
+
+The full structure of the config file looks like this:
+
+```json
+{
+  "generate": {
+    "clients": true,
+    "servers": true,
+    "messages": true
+  },
+  "generatedSource": {
+    "accessLevelOnImports": false,
+    "accessLevel": "internal"
+  },
+  "protoc": {
+    "executablePath": "/opt/homebrew/bin/protoc",
+    "importPaths": [
+      "../directory_1"
+    ]
+  }
+}
+```
+
+Each of the options are described below:
+
+| Name                                   | Possible Values                            | Default      | Description                                         |
+|----------------------------------------|--------------------------------------------|--------------|-----------------------------------------------------|
+| `generate.servers`                     | `true`, `false`                            | `true`       | Generate server stubs                               |
+| `generate.clients`                     | `true`, `false`                            | `true`       | Generate client stubs                               |
+| `generate.messages`                    | `true`, `false`                            | `true`       | Generate message stubs                              |
+| `generatedSource.accessLevelOnImports` | `true`, `false`                            | `false`      | Whether imports should have explicit access levels  |
+| `generatedSource.accessLevel`          | `"public"`, `"package"`, `"internal"`      | `"internal"` | Access level for generated stubs                    |
+| `protoc.executablePath`                | N/A                                        | `null`†      | Path to the `protoc` executable                     |
+| `protoc.importPaths`                   | N/A                                        | `null`‡      | Import paths passed to `protoc`                     |
+
+† The Swift Package Manager build plugin infrastructure will attempt to discover
+the executable's location if you don't provide one.
+
+‡ If you don't provide any import paths then the path to the configuration file
+will be used on a per-source-file basis.
+
+Many of these options map to `protoc-gen-grpc-swift-2` and `protoc-gen-swift`
+options.
+
+If you require greater flexibility you may specify more than one configuration
+file. Configuration files apply to all `.proto` files equal to or below it in
+the file hierarchy. A configuration file lower in the file hierarchy supersedes
+one above it.

Sources/GRPCProtobuf/Documentation.docc/Articles/Code-generation-with-the-command-plugin.md

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+# Code generation with generate-grpc-code-from-protos
+
+This article describes how to use the `generate-grpc-code-from-protos` Swift
+Package Manager command plugin to generate gRPC Swift and Swift Protobuf code
+from your Protocol Buffers `.proto` files.
+
+This plugin is particularly useful for:
+
+- **Manual, on-demand code generation:** When you prefer to explicitly generate
+  code rather than relying on a build tool plugin.
+- **Libraries:** For Swift packages intended as libraries, where generated
+  source code should be checked into your repository to avoid external `protoc`
+  dependencies for your library's consumers.
+
+If you haven't installed `protoc` yet refer to <doc:Installing-protoc> for
+instructions.
+
+## Adding the Plugin to Your Package
+
+To use `generate-grpc-code-from-protos` your package needs to depend on
+`grpc-swift-protobuf`. You **don't** need to add a dependency on each target
+like you would for the build plugin.
+
+## Basic Usage
+
+Once your package depends on `grpc-swift-protobuf` you can invoke it using:
+
+```sh
+swift package generate-grpc-code-from-protos path/to/YourService.proto path/to/YourMessages.proto
+```
+
+If you've organised your protos within a single directory then you can
+pass the path of the directory as an argument instead, the plugin will find
+all `.proto` files nested within that directory:
+
+```sh
+swift package generate-grpc-code-from-protos Protos
+```
+
+By default the plugin generates code for gRPC servers, clients, and Protobuf
+messages into the current working directory. To change where the code is
+generated you can specify the `--output-path` option:
+
+
+```sh
+swift package generate-grpc-code-from-protos --output-path Sources/Generated -- Protos
+```
+
+> Important: The "`--`" separates options and inputs passed to the plugin.
+>
+> Everything after "`--`" is treated as an input (a `.proto` file or a
+> directory), everything before "`--`" is treated as an option with a value or
+> a flag. If there is no "`--`" then all arguments are treated as input.
+
+You should now have a basic understanding of how to use the plugin. You can
+configure how the code is generated via a number of options, a few commonly used
+ones are:
+- `--no-client` disables client code generation,
+- `--no-server` disables server code generation,
+- `--no-messages` disables message code generation,
+- `--access-level <access>` specifies the access level of the generated code,
+  (`<access>` must be one of "internal", "package", or "public").
+
+You can read about other options by referring to the `--help` text:
+
+```sh
+swift package generate-grpc-code-from-protos --help
+```
+
+### Permissions
+
+Swift Package Manager command plugins require permission to create files. You'll
+be prompted to give `generate-grpc-code-from-protos` permission when running it.
+To avoid being prompted you can grant permissions ahead of time by specifying
+`--allow-writing-to-package-directory` to the `swift package` command. For
+example:
+
+```sh
+swift package --allow-writing-to-package-directory generate-grpc-code-from-protos Protos
+```