Add note about SPM plugin known issues, provide better errors (#1629)

Plugins/GRPCSwiftPlugin/plugin.swift

@@ -20,13 +20,27 @@ import PackagePlugin
 @main
 struct GRPCSwiftPlugin {
   /// Errors thrown by the `GRPCSwiftPlugin`
-  enum PluginError: Error {
+  enum PluginError: Error, CustomStringConvertible {
     /// Indicates that the target where the plugin was applied to was not `SourceModuleTarget`.
-    case invalidTarget
+    case invalidTarget(Target)
     /// Indicates that the file extension of an input file was not `.proto`.
-    case invalidInputFileExtension
+    case invalidInputFileExtension(String)
     /// Indicates that there was no configuration file at the required location.
-    case noConfigFound
+    case noConfigFound(String)
+
+    var description: String {
+      switch self {
+      case let .invalidTarget(target):
+        return "Expected a SwiftSourceModuleTarget but got '\(type(of: target))'."
+      case let .invalidInputFileExtension(path):
+        return "The input file '\(path)' does not have a '.proto' extension."
+      case let .noConfigFound(path):
+        return """
+        No configuration file found named '\(path)'. The file must not be listed in the \
+        'exclude:' argument for the target in Package.swift.
+        """
+      }
+    }
   }
 
   /// The configuration of the plugin.
@@ -88,7 +102,7 @@ struct GRPCSwiftPlugin {
         $0.path.lastComponent == Self.configurationFileName
       }
     )?.path else {
-      throw PluginError.noConfigFound
+      throw PluginError.noConfigFound(Self.configurationFileName)
     }
 
     let data = try Data(contentsOf: URL(fileURLWithPath: "\(configurationFilePath)"))
@@ -208,7 +222,7 @@ struct GRPCSwiftPlugin {
     for invocation in configuration.invocations {
       for protoFile in invocation.protoFiles {
         if !protoFile.hasSuffix(".proto") {
-          throw PluginError.invalidInputFileExtension
+          throw PluginError.invalidInputFileExtension(protoFile)
         }
       }
     }
@@ -221,7 +235,7 @@ extension GRPCSwiftPlugin: BuildToolPlugin {
     target: Target
   ) async throws -> [Command] {
     guard let swiftTarget = target as? SwiftSourceModuleTarget else {
-      throw PluginError.invalidTarget
+      throw PluginError.invalidTarget(target)
     }
     return try self.createBuildCommands(
       pluginWorkDirectory: context.pluginWorkDirectory,

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

@@ -58,12 +58,12 @@ Here's an example file structure that looks like this:
 ```text
 Sources
 ├── main.swift
-├── ProtoBuf
+└── ProtoBuf
     ├── grpc-swift-config.json
     ├── foo.proto
     └── Bar
         └── Bar.proto
-```        
+```
 
 ```json
 {
@@ -103,19 +103,19 @@ the `keepMethodCasing` option to false, which means that the casing of the autog
 
 ### Defining the path to the protoc binary
 
-The plugin needs to be able to invoke the `protoc` binary to generate the Swift types. There are several ways to achieve this. 
+The plugin needs to be able to invoke the `protoc` binary to generate the Swift types. There are several ways to achieve this.
 
-First, by default, the package manager looks into the `$PATH` to find binaries named `protoc`. 
-This works immediately if you use `swift build` to build your package and `protoc` is installed 
+First, by default, the package manager looks into the `$PATH` to find binaries named `protoc`.
+This works immediately if you use `swift build` to build your package and `protoc` is installed
 in the `$PATH` (`brew` is adding it to your `$PATH` automatically).
 However, this doesn't work if you want to compile from Xcode since Xcode is not passed the `$PATH`.
 
-If compiling from Xcode, you have **three options** to set the path of `protoc` that the plugin is going to use: 
+If compiling from Xcode, you have **three options** to set the path of `protoc` that the plugin is going to use:
 
 * Set an environment variable `PROTOC_PATH` that gets picked up by the plugin. Here are two examples of how you can achieve this:
 
 ```shell
-# swift build
+# swift build
 env PROTOC_PATH=/opt/homebrew/bin/protoc swift build
 
 # To start Xcode (Xcode MUST NOT be running before invoking this)
@@ -134,9 +134,18 @@ env PROTOC_PATH=/opt/homebrew/bin/protoc xcodebuild <Here goes your command>
 }
 ```
 
-> Warning: The configuration file option only solves the problem for leaf packages that are using the Swift package manager
-plugin since there you can point the package manager to the right binary. The environment variable
-does solve the problem for transitive packages as well; however, it requires your users to set
-the variable now. In general we advise against adopting the plugin as a non-leaf package!
+* You can start Xcode by running `$ xed .` from the command line from the directory your project is located - this should make `$PATH` visible to Xcode.
+
+### Known Issues
+
+- The configuration file _must not_ be excluded from the list of sources for the
+  target in the package manifest (that is, it should not be present in the
+  `exclude` argument for the target). The build system does not have access to
+  the file if it is excluded, however, `swift build` will result in a warning
+  that the file should be excluded.
+- The plugin should only be used for leaf package. The configuration file option
+  only solves the problem for leaf packages that are using the Swift package
+  manager plugin since there you can point the package manager to the right
+  binary. The environment variable does solve the problem for transitive
+  packages as well; however, it requires your users to set the variable now.
 
-* You can start Xcode by running `$ xed .` from the command line from the directory your project is located - this should make `$PATH` visible to Xcode.