Move CaseIterable conformance to library base Enum type

Update generation to also add allCases (when needed) in that main decl for the
enum.

Progress on apple#1210

Sources/SwiftProtobuf/Enum.swift

@@ -16,7 +16,7 @@
 // -----------------------------------------------------------------------------
 
 /// Generated enum types conform to this protocol.
-public protocol Enum: RawRepresentable, Hashable, _ProtoSendable {
+public protocol Enum: RawRepresentable, Hashable, CaseIterable, _ProtoSendable {
   /// Creates a new instance of the enum initialized to its default value.
   init()
 

Sources/protoc-gen-swift/EnumGenerator.swift

@@ -80,33 +80,25 @@ class EnumGenerator {
     p.print("\n")
     generateRawValueProperty(printer: &p)
 
+    maybeGenerateCaseIterable(printer: &p)
+
     p.outdent()
     p.print("\n")
     p.print("}\n")
   }
 
-  func generateCaseIterable(printer p: inout CodePrinter) {
-    // NOTE: When we can assume Swift 4.2, this should move from an extension
-    // to being directly done when declaring the type.
+  func maybeGenerateCaseIterable(printer p: inout CodePrinter) {
+    guard enumDescriptor.hasUnknownPreservingSemantics else { return }
 
     let visibility = generatorOptions.visibilitySourceSnippet
-
     p.print("\n")
-    p.print("extension \(swiftFullName): CaseIterable {\n")
-    p.indent()
-    if enumDescriptor.hasUnknownPreservingSemantics {
-      p.print("// The compiler won't synthesize support with the \(unrecognizedCaseName) case.\n")
-      p.print("\(visibility)static var allCases: [\(swiftFullName)] = [\n")
-      for v in mainEnumValueDescriptors {
-        let dottedName = namer.dottedRelativeName(enumValue: v)
-        p.print("  \(dottedName),\n")
-      }
-      p.print("]\n")
-    } else {
-      p.print("// Support synthesized by the compiler.\n")
+    p.print("// The compiler won't synthesize support with the \(unrecognizedCaseName) case.\n")
+    p.print("\(visibility)static var allCases: [\(swiftFullName)] = [\n")
+    for v in mainEnumValueDescriptors {
+      let dottedName = namer.dottedRelativeName(enumValue: v)
+      p.print("  \(dottedName),\n")
     }
-    p.outdent()
-    p.print("}\n")
+    p.print("]\n")
   }
 
   func generateRuntimeSupport(printer p: inout CodePrinter) {

Sources/protoc-gen-swift/FileGenerator.swift

@@ -125,12 +125,10 @@ class FileGenerator {
 
         for e in enums {
             e.generateMainEnum(printer: &p)
-            e.generateCaseIterable(printer: &p)
         }
 
         for m in messages {
             m.generateMainStruct(printer: &p, parent: nil, errorString: &errorString)
-            m.generateEnumCaseIterable(printer: &p)
         }
 
         var sendablePrinter = CodePrinter()

Sources/protoc-gen-swift/MessageGenerator.swift

@@ -185,15 +185,6 @@ class MessageGenerator {
     p.print("}\n")
   }
 
-  func generateEnumCaseIterable(printer p: inout CodePrinter) {
-    for e in enums {
-      e.generateCaseIterable(printer: &p)
-    }
-    for m in messages {
-      m.generateEnumCaseIterable(printer: &p)
-    }
-  }
-
   func generateSendable(printer p: inout CodePrinter) {
     // Once our minimum supported version has Data be Sendable, @unchecked
     // will not be needed for all messages, provided that the extension types