Work around excessive stack space in non optimized builds during oneof isInitialized

Reference/unittest_swift_oneof_all_required.pb.swift

@@ -140,10 +140,22 @@ struct ProtobufUnittest_OneOfContainer {
     case option4(Int32)
 
     fileprivate var isInitialized: Bool {
+      // The use of inline closures is to circumvent an issue where the compiler
+      // allocates stack space for every case branch when no optimizations are
+      // enabled. https://github.com/apple/swift-protobuf/issues/1034
       switch self {
-      case .option1(let v): return v.isInitialized
-      case .option2(let v): return v.isInitialized
-      case .option3(let v): return v.isInitialized
+      case .option1: return {
+        guard case .option1(let v) = self else { preconditionFailure() }
+        return v.isInitialized
+      }()
+      case .option2: return {
+        guard case .option2(let v) = self else { preconditionFailure() }
+        return v.isInitialized
+      }()
+      case .option3: return {
+        guard case .option3(let v) = self else { preconditionFailure() }
+        return v.isInitialized
+      }()
       default: return true
       }
     }

Sources/protoc-gen-swift/OneofGenerator.swift

@@ -226,9 +226,18 @@ class OneofGenerator {
                   "guard case \(f.dottedSwiftName)(let v) = self else {return true}\n",
                   "return v.isInitialized\n")
           } else if fieldsToCheck.count > 1 {
-              p.print("switch self {\n")
+              p.print(
+                  "// The use of inline closures is to circumvent an issue where the compiler\n",
+                  "// allocates stack space for every case branch when no optimizations are\n",
+                  "// enabled. https://github.com/apple/swift-protobuf/issues/1034\n",
+                  "switch self {\n")
               for f in fieldsToCheck {
-                  p.print("case \(f.dottedSwiftName)(let v): return v.isInitialized\n")
+                  p.print("case \(f.dottedSwiftName): return {\n")
+                  p.indent()
+                  p.print("guard case \(f.dottedSwiftName)(let v) = self else { preconditionFailure() }\n")
+                  p.print("return v.isInitialized\n")
+                  p.outdent()
+                  p.print("}()\n")
               }
               // If there were other cases, add a default.
               if fieldsToCheck.count != fields.count {

Tests/SwiftProtobufTests/unittest_swift_oneof_all_required.pb.swift

@@ -140,10 +140,22 @@ struct ProtobufUnittest_OneOfContainer {
     case option4(Int32)
 
     fileprivate var isInitialized: Bool {
+      // The use of inline closures is to circumvent an issue where the compiler
+      // allocates stack space for every case branch when no optimizations are
+      // enabled. https://github.com/apple/swift-protobuf/issues/1034
       switch self {
-      case .option1(let v): return v.isInitialized
-      case .option2(let v): return v.isInitialized
-      case .option3(let v): return v.isInitialized
+      case .option1: return {
+        guard case .option1(let v) = self else { preconditionFailure() }
+        return v.isInitialized
+      }()
+      case .option2: return {
+        guard case .option2(let v) = self else { preconditionFailure() }
+        return v.isInitialized
+      }()
+      case .option3: return {
+        guard case .option3(let v) = self else { preconditionFailure() }
+        return v.isInitialized
+      }()
       default: return true
       }
     }