Work around excessive stack space in non optimized builds during oneo…

…f isInitialized

Progress on: apple#1034

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 {