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

…f enum equality.

Progress on: apple#1034

Sources/protoc-gen-swift/OneofGenerator.swift

@@ -219,9 +219,21 @@ class OneofGenerator {
         p.print(
             "\(visibility)static func ==(lhs: \(swiftFullName), rhs: \(swiftFullName)) -> Bool {\n")
         p.indent()
-        p.print("switch (lhs, rhs) {\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 (lhs, rhs) {\n")
         for f in fields {
-            p.print("case (\(f.dottedSwiftName)(let l), \(f.dottedSwiftName)(let r)): return l == r\n")
+            p.print(
+                "case (\(f.dottedSwiftName), \(f.dottedSwiftName)): return {\n")
+          p.indent()
+          p.print(
+                "guard case \(f.dottedSwiftName)(let l) = lhs, case \(f.dottedSwiftName)(let r) = rhs else { preconditionFailure() }\n",
+                "return l == r\n")
+          p.outdent()
+          p.print(
+                "}()\n")
         }
         if fields.count > 1 {
             // A tricky edge case: If the oneof only has a single case, then