Boxed OneOfs

Let’s implement an alternate form of oneof that doesn’t crash the compiler!

• Runtime API
• Kotlin generator
• Reflection-based adapters
• Java generator
• Redacted support
• Tests & coverage for NameAllocator / name collisions
• JSON support Kotlin
• JSON support Java
• Fix ordering so oneofs are interleaved with non-oneofs in generated code #1997
• Fix generator to not double readUnknownFields() calls
• Fix generator to use CONSTANT_CASE for oneof declarations and their keys
• Confirm we don't have circular deps: https://github.com/square/wire/pull/1856#discussion_r530633011

@damianw’s original report below:

Wire version: 3.1.0

For example, Perfetto's FtraceEvent generates this Kotlin file. Attempting to compile it (with Kotlin 1.3.70) results in:

e: java.lang.IllegalStateException: Backend Internal error: Exception during file facade code generation
File being compiled: file:///.../build/generated/source/wire/perfetto/protos/FtraceEvent.kt
The root cause java.lang.RuntimeException was thrown at: org.jetbrains.kotlin.codegen.ClassFileFactory$OutputClassFile.asByteArray(ClassFileFactory.java:315)
        at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException(CodegenUtil.kt:247)
        at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generate(PackageCodegenImpl.java:78)
        at org.jetbrains.kotlin.codegen.DefaultCodegenFactory.generatePackage(CodegenFactory.kt:88)
        at org.jetbrains.kotlin.codegen.DefaultCodegenFactory.generateModule(CodegenFactory.kt:67)
        ...
Caused by: java.lang.RuntimeException: Error generating class file perfetto/protos/FtraceEvent$Companion$ADAPTER$1.class (compiled from [/.../build/generated/source/wire/perfetto/protos/FtraceEvent.kt]): Method too large: perfetto/protos/FtraceEvent$Companion$ADAPTER$1.redact (Lperfetto/protos/FtraceEvent;)Lperfetto/protos/FtraceEvent;
        at org.jetbrains.kotlin.codegen.ClassFileFactory$OutputClassFile.asByteArray(ClassFileFactory.java:315)
        at org.jetbrains.kotlin.cli.common.output.OutputUtilsKt.writeAll(outputUtils.kt:32)
        at org.jetbrains.kotlin.cli.common.output.OutputUtilsKt.writeAll(outputUtils.kt:42)
        at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.writeOutput(KotlinToJVMBytecodeCompiler.kt:101)
        ...
Caused by: org.jetbrains.org.objectweb.asm.MethodTooLargeException: Method too large: perfetto/protos/FtraceEvent$Companion$ADAPTER$1.redact (Lperfetto/protos/FtraceEvent;)Lperfetto/protos/FtraceEvent;
        at org.jetbrains.org.objectweb.asm.MethodWriter.computeMethodInfoSize(MethodWriter.java:2089)
        at org.jetbrains.org.objectweb.asm.ClassWriter.toByteArray(ClassWriter.java:458)
        at org.jetbrains.kotlin.codegen.ClassBuilderFactories$2.asBytes(ClassBuilderFactories.java:118)
        at org.jetbrains.kotlin.codegen.DelegatingClassBuilderFactory.asBytes(DelegatingClassBuilderFactory.kt:36)
        ... 46 more