[wasm] [cleanup] Remove unused DecodeStruct type

It was used before as a placeholder in Result<DecodeStruct*> to
communicate that no value was returned. We actually only created a
Results holding {nullptr} when returning such values. Thus, the whole
struct is not needed, and we return Result<nullptr_t> instead, which
clearly communicates that this result does not hold any value.

An alternative would be to use Result<void>, but this would require
partial specialization of the Result template, which would be overkill
here.

R=ahaas@chromium.org

Change-Id: Ib07d2c4fe716c735839675d11146c47f97997d40
Reviewed-on: https://chromium-review.googlesource.com/509551Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45464}

src/wasm/function-body-decoder.cc

@@ -2130,7 +2130,7 @@ DecodeResult VerifyWasmCode(AccountingAllocator* allocator,
   Zone zone(allocator, ZONE_NAME);
   WasmFullDecoder decoder(&zone, module, body);
   decoder.Decode();
-  return decoder.toResult<DecodeStruct*>(nullptr);
+  return decoder.toResult(nullptr);
 }
 
 DecodeResult BuildTFGraph(AccountingAllocator* allocator, TFBuilder* builder,
@@ -2138,7 +2138,7 @@ DecodeResult BuildTFGraph(AccountingAllocator* allocator, TFBuilder* builder,
   Zone zone(allocator, ZONE_NAME);
   WasmFullDecoder decoder(&zone, builder, body);
   decoder.Decode();
-  return decoder.toResult<DecodeStruct*>(nullptr);
+  return decoder.toResult(nullptr);
 }
 
 unsigned OpcodeLength(const byte* pc, const byte* end) {

src/wasm/function-body-decoder.h

@@ -42,13 +42,11 @@ static inline FunctionBody FunctionBodyForTesting(const byte* start,
   return {nullptr, start, start, end};
 }
 
-struct DecodeStruct {
-  int unused;
-};
-typedef Result<DecodeStruct*> DecodeResult;
-inline std::ostream& operator<<(std::ostream& os, const DecodeStruct& tree) {
-  return os;
-}
+// A {DecodeResult} only stores the failure / success status, but no data. Thus
+// we use {nullptr_t} as data value, such that the only valid data stored in
+// this type is a nullptr.
+// Storing {void} would require template specialization.
+using DecodeResult = Result<std::nullptr_t>;
 
 V8_EXPORT_PRIVATE DecodeResult VerifyWasmCode(AccountingAllocator* allocator,
                                               const wasm::WasmModule* module,