[WASM] Use better error message when decoding function body fails.

src/wasm/module-decoder.cc

Index: src/wasm/module-decoder.cc
diff --git a/src/wasm/module-decoder.cc b/src/wasm/module-decoder.cc
index ecf4a160da234b8e856d654b2377b195992119c3..bfadb902789fc714611a0737909f57a6a781d28f 100644
--- a/src/wasm/module-decoder.cc
+++ b/src/wasm/module-decoder.cc
@@ -960,8 +960,9 @@ class ModuleDecoder : public Decoder {
         allocator, menv == nullptr ? nullptr : menv->module_env.module, body);
     if (result.failed()) {
       // Wrap the error message from the function decoder.
-      std::ostringstream str;
-      str << "in function " << func_name << ": " << result.error_msg();
+      std::ostringstream wrapped;
+      wrapped << "in function " << func_name << ": " << result.error_msg();
+      result.error(result.error_offset(), wrapped.str());

[Mircea Trofin, 2017/05/31 14:51:42]

Does str() flush?

[bbudge, 2017/06/01 00:53:26]

I think flush is a nop on string streams, since data can be written immediately.
The docs say that str() is a copy of the current contents of the stream.

 
       // Set error code and location, if this is the first error.
       if (intermediate_result_.ok()) {