Consider running tests with -D_LIBCPP_ENABLE_ASSERTIONS=1

There's a handful of instances across binaryen/wasm-opt where there is "technically C++ Undefined Behavior". These code defects may be causing nondeterminism, or random failures.

For example, https://github.com/WebAssembly/binaryen/blob/main/src/wasm/wasm-s-parser.cpp#L1154, str.size() is checked to be >= 3, and then str[3] is accessed. There's a few more instances in wasm-s-parser.cpp, for what it's worth. They should all be easy fixes :)

Building all of binaryen with -D_LIBCPP_ENABLE_ASSERTIONS=1 should point out these issues pretty quickly, or running the existing fuzzer after enabling this build macro. More info on this build macro: https://libcxx.llvm.org/DesignDocs/DebugMode.html (This is a clang-LLVM only thing).

Start by building Binaryen with -D_LIBCPP_ENABLE_ASSERTIONS=1, or run the existing fuzzer with that macro enabled. Inspect src/wasm/wasm-s-parser.cpp around line 1154 and its other similar cases, then address the undefined-behavior instances identified by the assertions; done means the affected tests or fuzzer run without triggering them.