@alexcrichton I'm not really sure, as the C and Rust allocators are different, right? Would you compile both with LeakSanitizer?
See:
https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html#leaksanitizer
https://clang.llvm.org/docs/LeakSanitizer.html

They should both bottom out in malloc and free by default, so I think ASAN might be enough (which IIRC also detects leaks in addition to other things). If LSAN works though that also seems reasonable to me

I recently hooked up wasmtime and a C api project (though not wasmtime's C api) to asan to find my own bugs of that sort. I found that I needed to:

• compile the rust with nightly and RUSTFLAGS=-Zsanitizer=address
• install and build the C with an llvm/clang toolchain that is relatively recent so the sanitizer is compatible with the information emitted by rustc, since my CI package manager's was too old I used LLVM-20.1 which is the same version rust is using in nightly. I don't have a sense for when rust will drift far enough from that to break something, but it works right now anyway.
• build the C sources with -fsanitize=address,undefined -fno-omit-frame-pointer cflags
• set ASAN_SYMBOLIZER_PATH=<install location of LLVM-20.1.0-Linux-X64>/bin/llvm-symbolizer to get symbols in asan's traces

Also, beware that running debug builds of cranelift under asan will be egregiously, obscenely slow.