I couldn't actually figure out whether this TCO being done "fairly well" was a guarantee or simply like Rust (I am referring to the native support of the language, not what crates allow)

When that SO answer was written, it was not a guarantee.

You can now get a guarantee by using non-standard compiler attributes:

https://clang.llvm.org/docs/AttributeReference.html#musttail

https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html...