And it's not a cut and dry issue to add. Function effects would add a lot of cognitive load to the developer along with more implicit bounds which increases accidental API break changes. You talk about the compiler implicitly adding the bounds to functions, but what happens when I now add a line in my function that allocates when before it didn't? I just broke my API unless I was also defensively testing all implicit bounds. And if I was testing all implicit bounds, can the language no longer add new bounds? Reversing that and requiring the callee to defensively declare all bounds is a borderline non-starter because it'd such a huge burden to write any function or refactor anything.

That's probably the least convincing of your examples. My understanding is that effects systems can get complicated fast, and there's no consensus yet on what a good general purpose implementation should look like, never mind a specific implementation for Rust.