It's surely not a showstopper when it's on one line, but again the problem in practice is that those slices may be other operations or hidden behind other APIs, and happening at different depths in a complicated call tree, and you get an error only up at the top when what you're doing looks "clearly correct".

Yes, sure, the rule in question may be simple in the abstract (it's a compiler, after all -- they're just software doing straightforward things). But the ability for the poor programmer to detect which rule is being violated where is a lot more limited than the compiler is designed for. Thus the user with the upthread complaint, which is hardly unique.

I mean, at the end of the day if Rust wants to be an everyday language for this kind of everyday problem, the analysis paradigm needs to be communicated much better to everyday hackers (via docs, error messages, whatever). When Rust was new this seemed like just a technical problem to be solved with software maturity. I guess at this point after several years of regularly returning to play with Rust and being frustrated every time, I've mostly given up.