I'll give you "more ergonomic" if you'll give me "less safe".

I'd argue it's no less safe than the status quo, just easier to use. The standard "assert" can be switched off. There's "__builtin_unreachable". My personal utility library has "assume" which switches between the two based on NDEBUG.

C is a knife. Knives are sharp. If that's a problem then C is the wrong language.

But people are looking at C3, Odin & Zig because they've determined that C is the wrong language for them; many have determined that it's too sharp. C3 has "safe" in its title, they're expecting fewer sharp edges.

I'm not asking for useful optimizations like constraints to go away, I'm asking for them to be properly communicated as being sharp. If you use "unsafe" incorrectly in your rust code, you invite UB. But because of the keyword they chose, it's hardly surprising.