Is there a place to report missed optimisation?

[mathstuf]

@cuviper

There's no guarantee at the Rust library level, so it's something that could possibly be specialized, but at LLVM's level the capacity is definitely an observable effect.

Right. Optimizations are usually couched in "as-if" language. The collect induces a reallocation which does not have a forced link to the old capacity, so that is "observable behavior" as far as the optimizer is concerned.

@robinm

For fun I also tested v.clone() , and it’s not optimized away, even if v is not re-used (godbolt).

AFAIU, Clone can only be optimized away if it is also Copy (though I'm not sure if this is actually happening). See these threads:

• Assuming `Clone::clone()` is equivalent to `Copy` semantics for all instances of `Copy`
• Optimized Clone of Copy Types?

There was a more recent one mentioning it too, but I'm not seeing it right now.