Auto merge of #3940 - rust-lang:refutable_slice, r=RalfJung

Prefer refutable slice patterns over len check + index op

Just something I noticed while reviewing other PRs

We do it for shim arguments almost everywhere, but when the size is not statically known, we didn't use the helpers as they rely on array ops. But we can avoid a len check followed by several index ops by just using a refutable slice pattern with `let else`.

The pattern is so common, it seems almost worth a dedicated macro

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