My impression from Klabnik's most recent comment is that this isn't the case. Which is fine! It lines up with my previous assumptions about Rust idiom.
Just as one example, consider a function I wrote. It accepts a &str as a parameter. Internally, among other things, it clones that &str into a String. If you asked me whether that was "idiomatic" devoid of any other context, I would say, "probably not? Why not just ask for the String or a Into<String> instead?" But it's not a certainty. Just a likelihood.
But now let's add context. The context is that that function I mentioned is Regex::new. Does that change my answer? You bet your ass it does. Because cloning that pattern string has now turned into a definitively negligible cost. (By many orders of magnitude.) And there is upside to accepting a &str: it's non-generic, a little easier to understand and is unlikely to ever run afoul of type inference. Those aren't huge upsides, but there is actually no downside here because of the context.
As others have pointed out, there is no one-size-fits-all answer to this question. There are models though, and those models (e.g., "don't worry about cloning while you're learning Rust") are very likely to serve you quite well. That might not be every path that everyone learning Rust always takes. But it's a decent bet in my experience.
Okay last comment in this sub-thread because I have to get back to my job here, which hilariously is adding some boxing and possibly cloning in order to improve a codebase.
I feel like I'm going crazy. I feel like this:
> if I can mostly ignore fussy ownership and borrowing stuff and just clone everything, all the time, getting rid of clones exclusively in places that I profile and learn to be in the hot path, and the resulting code will be idiomatic Rust
is exactly what my last comment said. This whole chain, in my experience, is me saying "clone is not a moral issue" and you saying "why are you saying that people that use clone should feel bad." So I need to bow out. I wish I could figure out where this disconnect is, but it's clear that this thread is not achieving that.