Returning strings: `String` vs. `&str`

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Here's my Rust 101 rules for this:

  1. "&str for parameters; String for return types". This always works, and has no borrowck complications, but is sometimes inefficient.

  2. the small tweak to add "If you always need to to_owned/to_string the &str parameter, change it to a String parameter". That's more efficient when your caller happens to already have a String, no less efficient if they don't, and still doesn't require thinking about lifetimes.

  3. if you're always returning a substring of one of the arguments, add a lifetime generic and return a &'a str, updating the corresponding parameter from &str to &'a str as well.

That covers the vast majority of cases, but there are of course more complicated scenarios that need something fancier.

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