What is a good mental model of borrow checker?

[quinedot]

A couple more earlier threads if you want to deep dive on the higher-ranked stuff

• Generic function failing to satisfy "any" lifetime on argument
• Rust Quiz 5: when will higher-ranked closure type be yielded?

But the boldtype-in-textbook summary is still

• &T is not a (single, concrete) type ^[1]
• Generic type parameters must resolve to a single, concrete type

And the small textbook-infobox-example is

• fn(&String) -> &String ^[2] does not satify a <I, O> Fn(&I) -> O ^[3] bound
  • Because O would have to match &String but that's not a type

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1. and there is no higher-ranked for<'any> &'any T type ↩︎

2. aka for<'any> fn(&'any String) -> &'any String ↩︎

3. aka <I, O> for<'any> Fn(&'any I) -> O ↩︎