As of Rust 1.63 (releasing next week), the "non-lexical lifetimes" (NLL) work will be enabled by default. NLL is the second iteration of Rust's borrow checker. The RFC actually does quite a nice job of highlighting some of the motivating examples. "But," I hear you saying, "wasn't NLL included in Rust 2018?" And yes, yes it was! But at that time, NLL was only enabled for Rust 2018 code, while Rust