Note that that's in-band lifetimes, which didn't stabilize. Only the impl TheTrait for TheType<'_> form stabilized. More details in https://rust-lang-nursery.github.io/edition-guide/rust-2018/ownership-and-lifetimes/lifetime-elision-in-impl.html or in Tracking issue for lifetime elision for impl headers (feature impl_header_lifetime_elision) · Issue #15872 · rust-lang/rust · GitHub
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