There is no cross-crate LTO with 'lto = false', but there is cross-crate thin LTO with 'lto = "thin"'. The codepaths might still be getting hit, but individual CGUs within a crate are generally invisible to the user, which can create the impression that LTO doesn't occur. (That is, if you operate under the mental model of the crate being the basic compilation unit, then 'lto = false' means you'll never see LTO.)

Oh I hadn’t realized Rust does that. Really cool.