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Togglebit

@togglebit.io

Oh Rust, you be fickle sometimes! Today during a live stream I was exploring a lifetime issue, and I came up with this:

gist.github.comFickle lifetimesFickle lifetimes. GitHub Gist: instantly share code, notes, and snippets.

13/11/25, 6:23 pm0 reposts4 likes

Togglebit

@togglebit.io

13 Nov 2025

At an initial glance it seems like both functions should compile, after all the only difference is that we are returning an alias, right? SPOILER AHEAD: So why does the function returning an alias work while the other one doesn't? The answer can be found here: doc.rust-lang.org/reference/li...

doc.rust-lang.orgLifetime elision - The Rust Reference

Togglebit

@togglebit.io

13 Nov 2025

In short these two are the same: type T1<'a> = Box<dyn Bar<'a>>; type T2<'a> = Box<dyn Bar<'a> + 'a>; So by returning the type alias we are not returning Box<dyn Trait<'a>> but Box<dyn Trait<'a> + 'a>.