"Lifetime placeholders", or elided lifetimes

A lifetime can be elided:

  • either implicitly, as is the case for &str or &mut i32 (behind a & always lurks a lifetime parameter);
  • or explicitly, using the special '_ syntax.

Across this book, I personally (overly) use the latter syntax, so as to show, clearly, all the elided lifetime parameters.

The result of an elided lifetime is thus a lifetime "hole", or lifetime placeholder, which will be the key thing to look at when thinking about lifetime elision rules. For instance, all these types involve lifetime holes:

  • &i32, &'_ i32, &mut i32, &'_ mut i32,
  • GenericTy<'_>1
    • e.g., Cow<'_, str>, Formatter<'_>, Context<'_>, BoxFuture<'_, R>, BorrowedFd<'_>
  • dyn '_ + Trait, dyn GenericTrait<'_>, and ditto for impl …,
    • e.g., Pin<Box<dyn '_ + Send + Future<Output = R>>>
1

Historically, it has been possible in Rust to use a lifetime-generic type without the <'_>. But this can be very confusing, which is why since the edition 2018_idioms, there is a elided_lifetimes_in_paths lint which can be enabled (e.g., set to warn), to catch these things. It is highly advisable to always set up this lint in any Rust project.

See the lifetime elision rules for more info.