Error code E0637
'_
lifetime name or &T
without an explicit lifetime name has been used
on illegal place.
Erroneous code example:
#![allow(unused)] fn main() { fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str { //^^ `'_` is a reserved lifetime name if str1.len() > str2.len() { str1 } else { str2 } } fn and_without_explicit_lifetime<T>() where T: Into<&u32>, //^ `&` without an explicit lifetime name { } }
First, '_
cannot be used as a lifetime identifier in some places
because it is a reserved for the anonymous lifetime. Second, &T
without an explicit lifetime name cannot also be used in some places.
To fix them, use a lowercase letter such as 'a
, or a series
of lowercase letters such as 'foo
. For more information about lifetime
identifier, see the book. For more information on using
the anonymous lifetime in Rust 2018, see the Rust 2018 blog post.
Corrected example:
#![allow(unused)] fn main() { fn underscore_lifetime<'a>(str1: &'a str, str2: &'a str) -> &'a str { if str1.len() > str2.len() { str1 } else { str2 } } fn and_without_explicit_lifetime<'foo, T>() where T: Into<&'foo u32>, { } }