src/librustc_error_codes/error_codes/E0637.md

   1 An underscore `_` character has been used as the identifier for a lifetime.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0106,E0637
   6 fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
   7          //^^ `'_` is a reserved lifetime name
   8     if str1.len() > str2.len() {
   9         str1
  10     } else {
  11         str2
  12     }
  13 }
  14 ```
  15
  16 `'_`, cannot be used as a lifetime identifier because it is a reserved for the
  17 anonymous lifetime. To fix this, use a lowercase letter such as 'a, or a series
  18 of lowercase letters such as `'foo`.  For more information, see [the
  19 book][bk-no].  For more information on using the anonymous lifetime in rust
  20 nightly, see [the nightly book][bk-al].
  21
  22 Corrected example:
  23
  24 ```
  25 fn longest<'a>(str1: &'a str, str2: &'a str) -> &'a str {
  26     if str1.len() > str2.len() {
  27         str1
  28     } else {
  29         str2
  30     }
  31 }
  32 ```
  33
  34 [bk-no]: https://doc.rust-lang.org/book/appendix-02-operators.html#non-operator-symbols
  35 [bk-al]: https://doc.rust-lang.org/nightly/edition-guide/rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html