error[E0637]: `'_` cannot be used here
  --> $DIR/E0637.rs:1:24
   |
LL | fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
   |                        ^^ `'_` is a reserved lifetime name

error[E0106]: missing lifetime specifier
  --> $DIR/E0637.rs:1:62
   |
LL | fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
   |                                  -------        -------      ^^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `str1` or `str2`
help: consider introducing a named lifetime parameter
   |
LL | fn underscore_lifetime<'a, '_>(str1: &'a str, str2: &'a str) -> &'a str {
   |                        +++            ~~             ~~          ~~

error[E0637]: `&` without an explicit lifetime name cannot be used here
  --> $DIR/E0637.rs:13:13
   |
LL |     T: Into<&u32>,
   |             ^ explicit lifetime name needed here

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0106, E0637.
For more information about an error, try `rustc --explain E0106`.