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`'_` lifetime name or `&T` without an explicit lifetime name has been used
on illegal place.
Erroneous code example:
```compile_fail,E0106,E0637
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][bk-no]. For more information on using
the anonymous lifetime in Rust 2018, see [the Rust 2018 blog post][blog-al].
Corrected example:
```
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>,
{
}
```
[bk-no]: https://doc.rust-lang.org/book/appendix-02-operators.html#non-operator-symbols
[blog-al]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#more-lifetime-elision-rules
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