compiler/rustc_error_codes/src/error_codes/E0637.md

`'_` lifetime name or `&T` without an explicit lifetime name has been used
in an 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 without_explicit_lifetime<T>()
where
    T: Iterator<Item = &u32>,
                     //^ `&` without an explicit lifetime name
{
}

fn without_hrtb<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 without_explicit_lifetime<'a, T>()
where
    T: Iterator<Item = &'a u32>,
{
}

fn without_hrtb<T>()
where
    T: for<'foo> 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