The type placeholder `_` was used within a type on an item's signature.
Erroneous code example:
```compile_fail,E0121
fn foo() -> _ { 5 } // error
static BAR: _ = "test"; // error
```
In those cases, you need to provide the type explicitly:
```
fn foo() -> i32 { 5 } // ok!
static BAR: &str = "test"; // ok!
```
The type placeholder `_` can be used outside item's signature as follows:
```
let x = "a4a".split('4')
.collect::<Vec<_>>(); // No need to precise the Vec's generic type.
```