compiler/rustc_error_codes/src/error_codes/E0121.md

   1 The type placeholder `_` was used within a type on an item's signature.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0121
   6 fn foo() -> _ { 5 } // error
   7
   8 static BAR: _ = "test"; // error
   9 ```
  10
  11 In those cases, you need to provide the type explicitly:
  12
  13 ```
  14 fn foo() -> i32 { 5 } // ok!
  15
  16 static BAR: &str = "test"; // ok!
  17 ```
  18
  19 The type placeholder `_` can be used outside item's signature as follows:
  20
  21 ```
  22 let x = "a4a".split('4')
  23     .collect::<Vec<_>>(); // No need to precise the Vec's generic type.
  24 ```