src/librustc_error_codes/error_codes/E0106.md

   1 This error indicates that a lifetime is missing from a type. If it is an error
   2 inside a function signature, the problem may be with failing to adhere to the
   3 lifetime elision rules (see below).
   4
   5 Erroneous code examples:
   6
   7 ```compile_fail,E0106
   8 struct Foo1 { x: &bool }
   9               // ^ expected lifetime parameter
  10 struct Foo2<'a> { x: &'a bool } // correct
  11
  12 struct Bar1 { x: Foo2 }
  13               // ^^^^ expected lifetime parameter
  14 struct Bar2<'a> { x: Foo2<'a> } // correct
  15
  16 enum Baz1 { A(u8), B(&bool), }
  17                   // ^ expected lifetime parameter
  18 enum Baz2<'a> { A(u8), B(&'a bool), } // correct
  19
  20 type MyStr1 = &str;
  21            // ^ expected lifetime parameter
  22 type MyStr2<'a> = &'a str; // correct
  23 ```
  24
  25 Lifetime elision is a special, limited kind of inference for lifetimes in
  26 function signatures which allows you to leave out lifetimes in certain cases.
  27 For more background on lifetime elision see [the book][book-le].
  28
  29 The lifetime elision rules require that any function signature with an elided
  30 output lifetime must either have:
  31
  32  - exactly one input lifetime
  33  - or, multiple input lifetimes, but the function must also be a method with a
  34    `&self` or `&mut self` receiver
  35
  36 In the first case, the output lifetime is inferred to be the same as the unique
  37 input lifetime. In the second case, the lifetime is instead inferred to be the
  38 same as the lifetime on `&self` or `&mut self`.
  39
  40 Here are some examples of elision errors:
  41
  42 ```compile_fail,E0106
  43 // error, no input lifetimes
  44 fn foo() -> &str { }
  45
  46 // error, `x` and `y` have distinct lifetimes inferred
  47 fn bar(x: &str, y: &str) -> &str { }
  48
  49 // error, `y`'s lifetime is inferred to be distinct from `x`'s
  50 fn baz<'a>(x: &'a str, y: &str) -> &str { }
  51 ```
  52
  53 [book-le]: https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html#lifetime-elision