src/doc/reference/src/items/constant-items.md

   1 # Constant items
   2
   3 A *constant item* is a named _[constant value]_ which is not associated with a
   4 specific memory location in the program. Constants are essentially inlined
   5 wherever they are used, meaning that they are copied directly into the relevant
   6 context when used. References to the same constant are not necessarily
   7 guaranteed to refer to the same memory address.
   8
   9 Constant values must not have destructors, and otherwise permit most forms of
  10 data. Constants may refer to the address of other constants, in which case the
  11 address will have elided lifetimes where applicable, otherwise – in most cases
  12 – defaulting to the `static` lifetime. (See below on [static lifetime
  13 elision].) The compiler is, however, still at liberty to translate the constant
  14 many times, so the address referred to may not be stable.
  15
  16 Constants must be explicitly typed. The type may be any type that doesn't
  17 implement [`Drop`] and has a `'static` lifetime: any references it contains
  18 must have `'static` lifetimes.
  19
  20 ```rust
  21 const BIT1: u32 = 1 << 0;
  22 const BIT2: u32 = 1 << 1;
  23
  24 const BITS: [u32; 2] = [BIT1, BIT2];
  25 const STRING: &'static str = "bitstring";
  26
  27 struct BitsNStrings<'a> {
  28     mybits: [u32; 2],
  29     mystring: &'a str,
  30 }
  31
  32 const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
  33     mybits: BITS,
  34     mystring: STRING,
  35 };
  36 ```
  37
  38 [constant value]: expressions.html#constant-expressions
  39 [static lifetime elision]: items/static-items.html#static-lifetime-elision
  40 [`Drop`]: the-drop-trait.html