const_eval.md

Constant evaluation

Constant evaluation is the process of computing the result of expressions during compilation. Only a subset of all expressions can be evaluated at compile-time.

Constant expressions

Certain forms of expressions, called constant expressions, can be evaluated at compile time. In const contexts, these are the only allowed expressions, and are always evaluated at compile time. In other places, such as let statements, constant expressions may be, but are not guaranteed to be, evaluated at compile time. Behaviors such as out of bounds array indexing or overflow are compiler errors if the value must be evaluated at compile time (i.e. in const contexts). Otherwise, these behaviors are warnings, but will likely panic at run-time.

The following expressions are constant expressions, so long as any operands are also constant expressions and do not cause any Drop::drop calls to be run.

• Literals.
• Paths to functions and constants. Recursively defining constants is not allowed.
• Tuple expressions.
• Array expressions.
• Struct expressions.
• Enum variant expressions.
• Block expressions, including unsafe blocks.
  • let statements and thus irrefutable patterns, with the caveat that until if and match are implemented, one cannot use both short circuiting operators (&& and ||) and let statements within the same constant.
  • assignment expressions
  • assignment operator expressions
  • expression statements
• Field expressions.
• Index expressions, array indexing or slice with a usize.
• Range expressions.
• Closure expressions which don't capture variables from the environment.
• Built in negation, arithmetic, logical, comparison or lazy boolean operators used on integer and floating point types, bool and char.
• Shared borrows, except if applied to a type with interior mutability.
• The dereference operator.
• Grouped expressions.
• Cast expressions, except pointer to address and function pointer to address casts.
• Calls of const functions and const methods.

Const context

A const context is one of the following:

• Array type length expressions
• Repeat expression length expressions
• The initializer of
  • constants
  • statics
  • enum discriminants