Constant Evaluation - The Rust Reference

# 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 types of expressions can be evaluated at compile time. These are called constant expressions and are required in const contexts. In other places, such as in `let` statements, constant expressions may be evaluated at compile time. If errors, such as out of bounds array indexing or overflow occurs, then it is a compiler error if the value must be evaluated at compile time, otherwise it is just a warning, but the code will most likely panic when run.

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 ran.

- 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, which only contain items and possibly a constant tail expression.
- 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