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.
- Const parameters.
- Paths to functions and constants. Recursively defining constants is not allowed.
- Paths to statics. These are only allowed within the initializer of a static.
- Tuple expressions.
- Array expressions.
- Struct expressions.
- Block expressions, including
unsafe
andconst
blocks.- let statements and thus irrefutable patterns, including mutable bindings
- assignment expressions
- compound assignment 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
, andchar
. - Shared borrows, except if applied to a type with interior mutability.
- The dereference operator except for raw pointers.
- Grouped expressions.
- Cast expressions, except
- pointer to address casts and
- function pointer to address casts.
- Calls of const functions and const methods.
- loop, while and
while let
expressions. - if,
if let
and match expressions.
Const context
A const context is one of the following:
- Array type length expressions
- Array repeat length expressions
- The initializer of
- A const generic argument
- A const block
Const contexts that are used as parts of types (array type and repeat length expressions as well as const generic arguments) can only make restricted use of surrounding generic parameters: such an expression must either be a single bare const generic parameter, or an arbitrary expression not making use of any generics.
Const Functions
A const fn is a function that one is permitted to call from a const context. Declaring a function
const
has no effect on any existing uses, it only restricts the types that arguments and the
return type may use, and restricts the function body to constant expressions.
When called from a const context, the function is interpreted by the
compiler at compile time. The interpretation happens in the
environment of the compilation target and not the host. So usize
is
32
bits if you are compiling against a 32
bit system, irrelevant
of whether you are building on a 64
bit or a 32
bit system.