Constant Evaluation

Constant evaluation is the process of running Rust code at compile-time in order to use the result e.g. to set the discriminant of enum variants or as an array length.

Examples:

enum Foo {
    A = 5,
    B = 7 - 3,
}
type NineStrings = [&str, 3 * 3];

The Rust compiler runs the MIR in the MIR interpreter (miri), which sort of is a virtual machine using MIR as "bytecode".

Table of Contents

• Const Safety
• The three "kinds" of compile-time evaluated data:
  • Statics (static, static mut)
  • Constants (const, array sizes, non-Copy array initializers)
  • Promoteds (rvalue promotion)

Related RFCs

Const Promotion

RFC 1414 injects a hidden static for any &foo expression as long as foo follows a certain set of rules. These rules are discussed here

Drop types in constants/statics

RFC 1440 allows using types that implement Drop (or have fields that do so) inside constants and statics. It is guaranteed that the Drop::drop method will never be called on the static/const object.

Constants in Repeat expressions

RFC 2203 allows the use !Copy types for the initializer of repeat expressions, as long as that value is constant.

This permits e.g. [Vec::new(); 42].

Statically known bugs and panics in runtime code are warnings

RFC 1229 formalized the concept that

let x: usize = 1 - 2;

is allowed to produce a lint but not an error. This allows us to make these analyses more powerful without suddenly breaking compilation. The corresponding lint is the const_err lint, which is deny by default and will thus break compilation of the crate currently being built, even if it does not break the compilation of the current crate's dependencies.

Various new const-eval features

• loop
• if and match
• panic!
• locals and destructuring

Some of these features interact. E.g.

• match + loop yields while
• panic! + if + locals yields assert!