RFC 2341: const-locals

lang (machine | const-eval)

Feature Name: const_locals
Start Date: 2018-01-11
RFC PR: rust-lang/rfcs#2341
Rust Issue: rust-lang/rust#48821

Summary

Allow let bindings in the body of constants and const fns. Additionally enable destructuring in let bindings and const fn arguments.

Motivation

It makes writing const fns much more like writing regular functions and is not possible right now because the old constant evaluator was a constant folder that could only process expressions. With the miri const evaluator this feature exists but is still disallowed.

Guide-level explanation

let bindings in constants and const fn work just like let bindings everywhere else. Historically these did not exist in constants and const fn because it would have been very hard to support them in the old const evaluator.

This means that you can only move out of any let binding once, even though in a const environment obtaining a copy of the object could be done by executing the code twice, side effect free. All invariants held by runtime code are also upheld by constant evaluation.

Reference-level explanation

Expressions like a + b + c are already transformed to

let tmp = a + b;
tmp + c

With this RFC we can create bindings ourselves instead of only allowing compiler generated bindings.

Drawbacks

You can create mutable locals in constants and then actually modify them. This has no real impact on the constness, as the mutation happens entirely at compile time and results in an immutable value.

Rationale and alternatives

The backend already supports this 100%. This is essentially just disabling a check

Why is this design the best in the space of possible designs?

Being the only design makes it the best design by definition

What is the impact of not doing this?

Not having locals and destructuring severely limits the functions that can be turned into const fn and generally leads to unreadable const fns.