Constants in the type system

Constants used in the type system are represented as ty::Const. The variants of their ty::ConstKind mostly mirror the variants of ty::TyKind with the two additional variants being ConstKind::Value and ConstKind::Unevaluated.

WithOptConstParam and dealing with the query system

To typecheck constants used in the type system, we have to know their expected type. For const arguments in type dependent paths, e.g. x.foo::<{ 3 + 4 }>(), we don't know the expected type for { 3 + 4 } until we are typechecking the containing function.

As we may however have to evaluate that constant during this typecheck, we would get a cycle error. For more details, you can look at this document.

Unevaluated constants

This section talks about what's happening with feature(generic_const_exprs) enabled. On stable we do not yet supply any generic parameters to anonymous constants, avoiding most of the issues mentioned here.

Unless a constant is either a simple literal, e.g. [u8; 3] or foo::<{ 'c' }>(), or a generic parameter, e.g. [u8; N], converting a constant to its ty::Const representation returns an unevaluated constant. Even fully concrete constants which do not depend on generic parameters are not evaluated right away.

Anonymous constants are typechecked separately from their containing item, e.g.


#![allow(unused)]
fn main() {
fn foo<const N: usize>() -> [u8; N + 1] {
    [0; N + 1]
}
}

is treated as


#![allow(unused)]
fn main() {
const ANON_CONST_1<const N: usize> = N + 1;
const ANON_CONST_2<const N: usize> = N + 1;
fn foo<const N: usize>() -> [u8; ANON_CONST_1::<N>] {
    [0; ANON_CONST_2::<N>]
}
}

Unifying constants

For the compiler, ANON_CONST_1 and ANON_CONST_2 are completely different, so we have to somehow look into unevaluated constants to check whether they should unify.

For this we use InferCtxt::try_unify_abstract_consts. This builds a custom AST for the two inputs from their THIR. This is then used for the actual comparison.

Lazy normalization for constants

We do not eagerly evaluate constant as they can be used in the where-clauses of their parent item, for example:


#![allow(unused)]
fn main() {
#[feature(generic_const_exprs)]
fn foo<T: Trait>()
where
    [u8; <T as  Trait>::ASSOC + 1]: SomeOtherTrait,
{}
}

The constant <T as Trait>::ASSOC + 1 depends on the T: Trait bound of its parents caller bounds, but is also part of another bound itself. If we were to eagerly evaluate this constant while computing its parents bounds this would cause a query cycle.

Unused generic arguments of anonymous constants

Anonymous constants inherit the generic parameters of their parent, which is why the array length in foo<const N: usize>() -> [u8; N + 1] can use N.

Without any manual adjustments, this causes us to include parameters even if the constant doesn't use them in any way. This can cause some interesting errors and breaks some already stable code.