Note: all of this describes the implementation of the unstable
const_trait_impl features. None of this implementation is usable or visible from
The implementation of const traits and
~const bounds is a limited effect system.
It is used to allow trait bounds on
const fn to be used within the
const fn for
method calls. Within the function, in order to know whether a method on a trait
const, we need to know whether there is a
~const bound for the trait.
In order to know whether we can instantiate a
~const bound on a
const fn, we
need to know whether there is a
const_trait impl for the type and trait being
used (or whether the
const fn is used at runtime, then any type implementing the
trait is ok, just like with other bounds).
We perform these checks via a const generic boolean that gets attached to all
const fn and
const trait. The following sections will explain the desugarings
and the way we perform the checks at call sites.
The const generic boolean is inverted to the meaning of
const. In the compiler
it is called
host, because it enables "host APIs" like
static items, network
access, disk access, random numbers and everything else that isn't available in
const contexts. So
false means "const",
true means "not const" and if it's
a generic parameter, it means "maybe const" (meaning we're in a const fn or const
const fn have a
#[rustc_host] const host: bool generic parameter that is
hidden from users. Any
~const Trait bounds in the generics list or
const fn get converted to
Trait<host> + Trait<true> bounds. The
exists so that associated types of the generic param can be used from projections
<T as Trait>::Assoc, because there are no
<T as ~const Trait> projections for now.
#[const_trait] attribute gives the marked trait a
#[rustc_host] const host: bool
generic parameter. All functions of the trait "inherit" this generic parameter, just like
they have all the regular generic parameters of the trait. Any
~const Trait super-trait
bounds get desugared to
Trait<host> + Trait<true> in order to allow using associated
types and consts of the super traits in the trait declaration. This is necessary, because
<Self as SuperTrait>::Assoc is always
<Self as SuperTrait<true>>::Assoc as there is
<Self as ~const SuperTrait> syntax.
When generic parameters are instantiated for any items, the
host generic parameter
is always instantiated as an inference variable. This is a special kind of inference var
that is not part of the type or const inference variables, similar to how we have
special inference variables for type variables that we know to be an integer, but not
yet which one. These separate inference variables fall back to
the end of typeck (in
fallback_effects) to ensure that
let _ = some_fn_item_name;
will keep compiling.
All actually used (in function calls, casts, or anywhere else) function items, will
enforce_context_effects method invoked.
It trivially returns if the function being called has no
host generic parameter.
In order to error if a non-const function is called in a const context, we have not
yet disabled the const-check logic that happens on MIR, because
enforce_context_effects does not yet perform this check.
The function call's
host parameter is then equated to the context's
which almost always trivially succeeds, as it was an inference var. If the inference
var has already been bound (since the function item is invoked twice), the second
invocation checks it against the first.