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fmease merged 1 commit into from
Apr 17, 2026
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effects.md: update const trait syntax #2838

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28 changes: 14 additions & 14 deletions src/effects.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2,16 +2,16 @@

## The `HostEffect` predicate

[`HostEffectPredicate`]s are a kind of predicate from `~const Tr` or `const Tr` bounds.
[`HostEffectPredicate`]s are a kind of predicate from `[const] Tr` or `const Tr` bounds.
It has a trait reference, and a `constness` which could be `Maybe` or
`Const` depending on the bound.
Because `~const Tr`, or rather `Maybe` bounds
Because `[const] Tr`, or rather `Maybe` bounds
apply differently based on whichever contexts they are in, they have different
behavior than normal bounds.
Where normal trait bounds on a function such as
`T: Tr` are collected within the [`predicates_of`] query to be proven when a
function is called and to be assumed within the function, bounds such as
`T: ~const Tr` will behave as a normal trait bound and add `T: Tr` to the result
`T: [const] Tr` will behave as a normal trait bound and add `T: Tr` to the result
from `predicates_of`, but also adds a `HostEffectPredicate` to the [`const_conditions`] query.

On the other hand, `T: const Tr` bounds do not change meaning across contexts,
Expand All @@ -37,7 +37,7 @@ In a similar vein,
an item *in const contexts*. If we adjust the example above to use `const` trait bounds:

```rust
const fn foo<T>() where T: ~const Default {}
const fn foo<T>() where T: [const] Default {}
```

Then `foo` would get a `HostEffect(T: Default, maybe)` in the `const_conditions`
Expand All @@ -55,7 +55,7 @@ Note that we don't check
if the function is only referred to but not called, as the following code needs to compile:

```rust
const fn hi<T: ~const Default>() -> T {
const fn hi<T: [const] Default>() -> T {
T::default()
}
const X: fn() -> u32 = hi::<u32>;
Expand All @@ -69,7 +69,7 @@ Here's an example:

```rust
const trait Bar {}
const trait Foo: ~const Bar {}
const trait Foo: [const] Bar {}
// `const_conditions` contains `HostEffect(Self: Bar, maybe)`

impl const Bar for () {}
Expand All @@ -86,13 +86,13 @@ We do the same for `const_conditions`:

```rust
const trait Foo {
fn hi<T: ~const Default>();
fn hi<T: [const] Default>();
}

impl<T: ~const Clone> Foo for Vec<T> {
fn hi<T: ~const PartialEq>();
// ^ we can't prove `T: ~const PartialEq` given `T: ~const Clone` and
// `T: ~const Default`, therefore we know that the method on the impl
impl<T: [const] Clone> Foo for Vec<T> {
fn hi<T: [const] PartialEq>();
// ^ we can't prove `T: [const] PartialEq` given `T: [const] Clone` and
// `T: [const] Default`, therefore we know that the method on the impl
// is stricter than the method on the trait.
}
```
Expand All @@ -117,11 +117,11 @@ Bounds on associated types, opaque types, and supertraits such as the following
have their bounds represented differently:

```rust
trait Foo: ~const PartialEq {
type X: ~const PartialEq;
trait Foo: [const] PartialEq {
type X: [const] PartialEq;
}

fn foo() -> impl ~const PartialEq {
fn foo() -> impl [const] PartialEq {
// ^ unimplemented syntax
}
```
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