effects.md: update const trait syntax

`~const` was replaced with `[const]`, making uses of the former confusing

Author: DanielEScherzer

src/effects.md

@@ -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,
@@ -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`
@@ -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>;
@@ -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 () {}
@@ -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.
 }
 ```
@@ -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
 }
 ```