Auto merge of #132479 - compiler-errors:fx-feat-yeet, r=fee1-dead

Yeet the `effects` feature, move it onto `const_trait_impl`

This PR merges the `effects` feature into the `const_trait_impl` feature. There's really no need to have two feature gates for one feature.

After this PR, if `const_trait_impl` **is** enabled:
* Users can use and define const traits
* `HostEffect` const conditions will be enforced on the HIR
* We re-check the predicates in MIR just to make sure that we don't "leak" anything during MIR lowering

And if `const_trait_impl` **is not** enabled:
* Users cannot use nor define const traits
* `HostEffect` const conditions are not enforced on the HIR
* We will raise a const validation error if we call a function that has any const conditions (i.e. const traits and functions with any `~const` in their where clasues)

This should be the last step for us to be able to enable const traits in the standard library. We still need to re-constify `Drop` and `Destruct` and stuff for const traits to be particularly *useful* for some cases, but this is a good step :D

r? fee1-dead
cc `@rust-lang/project-const-traits`

1 files changed, 3 insertions, 3 deletions

diff --git a/library/core/src/ops/function.rs b/library/core/src/ops/function.rs
index 3a3d3fcf1da..e9014458b48 100644
--- a/library/core/src/ops/function.rs
+++ b/library/core/src/ops/function.rs
@@ -72,7 +72,7 @@ use crate::marker::Tuple;
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-// FIXME(effects) #[const_trait]
+// FIXME(const_trait_impl) #[const_trait]
 pub trait Fn<Args: Tuple>: FnMut<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
@@ -159,7 +159,7 @@ pub trait Fn<Args: Tuple>: FnMut<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-// FIXME(effects) #[const_trait]
+// FIXME(const_trait_impl) #[const_trait]
 pub trait FnMut<Args: Tuple>: FnOnce<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
@@ -238,7 +238,7 @@ pub trait FnMut<Args: Tuple>: FnOnce<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-// FIXME(effects) #[const_trait]
+// FIXME(const_trait_impl) #[const_trait]
 pub trait FnOnce<Args: Tuple> {
     /// The returned type after the call operator is used.
     #[lang = "fn_once_output"]