Auto merge of rust-lang#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`

Author: bors

core/src/ffi/c_str.rs

@@ -510,7 +510,7 @@ impl CStr {
     #[inline]
     #[must_use]
     const fn as_non_null_ptr(&self) -> NonNull<c_char> {
-        // FIXME(effects) replace with `NonNull::from`
+        // FIXME(const_trait_impl) replace with `NonNull::from`
         // SAFETY: a reference is never null
         unsafe { NonNull::new_unchecked(&self.inner as *const [c_char] as *mut [c_char]) }
             .as_non_null_ptr()

core/src/ops/drop.rs

@@ -203,7 +203,7 @@
 /// [nomicon]: ../../nomicon/phantom-data.html#an-exception-the-special-case-of-the-standard-library-and-its-unstable-may_dangle
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
-// FIXME(effects) #[const_trait]
+// FIXME(const_trait_impl) #[const_trait]
 pub trait Drop {
     /// Executes the destructor for this type.
     ///

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"]

core/src/slice/sort/shared/smallsort.rs

@@ -102,7 +102,7 @@ impl<T: FreezeMarker> UnstableSmallSortTypeImpl for T {
     }
 }
 
-/// FIXME(effects) use original ipnsort approach with choose_unstable_small_sort,
+/// FIXME(const_trait_impl) use original ipnsort approach with choose_unstable_small_sort,
 /// as found here <https://github.com/Voultapher/sort-research-rs/blob/438fad5d0495f65d4b72aa87f0b62fc96611dff3/ipnsort/src/smallsort.rs#L83C10-L83C36>.
 pub(crate) trait UnstableSmallSortFreezeTypeImpl: Sized + FreezeMarker {
     fn small_sort_threshold() -> usize;