Rollup merge of #126552 - fee1-dead-contrib:rmfx, r=compiler-errors

Remove use of const traits (and `feature(effects)`) from stdlib

The current uses are already unsound because they are using non-const impls in const contexts. We can reintroduce them by reverting the commit in this PR, after #120639 lands.

Also, make `effects` an incomplete feature.

cc `@rust-lang/project-const-traits`
r? `@compiler-errors`

Author: matthiaskrgr

tests/ui/const-generics/const_trait_fn-issue-88433.rs

@@ -1,6 +1,6 @@
 //@ build-pass
 
-#![feature(const_trait_impl, effects)]
+#![feature(const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
 
 #[const_trait]
 trait Func<T> {

tests/ui/const-generics/const_trait_fn-issue-88433.stderr

@@ -0,0 +1,11 @@
+warning: the feature `effects` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/const_trait_fn-issue-88433.rs:3:30
+   |
+LL | #![feature(const_trait_impl, effects)]
+   |                              ^^^^^^^
+   |
+   = note: see issue #102090 <https://github.com/rust-lang/rust/issues/102090> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/consts/auxiliary/closure-in-foreign-crate.rs

@@ -1,5 +1,5 @@
 #![crate_type = "lib"]
-#![feature(const_closures, const_trait_impl, effects)]
+#![feature(const_closures, const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
 
 pub const fn test() {
     let cl = const || {};