Auto merge of rust-lang#127176 - fee1-dead-contrib:fx-requires-next-solver, r=compiler-errors

Make `feature(effects)` require `-Znext-solver`

Per rust-lang#120639 (review)

I made this a hard error because otherwise it should be a lint and that seemed more complicated. Not sure if this is the best place to put the error though.

r? project-const-traits

Author: bors

tests/ui/rfcs/rfc-2632-const-trait-impl/auxiliary/staged-api.rs

@@ -1,4 +1,6 @@
-#![feature(const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
+//@ compile-flags: -Znext-solver
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
 #![feature(staged_api)]
 #![stable(feature = "rust1", since = "1.0.0")]
 

tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr

@@ -7,6 +7,11 @@ 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
 
+error: using `#![feature(effects)]` without enabling next trait solver globally
+   |
+   = note: the next trait solver must be enabled globally for the effects feature to work correctly
+   = help: use `-Znext-solver` to enable
+
 error[E0277]: the trait bound `Runtime: ~const Compat` is not satisfied
   --> $DIR/call-const-trait-method-fail.rs:25:5
    |
@@ -23,6 +28,6 @@ LL | pub trait Plus {
 LL |     fn plus(self, rhs: Self) -> Self;
    |        ---- required by a bound in this associated function
 
-error: aborting due to 1 previous error; 1 warning emitted
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.