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

compiler/rustc_hir_analysis/messages.ftl

@@ -120,6 +120,10 @@ hir_analysis_drop_impl_reservation = reservation `Drop` impls are not supported
 hir_analysis_duplicate_precise_capture = cannot capture parameter `{$name}` twice
     .label = parameter captured again here
 
+hir_analysis_effects_without_next_solver = 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
+
 hir_analysis_empty_specialization = specialization impl does not specialize any associated items
     .note = impl is a specialization of this impl
 

compiler/rustc_hir_analysis/src/errors.rs

@@ -1699,3 +1699,9 @@ pub struct InvalidReceiverTy<'tcx> {
     pub span: Span,
     pub receiver_ty: Ty<'tcx>,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_effects_without_next_solver)]
+#[note]
+#[help]
+pub struct EffectsWithoutNextSolver;

compiler/rustc_hir_analysis/src/lib.rs

@@ -151,6 +151,12 @@ pub fn provide(providers: &mut Providers) {
 pub fn check_crate(tcx: TyCtxt<'_>) {
     let _prof_timer = tcx.sess.timer("type_check_crate");
 
+    // FIXME(effects): remove once effects is implemented in old trait solver
+    // or if the next solver is stabilized.
+    if tcx.features().effects && !tcx.next_trait_solver_globally() {
+        tcx.dcx().emit_err(errors::EffectsWithoutNextSolver);
+    }
+
     tcx.sess.time("coherence_checking", || {
         tcx.hir().par_for_each_module(|module| {
             let _ = tcx.ensure().check_mod_type_wf(module);

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/with-without-next-solver.coherence.stderr

@@ -0,0 +1,7 @@
+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: aborting due to 1 previous error
+

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/with-without-next-solver.rs

@@ -0,0 +1,10 @@
+// test that we error correctly when effects is used without the next-solver flag.
+//@ revisions: stock coherence full
+//@[coherence] compile-flags: -Znext-solver=coherence
+//@[full] compile-flags: -Znext-solver
+//@[full] check-pass
+
+#![feature(effects)]
+#![allow(incomplete_features)]
+
+fn main() {}

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/with-without-next-solver.stock.stderr

@@ -0,0 +1,7 @@
+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: aborting due to 1 previous error
+