fatal error if both -Znext-solver and GCE are enabled

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -746,8 +746,10 @@ fn check_new_solver_banned_features(sess: &Session, features: &Features) {
         .find(|feat| feat.gate_name == sym::generic_const_exprs)
         .map(|feat| feat.attr_sp)
     {
+        // Abort immediately, otherwise GCE can lower to `ConstKind::Expr`,
+        // which the new solver intentionally does not support.
         #[allow(rustc::symbol_intern_string_literal)]
-        sess.dcx().emit_err(diagnostics::IncompatibleFeatures {
+        sess.dcx().emit_fatal(diagnostics::IncompatibleFeatures {
             spans: vec![gce_span],
             f1: Symbol::intern("-Znext-solver=globally"),
             f2: sym::generic_const_exprs,

tests/ui/const-generics/generic_const_exprs/next-solver-gce-incompatible-issue-158428.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: -Znext-solver=globally
+
+#![feature(min_generic_const_args)]
+#![feature(generic_const_args)]
+#![feature(generic_const_exprs)]
+//~^ ERROR `-Znext-solver=globally` and `generic_const_exprs` are incompatible
+//@ normalize-stderr: "(--> ).*/tests/ui/const-generics/generic_const_exprs" -> "$1$$DIR"
+
+use std::mem::size_of;
+
+union AsBytes<T> {
+    as_bytes: [u8; const { size_of::<T>() }],
+}
+
+fn main() {}

tests/ui/const-generics/generic_const_exprs/next-solver-gce-incompatible-issue-158428.stderr

@@ -0,0 +1,10 @@
+error: `-Znext-solver=globally` and `generic_const_exprs` are incompatible, using them at the same time is not allowed
+  --> $DIR/next-solver-gce-incompatible-issue-158428.rs:5:12
+   |
+LL | #![feature(generic_const_exprs)]
+   |            ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: remove one of these features
+
+error: aborting due to 1 previous error
+

tests/ui/traits/const-traits/unsatisfied-const-trait-bound.rs

@@ -1,5 +1,6 @@
 //@ known-bug: unknown
-// Ensure that we print unsatisfied always-const trait bounds as `const Trait` in diagnostics.
+// This used to ensure that the next solver prints unsatisfied always-const trait bounds as
+// `const Trait`, but no longer does because GCE is incompatible with the next solver.
 //@ compile-flags: -Znext-solver
 
 #![feature(const_trait_impl, generic_const_exprs)]

tests/ui/traits/const-traits/unsatisfied-const-trait-bound.stderr

@@ -1,45 +1,10 @@
 error: `-Znext-solver=globally` and `generic_const_exprs` are incompatible, using them at the same time is not allowed
-  --> $DIR/unsatisfied-const-trait-bound.rs:5:30
+  --> $DIR/unsatisfied-const-trait-bound.rs:6:30
    |
 LL | #![feature(const_trait_impl, generic_const_exprs)]
    |                              ^^^^^^^^^^^^^^^^^^^
    |
    = help: remove one of these features
 
-error[E0391]: cycle detected when evaluating type-level constant
-  --> $DIR/unsatisfied-const-trait-bound.rs:28:35
-   |
-LL | fn accept0<T: Trait>(_: Container<{ T::make() }>) {}
-   |                                   ^^^^^^^^^^^^^
-   |
-   = note: ...which requires const-evaluating + checking `accept0::{constant#0}`...
-   = note: ...which requires checking if `accept0::{constant#0}` is a trivial const...
-   = note: ...which requires building MIR for `accept0::{constant#0}`...
-   = note: ...which requires building an abstract representation for `accept0::{constant#0}`...
-   = note: ...which requires building THIR for `accept0::{constant#0}`...
-   = note: ...which requires type-checking `accept0::{constant#0}`...
-   = note: ...which requires computing normalized predicates of `accept0::{constant#0}`...
-   = note: ...which again requires evaluating type-level constant, completing the cycle
-   = note: cycle used when computing normalized predicates of `accept0`
-   = note: for more information, see <https://rustc-dev-guide.rust-lang.org/overview.html#queries> and <https://rustc-dev-guide.rust-lang.org/query.html>
-
-error[E0391]: cycle detected when checking if `accept1::{constant#0}` is a trivial const
-  --> $DIR/unsatisfied-const-trait-bound.rs:32:49
-   |
-LL | const fn accept1<T: [const] Trait>(_: Container<{ T::make() }>) {}
-   |                                                 ^^^^^^^^^^^^^
-   |
-   = note: ...which requires building MIR for `accept1::{constant#0}`...
-   = note: ...which requires building an abstract representation for `accept1::{constant#0}`...
-   = note: ...which requires building THIR for `accept1::{constant#0}`...
-   = note: ...which requires type-checking `accept1::{constant#0}`...
-   = note: ...which requires computing normalized predicates of `accept1::{constant#0}`...
-   = note: ...which requires evaluating type-level constant...
-   = note: ...which requires const-evaluating + checking `accept1::{constant#0}`...
-   = note: ...which again requires checking if `accept1::{constant#0}` is a trivial const, completing the cycle
-   = note: cycle used when const-evaluating + checking `accept1::{constant#0}`
-   = note: for more information, see <https://rustc-dev-guide.rust-lang.org/overview.html#queries> and <https://rustc-dev-guide.rust-lang.org/query.html>
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0391`.