Make well-formedness predicates no longer coinductive

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1242,7 +1242,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
                 self.infcx.tcx.trait_is_coinductive(data.def_id())
             }
-            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => true,
+            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => {
+                // FIXME(generic_const_exprs): GCE needs well-formedness predicates to be
+                // coinductive, but GCE is on the way out anyways, so this should eventually
+                // be replaced with `false`.
+                self.infcx.tcx.features().generic_const_exprs()
+            }
             _ => false,
         })
     }

tests/ui/associated-types/issue-64855-2.rs

@@ -1,5 +1,8 @@
-//@ check-pass
+// This was originally a test for a `ReEmpty` ICE, but became an unintentional test of
+// the coinductiveness of WF predicates. That behavior was removed, and thus this is
+// also inadvertently a test for the (non-)co-inductiveness of WF predicates.
 
 pub struct Bar<'a>(&'a Self) where Self: ;
+//~^ ERROR overflow evaluating the requirement `Bar<'a> well-formed`
 
 fn main() {}

tests/ui/associated-types/issue-64855-2.stderr

@@ -0,0 +1,15 @@
+error[E0275]: overflow evaluating the requirement `Bar<'a> well-formed`
+  --> $DIR/issue-64855-2.rs:5:36
+   |
+LL | pub struct Bar<'a>(&'a Self) where Self: ;
+   |                                    ^^^^
+   |
+note: required by a bound in `Bar`
+  --> $DIR/issue-64855-2.rs:5:36
+   |
+LL | pub struct Bar<'a>(&'a Self) where Self: ;
+   |                                    ^^^^ required by this bound in `Bar`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/associated-types/issue-64855.rs

@@ -1,8 +1,13 @@
+// This was originally a test for a `ReEmpty` ICE, but became an unintentional test of
+// the coinductiveness of WF predicates. That behavior was removed, and thus this is
+// also inadvertently a test for the (non-)co-inductiveness of WF predicates.
+
 pub trait Foo {
     type Type;
 }
 
 pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
 //~^ ERROR the trait bound `Bar<T>: Foo` is not satisfied
+//~| ERROR overflow evaluating the requirement `Bar<T> well-formed`
 
 fn main() {}

tests/ui/associated-types/issue-64855.stderr

@@ -1,15 +1,28 @@
 error[E0277]: the trait bound `Bar<T>: Foo` is not satisfied
-  --> $DIR/issue-64855.rs:5:19
+  --> $DIR/issue-64855.rs:9:19
    |
 LL | pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
    |                   ^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `Bar<T>`
    |
 help: this trait has no implementations, consider adding one
-  --> $DIR/issue-64855.rs:1:1
+  --> $DIR/issue-64855.rs:5:1
    |
 LL | pub trait Foo {
    | ^^^^^^^^^^^^^
 
-error: aborting due to 1 previous error
+error[E0275]: overflow evaluating the requirement `Bar<T> well-formed`
+  --> $DIR/issue-64855.rs:9:46
+   |
+LL | pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
+   |                                              ^^^^
+   |
+note: required by a bound in `Bar`
+  --> $DIR/issue-64855.rs:9:46
+   |
+LL | pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
+   |                                              ^^^^ required by this bound in `Bar`
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0275, E0277.
+For more information about an error, try `rustc --explain E0275`.

tests/ui/higher-ranked/trait-bounds/issue-95230.rs

@@ -1,11 +1,10 @@
-//@ revisions: old next
-//@[next] compile-flags: -Znext-solver
-//@[old] check-pass
-//@[next] known-bug: #109764
-
+// This used to be a test for overflow handling + higher-ranked outlives
+// in the new solver, but this test isn't expected to pass since WF preds
+// are not coinductive anymore.
 
 pub struct Bar
 where
     for<'a> &'a mut Self:;
+//~^ ERROR overflow evaluating the requirement `for<'a> &'a mut Bar well-formed`
 
 fn main() {}

tests/ui/higher-ranked/trait-bounds/issue-95230.stderr

@@ -0,0 +1,18 @@
+error[E0275]: overflow evaluating the requirement `for<'a> &'a mut Bar well-formed`
+  --> $DIR/issue-95230.rs:7:13
+   |
+LL |     for<'a> &'a mut Self:;
+   |             ^^^^^^^^^^^^
+   |
+note: required by a bound in `Bar`
+  --> $DIR/issue-95230.rs:7:13
+   |
+LL | pub struct Bar
+   |            --- required by a bound in this struct
+LL | where
+LL |     for<'a> &'a mut Self:;
+   |             ^^^^^^^^^^^^ required by this bound in `Bar`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.