Rollup merge of rust-lang#140208 - compiler-errors:wf-coinductive, r=lcnr

Make well-formedness predicates no longer coinductive

This PR makes well-formedness no longer coinductive. It was made coinductive in rust-lang#98542, but AFAICT this was only to fix UI tests since we stopped lowering `where Ty:` to an empty-region outlives predicate but to a WF predicate instead.

Arguably it should lower to something completely different, something like a "type mentioned no-op predicate", but well-formedness serves this purpose fine today, and since no code (according to crater) relies on this coinductive behavior, we'd like to avoid having to emulate it in the new solver.

Fixes rust-lang#123456 (I didn't want to add a test since it seems low-value to have a ICE test for a fuzzer minimization that is basically garbage code.)

Fixes #109764 (not sure if this behavior is emulatable w/o coinductive WF?)

Fixes rust-lang/trait-system-refactor-initiative#169

r? lcnr

Author: matthiaskrgr

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

@@ -1241,7 +1241,12 @@ 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/crashes/123456.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.rs

@@ -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-64855-2.stderr

@@ -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.rs

@@ -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/associated-types/issue-64855.stderr

@@ -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.rs

@@ -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`.