Add regression test

Author: oli-obk

Diff for: tests/ui/impl-trait/rpit/const_check_false_cycle.rs

@@ -0,0 +1,11 @@
+//! This test causes a cycle error when checking whether the
+//! return type is `Freeze` during const checking, even though
+//! the information is readily available.
+
+const fn f() -> impl Eq {
+    //~^ ERROR cycle detected
+    g()
+}
+const fn g() {}
+
+fn main() {}

Diff for: tests/ui/impl-trait/rpit/const_check_false_cycle.stderr

@@ -0,0 +1,34 @@
+error[E0391]: cycle detected when computing type of opaque `f::{opaque#0}`
+  --> $DIR/const_check_false_cycle.rs:5:17
+   |
+LL | const fn f() -> impl Eq {
+   |                 ^^^^^^^
+   |
+note: ...which requires borrow-checking `f`...
+  --> $DIR/const_check_false_cycle.rs:5:1
+   |
+LL | const fn f() -> impl Eq {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires promoting constants in MIR for `f`...
+  --> $DIR/const_check_false_cycle.rs:5:1
+   |
+LL | const fn f() -> impl Eq {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `f`...
+  --> $DIR/const_check_false_cycle.rs:5:1
+   |
+LL | const fn f() -> impl Eq {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `f::{opaque#0}` is freeze...
+   = note: ...which requires evaluating trait selection obligation `f::{opaque#0}: core::marker::Freeze`...
+   = note: ...which again requires computing type of opaque `f::{opaque#0}`, completing the cycle
+note: cycle used when computing type of `f::{opaque#0}`
+  --> $DIR/const_check_false_cycle.rs:5:17
+   |
+LL | const fn f() -> impl Eq {
+   |                 ^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0391`.