Add regression test

Author: oli-obk

tests/ui/consts/const-closure-in-trait-impl.rs

@@ -0,0 +1,25 @@
+#![feature(const_closures, const_destruct, const_trait_impl)]
+
+use std::marker::Destruct;
+use std::num::NonZero;
+
+const trait T {
+    fn a(&mut self, f: impl [const] Fn() + [const] Destruct);
+    fn b(&mut self);
+}
+
+struct S;
+
+impl const T for S {
+    fn a(&mut self, f: impl [const] Fn() + [const] Destruct) {
+        f()
+    }
+
+    fn b(&mut self) {
+        self.a(const || {});
+        //~^ ERROR: cannot use `const` closures outside of const contexts
+        //~| ERROR: [const] Fn()` is not satisfied
+    }
+}
+
+fn main() {}

tests/ui/consts/const-closure-in-trait-impl.stderr

@@ -0,0 +1,23 @@
+error: cannot use `const` closures outside of const contexts
+  --> $DIR/const-closure-in-trait-impl.rs:19:16
+   |
+LL |         self.a(const || {});
+   |                ^^^^^
+
+error[E0277]: the trait bound `{closure@$DIR/const-closure-in-trait-impl.rs:19:16: 19:24}: [const] Fn()` is not satisfied
+  --> $DIR/const-closure-in-trait-impl.rs:19:16
+   |
+LL |         self.a(const || {});
+   |              - ^^^^^^^^^^^
+   |              |
+   |              required by a bound introduced by this call
+   |
+note: required by a bound in `T::a`
+  --> $DIR/const-closure-in-trait-impl.rs:7:29
+   |
+LL |     fn a(&mut self, f: impl [const] Fn() + [const] Destruct);
+   |                             ^^^^^^^^^^^^ required by this bound in `T::a`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.