add test involving opaque types

Author: RalfJung

tests/ui/consts/required-consts/collect-in-dead-fn-behind-opaque-type.noopt.stderr

@@ -0,0 +1,20 @@
+error[E0080]: evaluation of `m::Fail::<i32>::C` failed
+  --> $DIR/collect-in-dead-fn-behind-opaque-type.rs:10:23
+   |
+LL |         const C: () = panic!();
+   |                       ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/collect-in-dead-fn-behind-opaque-type.rs:10:23
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/collect-in-dead-fn-behind-opaque-type.rs:17:21
+   |
+LL |             let _ = Fail::<T>::C;
+   |                     ^^^^^^^^^^^^
+
+note: the above error was encountered while instantiating `fn m::not_called::<i32>`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/required-consts/collect-in-dead-fn-behind-opaque-type.opt.stderr

@@ -0,0 +1,23 @@
+error[E0080]: evaluation of `m::Fail::<i32>::C` failed
+  --> $DIR/collect-in-dead-fn-behind-opaque-type.rs:10:23
+   |
+LL |         const C: () = panic!();
+   |                       ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/collect-in-dead-fn-behind-opaque-type.rs:10:23
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/collect-in-dead-fn-behind-opaque-type.rs:17:21
+   |
+LL |             let _ = Fail::<T>::C;
+   |                     ^^^^^^^^^^^^
+
+note: the above error was encountered while instantiating `fn m::not_called::<i32>`
+  --> $DIR/collect-in-dead-fn-behind-opaque-type.rs:28:42
+   |
+LL |         let x: m::NotCalledFn = unsafe { std::mem::transmute(()) };
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/required-consts/collect-in-dead-fn-behind-opaque-type.rs

@@ -0,0 +1,31 @@
+//@revisions: noopt opt
+//@ build-fail
+//@[opt] compile-flags: -O
+//! This fails without optimizations, so it should also fail with optimizations.
+#![feature(type_alias_impl_trait)]
+
+mod m {
+    struct Fail<T>(T);
+    impl<T> Fail<T> {
+        const C: () = panic!(); //~ERROR evaluation of `m::Fail::<i32>::C` failed
+    }
+
+    pub type NotCalledFn = impl Fn();
+    
+    fn not_called<T>() {
+        if false {
+            let _ = Fail::<T>::C;
+        }
+    }
+
+    fn mk_not_called() -> NotCalledFn {
+        not_called::<i32>
+    }
+}
+
+fn main() {
+    if false {
+        let x: m::NotCalledFn = unsafe { std::mem::transmute(()) };
+        x();
+    }
+}