add test for const-eval error in dead code during monomorphization

Author: RalfJung

tests/ui/consts/const-eval/unused-broken-const-late.rs

@@ -0,0 +1,19 @@
+// build-fail
+//! Make sure we detect erroneous constants post-monomorphization even when they are unused.
+//! This is crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
+
+struct PrintName<T>(T);
+impl<T> PrintName<T> {
+    const VOID: () = panic!(); //~ERROR evaluation of `PrintName::<i32>::VOID` failed
+}
+
+fn no_codegen<T>() {
+    // Any function that is called is guaranteed to have all consts that syntactically
+    // appear in its body evaluated, even if they only appear in dead code.
+    if false {
+        let _ = PrintName::<T>::VOID;
+    }
+}
+pub fn main() {
+    no_codegen::<i32>(); //~ encountered
+}

tests/ui/consts/const-eval/unused-broken-const-late.stderr

@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `PrintName::<i32>::VOID` failed
+  --> $DIR/unused-broken-const-late.rs:7:22
+   |
+LL |     const VOID: () = panic!();
+   |                      ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/unused-broken-const-late.rs:7:22
+   |
+   = note: this error originates in the macro `$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: the above error was encountered while instantiating `fn no_codegen::<i32>`
+  --> $DIR/unused-broken-const-late.rs:18:5
+   |
+LL |     no_codegen::<i32>();
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.