dbg_macro: Debug-required test.

Author: Centril

src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.rs

@@ -0,0 +1,9 @@
+// Test ensuring that `dbg!(expr)` requires the passed type to implement `Debug`.
+
+#![feature(dbg_macro)]
+
+struct NotDebug;
+
+fn main() {
+    let _: NotDebug = dbg!(NotDebug);
+}

src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr

@@ -0,0 +1,15 @@
+error[E0277]: `NotDebug` doesn't implement `std::fmt::Debug`
+  --> $DIR/dbg-macro-requires-debug.rs:8:23
+   |
+LL |     let _: NotDebug = dbg!(NotDebug);
+   |                       ^^^^^^^^^^^^^^ `NotDebug` cannot be formatted using `{:?}`
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `NotDebug`
+   = note: add `#[derive(Debug)]` or manually implement `std::fmt::Debug`
+   = note: required because of the requirements on the impl of `std::fmt::Debug` for `&NotDebug`
+   = note: required by `std::fmt::Debug::fmt`
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.