Improve non_fmt_panic lint.

src/test/ui/non-fmt-panic.rs

@@ -29,6 +29,12 @@ fn main() {
     fancy_panic::fancy_panic!(S);
     //~^ WARN panic message is not a string literal
 
+    macro_rules! a {
+        () => { 123 };
+    }
+
+    panic!(a!()); //~ WARN panic message is not a string literal
+
     // Check that the lint only triggers for std::panic and core::panic,
     // not any panic macro:
     macro_rules! panic {

src/test/ui/non-fmt-panic.stderr

@@ -183,5 +183,21 @@ LL |     fancy_panic::fancy_panic!(S);
    |
    = note: this is no longer accepted in Rust 2021
 
-warning: 14 warnings emitted
+warning: panic message is not a string literal
+  --> $DIR/non-fmt-panic.rs:36:12
+   |
+LL |     panic!(a!());
+   |            ^^^^
+   |
+   = note: this is no longer accepted in Rust 2021
+help: add a "{}" format string to Display the message
+   |
+LL |     panic!("{}", a!());
+   |            ^^^^^
+help: or use std::panic::panic_any instead
+   |
+LL |     std::panic::panic_any(a!());
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+warning: 15 warnings emitted