Add test for panic!(format!(..)) lint.

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

@@ -35,6 +35,8 @@ fn main() {
 
     panic!(a!()); //~ WARN panic message is not a string literal
 
+    panic!(format!("{}", 1)); //~ 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

@@ -199,5 +199,18 @@ help: or use std::panic::panic_any instead
 LL |     std::panic::panic_any(a!());
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
-warning: 15 warnings emitted
+warning: panic message is not a string literal
+  --> $DIR/non-fmt-panic.rs:38:12
+   |
+LL |     panic!(format!("{}", 1));
+   |            ^^^^^^^^^^^^^^^^
+   |
+   = note: this is no longer accepted in Rust 2021
+   = note: the panic!() macro supports formatting, so there's no need for the format!() macro here
+help: remove the `format!(..)` macro call
+   |
+LL |     panic!("{}", 1);
+   |           --     --
+
+warning: 16 warnings emitted