What it does
assert! and panic! have built-in support for formatting, but they also accept arguments of type String, so one could use format! instead.
Using format! here is entirely unnecessary, and yet happens in the wild, so this seems like a useful target for a Clippy lint. Also the long-term plan is to not support String arguments any more, so such code might not work in future editions.
Cc @m-ou-se
Example
panic!(format!("hello {}", var))
assert!(x, format!("hello {}", var))
Could be written as:
panic!("hello {}", var)
assert!(x, "hello {}", var)