マクロの引数を間違えた場合

message.txt

$ rustc test.rs
error[E0308]: mismatched types
 --> test.rs:2:35
  |
2 |     let r: Result<(), &str> = Err(format!("hello!"));
  |                                   ^^^^^^^^^^^^^^^^^
  |                                   |
  |                                   expected &str, found struct `std::string::String`
  |                                   help: consider borrowing here: `&format!("hello!")`
  |
  = note: expected type `&str`
             found type `std::string::String`
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

test.rs

fn main() {
    let r: Result<(), &str> = Err(format!("hello!"));
    println!("{:?}", r);
}