$ cargo run
   Compiling error-handling v0.1.0 (file:///projects/error-handling)
error[E0308]: mismatched types
 --> src/main.rs:4:18
  |
4 |     let f: u32 = File::open("hello.txt");
  |            ---   ^^^^^^^^^^^^^^^^^^^^^^^ expected `u32`, found enum `Result`
  |            |
  |            expected due to this
  |
  = note: expected type `u32`
             found enum `Result<File, std::io::Error>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `error-handling` due to previous error
