RustのCLI作成チュートリアルやっている中のメモ書き

/// Search for a pattern in a file and display the lines that contain it.
#[derive(Parser)]
struct Cli {
    /// The pattern to look for
    pattern: String,
    /// The path to the file to read
    #[clap(parse(from_os_str))]
    path: std::path::PathBuf,
}

trait Trait {}

impl Trait for i32 {}

// old
// いままで
fn function1() -> Box<Trait> {
}

// new
// これから
fn function2() -> Box<dyn Trait> {
}

error[E0277]: `PathBuf` doesn't implement `std::fmt::Display`
  --> src/main.rs:17:84
   |
17 |         File::open(&args.path).with_context(|| format!("could not read file `{}`", &args.path));
   |                                                                                    ^^^^^^^^^^ `PathBuf` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `PathBuf`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: this error originates in the macro `$crate::__export::format_args` (in Nightly builds, run with -Z macro-backtrace for more info)

#![allow(unused)]
fn main() {
use std::io::{self, Write};

let stdout = io::stdout(); // get the global stdout entity
let mut handle = io::BufWriter::new(stdout); // optional: wrap that handle in a buffer
writeln!(handle, "foo: {}", 42); // add `?` if you care about errors here
}

#![allow(unused)]
fn main() {
use std::io::{self, Write};

let stdout = io::stdout(); // get the global stdout entity
let mut handle = stdout.lock(); // acquire a lock on it
writeln!(handle, "foo: {}", 42); // add `?` if you care about errors here
}

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
  --> src/main.rs:27:38
   |
15 | / fn main() {
16 | |     let args = Cli::parse();
17 | |     let content = File::open(&args.path)
18 | |         .with_context(|| format!("could not read file `{}`", &args.path.as_path().display()));
...  |
27 | |             writeln!(handle, "{}", l)?;
   | |                                      ^ cannot use the `?` operator in a function that returns `()`
28 | |         }
29 | |     }
30 | | }
   | |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`

[package]
name = "grrs"