trait Geometry<T>: std::fmt::Debug {
    fn area(&self) -> T;

    // name のデフォルト実装
    fn name(&self) -> &str {
        "Geometry"
    }
}

trait Geometry<T> {
    fn area(&self) -> T;

    // name のデフォルト実装
    fn name(&self) -> &str {
        "Geometry"
    }
}

➜ cargo run
   Compiling rust-playground v0.1.0 (/Users/nukopy/Projects/Rust/rust-playground)
error[E0277]: `dyn Geometry<f64>` doesn't implement `Debug`
  --> src/trait_practice.rs:97:25
   |
97 |     println!("r: {:?}", r);
   |                         ^ `dyn Geometry<f64>` cannot be formatted using `{:?}` because it doesn't implement `Debug`
   |
   = help: the trait `Debug` is not implemented for `dyn Geometry<f64>`
   = help: the following other types implement trait `Debug`:
             (dyn Any + 'static)
             (dyn Any + Send + 'static)
             (dyn Any + Send + Sync + 'static)
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `dyn Geometry<f32>` doesn't implement `Debug`
  --> src/trait_practice.rs:98:25
   |
98 |     println!("t: {:?}", t);
   |                         ^ `dyn Geometry<f32>` cannot be formatted using `{:?}` because it doesn't implement `Debug`
   |
   = help: the trait `Debug` is not implemented for `dyn Geometry<f32>`
   = help: the following other types implement trait `Debug`:
             (dyn Any + 'static)
             (dyn Any + Send + 'static)
             (dyn Any + Send + Sync + 'static)
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rust-playground` due to 2 previous errors

fn draw<T>(geometry: &impl Geometry<T>) {
    println!("draw: {:?}", geometry);
}

fn draw1<T: Geometry<f64>>(geometry: &T) {
    println!("draw1: {:?}", geometry);
}

fn draw2<T>(geometry: &T)
where
    T: Geometry<f64>,
{
    println!("draw2: {:?}", geometry);
}

fn main() {
    let x = 10;
    let y = &x;

    println!("x: {}", x);
    print_val(y);
}

fn print_val(x: &i32) {
    println!("print_val: {}", x);
}

fn test_mutable_reference() {
    let mut x = 10;
    let y = &mut x;

    println!("x: {}", x);
}

error[E0502]: cannot borrow `x` as immutable because it is also borrowed as mutable
  --> src/raii.rs:26:23
   |
24 |     let y = &mut x;
   |             ------ mutable borrow occurs here
25 |
26 |     println!("x: {}", x);
   |                       ^ immutable borrow occurs here
27 |     print_val_mut(y);
   |                   - mutable borrow later used here
   |
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

fn test_mutable_reference() {
    let mut x = 10;
    {
        let y = &mut x;
        // y の生存期間はこのスコープ内に限られます。
    }
    println!("x: {}", x);  // y のスコープ（生存期間）が終わった後なのでエラーは発生しない
}

fn test_mutable_reference() {
    let mut x = 10;
    println!("x: {}", x);  // y が借用する前なのでエラーは発生しない
    let y = &mut x;
}

let x = 5;
let y = &x;
println!("{}", *y); // ここで参照外しを行なっているが、所有権は移動しません
println!("{}", x); // 所有権は移動しないためここで x を使うことができる

#[derive(Debug)]
struct Foo(i32);

let x = Foo(5);
let y = &x;
let z = *y; // この場合、所有権が z に移動します。
// ここで x を使おうとするとコンパイルエラーになります

fn test_deref() {
    let mut x: i32 = 10;
    println!("x: {}", x); // 不変参照

    let y: &mut i32 = &mut x; // 可変参照

    deref_of_immutable_ref(y);
    println!("y: {}", y);
    deref_of_mutable_ref(y);
    println!("y: {}", y);
}

fn deref_of_immutable_ref(x: &i32) {
    println!("x: {}", *x); // 参照外し
}

fn deref_of_mutable_ref(x: &mut i32) {
    *x += 1; // 参照外し
}