Rust 2024: update Ch. 20 for new unsafe rules

- `extern` now requires `unsafe extern`, but in turn allows functions
  within it to be marked as `safe`. Add examples to both effects within
  the text, and update listing numbers accordingly.
- `#[no_mangle]` now requires `#[unsafe(no_mangle)]`

These are all backwards-compatible, so we can opt the chapter into using
the new rules without issue.

listings/ch20-advanced-features/listing-20-08/src/main.rs

@@ -1,4 +1,4 @@
-extern "C" {
+unsafe extern "C" {
     fn abs(input: i32) -> i32;
 }
 

listings/ch20-advanced-features/listing-20-09/src/main.rs

@@ -1,5 +1,7 @@
-static HELLO_WORLD: &str = "Hello, world!";
+unsafe extern "C" {
+    safe fn abs(input: i32) -> i32;
+}
 
 fn main() {
-    println!("name is: {HELLO_WORLD}");
+    println!("Absolute value of -3 according to C: {}", abs(-3));
 }

listings/ch20-advanced-features/listing-20-10/src/main.rs

@@ -1,16 +1,5 @@
-static mut COUNTER: u32 = 0;
-
-/// SAFETY: Calling this from more than a single thread at a time is undefined
-/// behavior, so you *must* guarantee you only call it from a single thread at
-/// a time.
-unsafe fn add_to_count(inc: u32) {
-    COUNTER += inc;
-}
+static HELLO_WORLD: &str = "Hello, world!";
 
 fn main() {
-    unsafe {
-        // SAFETY: This is only called from a single thread in `main`.
-        add_to_count(3);
-        println!("COUNTER: {}", COUNTER);
-    }
+    println!("name is: {HELLO_WORLD}");
 }

listings/ch20-advanced-features/listing-20-11/src/main.rs

@@ -1,9 +1,16 @@
-unsafe trait Foo {
-    // methods go here
-}
+static mut COUNTER: u32 = 0;
 
-unsafe impl Foo for i32 {
-    // method implementations go here
+/// SAFETY: Calling this from more than a single thread at a time is undefined
+/// behavior, so you *must* guarantee you only call it from a single thread at
+/// a time.
+unsafe fn add_to_count(inc: u32) {
+    COUNTER += inc;
 }
 
-fn main() {}
+fn main() {
+    unsafe {
+        // SAFETY: This is only called from a single thread in `main`.
+        add_to_count(3);
+        println!("COUNTER: {}", COUNTER);
+    }
+}

listings/ch20-advanced-features/listing-20-12/Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "traits-example"
+name = "unsafe-example"
 version = "0.1.0"
 edition = "2021"
 

listings/ch20-advanced-features/listing-20-12/src/main.rs

@@ -0,0 +1,9 @@
+unsafe trait Foo {
+    // methods go here
+}
+
+unsafe impl Foo for i32 {
+    // method implementations go here
+}
+
+fn main() {}

listings/ch20-advanced-features/listing-20-13/src/lib.rs

@@ -1,3 +1,5 @@
-pub trait Iterator<T> {
-    fn next(&mut self) -> Option<T>;
+pub trait Iterator {
+    type Item;
+
+    fn next(&mut self) -> Option<Self::Item>;
 }

listings/ch20-advanced-features/listing-20-14/src/lib.rs

@@ -0,0 +1,3 @@
+pub trait Iterator<T> {
+    fn next(&mut self) -> Option<T>;
+}

listings/ch20-advanced-features/listing-20-17/src/main.rs

@@ -1,3 +1,4 @@
+// ANCHOR: here
 trait Pilot {
     fn fly(&self);
 }
@@ -25,10 +26,6 @@ impl Human {
         println!("*waving arms furiously*");
     }
 }
-
-// ANCHOR: here
-fn main() {
-    let person = Human;
-    person.fly();
-}
 // ANCHOR_END: here
+
+fn main() {}

listings/ch20-advanced-features/listing-20-18/src/main.rs

@@ -29,8 +29,6 @@ impl Human {
 // ANCHOR: here
 fn main() {
     let person = Human;
-    Pilot::fly(&person);
-    Wizard::fly(&person);
     person.fly();
 }
 // ANCHOR_END: here

listings/ch20-advanced-features/listing-20-19/output.txt

@@ -1,5 +1,7 @@
 $ cargo run
    Compiling traits-example v0.1.0 (file:///projects/traits-example)
-    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.54s
+    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.46s
      Running `target/debug/traits-example`
-A baby dog is called a Spot
+This is your captain speaking.
+Up!
+*waving arms furiously*

listings/ch20-advanced-features/listing-20-19/src/main.rs

@@ -1,21 +1,36 @@
-trait Animal {
-    fn baby_name() -> String;
+trait Pilot {
+    fn fly(&self);
 }
 
-struct Dog;
+trait Wizard {
+    fn fly(&self);
+}
+
+struct Human;
+
+impl Pilot for Human {
+    fn fly(&self) {
+        println!("This is your captain speaking.");
+    }
+}
 
-impl Dog {
-    fn baby_name() -> String {
-        String::from("Spot")
+impl Wizard for Human {
+    fn fly(&self) {
+        println!("Up!");
     }
 }
 
-impl Animal for Dog {
-    fn baby_name() -> String {
-        String::from("puppy")
+impl Human {
+    fn fly(&self) {
+        println!("*waving arms furiously*");
     }
 }
 
+// ANCHOR: here
 fn main() {
-    println!("A baby dog is called a {}", Dog::baby_name());
+    let person = Human;
+    Pilot::fly(&person);
+    Wizard::fly(&person);
+    person.fly();
 }
+// ANCHOR_END: here

listings/ch20-advanced-features/listing-20-20/output.txt

@@ -1,18 +1,5 @@
 $ cargo run
    Compiling traits-example v0.1.0 (file:///projects/traits-example)
-error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
-  --> src/main.rs:20:43
-   |
-2  |     fn baby_name() -> String;
-   |     ------------------------- `Animal::baby_name` defined here
-...
-20 |     println!("A baby dog is called a {}", Animal::baby_name());
-   |                                           ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
-   |
-help: use the fully-qualified path to the only available implementation
-   |
-20 |     println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
-   |                                           +++++++       +
-
-For more information about this error, try `rustc --explain E0790`.
-error: could not compile `traits-example` (bin "traits-example") due to 1 previous error
+    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.54s
+     Running `target/debug/traits-example`
+A baby dog is called a Spot

listings/ch20-advanced-features/listing-20-20/src/main.rs

@@ -16,8 +16,6 @@ impl Animal for Dog {
     }
 }
 
-// ANCHOR: here
 fn main() {
-    println!("A baby dog is called a {}", Animal::baby_name());
+    println!("A baby dog is called a {}", Dog::baby_name());
 }
-// ANCHOR_END: here

listings/ch20-advanced-features/listing-20-21/output.txt

@@ -1,5 +1,18 @@
 $ cargo run
    Compiling traits-example v0.1.0 (file:///projects/traits-example)
-    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s
-     Running `target/debug/traits-example`
-A baby dog is called a puppy
+error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
+  --> src/main.rs:20:43
+   |
+2  |     fn baby_name() -> String;
+   |     ------------------------- `Animal::baby_name` defined here
+...
+20 |     println!("A baby dog is called a {}", Animal::baby_name());
+   |                                           ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
+   |
+help: use the fully-qualified path to the only available implementation
+   |
+20 |     println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
+   |                                           +++++++       +
+
+For more information about this error, try `rustc --explain E0790`.
+error: could not compile `traits-example` (bin "traits-example") due to 1 previous error

listings/ch20-advanced-features/listing-20-21/src/main.rs

@@ -18,6 +18,6 @@ impl Animal for Dog {
 
 // ANCHOR: here
 fn main() {
-    println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
+    println!("A baby dog is called a {}", Animal::baby_name());
 }
 // ANCHOR_END: here

listings/ch20-advanced-features/listing-20-18/output.txt → listings/ch20-advanced-features/listing-20-22/output.txt

@@ -1,7 +1,5 @@
 $ cargo run
    Compiling traits-example v0.1.0 (file:///projects/traits-example)
-    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.46s
+    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s
      Running `target/debug/traits-example`
-This is your captain speaking.
-Up!
-*waving arms furiously*
+A baby dog is called a puppy

listings/ch20-advanced-features/listing-20-22/src/main.rs

@@ -1,17 +1,23 @@
-// ANCHOR: here
-use std::fmt;
+trait Animal {
+    fn baby_name() -> String;
+}
+
+struct Dog;
 
-trait OutlinePrint: fmt::Display {
-    fn outline_print(&self) {
-        let output = self.to_string();
-        let len = output.len();
-        println!("{}", "*".repeat(len + 4));
-        println!("*{}*", " ".repeat(len + 2));
-        println!("* {output} *");
-        println!("*{}*", " ".repeat(len + 2));
-        println!("{}", "*".repeat(len + 4));
+impl Dog {
+    fn baby_name() -> String {
+        String::from("Spot")
+    }
+}
+
+impl Animal for Dog {
+    fn baby_name() -> String {
+        String::from("puppy")
     }
 }
-// ANCHOR_END: here
 
-fn main() {}
+// ANCHOR: here
+fn main() {
+    println!("A baby dog is called a {}", <Dog as Animal>::baby_name());
+}
+// ANCHOR_END: here

listings/ch20-advanced-features/listing-20-23/src/main.rs

@@ -1,14 +1,17 @@
+// ANCHOR: here
 use std::fmt;
 
-struct Wrapper(Vec<String>);
-
-impl fmt::Display for Wrapper {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "[{}]", self.0.join(", "))
+trait OutlinePrint: fmt::Display {
+    fn outline_print(&self) {
+        let output = self.to_string();
+        let len = output.len();
+        println!("{}", "*".repeat(len + 4));
+        println!("*{}*", " ".repeat(len + 2));
+        println!("* {output} *");
+        println!("*{}*", " ".repeat(len + 2));
+        println!("{}", "*".repeat(len + 4));
     }
 }
+// ANCHOR_END: here
 
-fn main() {
-    let w = Wrapper(vec![String::from("hello"), String::from("world")]);
-    println!("w = {w}");
-}
+fn main() {}

listings/ch20-advanced-features/listing-20-24/Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "types-example"
+name = "traits-example"
 version = "0.1.0"
 edition = "2021"
 

listings/ch20-advanced-features/listing-20-24/src/main.rs

@@ -1,16 +1,14 @@
-fn main() {
-    // ANCHOR: here
-    let f: Box<dyn Fn() + Send + 'static> = Box::new(|| println!("hi"));
+use std::fmt;
 
-    fn takes_long_type(f: Box<dyn Fn() + Send + 'static>) {
-        // --snip--
-    }
+struct Wrapper(Vec<String>);
 
-    fn returns_long_type() -> Box<dyn Fn() + Send + 'static> {
-        // --snip--
-        // ANCHOR_END: here
-        Box::new(|| ())
-        // ANCHOR: here
+impl fmt::Display for Wrapper {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "[{}]", self.0.join(", "))
     }
-    // ANCHOR_END: here
+}
+
+fn main() {
+    let w = Wrapper(vec![String::from("hello"), String::from("world")]);
+    println!("w = {w}");
 }