feat: add async exercises

solutions/23_clippy/clippy1.rs

@@ -0,0 +1,17 @@
+// The Clippy tool is a collection of lints to analyze your code so you can
+// catch common mistakes and improve your Rust code.
+//
+// For these exercises, the code will fail to compile when there are Clippy
+// warnings. Check Clippy's suggestions from the output to solve the exercise.
+
+use std::f32::consts::PI;
+
+fn main() {
+    // Use the more accurate `PI` constant.
+    let pi = PI;
+    let radius: f32 = 5.0;
+
+    let area = pi * radius.powi(2);
+
+    println!("The area of a circle with radius {radius:.2} is {area:.5}");
+}

solutions/23_clippy/clippy2.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let mut res = 42;
+    let option = Some(12);
+    // Use `if-let` instead of iteration.
+    if let Some(x) = option {
+        res += x;
+    }
+
+    println!("{res}");
+}

solutions/23_clippy/clippy3.rs

@@ -0,0 +1,31 @@
+use std::mem;
+
+#[rustfmt::skip]
+#[allow(unused_variables, unused_assignments)]
+fn main() {
+    let my_option: Option<&str> = None;
+    // `unwrap` of an `Option` after checking if it is `None` will panic.
+    // Use `if-let` instead.
+    if let Some(value) = my_option {
+        println!("{value}");
+    }
+
+    // A comma was missing.
+    let my_arr = &[
+        -1, -2, -3,
+        -4, -5, -6,
+    ];
+    println!("My array! Here it is: {my_arr:?}");
+
+    let mut my_empty_vec = vec![1, 2, 3, 4, 5];
+    // `resize` mutates a vector instead of returning a new one.
+    // `resize(0, …)` clears a vector, so it is better to use `clear`.
+    my_empty_vec.clear();
+    println!("This Vec is empty, see? {my_empty_vec:?}");
+
+    let mut value_a = 45;
+    let mut value_b = 66;
+    // Use `mem::swap` to correctly swap two values.
+    mem::swap(&mut value_a, &mut value_b);
+    println!("value a: {value_a}; value b: {value_b}");
+}