feat: Add exercises for understanding borrowing

Exercises to explore immutable borrowing, mutable borrowing, and "aliasing xor mutability".

exercises/borrowing/borrowing1.rs

@@ -0,0 +1,18 @@
+// borrowing1.rs
+
+// If you want to let a function borrow a variable but not move it,
+// you can use references.  This allows you to use the variable later.
+// Make this compile without cloning `hello`.
+
+// I AM NOT DONE
+
+fn main() {
+    let hello = String::from("Hello world!");
+    let length = string_size(hello);
+
+    println!("The string `{}` has {} characters", hello, length);
+}
+
+fn string_size(s: String) -> usize {
+    s.len()
+}

exercises/borrowing/borrowing2.rs

@@ -0,0 +1,18 @@
+// borrowing2.rs
+//
+// Rust has strong guarantees about immutability.  In order for something
+// to be mutated, it must be marked explicitly.  Make this compile only
+// by changing the mutabilities of the calls.
+
+// I AM NOT DONE
+
+fn main() {
+    let hello = String::from("Hello ");
+    append_world(&hello);
+
+    assert_eq!(&hello, "Hello world!");
+}
+
+fn append_world(s: &String) {
+    s.push_str("world!");
+}

exercises/borrowing/borrowing3.rs

@@ -0,0 +1,18 @@
+// borrowing3.rs
+//
+// A core safety feature of Rust is that it may not have mutable and immutable
+// borrows.  Make this compile only by changing the order of the lines.
+
+// I AM NOT DONE
+
+fn main() {
+    let mut hello = String::from("Hello ");
+    let hello_ref = &hello;
+    append_world(&mut hello);
+
+    println!("{}", hello_ref);
+}
+
+fn append_world(s: &mut String) {
+    s.push_str("world!");
+}

info.toml

@@ -453,6 +453,45 @@ path = "exercises/quiz2.rs"
 mode = "compile"
 hint = "No hints this time ;)"
 
+# BORROWING
+
+[[exercises]]
+name = "borrowing1"
+path = "exercises/borrowing/borrowing1.rs"
+mode = "compile"
+hint = """
+To let a function borrow a variable, you must use a reference.
+
+Have a look in The Book, to find out more:
+https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
+"""
+
+[[exercises]]
+name = "borrowing2"
+path = "exercises/borrowing/borrowing2.rs"
+mode = "compile"
+hint = """
+In order for a variable to be mutated, it must be declared mutable.
+
+In order for a borrowed variable to mutated, it must be a mutable reference.
+
+Have a look in The Book, to find out more:
+https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
+"""
+
+
+[[exercises]]
+name = "borrowing3"
+path = "exercises/borrowing/borrowing3.rs"
+mode = "compile"
+hint = """
+Rust won't let you have multiple mutable borrows.  You can either have
+one mutable borrow, or multiple immutable borrows.
+
+Have a look in The Book, to find out more:
+https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
+"""
+
 # ERROR HANDLING
 
 [[exercises]]