Add exercise involving lifetime elision

Author: sgasse

dev/Cargo.toml

@@ -134,6 +134,8 @@ bin = [
   { name = "lifetimes2_sol", path = "../solutions/16_lifetimes/lifetimes2.rs" },
   { name = "lifetimes3", path = "../exercises/16_lifetimes/lifetimes3.rs" },
   { name = "lifetimes3_sol", path = "../solutions/16_lifetimes/lifetimes3.rs" },
+  { name = "lifetimes4", path = "../exercises/16_lifetimes/lifetimes4.rs" },
+  { name = "lifetimes4_sol", path = "../solutions/16_lifetimes/lifetimes4.rs" },
   { name = "tests1", path = "../exercises/17_tests/tests1.rs" },
   { name = "tests1_sol", path = "../solutions/17_tests/tests1.rs" },
   { name = "tests2", path = "../exercises/17_tests/tests2.rs" },

exercises/16_lifetimes/lifetimes4.rs

@@ -0,0 +1,50 @@
+// Sometimes, we have structs which hold on to data temporarily. A use-case of
+// this could be a routing component which accepts a connection and returns it to
+// another recipient. To avoid copying the data, we just accept a reference with
+// lifetime and return this reference later.
+//
+// In the example below, we create a `Router` instance in a limited scope. It
+// accepts a connection reference created in the enclosing scope and returns it.
+// In theory, this should be possible given that the connection reference outlives
+// the scope from which it is returned. However, the borrow checker does not
+// seem to understand it. What can we do about that?
+
+// TODO: Fix the compiler error about `router` not living long enough without changing `main`
+
+struct Router<'a> {
+    connection: Option<&'a u64>,
+}
+
+impl<'a> Router<'a> {
+    fn new() -> Self {
+        Self { connection: None }
+    }
+
+    fn accept_connection(&mut self, connection: &'a u64) {
+        self.connection = Some(connection);
+    }
+
+    fn return_connection(&mut self) -> Option<&u64> {
+        self.connection.take()
+    }
+}
+
+// Do not change `main`
+fn main() {
+    let connection = &123;
+
+    let returned_connection = {
+        // Create router within scope
+        let mut router = Router::new();
+
+        // Accept connection which lives longer than the router
+        router.accept_connection(connection);
+
+        // Return connection which **should** live longer than the router
+        router.return_connection()
+    };
+
+    if let Some(connection) = returned_connection {
+        println!("The connection is {connection}");
+    }
+}

rustlings-macros/info.toml

@@ -832,6 +832,21 @@ dir = "16_lifetimes"
 test = false
 hint = """Let the compiler guide you :)"""
 
+[[exercises]]
+name = "lifetimes4"
+dir = "16_lifetimes"
+test = false
+hint = """
+Lifetimes can be elided if the compiler can infer a sensible default choice:
+https://doc.rust-lang.org/reference/lifetime-elision.html
+
+In most cases, the default choice is correct.
+But occasionally, the default choice does not follow our intent.
+Can you spot an elided lifetime which might not be correct?
+You can think about what the compiler's default choice is for every elided lifetime.
+Where could you add an explicit lifetime annotation to better express what we want?
+"""
+
 # TESTS
 
 [[exercises]]

solutions/16_lifetimes/lifetimes4.rs

@@ -0,0 +1,60 @@
+// Sometimes, we have structs which hold on to data temporarily. A use-case of
+// this could be a routing component which accepts a connection and returns it to
+// another recipient. To avoid copying the data, we just accept a reference with
+// lifetime and return this reference later.
+//
+// In the example below, we create a `Router` instance in a limited scope. It
+// accepts a connection reference created in the enclosing scope and returns it.
+// In theory, this should be possible given that the connection reference outlives
+// the scope from which it is returned. However, the borrow checker does not
+// seem to understand it. What can we do about that?
+
+struct Router<'a> {
+    connection: Option<&'a u64>,
+    //                  ^^ lifetime of the connection reference
+    //                     which may outlive the `Router` itself
+}
+
+impl<'a> Router<'a> {
+    fn new() -> Self {
+        Self { connection: None }
+    }
+
+    fn accept_connection(&mut self, connection: &'a u64) {
+        self.connection = Some(connection);
+    }
+
+    fn return_connection(&mut self) -> Option<&'a u64> {
+        //           added lifetime annotation ^^
+        //
+        //           Without annotation, the compiler infers the output reference
+        //           to have the lifetime of the only input reference
+        //           -> the lifetime of `&mut self`.
+        self.connection.take()
+    }
+}
+
+fn main() {
+    let connection = &123;
+
+    let returned_connection = {
+        // Create router within scope
+        let mut router = Router::new();
+
+        // Accept connection which lives longer than the router
+        router.accept_connection(connection);
+
+        // Return connection which **should** live longer than the router
+        router.return_connection()
+        //    ^^^^^^^^^^^^^^^^^^^^
+        //
+        //    Without the explicit lifetime annotation in `return_connection`,
+        //    the reference from `return_connection` has the lifetime of `router`.
+        //    We are returning the reference from the scope, requiring it to outlive it,
+        //    so the compiler complains about `router` not living long enough.
+    };
+
+    if let Some(connection) = returned_connection {
+        println!("The connection is {connection}");
+    }
+}