feat: improved closure docs and exercises by using into_iter()

exercises/closures/closures5.rs

@@ -21,6 +21,9 @@ fn sum_letters(animals: &Vec<&str>) -> usize {
     // pay close attention to the where clause in the function signatures
     // https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.reduce
     // https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.fold
+    // and the return types of the different ways of getting an iterator
+    // https://doc.rust-lang.org/stable/std/iter/
+    // https://doc.rust-lang.org/std/iter/trait.IntoIterator.html#tymethod.into_iter
     // TODO: change the next 2 lines to compile and pass the test.
     let sum_closure = |x: &usize, y: &usize| &(x + y);
     animals.iter().reduce(sum_closure).unwrap().to_owned()

exercises/closures/closures7.rs

@@ -20,7 +20,7 @@ fn main() {
     let maybe = Some("yes");
     let list = vec![maybe];
 
-    list.iter().inspect(|&Some(value)| {
+    list.into_iter().inspect(|Some(value)| {
         println!("maybe {:?}", value);
     });
 }

info.toml

@@ -178,13 +178,15 @@ path = "exercises/closures/closures5.rs"
 mode = "test"
 hint = """
 Getting the signatures of closures right can be tough. I don't think you can write
-any version that fulfills the tests using 'reduce()': you can never return a borrow
+any version that fulfills the tests using 'reduce()' with 'iter()': you can never return a borrow
 of a value owned by a called function.
-You need to replace 'reduce()' with a different function call, update the closure
+You can replace 'reduce()' with a different function call. Update the closure
 so that it matches the signature of this new call, and change the code to return
 the correct type from the sum_letters function. (Using 'sum()' is cheating!)
+OR you can change the sort of iterator you get, so that you don't have to return a reference.
 Pay close attention to the documentation linked to in the exercise: which type
-parameters are the same in the function signatures, and which are different."""
+parameters are the same in the function signatures, and which are different. And
+how do the return types of the variations of getting an iterator differ from each other?"""
 
 [[exercises]]
 name = "closures6"