Merge pull request #1265 from andymac-2/closures-capturing

Capturing changes

Author: marioidival

src/fn/closures/capture.md

@@ -18,37 +18,51 @@ fn main() {
     
     let color = "green";
 
-    // A closure to print `color` which immediately borrows (`&`)
-    // `color` and stores the borrow and closure in the `print`
-    // variable. It will remain borrowed until `print` goes out of
-    // scope. `println!` only requires `by reference` so it doesn't
+    // A closure to print `color` which immediately borrows (`&`) `color` and
+    // stores the borrow and closure in the `print` variable. It will remain
+    // borrowed until `print` is used the last time. 
+    //
+    // `println!` only requires arguments by immutable reference so it doesn't
     // impose anything more restrictive.
     let print = || println!("`color`: {}", color);
 
     // Call the closure using the borrow.
     print();
+
+    // `color` can be borrowed immutably again, because the closure only holds
+    // an immutable reference to `color`. 
+    let _reborrow = &color;
     print();
 
-    let mut count = 0;
+    // A move or reborrow is allowed after the final use of `print`
+    let _color_moved = color;
 
-    // A closure to increment `count` could take either `&mut count`
-    // or `count` but `&mut count` is less restrictive so it takes
-    // that. Immediately borrows `count`.
+
+    let mut count = 0;
+    // A closure to increment `count` could take either `&mut count` or `count`
+    // but `&mut count` is less restrictive so it takes that. Immediately
+    // borrows `count`.
     //
-    // A `mut` is required on `inc` because a `&mut` is stored inside.
-    // Thus, calling the closure mutates the closure which requires
-    // a `mut`.
+    // A `mut` is required on `inc` because a `&mut` is stored inside. Thus,
+    // calling the closure mutates the closure which requires a `mut`.
     let mut inc = || {
         count += 1;
         println!("`count`: {}", count);
     };
 
-    // Call the closure.
-    inc();
+    // Call the closure using a mutable borrow.
     inc();
 
-    //let _reborrow = &mut count;
+    // The closure still mutably borrows `count` because it is called later.
+    // An attempt to reborrow will lead to an error.
+    // let _reborrow = &count; 
     // ^ TODO: try uncommenting this line.
+    inc();
+
+    // The closure no longer needs to borrow `&mut count`. Therefore, it is
+    // possible to reborrow without an error
+    let _count_reborrowed = &mut count; 
+
     
     // A non-copy type.
     let movable = Box::new(3);
@@ -64,7 +78,7 @@ fn main() {
 
     // `consume` consumes the variable so this can only be called once.
     consume();
-    //consume();
+    // consume();
     // ^ TODO: Try uncommenting this line.
 }
 ```
@@ -82,7 +96,7 @@ fn main() {
     println!("{}", contains(&1));
     println!("{}", contains(&4));
 
-    // `println!("There're {} elements in vec", haystack.len());`
+    // println!("There're {} elements in vec", haystack.len());
     // ^ Uncommenting above line will result in compile-time error
     // because borrow checker doesn't allow re-using variable after it
     // has been moved.