fix the patching monkeys

[rust-101.git] / src / part12.rs

diff --git a/src/part12.rs b/src/part12.rs
index c749865dcae7cf366d9b8f40b0c7884043a9dfe3..f186b2aea2267f940c138ccec61842e447baae10 100644 (file)
--- a/src/part12.rs
+++ b/src/part12.rs
@@ -9,6 +9,7 @@ use std::cell::{Cell, RefCell};
 //@ (There's not even an automatic derivation happening for the cases where it would be possible.)
 //@ This restriction propagates up to `Callbacks` itself. What could we do about this?
 
+//@ ## `Rc`
 //@ The solution is to find some way of cloning `Callbacks` without cloning the environments. This can be achieved with
 //@ `Rc<T>`, a *reference-counted* pointer. This is is another example of a smart pointer. You can `clone` an `Rc` as often
 //@ as you want, that doesn't affect the data it contains. It only creates more references to the same data. Once all the
@@ -26,7 +27,7 @@ struct Callbacks {
 
 impl Callbacks {
     pub fn new() -> Self {
-        Callbacks { callbacks: Vec::new() }                         /*@*/
+        Callbacks { callbacks: Vec::new() }
     }
 
     // Registration works just like last time, except that we are creating an `Rc` now.
@@ -37,7 +38,7 @@ impl Callbacks {
     pub fn call(&self, val: i32) {
         // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
         for callback in self.callbacks.iter() {
-            callback(val);                                      /*@*/
+            callback(val);                                          /*@*/
         }
     }
 }
@@ -63,7 +64,7 @@ pub fn main() {
 //@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through
 //@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think
 //@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was
-//@ about replacing one piece of data by something else of the same type. This is called *exterior mutability*. <br/>
+//@ about replacing one piece of data by something else of the same type. This is called *inherited mutability*. <br/>
 //@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key to why this is safe.
 
 // So, let us put our counter in a `Cell`, and replicate the example from the previous part.
@@ -108,11 +109,11 @@ struct CallbacksMut {
 
 impl CallbacksMut {
     pub fn new() -> Self {
-        CallbacksMut { callbacks: Vec::new() }                      /*@*/
+        CallbacksMut { callbacks: Vec::new() }
     }
 
     pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
-        let cell = Rc::new(RefCell::new(callback));
+        let cell = Rc::new(RefCell::new(callback));                 /*@*/
         self.callbacks.push(cell);                                  /*@*/
     }
 
@@ -126,12 +127,13 @@ impl CallbacksMut {
             //@ appropriately updates the number of active borrows.
             //@ 
             //@ Since `call` is the only place that borrows the environments of the closures, we should expect that
-            //@ the check will always succeed. However, this function would still typecheck with an immutable borrow of `self` (since we are
-            //@ relying on the interior mutability of `RefCell`). Under this condition, it could happen that a callback
-            //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself.
-            //@ This is called reentrancy. It would imply that we borrow the closure a second time, and
-            //@ panic at run-time. I hope this also makes it clear that there's absolutely no hope of Rust
-            //@ performing these checks statically, at compile-time: It would have to detect reentrancy!
+            //@ the check will always succeed. However, this is not actually true. Several different `CallbacksMut` could share
+            //@ a callback (as they were created with `clone`), and calling one callback here could trigger calling
+            //@ all callbacks of the other `CallbacksMut`, which would end up calling the initial callback again. This issue is called *reentrancy*,
+            //@ and it can lead to subtle bugs. Here, it would mean that the closure runs twice, each time thinking it has the only
+            //@ mutable borrow of its environment - so it may end up dereferencing a dangling pointer. Ouch! Lucky enough,
+            //@ Rust detects this at run-time and panics once we try to borrow the same environment again. I hope this also makes it
+            //@ clear that there's absolutely no hope of Rust performing these checks statically, at compile-time: It would have to detect reentrancy!
             let mut closure = callback.borrow_mut();
             // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
             // dereference the smart pointer and obtain a mutable borrow of the content.
@@ -156,8 +158,7 @@ fn demo_mut(c: &mut CallbacksMut) {
     c.call(1); c.clone().call(2);
 }
 
-// **Exercise 12.1**: Change the type of `call` to ask only for a shared borrow. Then write some piece of code using only the available, public
-// interface of `CallbacksMut` such that a reentrant call to `call` is happening, and the program aborts because the `RefCell` refuses to hand
-// out a second mutable borrow to its content.
+// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure
+// is happening, and the program aborts because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.
 
 //@ [index](main.html) | [previous](part11.html) | [next](part13.html)