Merge pull request #190 from hydra/add-pinning-compilation-error-details

Pinning - Add details of the compiler errors that occur when trying to use get_mut().

examples/06_03_select/src/lib.rs

@@ -140,13 +140,6 @@ async fn get_new_num() -> u8 { /* ... */ 5 }
 
 async fn run_on_new_num(_: u8) -> u8 { /* ... */ 5 }
 
-// Runs `run_on_new_num` with the latest number
-// retrieved from `get_new_num`.
-//
-// `get_new_num` is re-run every time a timer elapses,
-// immediately cancelling the currently running
-// `run_on_new_num` and replacing it with the newly
-// returned value.
 async fn run_loop(
     mut interval_timer: impl Stream<Item = ()> + FusedStream + Unpin,
     starting_num: u8,

src/04_pinning/01_chapter.md

@@ -480,7 +480,27 @@ pub fn main() {
 # }
 ```
 
-The type system prevents us from moving the data.
+The type system prevents us from moving the data, as follows:
+
+```
+error[E0277]: `PhantomPinned` cannot be unpinned
+   --> src\test.rs:56:30
+    |
+56  |         std::mem::swap(test1.get_mut(), test2.get_mut());
+    |                              ^^^^^^^ within `test1::Test`, the trait `Unpin` is not implemented for `PhantomPinned`
+    |
+    = note: consider using `Box::pin`
+note: required because it appears within the type `test1::Test`
+   --> src\test.rs:7:8
+    |
+7   | struct Test {
+    |        ^^^^
+note: required by a bound in `std::pin::Pin::<&'a mut T>::get_mut`
+   --> <...>rustlib/src/rust\library\core\src\pin.rs:748:12
+    |
+748 |         T: Unpin,
+    |            ^^^^^ required by this bound in `std::pin::Pin::<&'a mut T>::get_mut`
+```
 
 > It's important to note that stack pinning will always rely on guarantees
 > you give when writing `unsafe`. While we know that the _pointee_ of `&'a mut T`