Merge #251

251: Note on transformation of static variables by attribute exception r=therealprof a=hyperslv

Hi All
The "// `COUNT` has type `&mut u32` and it's safe to use" comment confused me since I did not expect that transformation of the source code so deep comparing with the C language.
Finally I found how it (works)[https://docs.rs/cortex-m-rt-macros/0.1.5/src/cortex_m_rt_macros/lib.rs.html#447].
This PR is to provide a note about the transformation.
Thanks, Slava

Co-authored-by: hyperslv <hyper.slv@gmail.com>
Co-authored-by: hyperslv <57301860+hyperslv@users.noreply.github.com>

Author: bors[bot]

src/start/exceptions.md

@@ -31,7 +31,7 @@ This behavior is pretty much intended and it's required to provide a feature:
 fn SysTick() {
     static mut COUNT: u32 = 0;
 
-    // `COUNT` has type `&mut u32` and it's safe to use
+    // `COUNT` has transformed to type `&mut u32` and it's safe to use
     *COUNT += 1;
 }
 ```
@@ -47,6 +47,12 @@ use `static mut` variables. How is this possible? This is possible because
 `exception` handlers can *not* be called by software thus reentrancy is not
 possible.
 
+> Note that the `exception` attribute transforms definitions of static variables
+> inside the function by wrapping them into `unsafe` blocks and providing us
+> with new appropriate variables of type `&mut` of the same name.
+> Thus we can derefence the reference via `*` to access the values of the variables without
+> needing to wrap them in an `unsafe` block.
+
 ## A complete example
 
 Here's an example that uses the system timer to raise a `SysTick` exception