Auto merge of #6 - japaric:modify, r=japaric

`.modify()`: give access to readable bits of the register

closes #3

Author: homunkulus

README.md

@@ -134,7 +134,7 @@ This is signature of each of these methods:
 ``` rust
 impl Cr2 {
     pub fn modify<F>(&mut self, f: F)
-        where F: FnOnce(&mut Cr2W) -> &mut Cr2W
+        where for<'w> F: FnOnce(&Cr2R, &'w mut Cr2W) -> &'w mut Cr2W
     {
         ..
     }
@@ -209,15 +209,15 @@ Usage looks like this:
 i2c1.cr2.write(|w| w.sadd0(true).sadd1(0b0011110));
 ```
 
-Finally, the `modify` method performs a read-modify-write operation that involves at least a `LDR`
-instruction, a `STR` instruction plus extra instructions to modify the fetched value of the `CR2`
+Finally, the `modify` method performs a read-modify-write operation that involves at least one `LDR`
+instruction, one `STR` instruction plus extra instructions to modify the fetched value of the `CR2`
 register. This method accepts a closure that specifies how the `CR2` register will be modified.
 
 Usage looks like this:
 
 ``` rust
-// Sets the STOP bit of the CR2 register
-i2c1.cr2.modify(|r| r.stop(true));
+// Toggle the STOP bit of the CR2 register and set the START bit
+i2c1.cr2.modify(|r, w| w.stop(!r.stop()).start(true));
 ```
 
 ## License

src/lib.rs

@@ -156,11 +156,13 @@ pub fn gen_register(cx: &ExtCtxt, r: &Register, d: &Defaults) -> Vec<P<Item>> {
             items.push(quote_item!(cx,
                                    impl $name {
                                        pub fn modify<F>(&mut self, f: F)
-                                           where F: FnOnce(&mut $name_w) -> &mut $name_w,
+                                           where for<'w> F: FnOnce(&$name_r, &'w mut $name_w) -> &'w mut $name_w,
                                        {
-                                           let mut rw = $name_w { bits: self.register.read() };
-                                           f(&mut rw);
-                                           self.register.write(rw.bits);
+                                           let bits = self.register.read();
+                                           let r = $name_r { bits: bits };
+                                           let mut w = $name_w { bits: bits };
+                                           f(&r, &mut w);
+                                           self.register.write(w.bits);
                                        }
 
                                        pub fn read(&self) -> $name_r {