Demonstrate "unused variable" warnings in test

The macros may bring into scope constants called "mask" and "offset."
These symbols shadow a caller's similarly-named variables, and result in
a compiler warning. The tests suite promote this to an error to make it
prominent.

If they run, the tests will fail.

tests/macros.rs

@@ -49,3 +49,54 @@ fn register_modify() {
     ral::modify_reg!(self, &register_block, register, field_foo: 6);
     assert_eq!(6 << 10, register_block.register.read());
 }
+
+/// Demonstrates that a local variable, 'mask'
+/// doesn't affect the macro's behavior.
+///
+/// This is the same test as register_modify(), but
+/// with the number '6' in a variable called 'mask'.
+#[deny(warnings)] // Promotes "unused variable: mask" into error. Remove this, and the test fails.
+#[test]
+fn register_unused_mask_modify() {
+    let register_block = zeroed_register_block();
+    register_block.register.write(1 << 10);
+    let mask = 6;
+    ral::modify_reg!(self, &register_block, register, field_foo: mask);
+    assert_eq!(6 << 10, register_block.register.read());
+}
+
+/// Same test as above, but with a variable called
+/// 'offset' instead of 'mask'.
+#[deny(warnings)]
+#[test]
+fn register_unused_offset_modify() {
+    let register_block = zeroed_register_block();
+    register_block.register.write(1 << 10);
+    let offset = 6;
+    ral::modify_reg!(self, &register_block, register, field_foo: offset);
+    assert_eq!(6 << 10, register_block.register.read());
+}
+
+/// Same as above test, but using the 'write' macro
+/// instead of 'modify'.
+#[deny(warnings)]
+#[test]
+fn register_unused_mask_write() {
+    let register_block = zeroed_register_block();
+    register_block.register.write(1 << 10);
+    let mask = 6;
+    ral::write_reg!(self, &register_block, register, field_foo: mask);
+    assert_eq!(6 << 10, register_block.register.read());
+}
+
+/// Same test as above, but with a variable called
+/// 'offset' instead of 'mask'.
+#[deny(warnings)]
+#[test]
+fn register_unused_offset_write() {
+    let register_block = zeroed_register_block();
+    register_block.register.write(1 << 10);
+    let offset = 6;
+    ral::write_reg!(self, &register_block, register, field_foo: offset);
+    assert_eq!(6 << 10, register_block.register.read());
+}