Generate safe bits writers when possible

The 'bits' method of register writers can be safe if:
* there is a single field that covers the entire register
* that field can represent all values

Author: newAM

CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Bumped `xtensa-lx` and add `xtensa_lx::interrupt::InterruptNumber` implementation.
 - Don't use a mask when the width of the mask is the same as the width of the parent register.
 - Improved error handling
+- Registers with single fields that span the entire register now generate safe `bits` writers.
 
 ## [v0.19.0] - 2021-05-26
 

src/generate/register.rs

@@ -168,10 +168,28 @@ pub fn render(
 
         mod_items.extend(w_impl_items);
 
+        // the writer can be safe if:
+        // * there is a single field that covers the entire register
+        // * that field can represent all values
+        let unsafefn: Option<Ident> = register
+            .fields
+            .as_ref()
+            .and_then(|fields| fields.iter().next())
+            .and_then(|field| field.write_constraint)
+            .and_then(|constraint| match constraint {
+                WriteConstraint::Range(range)
+                    if range.min == 0 && range.max == 2_u64.pow(rsize) - 1 =>
+                {
+                    None
+                }
+                _ => Some(()),
+            })
+            .map(|_| Ident::new("unsafe", span));
+
         mod_items.extend(quote! {
             #[doc = "Writes raw bits to the register."]
             #[inline(always)]
-            pub unsafe fn bits(&mut self, bits: #rty) -> &mut Self {
+            pub #unsafefn fn bits(&mut self, bits: #rty) -> &mut Self {
                 self.0.bits(bits);
                 self
             }