update(generator): use u8 and u16 for register value when possible

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Improve error message from syn in generator. !15
 - Removed top level module (register_interface), and now it can have arbitrary name.
 - Use full path `RegisterSpec`s to avoid unused `use`
+- Use `u8` and `u16` for `T` of registers when the mask is 0xff or 0xffff, instead of defining new tuple struct.
 
 ## [0.3.1] - 2025-04-11
 

src/generator.rs

@@ -82,6 +82,21 @@ mod util {
             }
         }
 
+        /// Derive appropriate rust type for right aligned `mask`ed value.
+        pub(super) fn from_exact_mask(mask: u32) -> Option<RustUxTypes> {
+            if mask.trailing_zeros() != 0 {
+                return None;
+            }
+            match 32 - mask.leading_zeros() {
+                0 => panic!("mask cannot be 0"),
+                1 => Some(RustUxTypes::Bool),
+                8 => Some(RustUxTypes::U8),
+                16 => Some(RustUxTypes::U16),
+                32 => Some(RustUxTypes::U32),
+                _ => None,
+            }
+        }
+
         pub(super) fn to_rust_type_token(&self) -> proc_macro2::Ident {
             match self {
                 RustUxTypes::Bool => parse_to_ident("bool").unwrap(),

src/generator/codegen_register.rs

@@ -71,7 +71,12 @@ fn reg_type_def_masked(
         let x: RustUxTypes = basetype.into();
         x.to_rust_type_token()
     };
+    let type_t = RustUxTypes::from_exact_mask(mask);
 
+    // u8, u16 already implements Into and TryFrom with u32
+    // but bool doen't implement TryFrom<u32>, so it cannot be used as `T` here
+    match type_t {
+        None | Some(RustUxTypes::Bool) => {
             let out = quote! {
                 #[derive(Debug, Clone, Copy, Default)]
                 pub struct #upper_camel_name(pub #type_ux);
@@ -87,11 +92,15 @@ fn reg_type_def_masked(
                         value.0
                     }
                 }
-
             };
-
             (out, upper_camel_name.clone(), type_ux)
         }
+        Some(type_t) => {
+            let out = quote! {};
+            (out, type_t.to_rust_type_token(), type_ux)
+        }
+    }
+}
 
 /// Where `T` has fields.
 ///