From 1b0be30908983946b2492150ba99e8e34f128eb1 Mon Sep 17 00:00:00 2001
From: Wataru Otsubo <wataru.otsubo@cern.ch>
Date: Sat, 7 Jun 2025 05:04:39 +0900
Subject: [PATCH] update(generator): use u8 and u16 for register value when
 possible

---
 CHANGELOG.md                      |  1 +
 src/generator.rs                  | 15 +++++++++++
 src/generator/codegen_register.rs | 41 +++++++++++++++++++------------
 3 files changed, 41 insertions(+), 16 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0f7d9a0..b256eb0 100644
--- a/CHANGELOG.md
+++ b/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
 
diff --git a/src/generator.rs b/src/generator.rs
index 94ea55f..83d3dfd 100644
--- a/src/generator.rs
+++ b/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(),
diff --git a/src/generator/codegen_register.rs b/src/generator/codegen_register.rs
index 931ed3a..4936ad9 100644
--- a/src/generator/codegen_register.rs
+++ b/src/generator/codegen_register.rs
@@ -71,26 +71,35 @@ fn reg_type_def_masked(
         let x: RustUxTypes = basetype.into();
         x.to_rust_type_token()
     };
+    let type_t = RustUxTypes::from_exact_mask(mask);
 
-    let out = quote! {
-        #[derive(Debug, Clone, Copy, Default)]
-        pub struct #upper_camel_name(pub #type_ux);
-        impl TryFrom<#type_ux> for #upper_camel_name {
-            type Error = crate::register_spec::DataConversionError<#type_ux, Self>;
+    // 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);
+                impl TryFrom<#type_ux> for #upper_camel_name {
+                    type Error = crate::register_spec::DataConversionError<#type_ux, Self>;
 
-            fn try_from(value: #type_ux) -> Result<Self, Self::Error> {
-                Ok(Self(value & #mask))
-            }
+                    fn try_from(value: #type_ux) -> Result<Self, Self::Error> {
+                        Ok(Self(value & #mask))
+                    }
+                }
+                impl From<#upper_camel_name> for #type_ux {
+                    fn from(value: #upper_camel_name) -> Self {
+                        value.0
+                    }
+                }
+            };
+            (out, upper_camel_name.clone(), type_ux)
         }
-        impl From<#upper_camel_name> for #type_ux {
-            fn from(value: #upper_camel_name) -> Self {
-                value.0
-            }
+        Some(type_t) => {
+            let out = quote! {};
+            (out, type_t.to_rust_type_token(), type_ux)
         }
-
-    };
-
-    (out, upper_camel_name.clone(), type_ux)
+    }
 }
 
 /// Where `T` has fields.