fix compile stable

Author: burrbull

src/generate/register.rs

@@ -323,6 +323,7 @@ pub fn fields(
         let pc_r = &f.pc_r;
         let mut pc_w = &f.pc_r;
 
+        let mut base_pc_w = None;
         let mut evs_r = None;
 
         let _pc_r = &f._pc_r;
@@ -358,7 +359,7 @@ pub fn fields(
                     }
                 });
 
-                if let Some(base) = &base {
+                base_pc_w = base.as_ref().map(|base| {
                     let pc = base.field.to_sanitized_upper_case();
                     let base_pc_r = Ident::from(&*format!("{}_A", pc));
                     let base_pc_r = derive_from_base(mod_items, &base, &pc_r, &base_pc_r, f.name);
@@ -367,7 +368,9 @@ pub fn fields(
                         ///Reader of the field
                         pub type #_pc_r = crate::FR<#fty, #base_pc_r>;
                     });
-                }
+
+                    base_pc_r
+                });
 
                 if base.is_none() {
                     let has_reserved_variant = evs.values.len() != (1 << f.width);
@@ -485,7 +488,6 @@ pub fn fields(
                     unsafety = None;
                 }
 
-                let mut base_pc_w = None;
                 if Some(evs) != evs_r.as_ref() {
                     pc_w = &f.pc_w;