Fix unsynn parsing for unit structs in braid macro

The Fields::Unit enum variant was not parseable by unsynn because empty
enum variants have no associated parser. Changed to Fields::Unit(Nothing)
where Nothing is unsynn's type that always succeeds without consuming
tokens, allowing the enum parser to match unit structs like:

  #[braid(serde)]
  pub struct BasicExampleBuf;

Author: fasterthanlime

.gitignore

@@ -1,3 +1,4 @@
 /target
 .idea
 Cargo.lock
+.tracey/

crates/strid-macros/src/codegen/mod.rs

@@ -443,7 +443,7 @@ fn create_field_if_none(fields: &mut crate::grammar::Fields) {
     use crate::grammar::Fields;
 
     // If it's a unit struct, convert it to an unnamed tuple struct with String
-    if matches!(fields, Fields::Unit) {
+    if matches!(fields, Fields::Unit(_)) {
         // Parse a dummy struct to extract the fields structure
         let dummy_struct: proc_macro2::TokenStream = "struct Dummy(String);".parse().unwrap();
         let mut iter = dummy_struct.to_token_iter();
@@ -498,7 +498,7 @@ fn get_field_info<'a>(
             let field = &f.content[0].value;
             Ok((&field.ty, None, &field.attrs))
         }
-        Fields::Unit => {
+        Fields::Unit(_) => {
             Err("unit structs are not supported - struct must have at least one field".to_string())
         }
     }

crates/strid-macros/src/grammar.rs

@@ -121,9 +121,8 @@ unsynn! {
         Named(BraceGroupContaining<CommaDelimitedVec<NamedField>>),
         /// Unnamed fields: `(i32, i32)`
         Unnamed(ParenthesisGroupContaining<CommaDelimitedVec<UnnamedField>>),
-        /// Unit struct (no fields)
-        #[allow(dead_code)]
-        Unit,
+        /// Unit struct (no fields) - Nothing always succeeds without consuming tokens
+        Unit(Nothing),
     }
 
     /// A named field in a struct.
@@ -167,7 +166,7 @@ impl ItemStruct {
         match &self.fields {
             Fields::Named(f) => f.content.is_empty(),
             Fields::Unnamed(f) => f.content.is_empty(),
-            Fields::Unit => true,
+            Fields::Unit(_) => true,
         }
     }
 }