option default_by_default

Author: ModProg

macro/src/lib.rs

@@ -135,9 +135,11 @@ pub fn attribute_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStre
                     }
                 } else {
                     quote! {
-                        #ident: __options.#ident.map(|t| ::attribute_derive::ConvertParsed::convert(t)).ok_or_else(||
-                            #syn::Error::new(#pm2::Span::call_site(), #error)
-                        )??
+                        #ident: match __options.#ident.map(|t| ::attribute_derive::ConvertParsed::convert(t)) {
+                                Some(__option) => __option?,
+                                None if <#ty as ::attribute_derive::ConvertParsed>::default_by_default() => <#ty as ::attribute_derive::ConvertParsed>::default(),
+                                _ => #err(#syn::Error::new(#pm2::Span::call_site(), #error))?,
+                            }
                     }
                 });
 

src/lib.rs

@@ -8,8 +8,8 @@ use std::fmt::Display;
 use proc_macro2::{Literal, Span};
 pub use r#macro::Attribute;
 use syn::{
-    bracketed, parse::Parse, punctuated::Punctuated, Expr, Lit, LitBool,
-    LitByteStr, LitChar, LitFloat, LitInt, LitStr, Path, Result, Token, Type, __private::ToTokens,
+    bracketed, parse::Parse, punctuated::Punctuated, Expr, Lit, LitBool, LitByteStr, LitChar,
+    LitFloat, LitInt, LitStr, Path, Result, Token, Type, __private::ToTokens,
 };
 
 pub mod __private {
@@ -31,6 +31,12 @@ where
 {
     type Type;
     fn convert(value: Self::Type) -> Result<Self>;
+    fn default_by_default() -> bool {
+        false
+    }
+    fn default() -> Self {
+        unreachable!("default_by_default should only return true if this is overridden")
+    }
 }
 
 /// Helper trait to generate sensible errors
@@ -104,6 +110,14 @@ where
     fn convert(s: Parsed) -> Result<Self> {
         Ok(Some(ConvertParsed::convert(s)?))
     }
+
+    fn default_by_default() -> bool {
+        true
+    }
+
+    fn default() -> Self {
+        Default::default()
+    }
 }
 
 impl<Output, Parsed> ConvertParsed for Vec<Output>

tests/derive.rs

@@ -10,13 +10,11 @@ fn test() {
         // a: u8,
         b: LitStr,
         c: String,
-        #[attribute(default)]
         oc: Option<String>,
-        #[attribute(default)]
         od: Option<Type>,
         d: Type,
         e: Expr,
-        f: Vec<Type>
+        f: Vec<Type>,
     }
 
     let parsed = Test::from_attributes([
@@ -74,10 +72,10 @@ fn error() {
 fn error2() {
     #[derive(Attribute, Debug)]
     #[attribute(test)]
+    #[allow(dead_code)]
     struct Test {
-        #[allow(dead_code)]
         a: f32,
-        b: u8
+        b: u8,
     }
 
     assert_eq!(