Add #[config(partial_attr(...))] to add attributes for partial type

CC #17

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project will be documented in this file.
 
 
 ## [Unreleased]
+- Add `#[config(partial_attr(...))]` struct attribute to specify attributes for
+  the partial type.
 
 ## [0.2.4] - 2023-07-02
 - Fixed enum deserialization from env values

examples/simple.rs

@@ -15,6 +15,7 @@ struct Conf {
 
 /// Configuring the HTTP server of our app.
 #[derive(Debug, Config)]
+#[config(partial_attr(derive(Clone)))]
 struct Http {
     /// The port the server will listen on.
     #[config(env = "PORT")]

macro/src/gen/mod.rs

@@ -246,6 +246,8 @@ fn gen_partial_mod(input: &ir::Input) -> TokenStream {
         input.name,
     );
 
+    let partial_attrs = &input.partial_attrs;
+
     quote! {
         #[doc = #module_doc]
         #visibility mod #mod_name {
@@ -253,6 +255,7 @@ fn gen_partial_mod(input: &ir::Input) -> TokenStream {
             use super::*;
 
             #[derive(confique::serde::Deserialize)]
+            #( #[ #partial_attrs ])*
             #struct_visibility struct #struct_name {
                 #( #struct_fields, )*
             }

macro/src/ir.rs

@@ -1,10 +1,13 @@
 //! Definition of the intermediate representation.
 
+use proc_macro2::TokenStream;
+
 
 /// The parsed input to the `gen_config` macro.
 pub(crate) struct Input {
     pub(crate) doc: Vec<String>,
     pub(crate) visibility: syn::Visibility,
+    pub(crate) partial_attrs: Vec<TokenStream>,
     pub(crate) name: syn::Ident,
     pub(crate) fields: Vec<Field>,
 }

macro/src/parse.rs

@@ -1,3 +1,4 @@
+use proc_macro2::{TokenStream, Group, Delimiter, Ident};
 use syn::{Error, Token, parse::{Parse, ParseStream}, spanned::Spanned, punctuated::Punctuated};
 
 use crate::{
@@ -17,6 +18,7 @@ impl Input {
         };
 
         let doc = extract_doc(&mut input.attrs);
+        let partial_attrs = extract_struct_attrs(input.attrs)?;
         let fields = fields.named.into_iter()
             .map(Field::from_ast)
             .collect::<Result<Vec<_>, _>>()?;
@@ -25,12 +27,54 @@ impl Input {
         Ok(Self {
             doc,
             visibility: input.vis,
+            partial_attrs,
             name: input.ident,
             fields,
         })
     }
 }
 
+fn extract_struct_attrs(attrs: Vec<syn::Attribute>) -> Result<Vec<TokenStream>, Error> {
+    #[derive(Debug)]
+    enum StructAttr {
+        InternalAttr(TokenStream),
+    }
+
+    impl Parse for StructAttr {
+        fn parse(input: ParseStream) -> syn::Result<Self> {
+            let content;
+            syn::parenthesized!(content in input);
+            assert_empty_or_comma(input)?;
+
+            let name: Ident = content.parse()?;
+            match &*name.to_string() {
+                "partial_attr" => {
+                    let g: Group = content.parse()?;
+                    if g.delimiter() != Delimiter::Parenthesis {
+                        return Err(Error::new_spanned(g,
+                            "expected `(...)` but found different delimiter"));
+                    }
+                    assert_empty_or_comma(&content)?;
+                    Ok(Self::InternalAttr(g.stream()))
+                }
+                _ => Err(Error::new_spanned(name, "unknown attribute")),
+            }
+        }
+    }
+
+    let mut partial_attrs = Vec::new();
+    for attr in attrs {
+        if !attr.path.is_ident("config") {
+            continue;
+        }
+        match syn::parse2::<StructAttr>(attr.tokens)? {
+            StructAttr::InternalAttr(tokens) => partial_attrs.push(tokens),
+        }
+    }
+
+    Ok(partial_attrs)
+}
+
 impl Field {
     fn from_ast(mut field: syn::Field) -> Result<Self, Error> {
         let doc = extract_doc(&mut field.attrs);

src/lib.rs

@@ -326,6 +326,13 @@ pub use crate::{
 ///     the field.. Can only be present if the `env` attribute is present. Also
 ///     see [`env::parse`].
 ///
+/// There are also the following attributes on the struct itself:
+///
+/// - **`#[config(partial_attr(...))]`: specify attributes that should be
+///     attached to the partial struct definition. For example,
+///     `#[config(partial_attr(derive(Clone)))]` can be used to make the partial
+///     type implement `Clone`.
+///
 /// [serde-deser]: https://serde.rs/field-attrs.html#deserialize_with
 ///
 /// ## Special types for leaf fields