Use default attribute when constructing skipped values

crates/musli-macros/src/de.rs

@@ -7,7 +7,7 @@ use syn::Token;
 use crate::expander::{Result, TagMethod};
 use crate::internals::apply;
 use crate::internals::attr::{EnumTag, EnumTagging, Packing};
-use crate::internals::build::{Body, Build, BuildData, Enum, Field, FieldSkip, Variant};
+use crate::internals::build::{Body, Build, BuildData, Enum, Field, Variant};
 use crate::internals::tokens::Tokens;
 
 struct Ctxt<'a> {
@@ -765,11 +765,14 @@ fn decode_tagged(
         let decode_path = &f.decode_path.1;
 
         let expr = match &f.skip {
-            Some(FieldSkip::Default(span)) => {
+            Some(span) => {
                 let ty = f.ty;
-                syn::Expr::Verbatim(quote_spanned!(*span => #default_function::<#ty>()))
+
+                match &f.default_attr {
+                    Some((_, Some(path))) => syn::Expr::Verbatim(quote_spanned!(*span => #path())),
+                    _ => syn::Expr::Verbatim(quote_spanned!(*span => #default_function::<#ty>())),
+                }
             }
-            Some(FieldSkip::Expr(expr)) => expr.clone(),
             None => {
                 let formatted_tag = match &st.name_format_with {
                     Some((_, path)) => quote!(&#path(&#tag)),

crates/musli-macros/src/internals/attr.rs

@@ -12,8 +12,6 @@ use crate::expander::TagMethod;
 use crate::internals::ATTR;
 use crate::internals::{Ctxt, Mode};
 
-use super::build::FieldSkip;
-
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub(crate) enum Only {
     Encode,
@@ -575,7 +573,7 @@ layer! {
         /// Use a default value for the field if it's not available.
         is_default: Option<syn::Path>,
         /// Use a default value for the field if it's not available.
-        skip: FieldSkip,
+        skip: (),
         /// Use the alternate TraceDecode for the field.
         trace: (),
         /// Use the alternate EncodeBytes for the field.
@@ -706,15 +704,9 @@ pub(crate) fn field_attrs(cx: &Ctxt, attrs: &[syn::Attribute]) -> Field {
                 return Ok(());
             }
 
-            // parse #[musli(skip [= <path>])]
+            // parse #[musli(skip)]
             if meta.path.is_ident("skip") {
-                let skip = if meta.input.parse::<Option<Token![=]>>()?.is_some() {
-                    FieldSkip::Expr(meta.input.parse()?)
-                } else {
-                    FieldSkip::Default(meta.path.span())
-                };
-
-                new.skip.push((meta.path.span(), skip));
+                new.skip.push((meta.path.span(), ()));
                 return Ok(());
             }
 

crates/musli-macros/src/internals/build.rs

@@ -163,26 +163,18 @@ pub(crate) struct Variant<'a> {
     pub(crate) patterns: Punctuated<syn::FieldPat, Token![,]>,
 }
 
-/// Field skip configuration.
-pub(crate) enum FieldSkip {
-    /// Skip with a default value.
-    Default(Span),
-    /// Skip with an expression.
-    Expr(syn::Expr),
-}
-
 pub(crate) struct Field<'a> {
     pub(crate) span: Span,
     pub(crate) index: usize,
     pub(crate) encode_path: (Span, syn::Path),
     pub(crate) decode_path: (Span, syn::Path),
     pub(crate) tag: syn::Expr,
+    /// Skip field entirely and always initialize with the specified expresion,
+    /// or default value through `default_attr`.
+    pub(crate) skip: Option<Span>,
     pub(crate) skip_encoding_if: Option<&'a (Span, syn::Path)>,
     /// Fill with default value, if missing.
     pub(crate) default_attr: Option<(Span, Option<&'a syn::Path>)>,
-    /// Skip field entirely and always initialize with the specified expresion,
-    /// or default value if none is specified.
-    pub(crate) skip: Option<&'a FieldSkip>,
     pub(crate) self_access: syn::Expr,
     pub(crate) member: syn::Member,
     pub(crate) packing: Packing,
@@ -419,12 +411,12 @@ fn setup_field<'a>(
     let decode_path = data.attr.decode_path_expanded(mode, data.span);
     let (tag, tag_method) = data.expand_tag(e, mode, default_field)?;
     tag_methods.insert(data.span, tag_method);
+    let skip = data.attr.skip(mode).map(|&(s, ())| s);
     let skip_encoding_if = data.attr.skip_encoding_if(mode);
     let default_attr = data
         .attr
         .is_default(mode)
         .map(|(s, path)| (*s, path.as_ref()));
-    let skip = data.attr.skip(mode).map(|(_, skip)| skip);
 
     let member = match data.ident {
         Some(ident) => syn::Member::Named(ident.clone()),
@@ -508,9 +500,9 @@ fn setup_field<'a>(
         encode_path,
         decode_path,
         tag,
+        skip,
         skip_encoding_if,
         default_attr,
-        skip,
         self_access,
         member,
         packing,

crates/musli/src/derives.rs

@@ -533,10 +533,14 @@
 //! struct Struct {
 //!     #[musli(rename = "other")]
 //!     something: String,
-//!     #[musli(skip = 42)]
+//!     #[musli(skip, default = default_field)]
 //!     skipped_field: u32,
 //! }
 //!
+//! fn default_field() -> u32 {
+//!     42
+//! }
+//!
 //! #[derive(Encode, Decode)]
 //! #[musli(default_field = "name")]
 //! enum Enum {
@@ -563,17 +567,27 @@
 //!     name: String,
 //!     #[musli(skip)]
 //!     age: Option<u32>,
-//!     #[musli(skip = Some(String::from("Earth")))]
+//!     #[musli(skip, default = default_country)]
 //!     country: Option<String>,
 //! }
+//!
+//! fn default_country() -> Option<String> {
+//!     Some(String::from("Earth"))
+//! }
 //! ```
 //!
 //! <br>
 //!
-//! #### `#[musli(default)]`
+//! #### `#[musli(default [= <path>])]`
 //!
-//! This constructs the field using [`Default::default`] in case it's not
-//! available. This is only used when a field is missing during decoding.
+//! When a field is absent or disabled with `#[musli(skip)]`, this attribute
+//! specifies that a default value should be used instead.
+//!
+//! If `#[musli(default)]` is specified, the default value is constructed using
+//! [`Default::default`].
+//!
+//! If `#[musli(default = <path>)]` is specified, the default value is
+//! constructed by calling the function at `<path>`.
 //!
 //! ```
 //! use musli::{Encode, Decode};
@@ -585,11 +599,17 @@
 //!     age: Option<u32>,
 //!     #[musli(default = default_height)]
 //!     height: Option<u32>,
+//!     #[musli(skip, default = default_meaning)]
+//!     meaning: u32,
 //! }
 //!
 //! fn default_height() -> Option<u32> {
 //!     Some(180)
 //! }
+//!
+//! fn default_meaning() -> u32 {
+//!     42
+//! }
 //! ```
 //!
 //! <br>