Move generation of from_word body for unit variants

Author: TedDriggs

core/src/codegen/from_meta_impl.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use proc_macro2::TokenStream;
 use quote::{quote, quote_spanned, ToTokens};
 use syn::spanned::Spanned;
@@ -8,15 +10,15 @@ use crate::util::Callable;
 
 pub struct FromMetaImpl<'a> {
     pub base: TraitImpl<'a>,
-    pub from_word: Option<&'a Callable>,
+    pub from_word: Option<Cow<'a, Callable>>,
     pub from_none: Option<&'a Callable>,
 }
 
 impl ToTokens for FromMetaImpl<'_> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let base = &self.base;
 
-        let from_word = self.from_word.map(|body| {
+        let from_word = self.from_word.as_ref().map(|body| {
             quote_spanned! {body.span()=>
                 fn from_word() -> ::darling::Result<Self> {
                     ::darling::export::identity::<fn() -> ::darling::Result<Self>>(#body)()
@@ -113,23 +115,6 @@ impl ToTokens for FromMetaImpl<'_> {
                     }
                 };
 
-                let from_word_method = variants
-                    .iter()
-                    .find_map(|variant| {
-                        if variant.word {
-                            let ty_ident = variant.ty_ident;
-                            let variant_ident = variant.variant_ident;
-                            Some(quote! {
-                                fn from_word() -> ::darling::Result<Self> {
-                                    ::darling::export::Ok(#ty_ident::#variant_ident)
-                                }
-                            })
-                        } else {
-                            None
-                        }
-                    })
-                    .or(from_word);
-
                 let data_variants = variants.iter().map(Variant::as_data_match_arm);
 
                 quote!(
@@ -159,7 +144,7 @@ impl ToTokens for FromMetaImpl<'_> {
                         }
                     }
 
-                    #from_word_method
+                    #from_word
 
                     #from_none
                 )

core/src/codegen/variant.rs

@@ -26,10 +26,6 @@ pub struct Variant<'a> {
     /// Whether or not the variant should be skipped in the generated code.
     pub skip: bool,
 
-    /// Whether or not the variant should be used to create an instance for
-    /// `FromMeta::from_word`.
-    pub word: bool,
-
     pub allow_unknown_fields: bool,
 }
 

core/src/options/from_meta.rs

@@ -1,5 +1,8 @@
+use std::borrow::Cow;
+
 use proc_macro2::TokenStream;
 use quote::ToTokens;
+use syn::parse_quote;
 
 use crate::ast::Data;
 use crate::codegen::FromMetaImpl;
@@ -26,6 +29,29 @@ impl FromMetaOptions {
         .parse_attributes(&di.attrs)?
         .parse_body(&di.data)
     }
+
+    /// Get the `from_word` method body, if one exists. This can come from direct use of
+    /// `#[darling(from_word = ...)]` on the container or from use of `#[darling(word)]` on
+    /// a unit variant.
+    fn from_word(&self) -> Option<Cow<'_, Callable>> {
+        self.from_word.as_ref().map(Cow::Borrowed).or_else(|| {
+            if let Data::Enum(ref variants) = self.base.data {
+                // The first variant which has `word` set to `true`.
+                // This assumes that validation has prevented multiple variants
+                // from claiming `word`.
+                let variant = variants
+                    .iter()
+                    .find(|v| v.word.map(|x| *x).unwrap_or_default())?;
+                let variant_ident = &variant.ident;
+                let closure: syn::ExprClosure = parse_quote! {
+                    || ::darling::export::Ok(Self::#variant_ident)
+                };
+                Some(Cow::Owned(Callable::from(closure)))
+            } else {
+                None
+            }
+        })
+    }
 }
 
 impl ParseAttribute for FromMetaOptions {
@@ -109,7 +135,7 @@ impl<'a> From<&'a FromMetaOptions> for FromMetaImpl<'a> {
     fn from(v: &'a FromMetaOptions) -> Self {
         FromMetaImpl {
             base: (&v.base).into(),
-            from_word: v.from_word.as_ref(),
+            from_word: v.from_word(),
             from_none: v.from_none.as_ref(),
         }
     }

core/src/options/input_variant.rs

@@ -8,7 +8,7 @@ use crate::{Error, FromMeta, Result};
 
 #[derive(Debug, Clone)]
 pub struct InputVariant {
-    ident: syn::Ident,
+    pub ident: syn::Ident,
     attr_name: Option<String>,
     data: Fields<InputField>,
     skip: Option<bool>,
@@ -30,7 +30,6 @@ impl InputVariant {
                 .map_or_else(|| Cow::Owned(self.ident.to_string()), Cow::Borrowed),
             data: self.data.as_ref().map(InputField::as_codegen_field),
             skip: self.skip.unwrap_or_default(),
-            word: *self.word.unwrap_or_default(),
             allow_unknown_fields: self.allow_unknown_fields.unwrap_or_default(),
         }
     }

core/src/util/callable.rs

@@ -24,6 +24,22 @@ impl AsRef<syn::Expr> for Callable {
     }
 }
 
+impl From<syn::ExprPath> for Callable {
+    fn from(value: syn::ExprPath) -> Self {
+        Self {
+            call: syn::Expr::Path(value),
+        }
+    }
+}
+
+impl From<syn::ExprClosure> for Callable {
+    fn from(value: syn::ExprClosure) -> Self {
+        Self {
+            call: syn::Expr::Closure(value),
+        }
+    }
+}
+
 impl From<Callable> for syn::Expr {
     fn from(value: Callable) -> Self {
         value.call