Accept a closure for with in lieu of a path for fields

Fixes #309

core/src/codegen/field.rs

@@ -22,6 +22,9 @@ pub struct Field<'a> {
     /// The type of the field in the input.
     pub ty: &'a Type,
     pub default_expression: Option<DefaultExpression<'a>>,
+    /// Initial declaration for `with`; this is used if `with` was a closure,
+    /// to assign the closure to a local variable.
+    pub with_initializer: Option<syn::Stmt>,
     pub with_path: Cow<'a, Path>,
     pub post_transform: Option<&'a PostfixTransform>,
     pub skip: bool,
@@ -107,6 +110,10 @@ impl<'a> ToTokens for Declaration<'a> {
                 let mut __flatten: Vec<::darling::ast::NestedMeta> = vec![];
             });
         }
+
+        if let Some(stmt) = &self.0.with_initializer {
+            stmt.to_tokens(tokens);
+        }
     }
 }
 

core/src/options/input_field.rs

@@ -1,5 +1,6 @@
 use std::borrow::Cow;
 
+use quote::format_ident;
 use syn::{parse_quote_spanned, spanned::Spanned};
 
 use crate::codegen;
@@ -13,7 +14,7 @@ pub struct InputField {
     pub attr_name: Option<String>,
     pub ty: syn::Type,
     pub default: Option<DefaultExpression>,
-    pub with: Option<syn::Path>,
+    pub with: Option<With>,
 
     /// If `true`, generated code will not look for this field in the input meta item,
     /// instead always falling back to either `InputField::default` or `Default::default`.
@@ -34,7 +35,8 @@ impl InputField {
                 .map_or_else(|| Cow::Owned(self.ident.to_string()), Cow::Borrowed),
             ty: &self.ty,
             default_expression: self.as_codegen_default(),
-            with_path: self.with.as_ref().map_or_else(
+            with_initializer: self.with.as_ref().and_then(With::to_closure_declaration),
+            with_path: self.with.as_ref().map(|w| &w.path).map_or_else(
                 || {
                     Cow::Owned(
                         parse_quote_spanned!(self.ty.span()=> ::darling::FromMeta::from_meta),
@@ -149,7 +151,7 @@ impl ParseAttribute for InputField {
                 return Err(Error::duplicate_field_path(path).with_span(mi));
             }
 
-            self.with = Some(FromMeta::from_meta(mi)?);
+            self.with = Some(With::from_meta(&self.ident, mi)?);
 
             if self.flatten.is_present() {
                 return Err(
@@ -239,3 +241,56 @@ impl ParseAttribute for InputField {
         Ok(())
     }
 }
+
+#[derive(Debug, Clone)]
+pub struct With {
+    /// The path that generated code should use when calling this.
+    path: syn::Path,
+    /// If set, the closure that should be assigned to `path` locally.
+    closure: Option<syn::ExprClosure>,
+}
+
+impl With {
+    pub fn from_meta(field_name: &syn::Ident, meta: &syn::Meta) -> Result<Self> {
+        if let syn::Meta::NameValue(nv) = meta {
+            match &nv.value {
+                syn::Expr::Path(path) => Ok(Self::from(path.path.clone())),
+                syn::Expr::Closure(closure) => Ok(Self {
+                    path: format_ident!("__with_closure_for_{}", field_name).into(),
+                    closure: Some(closure.clone()),
+                }),
+                _ => Err(Error::unexpected_expr_type(&nv.value)),
+            }
+        } else {
+            Err(Error::unsupported_format("non-value"))
+        }
+    }
+
+    /// Create the statement that declares the closure as a function pointer.
+    fn to_closure_declaration(&self) -> Option<syn::Stmt> {
+        self.closure.as_ref().map(|c| {
+            let path = &self.path;
+            // An explicit annotation that the input is borrowed is needed here,
+            // or attempting to pass a closure will fail with an issue about a temporary
+            // value being dropped while still borrowed in the extractor loop.
+            //
+            // Making the parameter type explicit here avoids errors if the closure doesn't
+            // do enough to make the type clear to the compiler.
+            //
+            // The explicit return type is needed, or else using `Ok` and `?` in the closure
+            // body will produce an error about needing type annotations due to uncertainty
+            // about the error variant's type. `T` is left undefined so that postfix transforms
+            // still work as expected
+            parse_quote_spanned!(c.span()=> let #path: fn(&::syn::Meta) -> ::darling::Result<_> = #c;)
+        })
+    }
+}
+
+impl From<syn::Path> for With {
+    fn from(path: syn::Path) -> Self {
+        Self {
+            path,
+            closure: None,
+        }
+    }
+}

examples/expr_with.rs

@@ -1,19 +1,26 @@
-use darling::{util::parse_expr, FromDeriveInput};
+use darling::{util::parse_expr, FromDeriveInput, FromMeta};
 use syn::{parse_quote, Expr};
 
 #[derive(FromDeriveInput)]
 #[darling(attributes(demo))]
 pub struct Receiver {
     #[darling(with = parse_expr::preserve_str_literal, map = Some)]
     example1: Option<Expr>,
+    #[darling(
+        // A closure can be used in lieu of a path.
+        with = |m| Ok(String::from_meta(m)?.to_uppercase()),
+        default
+    )]
+    example2: String,
 }
 
 fn main() {
     let input = Receiver::from_derive_input(&parse_quote! {
-        #[demo(example1 = test::path)]
+        #[demo(example1 = test::path, example2 = "hello")]
         struct Example;
     })
     .unwrap();
 
     assert_eq!(input.example1, Some(parse_quote!(test::path)));
+    assert_eq!(input.example2, "HELLO".to_string());
 }

tests/meta_with.rs

@@ -0,0 +1,33 @@
+use darling::{util::parse_expr, FromDeriveInput, FromMeta};
+use syn::{parse_quote, Expr};
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(demo))]
+pub struct Receiver {
+    #[darling(with = parse_expr::preserve_str_literal, map = Some)]
+    example1: Option<Expr>,
+    #[darling(
+        with = |m| Ok(String::from_meta(m)?.to_uppercase()), 
+        map = Some
+    )]
+    example2: Option<String>,
+    // This is deliberately strange - it keeps the field name, and ignores
+    // the rest of the attribute. In normal operation, this is strongly discouraged.
+    // It's used here to verify that the parameter type is known even if it can't be
+    // inferred from usage within the closure.
+    #[darling(with = |m| Ok(m.path().clone()))]
+    example3: syn::Path,
+}
+
+#[test]
+fn handles_all_cases() {
+    let input = Receiver::from_derive_input(&parse_quote! {
+        #[demo(example1 = test::path, example2 = "hello", example3)]
+        struct Example;
+    })
+    .unwrap();
+
+    assert_eq!(input.example1, Some(parse_quote!(test::path)));
+    assert_eq!(input.example2, Some("HELLO".to_string()));
+    assert_eq!(input.example3, parse_quote!(example3));
+}