Repository: TedDriggs/darling (rust)

Issue:
Accept an arbitrary expression that returns a callable object in `#[darling(with)]`
It's not a big issue, but just an inconvenience. Here is an example of this problem that [I have](https://github.com/elastio/bon/blob/eb390012783b7fcf6f68223a30965b81b2b50f5a/bon-macros/src/builder/builder_gen/builder_params.rs#L9-L33).

I need to configure parsing with some parameterization necessary for validation and error messages. Currently to do that you always need to define a separate function item that accepts a `&syn::Meta` and returns a `Result`. It would be better if I could just write an expression that returns a closure to invoke in the `#[darling(with)]` directly like this:

```rust
use darling::FromMeta;

#[derive(FromMeta)]
struct Example {
    #[darling(with = parse_item_params(... config here ...))]
    finish_fn: ItemParams,
   
    // alternatively just pass a closure 
    #[darling(with = |meta| parse_item_params(meta, ... config here ...))]
    builder_type: ItemParams,
}

fn parse_item_params(config: ItemParamsParsing) -> impl FnOnce(&syn::Meta) -> Result<ItemParams> { /**/ }
fn parse_item_params_alt(meta: &syn::Meta, config: ItemParamsParsing) -> Result<ItemParams> { /**/ }
```

The main goal is to provide additional context for the parser function passed to `with`.

Pull request description:
Accept a closure for `with` in lieu of a path for fields
Fixes #309

This has some issues, so it's not yet ready to merge:

1. It doesn't work for all the places where `with` is used.
2. I'm not thrilled with the introduction of new locals to support this, and am wondering if there's a way to write it that better encapsulates the desired behavior.

Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6666686..5add02c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 -  Support `#[darling(with = ...)]` on the `data` field when deriving `FromDeriveInput`. This allows the use of simpler receiver types, such as a `Vec` of enum variants.
 -  Bump version of `proc-macro2` to 1.0.86.
+-  Accept closures for `#[darling(with = ...)]` on fields in `FromDeriveInput`, `FromMeta`, `FromField`, etc. [#309](https://github.com/TedDriggs/darling/issues/309)
 
 ## v0.20.10 (July 9, 2024)
 
diff --git a/Cargo.toml b/Cargo.toml
index 5ac3de0..d474d43 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -28,7 +28,7 @@ syn = "2.0.15"
 
 [target.'cfg(compiletests)'.dev-dependencies]
 rustversion = "1.0.9"
-trybuild = "1.0.38"
+trybuild = "1.0.89"
 
 [features]
 default = ["suggestions"]
diff --git a/core/src/codegen/field.rs b/core/src/codegen/field.rs
index d4f9369..a666422 100644
--- a/core/src/codegen/field.rs
+++ b/core/src/codegen/field.rs
@@ -2,7 +2,7 @@ use std::borrow::Cow;
 
 use proc_macro2::TokenStream;
 use quote::{quote, quote_spanned, ToTokens, TokenStreamExt};
-use syn::{spanned::Spanned, Ident, Path, Type};
+use syn::{spanned::Spanned, Ident, Type};
 
 use crate::codegen::{DefaultExpression, PostfixTransform};
 use crate::usage::{self, IdentRefSet, IdentSet, UsesTypeParams};
@@ -22,7 +22,10 @@ pub struct Field<'a> {
     /// The type of the field in the input.
     pub ty: &'a Type,
     pub default_expression: Option<DefaultExpression<'a>>,
-    pub with_path: Cow<'a, Path>,
+    /// An expression that will be wrapped in a call to [`core::convert::identity`] and
+    /// then used for converting a provided value into the field value _before_ postfix
+    /// transforms are called.
+    pub with_callable: Cow<'a, syn::Expr>,
     pub post_transform: Option<&'a PostfixTransform>,
     pub skip: bool,
     pub multiple: bool,
@@ -156,7 +159,7 @@ impl<'a> ToTokens for MatchArm<'a> {
 
         let name_str = &field.name_in_attr;
         let ident = field.ident;
-        let with_path = &field.with_path;
+        let with_callable = &field.with_callable;
         let post_transform = field.post_transform.as_ref();
 
         // Errors include the location of the bad input, so we compute that here.
@@ -171,7 +174,7 @@ impl<'a> ToTokens for MatchArm<'a> {
             quote!(#name_str)
         };
 
-        // Give darling's generated code the span of the `with_path` so that if the target
+        // Give darling's generated code the span of the `with_callable` so that if the target
         // type doesn't impl FromMeta, darling's immediate user gets a properly-spanned error.
         //
         // Within the generated code, add the span immediately on extraction failure, so that it's
@@ -179,7 +182,11 @@ impl<'a> ToTokens for MatchArm<'a> {
         // The behavior of `with_span` makes this safe to do; if the child applied an
         // even-more-specific span, our attempt here will not overwrite that and will only cost
         // us one `if` check.
-        let extractor = quote_spanned!(with_path.span()=>#with_path(__inner)#post_transform.map_err(|e| e.with_span(&__inner).at(#location)));
+        let extractor = quote_spanned!(with_callable.span()=>
+        ::darling::export::identity::<fn(&::syn::Meta) -> ::darling::Result<_>>(#with_callable)(__inner)
+            #post_transform
+            .map_err(|e| e.with_span(&__inner).at(#location))
+        );
 
         tokens.append_all(if field.multiple {
                 quote!(
diff --git a/core/src/options/input_field.rs b/core/src/options/input_field.rs
index 9b688d8..6569b83 100644
--- a/core/src/options/input_field.rs
+++ b/core/src/options/input_field.rs
@@ -13,7 +13,7 @@ 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 +34,7 @@ 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_callable: self.with.as_ref().map(|w| &w.call).map_or_else(
                 || {
                     Cow::Owned(
                         parse_quote_spanned!(self.ty.span()=> ::darling::FromMeta::from_meta),
@@ -149,7 +149,7 @@ 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(mi)?);
 
             if self.flatten.is_present() {
                 return Err(
@@ -239,3 +239,24 @@ impl ParseAttribute for InputField {
         Ok(())
     }
 }
+
+#[derive(Debug, Clone)]
+pub struct With {
+    /// The callable
+    call: syn::Expr,
+}
+
+impl With {
+    pub fn from_meta(meta: &syn::Meta) -> Result<Self> {
+        if let syn::Meta::NameValue(nv) = meta {
+            match &nv.value {
+                syn::Expr::Path(_) | syn::Expr::Closure(_) => Ok(Self {
+                    call: nv.value.clone(),
+                }),
+                _ => Err(Error::unexpected_expr_type(&nv.value)),
+            }
+        } else {
+            Err(Error::unsupported_format("non-value"))
+        }
+    }
+}
diff --git a/examples/expr_with.rs b/examples/expr_with.rs
index 037ce25..ed20e4f 100644
--- a/examples/expr_with.rs
+++ b/examples/expr_with.rs
@@ -1,4 +1,4 @@
-use darling::{util::parse_expr, FromDeriveInput};
+use darling::{util::parse_expr, FromDeriveInput, FromMeta};
 use syn::{parse_quote, Expr};
 
 #[derive(FromDeriveInput)]
@@ -6,14 +6,21 @@ use syn::{parse_quote, Expr};
 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());
 }
diff --git a/src/lib.rs b/src/lib.rs
index 42004f3..f9ba0ba 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -94,7 +94,7 @@ pub use darling_core::ToTokens;
 /// of the referenced types.
 #[doc(hidden)]
 pub mod export {
-    pub use core::convert::From;
+    pub use core::convert::{identity, From};
     pub use core::default::Default;
     pub use core::option::Option::{self, None, Some};
     pub use core::result::Result::{self, Err, Ok};


Test patch:
diff --git a/compiletests.sh b/compiletests.sh
index a8ce042..52dbc0e 100755
--- a/compiletests.sh
+++ b/compiletests.sh
@@ -1,1 +1,1 @@
-RUSTFLAGS="--cfg=compiletests" cargo +1.65.0 test --test compiletests
\ No newline at end of file
+RUSTFLAGS="--cfg=compiletests" cargo +1.77.0 test --test compiletests
\ No newline at end of file
diff --git a/tests/compile-fail/attrs_with_bad_fn.stderr b/tests/compile-fail/attrs_with_bad_fn.stderr
index a1b2672..c3d7906 100644
--- a/tests/compile-fail/attrs_with_bad_fn.stderr
+++ b/tests/compile-fail/attrs_with_bad_fn.stderr
@@ -15,6 +15,6 @@ note: method defined here
    |     pub fn handle<T>(&mut self, result: Result<T>) -> Option<T> {
    |            ^^^^^^
 help: try wrapping the expression in `Ok`
-   |
-11 |     #[darling(with = Ok(bad_converter))]
-   |                      +++             +
+    |
+11  |     #[darling(with = Ok(bad_converter))]
+    |                      +++             +
diff --git a/tests/compile-fail/with_closure_capture.rs b/tests/compile-fail/with_closure_capture.rs
new file mode 100644
index 0000000..8fada99
--- /dev/null
+++ b/tests/compile-fail/with_closure_capture.rs
@@ -0,0 +1,21 @@
+use darling::{FromDeriveInput, FromMeta};
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(demo))]
+pub struct Receiver {
+    example1: String,
+    #[darling(
+        // This should fail because `example1` is a local that's been captured
+        // from the `FromDeriveInput` impl. That's disallowed because exposing
+        // those internals would make any change to the derived method body a
+        // potentially-breaking change.
+        with = |m| Ok(
+            String::from_meta(m)?.to_uppercase()
+            + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+        ),
+        default
+    )]
+    example2: String,
+}
+
+fn main() {}
diff --git a/tests/compile-fail/with_closure_capture.stderr b/tests/compile-fail/with_closure_capture.stderr
new file mode 100644
index 0000000..fc36752
--- /dev/null
+++ b/tests/compile-fail/with_closure_capture.stderr
@@ -0,0 +1,30 @@
+error[E0308]: mismatched types
+  --> tests/compile-fail/with_closure_capture.rs:12:16
+   |
+12 |           with = |m| Ok(
+   |                  ^ arguments to this function are incorrect
+   |  ________________|
+   | |
+13 | |             String::from_meta(m)?.to_uppercase()
+14 | |             + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+15 | |         ),
+   | |_________^ expected fn pointer, found closure
+   |
+   = note: expected fn pointer `for<'a> fn(&'a syn::Meta) -> Result<String, darling::Error>`
+                 found closure `{closure@$DIR/tests/compile-fail/with_closure_capture.rs:12:16: 12:19}`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> tests/compile-fail/with_closure_capture.rs:14:15
+   |
+14 |             + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+   |               ^^^^^^^^ `example1` captured here
+help: the return type of this call is `{closure@$DIR/tests/compile-fail/with_closure_capture.rs:12:16: 12:19}` due to the type of the argument passed
+  --> tests/compile-fail/with_closure_capture.rs:12:16
+   |
+12 |           with = |m| Ok(
+   |  ________________^
+13 | |             String::from_meta(m)?.to_uppercase()
+14 | |             + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+15 | |         ),
+   | |_________- this argument influences the return type of `{{root}}`
+note: function defined here
+  --> $RUST/core/src/convert/mod.rs
diff --git a/tests/meta_with.rs b/tests/meta_with.rs
new file mode 100644
index 0000000..d4f2821
--- /dev/null
+++ b/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));
+}