Apply field doc-comments to builder method

Signed-off-by: Markus Mayer <widemeadows@gmail.com>

examples/example.rs

@@ -2,12 +2,17 @@ use typed_builder::TypedBuilder;
 
 #[derive(Debug, PartialEq, TypedBuilder)]
 struct Foo {
-    // Mandatory Field:
+    /// `x` value.
+    ///
+    /// This field is mandatory.
     x: i32,
 
     // #[builder(default)] without parameter - use the type's default
     // #[builder(setter(strip_option))] - wrap the setter argument with `Some(...)`
-    #[builder(default, setter(strip_option))]
+    #[builder(
+        default,
+        setter(strip_option, doc = "Set `y`. If you don't specify a value it'll default to no value.",)
+    )]
     y: Option<i32>,
 
     // Or you can set the default

tests/no_std.rs

@@ -52,10 +52,13 @@ fn test_into() {
 fn test_default() {
     #[derive(PartialEq, TypedBuilder)]
     struct Foo {
+        /// x value.
         #[builder(default, setter(strip_option))]
         x: Option<i32>,
         #[builder(default = 10)]
+        /// y value.
         y: i32,
+        /// z value.
         #[builder(default = [20, 30, 40])]
         z: [i32; 3],
     }

typed-builder-macro/src/field_info.rs

@@ -115,6 +115,7 @@ pub struct FieldBuilderAttr<'a> {
     pub default: Option<syn::Expr>,
     pub via_mutators: Option<ViaMutators>,
     pub deprecated: Option<&'a syn::Attribute>,
+    pub doc_comments: Vec<&'a syn::Expr>,
     pub setter: SetterSettings,
     /// Functions that are able to mutate fields in the builder that are already set
     pub mutators: Vec<Mutator>,
@@ -153,10 +154,20 @@ impl<'a> FieldBuilderAttr<'a> {
 
                     list
                 }
-                syn::Meta::Path(path) | syn::Meta::NameValue(syn::MetaNameValue { path, .. }) => {
-                    if path_to_single_string(path).as_deref() == Some("deprecated") {
-                        self.deprecated = Some(attr);
-                    };
+                syn::Meta::NameValue(syn::MetaNameValue { path, value, .. }) => {
+                    match path_to_single_string(path).as_deref() {
+                        Some("deprecated") => self.deprecated = Some(attr),
+                        Some("doc") => self.doc_comments.push(value),
+                        _ => continue,
+                    }
+
+                    continue;
+                }
+                syn::Meta::Path(path) => {
+                    match path_to_single_string(path).as_deref() {
+                        Some("deprecated") => self.deprecated = Some(attr),
+                        _ => continue,
+                    }
 
                     continue;
                 }

typed-builder-macro/src/struct_info.rs

@@ -251,11 +251,25 @@ impl<'a> StructInfo<'a> {
         target_generics.push(syn::GenericArgument::Type(target_generics_tuple.into()));
         ty_generics.push(syn::GenericArgument::Type(ty_generics_tuple.into()));
         let (impl_generics, _, where_clause) = generics.split_for_impl();
-        let doc = field.builder_attr.setter.doc.as_ref().map(|doc| quote!(#[doc = #doc]));
+        let doc = if let Some(doc) = field.builder_attr.setter.doc.as_ref() {
+            Some(quote!(#[doc = #doc]))
+        } else if !field.builder_attr.doc_comments.is_empty() {
+            Some(
+                field
+                    .builder_attr
+                    .doc_comments
+                    .iter()
+                    .map(|&line| quote!(#[doc = #line]))
+                    .collect(),
+            )
+        } else {
+            None
+        };
+
         let deprecated = &field.builder_attr.deprecated;
 
         // NOTE: both auto_into and strip_option affect `arg_type` and `arg_expr`, but the order of
-        // nesting is different so we have to do this little dance.
+        // nesting is different, so we have to do this little dance.
         let arg_type = if field.builder_attr.setter.strip_option.is_some() && field.builder_attr.setter.transform.is_none() {
             field
                 .type_from_inside_option()
@@ -320,6 +334,7 @@ impl<'a> StructInfo<'a> {
                 #[deprecated(
                     note = #repeated_fields_error_message
                 )]
+                #doc
                 pub fn #method_name (self, _: #repeated_fields_error_type_name) -> #builder_name <#target_generics> {
                     self
                 }
@@ -354,8 +369,8 @@ impl<'a> StructInfo<'a> {
             let mut generics = self.generics.clone();
             for f in self.included_fields() {
                 if f.builder_attr.default.is_some() || f.builder_attr.via_mutators.is_some() {
-                    // `f` is not mandatory - it does not have it's own fake `build` method, so `field` will need
-                    // to warn about missing `field` whether or not `f` is set.
+                    // `f` is not mandatory - it does not have its own fake `build` method, so `field` will need
+                    // to warn about missing `field` regardless of whether `f` is set.
                     assert!(
                         f.ordinal != field.ordinal,
                         "`required_field_impl` called for optional field {}",