From b9fcefa043746608e943ae01634332d2a8ee1f13 Mon Sep 17 00:00:00 2001
From: Andrii Kuteiko <kuteiko@amazon.com>
Date: Fri, 22 Jan 2021 23:55:58 +0100
Subject: [PATCH] Add an option to extend collection-like fields

---
 derive_builder/tests/setter_extend.rs         | 43 +++++++++++++++++++
 .../src/macro_options/darling_opts.rs         |  8 ++++
 derive_builder_core/src/setter.rs             | 29 ++++++++++++-
 3 files changed, 78 insertions(+), 2 deletions(-)
 create mode 100644 derive_builder/tests/setter_extend.rs

diff --git a/derive_builder/tests/setter_extend.rs b/derive_builder/tests/setter_extend.rs
new file mode 100644
index 00000000..c9e0ea20
--- /dev/null
+++ b/derive_builder/tests/setter_extend.rs
@@ -0,0 +1,43 @@
+#[macro_use]
+extern crate pretty_assertions;
+#[macro_use]
+extern crate derive_builder;
+
+use std::collections::HashMap;
+
+#[derive(Debug, PartialEq, Default, Builder, Clone)]
+struct Lorem {
+    #[builder(extend)]
+    foo: String,
+    #[builder(extend)]
+    bar: Vec<String>,
+    #[builder(extend)]
+    baz: HashMap<String, i32>,
+}
+
+#[test]
+fn generic_field() {
+    let x = LoremBuilder::default()
+        .foo("foo".into())
+        .bar_extend_one("bar".into())
+        .bar_extend_one("bar bar".into())
+        .bar_extend_one("bar bar bar".into())
+        .foo_extend_one('-')
+        .baz_extend_one(("baz".into(), 1))
+        .baz_extend_one(("bazz".into(), 2))
+        .baz_extend_one(("bazzz".into(), 3))
+        .foo_extend_one("foo")
+        .build()
+        .unwrap();
+
+    assert_eq!(
+        x,
+        Lorem {
+            foo: "foo-foo".into(),
+            bar: vec!["bar".into(), "bar bar".into(), "bar bar bar".into()],
+            baz: vec![("baz".into(), 1), ("bazz".into(), 2), ("bazzz".into(), 3)]
+                .into_iter()
+                .collect(),
+        }
+    );
+}
diff --git a/derive_builder_core/src/macro_options/darling_opts.rs b/derive_builder_core/src/macro_options/darling_opts.rs
index 5c59a729..8ab85816 100644
--- a/derive_builder_core/src/macro_options/darling_opts.rs
+++ b/derive_builder_core/src/macro_options/darling_opts.rs
@@ -238,6 +238,8 @@ pub struct Field {
     try_setter: Flag,
     #[darling(default)]
     field: FieldMeta,
+    #[darling(default)]
+    extend: Flag,
 }
 
 impl FlagVisibility for Field {
@@ -442,6 +444,11 @@ impl<'a> FieldWithDefaults<'a> {
             .unwrap_or(true)
     }
 
+    /// Check if this field should emit a setter for extend.
+    pub fn setter_extend_enabled(&self) -> bool {
+        self.field.extend.into()
+    }
+
     pub fn field_enabled(&self) -> bool {
         self.field
             .setter
@@ -555,6 +562,7 @@ impl<'a> FieldWithDefaults<'a> {
             generic_into: self.setter_into(),
             strip_option: self.setter_strip_option(),
             deprecation_notes: self.deprecation_notes(),
+            extend: self.setter_extend_enabled(),
         }
     }
 
diff --git a/derive_builder_core/src/setter.rs b/derive_builder_core/src/setter.rs
index 3f8fcb04..48c22f25 100644
--- a/derive_builder_core/src/setter.rs
+++ b/derive_builder_core/src/setter.rs
@@ -62,6 +62,8 @@ pub struct Setter<'a> {
     pub strip_option: bool,
     /// Emit deprecation notes to the user.
     pub deprecation_notes: &'a DeprecationNotes,
+    /// Emit extend
+    pub extend: bool,
 }
 
 impl<'a> ToTokens for Setter<'a> {
@@ -135,7 +137,8 @@ impl<'a> ToTokens for Setter<'a> {
                     let mut new = #self_into_return_ty;
                     new.#field_ident = ::derive_builder::export::core::option::Option::Some(#into_value);
                     new
-            }));
+                }
+            ));
 
             if self.try_setter {
                 let try_ty_params =
@@ -151,7 +154,28 @@ impl<'a> ToTokens for Setter<'a> {
                         let mut new = #self_into_return_ty;
                         new.#field_ident = ::derive_builder::export::core::option::Option::Some(converted);
                         Ok(new)
-                }));
+                    }
+                ));
+            }
+
+            if self.extend {
+                let ident_extend_one = format_ident!("{}_extend_one", self.ident);
+
+                tokens.append_all(quote!(
+                    #(#attrs)*
+                    #[allow(unused_mut)]
+                    #vis fn #ident_extend_one <ITEM>(#self_param, item: ITEM) -> #return_ty
+                    where
+                        #ty: ::derive_builder::export::core::default::Default + ::derive_builder::export::core::iter::Extend<ITEM>,
+                    {
+                        #deprecation_notes
+                        let mut new = #self_into_return_ty;
+                        new.#field_ident
+                            .get_or_insert_with(|| ::derive_builder::export::core::default::Default::default())
+                            .extend(::derive_builder::export::core::option::Option::Some(item));
+                        new
+                    }
+                ));
             }
         }
     }
@@ -222,6 +246,7 @@ macro_rules! default_setter {
             generic_into: false,
             strip_option: false,
             deprecation_notes: &Default::default(),
+            extend: false,
         };
     };
 }