From a08d9f1a1afa6ec11bcb99e29b030b1d3a44b63f Mon Sep 17 00:00:00 2001
From: Mica White <botahamec@outlook.com>
Date: Thu, 24 Sep 2026 20:59:43 -0400
Subject: Support base expressions

---
 src/lib.rs | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

(limited to 'src')

diff --git a/src/lib.rs b/src/lib.rs
index 314f813..8024048 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -293,6 +293,14 @@ pub fn derive_builder(input: TokenStream) -> TokenStream {
 
 		#builder_macro
 
+		impl core::convert::From<#struct_name> for #builder_name<#(#trues),*> {
+			fn from(s: #struct_name) -> Self {
+				Self {
+					#(#field_names: Some(s.#field_names),)*
+				}
+			}
+		}
+
 		impl<#(const #const_generics: bool),*> core::default::Default for #builder_name<#(#const_generics),*> {
 			fn default() -> Self {
 				Self {
@@ -305,6 +313,10 @@ pub fn derive_builder(input: TokenStream) -> TokenStream {
 			#builder_visibility fn builder() -> #builder_name<#(#falses),*> {
 				#builder_name::default()
 			}
+
+			#builder_visibility fn into_builder(self) -> #builder_name<#(#trues),*> {
+				#builder_name::from(self)
+			}
 		}
 
 		impl<#(const #const_generics: bool),*> #builder_name<#(#const_generics),*> {
@@ -360,17 +372,28 @@ pub fn derive_builder(input: TokenStream) -> TokenStream {
 pub fn build(input: TokenStream) -> TokenStream {
 	let structure = parse_macro_input!(input as ExprStruct);
 	let struct_name = &structure.path;
+	let base = &structure.rest;
 
 	let field_names = structure.fields.iter().map(|field| &field.member);
 	let field_values = structure.fields.iter().map(|field| &field.expr);
 
-	quote! {
-		#struct_name
-			::builder()
-			#(.#field_names(#field_values))*
-			.build()
+	if let Some(base) = base {
+		quote! {
+			#base
+				.into_builder()
+				#(.#field_names(#field_values))*
+				.build()
+		}
+		.into()
+	} else {
+		quote! {
+			#struct_name
+				::builder()
+				#(.#field_names(#field_values))*
+				.build()
+		}
+		.into()
 	}
-	.into()
 }
 
 /// Creates a declarative macro to construct a type that derives [`Builder`]
-- 
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