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

---
 Cargo.lock     | 35 +++++++++++++++++++++++------------
 devenv.nix     |  6 ------
 src/lib.rs     | 35 +++++++++++++++++++++++++++++------
 tests/basic.rs |  9 +++++++--
 4 files changed, 59 insertions(+), 26 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 2e23344..c98510c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -13,7 +13,7 @@ dependencies = [
  "manyhow",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.119",
 ]
 
 [[package]]
@@ -29,7 +29,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "quote-use",
- "syn",
+ "syn 2.0.119",
 ]
 
 [[package]]
@@ -40,13 +40,13 @@ checksum = "2550f75b8cfac212855f6b1885455df8eaee8fe8e246b647d69146142e016084"
 
 [[package]]
 name = "derive-where"
-version = "1.6.1"
+version = "1.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d08b3a0bcc0d079199cd476b2cae8435016ec11d1c0986c6901c5ac223041534"
+checksum = "2e2b94854e8576378ccda7c8de8a66ed8b4e8acbd2c50ec3418ea6c8aaf4b567"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 3.0.6",
 ]
 
 [[package]]
@@ -56,7 +56,7 @@ dependencies = [
  "attribute-derive",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.119",
 ]
 
 [[package]]
@@ -74,7 +74,7 @@ dependencies = [
  "manyhow-macros",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.119",
 ]
 
 [[package]]
@@ -136,14 +136,14 @@ dependencies = [
  "proc-macro-utils",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.119",
 ]
 
 [[package]]
 name = "smallvec"
-version = "1.15.2"
+version = "1.16.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8ed6a63f02c8539c91a8685a86f4099661ba3da017932f6ebbea6de3f0fa7c90"
+checksum = "ba467056f1b547ed52077911161fc86985becbc60e8e1857c8a144dab0def891"
 
 [[package]]
 name = "syn"
@@ -156,8 +156,19 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "syn"
+version = "3.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8593e8e72159ed2257d083c7a454a85cbf854f37a0966d8d483aff8c8a3ebcee"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
 [[package]]
 name = "unicode-ident"
-version = "1.0.24"
+version = "1.0.26"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
+checksum = "d245f478577f809a851594d02313b640fb437e0bb33866753cff937863096954"
diff --git a/devenv.nix b/devenv.nix
index 04cf216..e3ffe48 100644
--- a/devenv.nix
+++ b/devenv.nix
@@ -16,12 +16,6 @@ in
     cargo-seek
     cargo-nextest
   ];
-  scripts.watcher = {
-    exec = ''
-      watchexec -c -e rs "cargo clippy && cargo test && cargo run"
-    '';
-    packages = [ pkgs.watchexec ];
-  };
   enterShell = ''
       cargo update
     '';
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`]
diff --git a/tests/basic.rs b/tests/basic.rs
index ce3abf8..52212d0 100644
--- a/tests/basic.rs
+++ b/tests/basic.rs
@@ -16,6 +16,8 @@ mod foo {
 
 use foo::Foo;
 
+use crate::foo::FooBuild;
+
 fn main() {
 	let x = Foo::builder().baz(42).bar("hello").bat(5).build();
 	assert_eq!(
@@ -27,8 +29,11 @@ fn main() {
 		}
 	);
 
-	let bar = "hello";
-	let _: Foo = Foo! { bar };
+	let x: Foo = Foo! {
+		baz: 32,
+		bat: 64,
+		..x
+	};
 	assert_eq!(
 		x,
 		Foo {
-- 
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