libduckdb-sys: expose DEP_DUCKDB_INCLUDE for downstream C/C++ shims

crates/libduckdb-sys/Cargo.toml

@@ -8,6 +8,12 @@ homepage = { workspace = true }
 keywords = { workspace = true }
 readme = { workspace = true }
 build = "build.rs"
+# `links = "duckdb"` lets the build script publish metadata
+# (`cargo:include`, `cargo:lib_dir`) that downstream crates can read
+# via `DEP_DUCKDB_*` environment variables in their own build scripts.
+# Required for crates that need to compile their own C/C++ code that
+# `#include "duckdb.hpp"` against the bundled headers.
+links = "duckdb"
 categories = ["external-ffi-bindings", "database"]
 description = "Native bindings to the libduckdb library, C API"
 exclude = ["duckdb-sources"]

crates/libduckdb-sys/build.rs

@@ -120,6 +120,16 @@
         #[allow(unused_variables)]
         let header = find_duckdb(out_dir);
 
+        // Publish the resolved include directory so downstream crates
+        // that compile their own C/C++ code can read it from
+        // `DEP_DUCKDB_INCLUDE`. Wrapper-only mode (no explicit dir
+        // from env / vcpkg / pkg-config / download) intentionally
+        // skips the emission — there's no single directory to point
+        // at; the system header is on the default search path already.
+        if let Some(include_dir) = include_dir_for(&header) {
+            println!("cargo:include={include_dir}");
+        }
+
         #[cfg(not(feature = "buildtime_bindgen"))]
         {
             std::fs::copy(
@@ -138,6 +148,21 @@
         }
     }
 
+    /// The `HeaderLocation` enum string conversion appends a filename to
+    /// the path it resolved. For `cargo:include=...` we want the
+    /// directory itself, not a file path. Re-derive it.
+    fn include_dir_for(header: &HeaderLocation) -> Option<String> {
+        match header {
+            HeaderLocation::FromEnvironment => {
+                env::var("DUCKDB_INCLUDE_DIR")
+                    .or_else(|_| env::var("DUCKDB_LIB_DIR"))
+                    .ok()
+            }
+            HeaderLocation::FromPath(path) => Some(path.clone()),
+            HeaderLocation::Wrapper => None,
+        }
+    }
+
     fn link_directive() -> &'static str {
         // If the user specifies DUCKDB_STATIC, do static
         // linking, unless it's explicitly set to 0.

crates/libduckdb-sys/build_bundled_cc.rs

@@ -116,7 +116,15 @@ fn untar_archive(out_dir: &str) {
 pub fn main(out_dir: &str, out_path: &Path) {
     untar_archive(out_dir);
 
-    write_bindings(&Path::new(out_dir).join("duckdb/src/include"), out_path);
+    let include_path = Path::new(out_dir).join("duckdb/src/include");
+    write_bindings(&include_path, out_path);
+
+    // Publish the include directory so downstream crates that compile
+    // their own C/C++ code (e.g. extension shims that #include
+    // "duckdb.hpp") can pick it up from `DEP_DUCKDB_INCLUDE` without
+    // having to glob the target tree. Requires `links = "duckdb"` in
+    // the package manifest.
+    println!("cargo:include={}", include_path.display());
 
     let manifest_file = std::fs::File::open(format!("{out_dir}/duckdb/manifest.json")).expect("manifest file");
     let manifest: Manifest = serde_json::from_reader(manifest_file).expect("reading manifest file");

crates/libduckdb-sys/build_bundled_cmake.rs

@@ -37,7 +37,14 @@ pub fn main(_out_dir: &str, out_path: &Path) {
     println!("cargo:rerun-if-env-changed=CMAKE_CXX_COMPILER_LAUNCHER");
     println!("cargo:rerun-if-env-changed=MACOSX_DEPLOYMENT_TARGET");
 
-    write_bindings(&source_dir.join("src/include"), out_path);
+    let include_path = source_dir.join("src/include");
+    write_bindings(&include_path, out_path);
+
+    // Publish the include directory so downstream crates that compile
+    // their own C/C++ code can read it from `DEP_DUCKDB_INCLUDE`.
+    // See the matching emission in build_bundled_cc.rs.
+    println!("cargo:include={}", include_path.display());
+
     if let Some(configs) = env_var("DUCKDB_EXTENSION_CONFIGS") {
         if !configs.trim().is_empty() {
             panic!(