Add integration for libnixstore

This adds a conditional feature flag for loading data from libnixstore
exposed behind a feature flag dependency

Cargo.toml

@@ -7,5 +7,12 @@ repository = "https://im.badat.dev/bad/narinfo-rs"
 license = "MIT OR Apache-2.0"
 keywords = ["nix", "nixos"]
 
+[features]
+libnixstore = ["dep:libnixstore"]
+
 [dependencies]
 derive_builder = { version = "0.11.2", default-features = false }
+libnixstore = { version = "0.2.0", optional = true }
+
+[package.metadata.docs.rs]
+all-features = true

shell.nix

@@ -0,0 +1,10 @@
+with import <nixpkgs> { };
+mkShell {
+  nativeBuildInputs = [
+    nix
+    nlohmann_json
+    libsodium
+    boost
+    pkg-config
+  ];
+}

src/lib.rs

@@ -6,10 +6,13 @@ extern crate alloc;
 
 pub mod error;
 
-mod narinfo;
-mod nix_cache_info;
+pub mod narinfo;
+pub mod nix_cache_info;
 mod sig;
 
+#[cfg(feature = "libnixstore")]
+mod libnixstore;
+
 pub use crate::narinfo::NarInfo;
 pub use nix_cache_info::NixCacheInfo;
 pub use sig::Sig;

src/libnixstore.rs

@@ -0,0 +1,27 @@
+use crate::narinfo::NarInfoBuilder;
+use crate::Sig;
+use alloc::borrow::Cow;
+
+impl<'a> NarInfoBuilder<'a> {
+	/// Read and set the store dir from [`libnixstore`].
+	///
+	/// This function assumes that [`libnixstore::init`] has already been called
+	pub fn store_dir_from_libnixstore(&mut self) -> &Self {
+		let store_dir = libnixstore::get_store_dir();
+		self.store_path(Cow::Owned(store_dir));
+		self
+	}
+
+	/// Read and set `path`, `references` and `sigs` from a [`libnixstore::PathInfo`]
+	pub fn attr_from_libnixstore_path(&mut self, path: &'a libnixstore::PathInfo) -> &Self {
+		self.file_size(Some(path.size));
+		self.references(path.refs.iter().map(Cow::from).collect());
+		self.sigs(
+			path.sigs
+				.iter()
+				.map(|v| Sig::parse(v).expect("libnixstore provided signature should be in a valid format"))
+				.collect());
+
+		self
+	}
+}