EvalStateBuilder: Specify Nix version constraint

nix-bindings-expr/Cargo.toml

@@ -2,6 +2,7 @@
 name = "nix-bindings-expr"
 version = "0.1.0"
 edition = "2021"
+build = "build.rs"
 license = "LGPL-2.1"
 description = "Rust bindings to Nix expression evaluator"
 repository = "https://github.com/nixops4/nix-bindings-rust"
@@ -19,6 +20,10 @@ ctor = "0.2"
 tempfile = "3.10"
 cstr = "0.2"
 
+[build-dependencies]
+pkg-config = "0.3"
+nix-bindings-util = { path = "../nix-bindings-util" }
+
 [lints.rust]
 warnings = "deny"
 dead-code = "allow"

nix-bindings-expr/build.rs

@@ -0,0 +1,6 @@
+use nix_bindings_util::nix_version::emit_version_cfg;
+
+fn main() {
+    let nix_version = pkg_config::probe_library("nix-expr-c").unwrap().version;
+    emit_version_cfg(&nix_version, &["2.26"]);
+}

nix-bindings-expr/src/eval_state.rs

@@ -224,6 +224,8 @@ impl Drop for EvalStateRef {
 /// Provides advanced configuration options for evaluation context setup.
 /// Use [`EvalState::new`] for simple cases or this builder for custom configuration.
 ///
+/// Requires Nix 2.26.0 or later.
+///
 /// # Examples
 ///
 /// ```rust
@@ -244,11 +246,13 @@ impl Drop for EvalStateRef {
 /// # Ok(())
 /// # }
 /// ```
+#[cfg(nix_at_least = "2.26")]
 pub struct EvalStateBuilder {
     eval_state_builder: *mut raw::eval_state_builder,
     lookup_path: Vec<CString>,
     store: Store,
 }
+#[cfg(nix_at_least = "2.26")]
 impl Drop for EvalStateBuilder {
     fn drop(&mut self) {
         unsafe {
@@ -256,6 +260,7 @@ impl Drop for EvalStateBuilder {
         }
     }
 }
+#[cfg(nix_at_least = "2.26")]
 impl EvalStateBuilder {
     /// Creates a new [`EvalStateBuilder`].
     pub fn new(store: Store) -> Result<EvalStateBuilder> {