feat: Add nix-expr library, update nix

(cherry picked from commit bf1ddd1c9137578b86895577d5b80e0c5771b605)

rust/nix-expr/src/value.rs

@@ -0,0 +1,75 @@
+use nix_c_raw as raw;
+use nix_util::context::Context;
+use std::ptr::NonNull;
+
+// TODO: test: cloning a thunk does not duplicate the evaluation.
+
+/** The type of a value (or thunk) */
+#[derive(Eq, PartialEq)]
+pub enum ValueType {
+    AttrSet,
+    Bool,
+    External,
+    Float,
+    Function,
+    Int,
+    List,
+    Null,
+    Path,
+    String,
+    Thunk,
+    Unknown,
+}
+
+impl ValueType {
+    pub(crate) fn from_raw(raw: raw::ValueType) -> ValueType {
+        match raw {
+            raw::ValueType_NIX_TYPE_ATTRS => ValueType::AttrSet,
+            raw::ValueType_NIX_TYPE_BOOL => ValueType::Bool,
+            raw::ValueType_NIX_TYPE_EXTERNAL => ValueType::External,
+            raw::ValueType_NIX_TYPE_FLOAT => ValueType::Float,
+            raw::ValueType_NIX_TYPE_FUNCTION => ValueType::Function,
+            raw::ValueType_NIX_TYPE_INT => ValueType::Int,
+            raw::ValueType_NIX_TYPE_LIST => ValueType::List,
+            raw::ValueType_NIX_TYPE_NULL => ValueType::Null,
+            raw::ValueType_NIX_TYPE_PATH => ValueType::Path,
+            raw::ValueType_NIX_TYPE_STRING => ValueType::String,
+            raw::ValueType_NIX_TYPE_THUNK => ValueType::Thunk,
+            _ => ValueType::Unknown,
+        }
+    }
+}
+
+/* A pointer to a value or thunk, to be used with EvalState methods. */
+pub struct Value {
+    inner: NonNull<raw::Value>,
+}
+impl Value {
+    pub(crate) fn new(inner: *mut raw::Value) -> Self {
+        Value {
+            inner: NonNull::new(inner).unwrap(),
+        }
+    }
+    pub(crate) fn raw_ptr(&self) -> *mut raw::Value {
+        self.inner.as_ptr()
+    }
+}
+impl Drop for Value {
+    fn drop(&mut self) {
+        let context = Context::new();
+        unsafe {
+            raw::nix_gc_decref(context.ptr(), self.inner.as_ptr());
+        }
+        // ignore error from context, because drop should not panic
+    }
+}
+impl Clone for Value {
+    fn clone(&self) -> Self {
+        let context = Context::new();
+        unsafe { raw::nix_gc_incref(context.ptr(), self.inner.as_ptr()) };
+        context.check_err().unwrap();
+        Value { inner: self.inner }
+    }
+}
+
+// Tested in eval_state.rs