maint: Move to /rust/* to /

This makes it easier for tooling to find the Rust stuff.
Rust/non-rust is not a useful distinction in this repo anymore anyway.

nix-bindings-expr/src/value.rs

@@ -0,0 +1,126 @@
+pub mod __private;
+
+use nix_bindings_bindgen_raw as raw;
+use nix_bindings_util::{check_call, context::Context};
+use std::ptr::{null_mut, NonNull};
+
+// TODO: test: cloning a thunk does not duplicate the evaluation.
+
+pub type Int = i64;
+
+/// The type discriminator of a [`Value`] that has successfully evaluated to at least [weak head normal form](https://nix.dev/manual/nix/latest/language/evaluation.html?highlight=WHNF#values).
+///
+/// Typically acquired with [`EvalState::value_type`][`crate::eval_state::EvalState::value_type`]
+#[derive(Eq, PartialEq, Debug)]
+pub enum ValueType {
+    /// A Nix [attribute set](https://nix.dev/manual/nix/stable/language/types.html#type-attrs)
+    AttrSet,
+    /// A Nix [boolean](https://nix.dev/manual/nix/stable/language/types.html#type-bool)
+    Bool,
+    /// A Nix external value (mostly-opaque value for plugins, linked applications)
+    External,
+    /// A Nix [float](https://nix.dev/manual/nix/stable/language/types.html#type-float)
+    Float,
+    /// A Nix [function](https://nix.dev/manual/nix/stable/language/types.html#type-function)
+    Function,
+    /// A Nix [integer](https://nix.dev/manual/nix/stable/language/types.html#type-int)
+    Int,
+    /// A Nix [list](https://nix.dev/manual/nix/stable/language/types.html#type-list)
+    List,
+    /// A Nix [`null`](https://nix.dev/manual/nix/stable/language/types.html#type-null)
+    Null,
+    /// A Nix [path value](https://nix.dev/manual/nix/stable/language/types.html#type-path)
+    Path,
+    /// A Nix [string](https://nix.dev/manual/nix/stable/language/types.html#type-string)
+    String,
+    /// An unknown value, presumably from a new, partially unsupported version of Nix
+    Unknown,
+}
+
+impl ValueType {
+    /// Convert a raw value type to a [`ValueType`].
+    ///
+    /// Return [`None`] if the Value is still a thunk (i.e. not yet evaluated).
+    ///
+    /// Return `Some(`[`ValueType::Unknown`]`)` if the value type is not recognized.
+    pub(crate) fn from_raw(raw: raw::ValueType) -> Option<ValueType> {
+        match raw {
+            raw::ValueType_NIX_TYPE_ATTRS => Some(ValueType::AttrSet),
+            raw::ValueType_NIX_TYPE_BOOL => Some(ValueType::Bool),
+            raw::ValueType_NIX_TYPE_EXTERNAL => Some(ValueType::External),
+            raw::ValueType_NIX_TYPE_FLOAT => Some(ValueType::Float),
+            raw::ValueType_NIX_TYPE_FUNCTION => Some(ValueType::Function),
+            raw::ValueType_NIX_TYPE_INT => Some(ValueType::Int),
+            raw::ValueType_NIX_TYPE_LIST => Some(ValueType::List),
+            raw::ValueType_NIX_TYPE_NULL => Some(ValueType::Null),
+            raw::ValueType_NIX_TYPE_PATH => Some(ValueType::Path),
+            raw::ValueType_NIX_TYPE_STRING => Some(ValueType::String),
+
+            raw::ValueType_NIX_TYPE_THUNK => None,
+
+            // This would happen if a new type of value is added in Nix.
+            _ => Some(ValueType::Unknown),
+        }
+    }
+}
+
+/// A pointer to a [value](https://nix.dev/manual/nix/latest/language/types.html) or [thunk](https://nix.dev/manual/nix/2.31/language/evaluation.html?highlight=thunk#laziness), to be used with [`EvalState`][`crate::eval_state::EvalState`] methods.
+pub struct Value {
+    inner: NonNull<raw::Value>,
+}
+impl Value {
+    /// Take ownership of a new [`Value`].
+    ///
+    /// This does not call [`nix_c_raw::gc_incref`], but does call [`nix_c_raw::nix_gc_decref`] when [dropped][`Drop`].
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the provided `inner` has a positive reference count, and that `inner` is not used after the returned `Value` is dropped.
+    pub(crate) unsafe fn new(inner: *mut raw::Value) -> Self {
+        Value {
+            inner: NonNull::new(inner).unwrap(),
+        }
+    }
+
+    /// Borrow a reference to a [`Value`].
+    ///
+    /// This calls [`nix_c_raw::value_incref`], and the returned Value will call [`nix_c_raw::value_decref`] when dropped.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the provided `inner` has a positive reference count.
+    pub(crate) unsafe fn new_borrowed(inner: *mut raw::Value) -> Self {
+        let v = Value::new(inner);
+        unsafe { raw::value_incref(null_mut(), inner) };
+        v
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure that the returned pointer is not used after the `Value` is dropped.
+    pub(crate) unsafe fn raw_ptr(&self) -> *mut raw::Value {
+        self.inner.as_ptr()
+    }
+}
+impl Drop for Value {
+    fn drop(&mut self) {
+        unsafe {
+            // ignoring error because the only failure mode is leaking memory
+            raw::value_decref(null_mut(), self.inner.as_ptr());
+        }
+    }
+}
+impl Clone for Value {
+    fn clone(&self) -> Self {
+        // TODO: Is it worth allocating a new Context here? Ideally cloning is cheap.
+        //       this is very unlikely to error, and it is not recoverable
+        //       Maybe try without, and try again with context to report details?
+        unsafe {
+            check_call!(raw::value_incref(&mut Context::new(), self.inner.as_ptr())).unwrap();
+        }
+        // can't return an error here, but we don't want to ignore the error either as it means we could use-after-free
+        Value { inner: self.inner }
+    }
+}
+
+// Tested in eval_state.rs