Add API for adding new builtins

nix-bindings-expr/src/eval_state.rs

@@ -207,6 +207,12 @@ pub struct RealisedString {
 #[clippy::has_significant_drop]
 struct EvalStateRef {
     eval_state: NonNull<raw::EvalState>,
+    /// Whether we should call [`raw::state_free`] once this `EvalStateRef` is [`Drop`]ped.
+    ///
+    /// The reason this exists is because we might construct non-owning [`EvalState`]s by way of
+    /// using [`EvalState::from_raw`], in which case we do not want to call [`raw::state_free`] once
+    /// the constructed [`EvalState`] goes out of scope.
+    should_free: bool,
 }
 
 unsafe impl Send for EvalStateRef {}
@@ -224,8 +230,10 @@ impl EvalStateRef {
 
 impl Drop for EvalStateRef {
     fn drop(&mut self) {
-        unsafe {
-            raw::state_free(self.eval_state.as_ptr());
+        if self.should_free {
+            unsafe {
+                raw::state_free(self.eval_state.as_ptr());
+            }
         }
     }
 }
@@ -349,6 +357,7 @@ impl EvalStateBuilder {
                 eval_state: NonNull::new(eval_state).unwrap_or_else(|| {
                     panic!("nix_state_create returned a null pointer without an error")
                 }),
+                should_free: true,
             },
         })
     }
@@ -394,6 +403,7 @@ impl EvalState {
             eval_state: EvalStateRef {
                 eval_state: NonNull::new(ptr)
                     .expect("tried to construct a new `EvalState` from null pointer"),
+                should_free: false,
             },
         }
     }

nix-bindings-expr/src/primop.rs

@@ -92,7 +92,6 @@ impl PrimOp {
     /// does not affect `builtins`.
     #[doc(alias = "make_primop")]
     #[doc(alias = "create_function")]
-    #[doc(alias = "builtin")]
     pub fn new_value(self, eval_state: &mut EvalState) -> Result<Value, EvalStateError> {
         let value = eval_state.new_value_uninitialized()?;
         let mut ctx = Context::new();
@@ -101,6 +100,20 @@ impl PrimOp {
         };
         Ok(value)
     }
+
+    /// Creates a new [`function`](crate::value::ValueType::Function) Nix value implemented by a
+    /// Rust function, and adds it to the global `builtins` attribute set.
+    ///
+    /// Note that this `PrimOp` will only be accessible by [`EvalState`]s that are created _after_
+    /// this is called.
+    #[doc(alias = "builtin")]
+    pub fn new_builtin(self) -> Result<(), EvalStateError> {
+        let mut ctx = Context::new();
+        unsafe {
+            check_call!(raw::register_primop(&mut ctx, self.ptr))?;
+        }
+        Ok(())
+    }
 }
 
 /// The user_data for our Nix primops