reorganize compiler crates

ast/src/fold_helpers.rs

@@ -0,0 +1,66 @@
+use crate::constant;
+use crate::fold::Fold;
+
+pub(crate) trait Foldable<T, U> {
+    type Mapped;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error>;
+}
+
+impl<T, U, X> Foldable<T, U> for Vec<X>
+where
+    X: Foldable<T, U>,
+{
+    type Mapped = Vec<X::Mapped>;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error> {
+        self.into_iter().map(|x| x.fold(folder)).collect()
+    }
+}
+
+impl<T, U, X> Foldable<T, U> for Option<X>
+where
+    X: Foldable<T, U>,
+{
+    type Mapped = Option<X::Mapped>;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error> {
+        self.map(|x| x.fold(folder)).transpose()
+    }
+}
+
+impl<T, U, X> Foldable<T, U> for Box<X>
+where
+    X: Foldable<T, U>,
+{
+    type Mapped = Box<X::Mapped>;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error> {
+        (*self).fold(folder).map(Box::new)
+    }
+}
+
+macro_rules! simple_fold {
+    ($($t:ty),+$(,)?) => {
+        $(impl<T, U> $crate::fold_helpers::Foldable<T, U> for $t {
+            type Mapped = Self;
+            #[inline]
+            fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+                self,
+                _folder: &mut F,
+            ) -> Result<Self::Mapped, F::Error> {
+                Ok(self)
+            }
+        })+
+    };
+}
+
+simple_fold!(usize, String, bool, constant::Constant);