Feature fields: a type for each role (Expr, BodyName, FacePick…) (#1273)

Thanks @jijinbei.

crates/doc/src/points.rs

@@ -0,0 +1,61 @@
+//! The points a feature holds, by what they are: a point that picks a face, any other
+//! position, a direction. Each is a `Vec3` (and is saved as one), but a different type, so a
+//! direction can't be moved like a position, and a pick can't be mistaken for either.
+
+use std::ops::{Deref, DerefMut};
+
+use serde::{Deserialize, Serialize};
+use solidcraft_geom::Vec3;
+
+macro_rules! point_type {
+    ($(#[$doc:meta])* $name:ident) => {
+        $(#[$doc])*
+        #[derive(Clone, Copy, Debug, Default, PartialEq, Serialize, Deserialize)]
+        #[serde(transparent)]
+        pub struct $name(pub Vec3);
+
+        impl Deref for $name {
+            type Target = Vec3;
+            fn deref(&self) -> &Vec3 {
+                &self.0
+            }
+        }
+
+        impl DerefMut for $name {
+            fn deref_mut(&mut self) -> &mut Vec3 {
+                &mut self.0
+            }
+        }
+
+        impl From<Vec3> for $name {
+            fn from(v: Vec3) -> Self {
+                $name(v)
+            }
+        }
+
+        impl From<$name> for Vec3 {
+            fn from(p: $name) -> Vec3 {
+                p.0
+            }
+        }
+    };
+}
+
+point_type!(
+    /// A point that picks a face: it moves with the feature, and names the face it picks.
+    FacePick
+);
+point_type!(
+    /// A position (an edge pick, an axis origin, a fold line): it moves with the feature.
+    Position
+);
+point_type!(
+    /// A direction: it turns with the feature, and never moves.
+    Dir
+);
+
+/// The plain values of a list of picks, positions, directions or names (for code that takes
+/// `Vec3`s or `String`s).
+pub fn plain<T, P: Clone + Into<T>>(ps: &[P]) -> Vec<T> {
+    ps.iter().map(|p| p.clone().into()).collect()
+}