solid.mirror: a plane's {origin} takes length expressions and follows them (#1492) (#1501)

Thanks @erkexzcx.

crates/doc/src/document.rs

@@ -42,6 +42,14 @@ pub enum PlaneRef {
     Origin { name: String },
     /// An explicit plane (e.g. picked from a planar face).
     Custom { plane: Plane },
+    /// An explicit plane whose origin is given as length expressions
+    /// (`{origin: ["half", 0, 0], normal: [1, 0, 0]}`), so it follows parameter changes.
+    Placed {
+        #[serde(with = "point_expr")]
+        origin: PointExpr,
+        x_dir: Vec3,
+        y_dir: Vec3,
+    },
     /// Another plane offset along its normal by an expression.
     Offset { base: Box<PlaneRef>, distance: String },
     /// Another plane rotated about a line (in world coordinates) by an angle expression.
@@ -2938,6 +2946,15 @@ impl Document {
             PlaneRef::Custom { plane } | PlaneRef::Face { plane, .. } => {
                 Plane::new(plane.origin, plane.x, plane.y).ok_or_else(|| DocError::Invalid("degenerate plane".into()))
             }
+            PlaneRef::Placed { origin, x_dir, y_dir } => {
+                let [x, y, z] = origin;
+                let o =
+                    Vec3::new(Self::eval_in(vals, x, Kind::Length)?, Self::eval_in(vals, y, Kind::Length)?, Self::eval_in(vals, z, Kind::Length)?);
+                if ![o.x, o.y, o.z].iter().all(|c| c.is_finite() && c.abs() < 1e9) {
+                    return Err(DocError::Invalid("the plane's origin is out of range".into()));
+                }
+                Plane::new(o, *x_dir, *y_dir).ok_or_else(|| DocError::Invalid("degenerate plane".into()))
+            }
             PlaneRef::Offset { base, distance } => {
                 let b = self.resolve_plane_in(vals, base, depth + 1, st)?;
                 Ok(b.offset(Self::eval_in(vals, distance, Kind::Length)?))