Extrude: a taper composes with a start offset (#2312) (#2313)

Thanks @acbrazzl.

crates/doc/src/eval.rs

@@ -3195,21 +3195,30 @@ fn feature_tools_with_constructor(
             };
             if let Some(tp) = &extent.taper {
                 let taper = val(vals, tp, Kind::Angle)?;
-                if extent.distance2.is_some() || extent.start_offset.is_some() || extent.direction == crate::Direction::Symmetric {
-                    return Err(DocError::Invalid("not supported yet: taper with two-sided, symmetric or offset extents".into()));
+                if extent.distance2.is_some() || extent.direction == crate::Direction::Symmetric {
+                    return Err(DocError::Invalid("not supported yet: taper with two-sided or symmetric extents".into()));
                 }
                 let sign = if extent.to.is_none() && extent.direction == crate::Direction::Negative { -1.0 } else { 1.0 };
                 let (len, sign) = if d < 0.0 { (-d, -sign) } else { (d, sign) };
-                let start_plane = ss.plane.offset(from);
+                // Where the extrude starts: the From Object's level and a start offset along the
+                // normal from it. The offset moves the profile that far and the sides are drafted
+                // from there, so the profile stays the section at the start whatever the offset
+                // (`d` is measured from the start already, To Object included).
+                let base = from
+                    + match &extent.start_offset {
+                        Some(e) => val(vals, e, Kind::Length)?,
+                        None => 0.0,
+                    };
+                let start_plane = ss.plane.offset(base);
                 let caps = [
-                    Some(crate::lineage::extrude_cap_role(from)),
-                    Some(crate::lineage::extrude_cap_role(if sign < 0.0 { from - len } else { from + len })),
+                    Some(crate::lineage::extrude_cap_role(base)),
+                    Some(crate::lineage::extrude_cap_role(if sign < 0.0 { base - len } else { base + len })),
                 ];
                 // How far a tool grows past its profile at either end, at most.
                 let mut growth: f64 = 0.0;
                 // A face of the targets in the profile's plane (the same for every profile).
                 let plain_targets: Vec<String> = crate::plain(targets);
-                let on_face = matches!(operation, Operation::Join | Operation::Cut) && face_level_at(st, &ss.plane, from, &plain_targets);
+                let on_face = matches!(operation, Operation::Join | Operation::Cut) && face_level_at(st, &ss.plane, base, &plain_targets);
                 let surroundings = Surroundings::of(st, *operation, &plain_targets);
                 let tools: Vec<Traced> = regions
                     .iter()
@@ -3233,9 +3242,9 @@ fn feature_tools_with_constructor(
                             growth = growth.max(len * taper.tan().abs());
                             return Ok(roles.tool(exact, None));
                         }
-                        let (lo, hi) = if sign < 0.0 { (from - len, from) } else { (from, from + len) };
+                        let (lo, hi) = if sign < 0.0 { (base - len, base) } else { (base, base + len) };
                         let (l2, h2) = surroundings.as_ref().map_or((lo, hi), |sur| extend_for_coplanar(sur, &ss.plane, r, lo, hi));
-                        let (mut a, b) = if sign < 0.0 { (from - h2, from - l2) } else { (l2 - from, h2 - from) };
+                        let (mut a, b) = if sign < 0.0 { (base - h2, base - l2) } else { (l2 - base, h2 - base) };
                         // A taper that narrows the tool away from the face widens it as it runs on into it, past
                         // the profile (where the profile is the whole face, out through the body's sides, #1534):
                         // then it starts on the face instead.