Sketch drawing: points dropped on a curve stay on it ("on:<curve>:x,y" point arguments add a point-on-curve constraint; On hint while drawing)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/sketch_constraints_tests.rs

@@ -187,3 +187,16 @@ fn drawing_from_a_line_midpoint_keeps_it_there() {
     let b = sk.resolve_point(&format!("{l}.end")).and_then(|i| sk.point(i)).unwrap();
     assert!(st.dist((a + b) * 0.5) < 1e-7, "{st:?}");
 }
+
+#[test]
+fn drawing_onto_a_curve_keeps_the_point_on_it() {
+    let mut s = new_sketch();
+    let c = ids(&run(&mut s, "CircleCenterRadius", json!({"center": [0, 0], "radius": 10}))["curves"])[0].clone();
+    let r = run(&mut s, "DrawPolyline", json!({"points": [format!("on:{c}:7.2,7.1"), [30, 30]]}));
+    let n = ids(&r["curves"])[0].clone();
+    run(&mut s, "SketchDimension", json!({"entities": [c], "value": 16}));
+    let sk = sketch(&s);
+    let st = sk.resolve_point(&format!("{n}.start")).and_then(|i| sk.point(i)).unwrap();
+    let cc = sk.center(sk.curve_index(&c).unwrap()).unwrap();
+    assert!((st.dist(cc) - 8.0).abs() < 1e-7, "{st:?} {cc:?}");
+}

crates/engine/src/cmd/sketch_project.rs

@@ -347,7 +347,16 @@ pub(super) fn snap_points(s: &mut Session, p: &Value, cmd: &str) -> Result<Value
             }
         });
         let mid = v.as_str().and_then(|x| x.strip_prefix("mid:")).map(str::to_string);
-        if vertex.is_none() && edge.is_none() && mid.is_none() {
+        // "on:<curve>:x,y": a point on a curve of the sketch near x,y.
+        let on = v.as_str().and_then(|x| x.strip_prefix("on:")).and_then(|t| {
+            let (c, xy) = t.rsplit_once(':')?;
+            let n: Vec<f64> = xy.split(',').filter_map(|x| x.trim().parse().ok()).collect();
+            match n[..] {
+                [x, y] if x.is_finite() && y.is_finite() => Some((c.to_string(), solvecraft_geom::Vec2::new(x, y))),
+                _ => None,
+            }
+        });
+        if vertex.is_none() && edge.is_none() && mid.is_none() && on.is_none() {
             return Ok(());
         }
         if doc.is_none() {
@@ -358,7 +367,16 @@ pub(super) fn snap_points(s: &mut Session, p: &Value, cmd: &str) -> Result<Value
         }
         let Some((d, sk)) = doc.as_mut() else { return Ok(()) };
         let body = str_(v, "body").unwrap_or("").to_string();
-        let pid = if let Some(line) = mid {
+        let pid = if let Some((curve, near)) = on {
+            let c = sk.curve_index(&curve).ok_or_else(|| bad(cmd, format!("unknown curve `{curve}`")))?;
+            let at = match sk.shape(c) {
+                Some(sh) => sh.project(near),
+                None => near,
+            };
+            let pi = sk.add_point(at, None)?;
+            sk.add_constraint(ConstraintKind::PointOnCurve { p: pi, c }, None)?;
+            sk.points.get(pi).map(|q| q.id.clone()).unwrap_or_default()
+        } else if let Some(line) = mid {
             // "mid:<line>": a new point at the line's midpoint, kept there.
             let l = sk.curve_index(&line).ok_or_else(|| bad(cmd, format!("unknown curve `{line}`")))?;
             let Some(solvecraft_sketch::Shape::Line { a, b }) = sk.shape(l) else { return Err(bad(cmd, format!("`{line}` is not a line"))) };