Unit-less parameters used as lengths read in the design's units (feature evaluation and parameter values; bare numbers stored in inputs stay mm); document.units checks what the geometry reads (Document::length_inputs) and writes the old unit (with it as the parameter's unit) into bare unit-less parameters that would move; cached results keyed on the design's units

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

crates/engine/src/cmd/browser.rs

@@ -165,14 +165,17 @@ fn document_units(s: &mut Session, p: &Value) -> Result<Value> {
     if old == u {
         return Ok(json!({ "units": u, "kept": [] }));
     }
-    let (before, _) = s.doc.param_values();
+    // What the geometry reads: every feature's length inputs.
+    let before = s.doc.length_inputs();
     s.doc_mut().units = u.to_string();
     // Unit-less user parameters holding a bare number read it in the design's units where they
-    // stand for lengths. If any value moved, those numbers keep the old unit, written out.
-    let (after, _) = s.doc.param_values();
-    let moved = before.iter().any(|(k, b)| after.get(k).is_none_or(|a| (a.v - b.v).abs() > 1e-9 * b.v.abs().max(1.0)));
+    // stand for lengths. If any input moved, those numbers keep the old unit, written out.
+    let same = |a: &[Option<f64>]| {
+        a.iter().zip(&before).filter(|(x, y)| matches!((x, y), (Some(x), Some(y)) if (x - y).abs() <= 1e-9 * y.abs().max(1.0))).count()
+    };
+    let after = s.doc.length_inputs();
     let mut kept = Vec::new();
-    if moved {
+    if same(&after) < before.len() {
         let bare: Vec<(String, String)> = s
             .doc
             .params
@@ -181,16 +184,15 @@ fn document_units(s: &mut Session, p: &Value) -> Result<Value> {
             .map(|p| (p.name.clone(), p.expr.trim().to_string()))
             .collect();
         for (name, e) in bare {
+            let now = same(&s.doc.length_inputs());
             let probe = {
                 let mut d = (*s.doc).clone();
-                let _ = d.change_param(&name, &format!("{e} {old}"), None, None);
-                d.param_values().0
+                let _ = d.change_param(&name, &format!("{e} {old}"), Some(&old), None);
+                same(&d.length_inputs())
             };
-            // Only when writing the old unit puts the values back where they were.
-            let fixes = before.iter().filter(|(k, b)| probe.get(*k).is_some_and(|a| (a.v - b.v).abs() <= 1e-9 * b.v.abs().max(1.0))).count()
-                > before.iter().filter(|(k, b)| after.get(*k).is_some_and(|a| (a.v - b.v).abs() <= 1e-9 * b.v.abs().max(1.0))).count();
-            if fixes {
-                s.doc_mut().change_param(&name, &format!("{e} {old}"), None, None)?;
+            // Only when writing the old unit puts more inputs back where they were.
+            if probe > now {
+                s.doc_mut().change_param(&name, &format!("{e} {old}"), Some(&old), None)?;
                 kept.push(name);
             }
         }

crates/engine/src/tests.rs

@@ -1467,3 +1467,15 @@ fn booleans_stay_in_their_component() {
     assert!((vol(&mut s, "BBox") - (2000.0 - 8.0 * 6.0 * 10.0)).abs() < 1e-6, "{}", vol(&mut s, "BBox"));
     assert!(s.execute("Extrude", &json!({"sketch": "Slot", "distance": 1, "operation": "cut", "participants": ["Nope"]})).is_err());
 }
+
+/// A unit-less parameter used as a length means the design's units.
+#[test]
+fn unitless_parameters_used_as_lengths_take_the_design_units() {
+    let mut s = Session::default();
+    s.doc_mut().units = "in".into();
+    run(&mut s, "parameters.add", json!({"name": "w", "expression": "2", "unit": ""}));
+    run(&mut s, "PrimitiveBox", json!({"length": "w", "width": "w * 1", "height": "10 mm"}));
+    let m = run(&mut s, "MeasureCommand", json!({}));
+    let max = |i: usize| m["bodies"][0]["bbox"]["max"][i].as_f64().unwrap_or(0.0);
+    assert!((max(0) - 50.8).abs() < 1e-9 && (max(1) - 50.8).abs() < 1e-9 && (max(2) - 10.0).abs() < 1e-9, "{m}");
+}