fix: a size worked out from a bare diameter keeps its unit when the design's units change (#716) (#779)

A size typed as a bare diameter now keeps the design's length unit, so it no longer changes when the design's units change. Fixes #716.

crates/doc/src/expr.rs

@@ -350,6 +350,17 @@ pub fn is_literal(s: &str) -> bool {
     }
 }
 
+/// Is the expression a bare number without a unit, like `20` or `-1.5`?
+pub fn is_bare_number(s: &str) -> bool {
+    is_literal(s) && s.trim().trim_start_matches(['-', '+']).trim().parse::<f64>().is_ok()
+}
+
+/// The length unit of a design with these units: `mm` unless they name another length unit.
+pub fn length_unit_or_mm(units: &str) -> &str {
+    let u = units.trim();
+    if unit_info(u).is_some_and(|(k, _)| k == Kind::Length) { u } else { "mm" }
+}
+
 struct Parser<'a> {
     toks: Vec<Tok>,
     pos: usize,
@@ -942,5 +953,7 @@ mod tests {
         assert_eq!(rename_reference("10 mm", "mm", "x"), "10 mm");
         assert!(is_literal("20") && is_literal("-1.5 in") && is_literal("3 deg"));
         assert!(!is_literal("d1") && !is_literal("2 * 3") && !is_literal("2 width"));
+        assert!(is_bare_number(" -1.5 ") && is_bare_number("+2") && !is_bare_number("20 mm") && !is_bare_number("(2)"));
+        assert_eq!((length_unit_or_mm(" in "), length_unit_or_mm("deg"), length_unit_or_mm("")), ("in", "mm", "mm"));
     }
 }

crates/doc/src/params.rs

@@ -466,7 +466,7 @@ impl Document {
     /// in (feature inputs are stored in mm otherwise). Anything else is returned unchanged.
     pub fn with_design_unit(&self, e: &str, kind: Kind) -> String {
         let u = self.units.trim();
-        let bare = expr::is_literal(e) && e.trim().trim_start_matches(['-', '+']).trim().parse::<f64>().is_ok();
+        let bare = expr::is_bare_number(e);
         if kind == Kind::Length && bare && !u.is_empty() && u != "mm" && expr::unit_info(u).is_some_and(|(k, _)| k == Kind::Length) {
             format!("{} {u}", e.trim())
         } else {

crates/engine/src/cmd/features.rs

@@ -616,14 +616,36 @@ fn revolve(s: &mut Session, p: &Value) -> Result<Value> {
     add_feature(s, p, kind)
 }
 
-fn radius_expr(p: &Value, cmd: &str) -> Result<String> {
+fn radius_expr(s: &Session, p: &Value, cmd: &str) -> Result<String> {
     match (expr(p, "radius"), expr(p, "diameter")) {
         (Some(r), _) => Ok(r),
-        (None, Some(d)) => Ok(format!("({d}) / 2")),
+        (None, Some(d)) => {
+            let [d] = length_operands(s, [(&d, &typed_length_unit(s))]);
+            Ok(format!("({d}) / 2"))
+        }
         _ => Err(bad(cmd, "needs `radius` or `diameter`")),
     }
 }
 
+/// The unit a bare length typed into a command means: the design's units. (A point's bare
+/// coordinates, and a bare number already stored in a feature, are millimetres.)
+pub(super) fn typed_length_unit(s: &Session) -> String {
+    solidcraft_doc::expr::length_unit_or_mm(&s.doc.units).to_string()
+}
+
+/// The lengths a command works one expression out from (a radius from a diameter, a pitch from
+/// a height), each with the unit a bare number means where it came from. Bare numbers get that
+/// unit written in, so the expression keeps the meaning they had: inside a larger expression a
+/// bare number is read in the design's units of the moment, and changing them would resize the
+/// part (#716). Next to a unit-less expression that is not a bare number (`2 * 5`, a unit-less
+/// parameter), which is read in the design's units itself, they stay bare and are read with it,
+/// as before.
+pub(super) fn length_operands<const N: usize>(s: &Session, operands: [(&str, &str); N]) -> [String; N] {
+    use solidcraft_doc::expr::is_bare_number;
+    let follows = operands.iter().any(|(e, _)| !is_bare_number(e) && s.doc.eval(e, Kind::Unitless).is_ok());
+    operands.map(|(e, unit)| if !follows && is_bare_number(e) { format!("{} {unit}", e.trim()) } else { e.to_string() })
+}
+
 /// A placement point: `[x, y, z]` (or `{x, y, z}`) of numbers (mm) or length expressions;
 /// the origin when it is not given. Anything else is refused (never read as the origin).
 pub(super) fn point_param(s: &Session, p: &Value, key: &str, cmd: &str) -> Result<Option<PointExpr>> {
@@ -668,11 +690,17 @@ fn prim_box(s: &mut Session, p: &Value) -> Result<Value> {
     }
     let corner = match (point_param(s, p, "corner", cmd)?, point_param(s, p, "center", cmd)?) {
         (Some(c), _) => c,
-        // The corner half a size back from the centre (numbers stay numbers).
+        // The corner half a size back from the centre (numbers stay numbers, in mm). The centre's
+        // bare numbers are millimetres, the sizes' the design's units.
         (None, Some([cx, cy, cz])) => {
+            let unit = typed_length_unit(s);
+            let mm_per_unit = solidcraft_doc::expr::unit_info(&unit).map_or(1.0, |(_, k)| k);
             let back = |c: String, size: &String| match (c.parse::<f64>(), size.parse::<f64>()) {
-                (Ok(a), Ok(b)) => format!("{}", a - b / 2.0),
-                _ => format!("({c}) - ({size}) / 2"),
+                (Ok(a), Ok(b)) => format!("{}", a - b * mm_per_unit / 2.0),
+                _ => {
+                    let [c, size] = length_operands(s, [(&c, "mm"), (size, &unit)]);
+                    format!("({c}) - ({size}) / 2")
+                }
             };
             [back(cx, &l), back(cy, &w), back(cz, &h)]
         }
@@ -683,7 +711,7 @@ fn prim_box(s: &mut Session, p: &Value) -> Result<Value> {
 
 fn prim_cylinder(s: &mut Session, p: &Value) -> Result<Value> {
     let cmd = "solid.cylinder";
-    let radius = radius_expr(p, cmd)?;
+    let radius = radius_expr(s, p, cmd)?;
     let height = req_expr(cmd, p, "height")?;
     check_expr(s, &radius, Kind::Length, cmd, "radius")?;
     check_expr(s, &height, Kind::Length, cmd, "height")?;
@@ -694,7 +722,7 @@ fn prim_cylinder(s: &mut Session, p: &Value) -> Result<Value> {
 
 fn prim_sphere(s: &mut Session, p: &Value) -> Result<Value> {
     let cmd = "solid.sphere";
-    let radius = radius_expr(p, cmd)?;
+    let radius = radius_expr(s, p, cmd)?;
     check_expr(s, &radius, Kind::Length, cmd, "radius")?;
     let center = point_param(s, p, "center", cmd)?.unwrap_or_else(|| point_expr(Vec3::ZERO));
     add_feature(s, p, FeatureKind::Sphere { center, radius, operation: operation(p, cmd)? })
@@ -758,7 +786,9 @@ fn press_pull(s: &mut Session, p: &Value) -> Result<Value> {
                 FeatureKind::Chamfer { distance, .. } => ("distance", distance.clone()),
                 _ => return Err(bad(cmd, "that face is not a fillet's or a chamfer's")),
             };
-            let new = format!("({old}) + ({distance})");
+            // A bare number stored in the feature is millimetres; the distance's, the design's units.
+            let [old, more] = length_operands(s, [(&old, "mm"), (&distance, &typed_length_unit(s))]);
+            let new = format!("({old}) + ({more})");
             let r = super::find_command("timeline.edit").ok_or_else(|| bad(cmd, "timeline.edit"))?;
             let out = (r.run)(s, &json!({"feature": f.id, "set": {key: new}}))?;
             return Ok(json!({"feature": f.id, "edited": f.name, key: new, "recomputed": out.get("recomputed"), "restored": out.get("restored")}));
@@ -767,7 +797,13 @@ fn press_pull(s: &mut Session, p: &Value) -> Result<Value> {
         let r = super::find_command("solid.offset_face").ok_or_else(|| bad(cmd, "solid.offset_face"))?;
         return (r.run)(s, &json!({"faces": [face], "distance": distance}));
     }
-    let (dist, dir, op) = if d > 0.0 { (distance, "positive", "join") } else { (format!("-({distance})"), "negative", "cut") };
+    // Pushed in: the extrude cuts the other way by as much (a bare number stays one, so it keeps
+    // being read as a bare number typed into the extrude).
+    let flipped = match distance.trim().parse::<f64>() {
+        Ok(v) if solidcraft_doc::expr::is_bare_number(&distance) => format!("{}", -v),
+        _ => format!("-({distance})"),
+    };
+    let (dist, dir, op) = if d > 0.0 { (distance, "positive", "join") } else { (flipped, "negative", "cut") };
     extrude(s, &json!({"face": face, "distance": dist, "direction": dir, "operation": op}))
 }
 

crates/engine/src/cmd/features_more.rs

@@ -6,7 +6,9 @@ use solidcraft_doc::expr::Kind;
 use solidcraft_geom::Vec3;
 
 use super::CommandSpec;
-use super::features::{add_feature, axis_dir_param, check_expr, feature_sketch, plane_param, point_param, profiles};
+use super::features::{
+    add_feature, axis_dir_param, check_expr, feature_sketch, length_operands, plane_param, point_param, profiles, typed_length_unit,
+};
 use crate::params::{bad, bool_, expr, req_expr, str_, string_list, vec3};
 use crate::{Result, Session};
 
@@ -49,10 +51,18 @@ fn coil(s: &mut Session, p: &Value) -> Result<Value> {
     check_expr(s, &diameter, Kind::Length, cmd, "diameter")?;
     let turns = expr(p, "revolutions").or_else(|| expr(p, "turns"));
     let (height, pitch) = (expr(p, "height"), expr(p, "pitch"));
+    // The pitch or the revolutions from the height keep the units the lengths were given in.
+    let unit = typed_length_unit(s);
     let (turns, pitch) = match (turns, height, pitch) {
         (Some(n), _, Some(pt)) => (n, pt),
-        (Some(n), Some(h), None) => (n.clone(), format!("({h}) / ({n})")),
-        (None, Some(h), Some(pt)) => (format!("({h}) / ({pt})"), pt),
+        (Some(n), Some(h), None) => {
+            let [h] = length_operands(s, [(&h, &unit)]);
+            (n.clone(), format!("({h}) / ({n})"))
+        }
+        (None, Some(h), Some(pt)) => {
+            let [h, by] = length_operands(s, [(&h, &unit), (&pt, &unit)]);
+            (format!("({h}) / ({by})"), pt)
+        }
         _ => return Err(bad(cmd, "give two of `revolutions`, `height` and `pitch`")),
     };
     check_expr(s, &turns, Kind::Unitless, cmd, "revolutions")?;

crates/engine/src/derived_units_tests.rs

@@ -0,0 +1,230 @@
+//! Sizes a command works out from what it was given keep the meaning those inputs had when the
+//! design's units change (#716): a sphere or a cylinder made with a bare `diameter`, a coil's
+//! pitch or revolutions from its height, a box placed by its centre, a Press Pull. A radius from
+//! a bare diameter used to be stored as `(20) / 2`: a unit-less expression that is not a bare
+//! number is read in the design's units of the moment, so switching a millimetre design to
+//! inches made such a sphere 25.4 times larger.
+
+use std::f64::consts::PI;
+
+use serde_json::{Value, json};
+
+use crate::Session;
+
+fn run(s: &mut Session, id: &str, p: Value) -> Value {
+    match s.execute(id, &p) {
+        Ok(v) => v,
+        Err(e) => panic!("{id} {p}: {e}"),
+    }
+}
+
+fn rel(a: f64, b: f64) -> f64 {
+    (a - b).abs() / b.abs().max(1e-12)
+}
+
+/// A body's volume (mm³) and bounding box (min x, y, z, max x, y, z in mm).
+#[derive(Debug, Clone, Copy)]
+struct Shape {
+    volume: f64,
+    bbox: [f64; 6],
+}
+
+impl Shape {
+    fn size(&self, axis: usize) -> f64 {
+        self.bbox[axis + 3] - self.bbox[axis]
+    }
+    fn same(&self, other: &Shape) -> bool {
+        rel(self.volume, other.volume) < 1e-9 && self.bbox.iter().zip(&other.bbox).all(|(a, b)| (a - b).abs() < 1e-7)
+    }
+}
+
+/// Every body by name.
+fn shapes(s: &mut Session) -> Vec<(String, Shape)> {
+    measured(s, json!({}))
+}
+
+fn measured(s: &mut Session, p: Value) -> Vec<(String, Shape)> {
+    let m = run(s, "inspect.measure", p);
+    let bodies = m["bodies"].as_array().cloned().unwrap_or_default();
+    bodies
+        .iter()
+        .map(|b| {
+            let c = |k: &str, i: usize| b["bbox"][k][i].as_f64().unwrap_or(f64::NAN);
+            let bbox = [c("min", 0), c("min", 1), c("min", 2), c("max", 0), c("max", 1), c("max", 2)];
+            (b["name"].as_str().unwrap_or_default().to_string(), Shape { volume: b["volume_mm3"].as_f64().unwrap_or(f64::NAN), bbox })
+        })
+        .collect()
+}
+
+/// One body, measured alone.
+fn shape(s: &mut Session, body: &str) -> Shape {
+    measured(s, json!({ "bodies": [body] })).into_iter().next().map(|(_, x)| x).unwrap_or_else(|| panic!("no body {body}"))
+}
+
+/// Switch the design to each unit in turn; every body keeps its volume and its bounding box.
+fn keeps_shape_through(s: &mut Session, units: &[&str]) {
+    let before = shapes(s);
+    assert!(!before.is_empty());
+    for u in units {
+        let r = run(s, "document.units", json!({ "units": u }));
+        let now = shapes(s);
+        assert_eq!(now.len(), before.len(), "{u}: {r}");
+        for ((name, a), (_, b)) in before.iter().zip(&now) {
+            assert!(a.same(b), "{name} after switching to {u} ({r}): {a:?} became {b:?}");
+        }
+    }
+}
+
+/// The expressions of a feature's inputs.
+fn inputs(s: &Session, feature: usize) -> Vec<String> {
+    s.doc.features.get(feature).map(|f| f.kind.inputs().into_iter().map(|(_, e, _)| e).collect()).unwrap_or_default()
+}
+
+#[test]
+fn bare_diameters_keep_their_size_when_the_units_change() {
+    let mut s = Session::default();
+    run(&mut s, "solid.sphere", json!({"diameter": 20, "body_name": "Ball"}));
+    run(&mut s, "solid.cylinder", json!({"diameter": 20, "height": 10, "base": [50, 0, 0], "body_name": "Can"}));
+    let ball = shape(&mut s, "Ball");
+    assert!(rel(ball.volume, 4.0 / 3.0 * PI * 1000.0) < 1e-3, "{ball:?}");
+    // The bounding box comes from the tessellation: a little inside the surface.
+    assert!((ball.size(0) - 20.0).abs() < 1e-3 && (ball.bbox[0] + 10.0).abs() < 1e-3, "{ball:?}");
+    let can = shape(&mut s, "Can");
+    assert!(rel(can.volume, PI * 100.0 * 10.0) < 1e-3, "{can:?}");
+    assert!((can.bbox[0] - 40.0).abs() < 1e-3 && (can.bbox[3] - 60.0).abs() < 1e-3 && (can.size(2) - 10.0).abs() < 1e-6, "{can:?}");
+    keeps_shape_through(&mut s, &["in", "cm", "ft", "mm", "in"]);
+    // The radius says what the diameter was, with the unit it was typed in.
+    assert_eq!(inputs(&s, 0), ["(20 mm) / 2"]);
+    assert_eq!(inputs(&s, 1), ["(20 mm) / 2", "10"]);
+}
+
+#[test]
+fn bare_diameters_typed_in_an_inch_design_mean_inches() {
+    let mut s = Session::default();
+    run(&mut s, "document.units", json!({"units": "in"}));
+    run(&mut s, "solid.sphere", json!({"diameter": 2, "body_name": "Ball"}));
+    // The base is a point: its bare numbers are millimetres.
+    run(&mut s, "solid.cylinder", json!({"diameter": 1, "height": 2, "base": [100, 0, 0], "body_name": "Can"}));
+    let ball = shape(&mut s, "Ball");
+    assert!(rel(ball.volume, 4.0 / 3.0 * PI * 25.4f64.powi(3)) < 1e-3, "{ball:?}");
+    assert!((ball.size(0) - 50.8).abs() < 1e-3, "{ball:?}");
+    let can = shape(&mut s, "Can");
+    assert!(rel(can.volume, PI * 12.7 * 12.7 * 50.8) < 1e-3, "{can:?}");
+    assert!((can.bbox[0] - 87.3).abs() < 1e-3 && (can.size(2) - 50.8).abs() < 1e-6, "{can:?}");
+    keeps_shape_through(&mut s, &["mm", "cm", "in"]);
+}
+
+/// A body's extent: the diagonal of its vertices' bounding box. Measuring a coil takes seconds;
+/// this does not.
+fn extent(s: &Session, body: &str) -> f64 {
+    s.model.state().body(body).map_or(f64::NAN, |b| b.body.size())
+}
+
+#[test]
+fn coil_pitch_and_revolutions_keep_their_size_when_the_units_change() {
+    use std::f64::consts::TAU;
+    let mut s = Session::default();
+    // Height and revolutions give the pitch.
+    run(&mut s, "solid.coil", json!({"diameter": 40, "revolutions": 2, "height": 16, "section_size": 4, "body_name": "A"}));
+    // Height and pitch give the revolutions: a height with its unit next to a bare pitch, both
+    // millimetres here.
+    run(&mut s, "solid.coil", json!({"diameter": 40, "height": "16 mm", "pitch": 8, "section_size": 4, "base": [100, 0, 0], "body_name": "B"}));
+    let made = shapes(&mut s);
+    let want = PI * 4.0 * TAU * 20.0 * 2.0;
+    for (body, x) in &made {
+        assert!(rel(x.volume, want) < 1e-3, "{body}: {x:?}");
+        assert!((x.size(2) - made[0].1.size(2)).abs() < 1e-6, "{body} is as tall as A: {made:?}");
+    }
+    assert_eq!(inputs(&s, 0)[1], "(16 mm) / (2)", "A's pitch");
+    assert_eq!(inputs(&s, 1)[2], "(16 mm) / (8 mm)", "B's revolutions");
+    let sizes = |s: &Session| [extent(s, "A"), extent(s, "B")];
+    let before = sizes(&s);
+    for u in ["in", "cm", "mm"] {
+        run(&mut s, "document.units", json!({ "units": u }));
+        let now = sizes(&s);
+        assert!(now.iter().zip(&before).all(|(a, b)| rel(*a, *b) < 1e-9), "{u}: {before:?} became {now:?}");
+    }
+    // In an inch design the same bare numbers are inches.
+    let mut s = Session::default();
+    run(&mut s, "document.units", json!({"units": "in"}));
+    run(&mut s, "solid.coil", json!({"diameter": 2, "revolutions": 2, "height": 0.8, "section_size": 0.2, "body_name": "A"}));
+    let a = shape(&mut s, "A");
+    assert!(rel(a.volume, PI * 2.54f64.powi(2) * TAU * 25.4 * 2.0) < 1e-3, "{a:?}");
+    let before = extent(&s, "A");
+    run(&mut s, "document.units", json!({"units": "mm"}));
+    assert!(rel(extent(&s, "A"), before) < 1e-9, "{before} became {}", extent(&s, "A"));
+}
+
+#[test]
+fn a_box_placed_by_its_centre_reads_its_sizes_in_the_design_units() {
+    // An inch design: bare sizes are inches, the centre's bare numbers millimetres.
+    let mut s = Session::default();
+    run(&mut s, "document.units", json!({"units": "in"}));
+    run(&mut s, "solid.box", json!({"length": 2, "width": 1, "height": 1, "center": [0, 0, 0], "body_name": "A"}));
+    run(&mut s, "solid.box", json!({"length": 2, "width": 1, "height": 1, "center": ["10 mm", 100, 0], "body_name": "B"}));
+    let a = shape(&mut s, "A");
+    assert!((a.bbox[0] + 25.4).abs() < 1e-6 && (a.bbox[3] - 25.4).abs() < 1e-6 && (a.bbox[1] + 12.7).abs() < 1e-6, "{a:?}");
+    let b = shape(&mut s, "B");
+    assert!((b.bbox[0] + 15.4).abs() < 1e-6 && (b.bbox[3] - 35.4).abs() < 1e-6 && (b.bbox[1] - 87.3).abs() < 1e-6, "{b:?}");
+    keeps_shape_through(&mut s, &["mm", "in"]);
+    // A millimetre design: a centre in inches and a bare size in millimetres.
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 1, "width": 1, "height": 1, "center": ["0.5 in", 0, 0], "body_name": "C"}));
+    let c = shape(&mut s, "C");
+    assert!((c.bbox[0] - 12.2).abs() < 1e-6 && (c.bbox[3] - 13.2).abs() < 1e-6, "{c:?}");
+    keeps_shape_through(&mut s, &["in", "mm"]);
+}
+
+#[test]
+fn press_pull_keeps_its_distance_when_the_units_change() {
+    // A face pushed in: the cut's depth is the distance's size.
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 40, "width": 30, "height": 20, "body_name": "Block"}));
+    run(&mut s, "solid.press_pull", json!({"face": [20, 15, 20], "distance": -5}));
+    assert!(rel(shape(&mut s, "Block").volume, 40.0 * 30.0 * 15.0) < 1e-9);
+    keeps_shape_through(&mut s, &["in", "mm"]);
+    // A fillet's face: its radius grows by the distance.
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 40, "width": 30, "height": 20, "body_name": "Block"}));
+    run(&mut s, "solid.fillet", json!({"edges": [[40, 0, 10]], "radius": 3, "name": "Round"}));
+    let corner = |r: f64| (4.0 - PI) * r * r / 4.0 * 20.0;
+    let on = |r: f64| {
+        let d = r - r / 2f64.sqrt();
+        json!([40.0 - d, d, 10.0])
+    };
+    run(&mut s, "solid.press_pull", json!({"face": on(3.0), "distance": 2}));
+    assert!(rel(shape(&mut s, "Block").volume, 24000.0 - corner(5.0)) < 1e-4);
+    keeps_shape_through(&mut s, &["in", "cm", "mm"]);
+    // Pulled again in an inch design: 0.1 more is 2.54 mm more.
+    run(&mut s, "document.units", json!({"units": "in"}));
+    run(&mut s, "solid.press_pull", json!({"face": on(5.0), "distance": 0.1}));
+    assert!(rel(shape(&mut s, "Block").volume, 24000.0 - corner(7.54)) < 1e-4, "{:?}", inputs(&s, 1));
+    keeps_shape_through(&mut s, &["mm", "in"]);
+}
+
+/// A diameter that refers to a unit-less parameter is left as typed: the parameter is read in
+/// the design's units, and when they change it keeps the old one written in, so the part keeps
+/// its size as it did before.
+#[test]
+fn a_diameter_from_a_unitless_parameter_is_left_as_typed() {
+    let mut s = Session::default();
+    run(&mut s, "parameters.add", json!({"name": "w", "expression": "20", "unit": ""}));
+    run(&mut s, "solid.sphere", json!({"diameter": "w", "body_name": "Ball"}));
+    assert_eq!(inputs(&s, 0), ["(w) / 2"]);
+    assert!(rel(shape(&mut s, "Ball").volume, 4.0 / 3.0 * PI * 1000.0) < 1e-3);
+    keeps_shape_through(&mut s, &["in", "mm"]);
+}
+
+/// Beside a unit-less expression that is not a bare number, a bare number stays bare, as it
+/// did: the whole expression is read in the design's units.
+#[test]
+fn bare_numbers_beside_a_unitless_expression_stay_bare() {
+    let mut s = Session::default();
+    run(&mut s, "solid.coil", json!({"diameter": 40, "height": 16, "pitch": "2 * 4", "section_size": 4, "body_name": "A"}));
+    assert_eq!(inputs(&s, 0)[2], "(16) / (2 * 4)", "the revolutions");
+    assert!(s.model.result(s.doc.features[0].id).is_some_and(|r| r.error.is_none()));
+    run(&mut s, "solid.box", json!({"length": 20, "width": 10, "height": 10, "center": ["2 * 5", 0, 0], "body_name": "B"}));
+    assert_eq!(s.doc.features[1].kind.point_expr().map(|p| p[0].clone()), Some("(2 * 5) - (20) / 2".to_string()));
+    let b = shape(&mut s, "B");
+    assert!(b.bbox[0].abs() < 1e-6 && (b.bbox[3] - 20.0).abs() < 1e-6, "{b:?}");
+}

crates/engine/src/lib.rs

@@ -803,6 +803,8 @@ mod construct_tests;
 #[cfg(test)]
 mod delete_face_tests;
 #[cfg(test)]
+mod derived_units_tests;
+#[cfg(test)]
 mod display_mesh_tests;
 #[cfg(test)]
 mod extent_tests;

docs/book/src/guide/parameters.md

@@ -25,7 +25,11 @@ in use can't be deleted (the error lists its users).
 The design's units (Document Settings in the browser: mm, cm, m, in or ft) set how a bare number
 typed into a feature is read. In an inch design `2` means 2 in. The Box, Cylinder, Sphere, Torus
 and Coil dialogs start with their sizes in the design's units, unit shown (`20 mm`, `20 in`).
-Changing the units never moves existing geometry.
+Changing the units never moves existing geometry. A size worked out from a bare number keeps
+the unit the number was given in: a sphere with a bare diameter of 20 has the radius
+`(20 mm) / 2` in a millimetre design. Beside a unit-less expression such as `2 * 5` or a
+unit-less parameter, which is read in the design's units, the number stays bare and is read
+with it.
 
 Parameters export to and import from CSV or JSON (`parameters.export`, `parameters.import`).
 **Configurations** (`config.*`) keep a table of rows, each with its own parameter values and