Clippy: the kernel's and engine's warnings on develop

A collapsible if (assemble.rs), two manual assign operations (sew.rs), a range loop indexing
the placement inverse's translation row (step_in), and two needless borrows (engine features),
which failed CI's clippy -D warnings. No behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/features.rs

@@ -238,7 +238,7 @@ fn auto_operation_in(s: &Session, p: &Value, st: &solidcraft_doc::ModelState) ->
     } else if let Some(d) = extrude_reach(s, p) {
         if p.get("to").is_some() { d - start } else { d }
     } else {
-        through_all_reach(&st, p, base)?
+        through_all_reach(st, p, base)?
     };
     if st.bodies.is_empty() || !d.is_finite() {
         return Some("new");
@@ -254,7 +254,7 @@ fn auto_operation_in(s: &Session, p: &Value, st: &solidcraft_doc::ModelState) ->
     } else if implicit_all {
         st.bodies.iter().collect()
     } else {
-        active_scope(s, &st)
+        active_scope(s, st)
     };
     if parts.is_empty()
         && let Some(body) = started_on.as_deref().and_then(|name| st.body(name))

crates/kernel/src/assemble.rs

@@ -281,10 +281,10 @@ impl Assembly {
             let frame = if *takes { form::frame_of(&f.surface) } else { None };
             for u in f.loops.iter().flatten() {
                 let e = u.edge as usize;
-                if u.given.is_some() {
-                    if let Some(g) = given.get_mut(e) {
-                        *g = true;
-                    }
+                if u.given.is_some()
+                    && let Some(g) = given.get_mut(e)
+                {
+                    *g = true;
                 }
                 if !on_form && let Some(o) = open.get_mut(e) {
                     *o = false;

crates/kernel/src/sew.rs

@@ -376,7 +376,7 @@ fn curve_derivs(knots: &[f64], points: &[[f64; 4]], t: f64) -> Option<[Vec3; 4]>
         let h = points.get(first + i)?;
         for k in 0..4 {
             let n = d.get(k).copied().unwrap_or(0.0);
-            a[k] = a[k] + Vec3::new(h[0], h[1], h[2]) * n;
+            a[k] += Vec3::new(h[0], h[1], h[2]) * n;
             w[k] += h[3] * n;
         }
     }
@@ -388,7 +388,7 @@ fn curve_derivs(knots: &[f64], points: &[[f64; 4]], t: f64) -> Option<[Vec3; 4]>
         let mut num = a[k];
         for j in 1..=k {
             let binom = [[1.0, 0.0, 0.0, 0.0], [1.0, 1.0, 0.0, 0.0], [1.0, 2.0, 1.0, 0.0], [1.0, 3.0, 3.0, 1.0]][k][j];
-            num = num - c[k - j] * (binom * w[j]);
+            num -= c[k - j] * (binom * w[j]);
         }
         c[k] = num * (1.0 / w[0]);
     }

crates/kernel/src/step_in/mod.rs

@@ -163,9 +163,9 @@ fn placement_inverse(m: &M4) -> M4 {
             col[i] = row[j] / det;
         }
     }
-    for i in 0..3 {
-        out[3][i] = -(0..3).map(|k| out[k][i] * m[3][k]).sum::<f64>();
-    }
+    // The translation taken back through the inverted linear part.
+    let back: [f64; 3] = std::array::from_fn(|i| -(0..3).map(|k| out[k][i] * m[3][k]).sum::<f64>());
+    out[3][..3].copy_from_slice(&back);
     out
 }