Safe spread optimizations

src/transforms/safe.lsc

@@ -100,6 +100,24 @@ export transformExistentialExpression(path) ->
   )
 
 export transformSafeSpreadElement(path) ->
+  { node: { argument } } = path
+  // Optimization: if the thing being spread is already an array, elide
+  if argument~isa("ArrayExpression"):
+    path.node._safe = true
+    return
+
+  // Optimization: if spreading an `IfExpr` without an alternate, just add
+  // `[]` as an alternate if consequent is an array
+  if (
+    argument~isa("IfExpression") and
+    argument.consequent~isa("ArrayExpression") and
+    (not argument.alternate)
+  ):
+    argument.alternate = t.arrayExpression()
+    path.get("argument").replaceWith(argument)
+    path.node._safe = true
+    return
+
   { ref, assign, isComplex } = hoistRef(path, path.node.argument, "ref")
 
   cond = t.conditionalExpression(

test/fixtures/comprehensions/multi/expected.js

@@ -1,11 +1,9 @@
 (() => {
-  var _ref;
-
   const _arr = [1, 2];
 
   for (let _arr2 = ["buckle", "my", "shoe"], _i = 0, _len = _arr2.length; _i < _len; _i++) {
     const e = _arr2[_i];_arr.push(e);
-  }_arr.push(...(_ref = true ? [3] : void 0, _ref === void 0 ? [] : _ref), 4);
+  }_arr.push(...(true ? [3] : []), 4);
   for (let _arr3 = ["shut", "the", "door"], _i2 = 0, _len2 = _arr3.length; _i2 < _len2; _i2++) {
     const e = _arr3[_i2];_arr.push(e);
   }_arr.push(5, 6, "pickup sticks");

test/fixtures/safe-spread/kitchen-sink/actual.js

@@ -0,0 +1,11 @@
+[...expr]
+
+[...[arrayExpr]]
+
+[...if test: someFunction()]
+
+[...if test: [explicitValue]]
+
+[...if test: consequent else: alternate]
+
+[...if test: [consequent] else: [alternate]]

test/fixtures/safe-spread/kitchen-sink/expected.js

@@ -0,0 +1,13 @@
+var _ref, _ref2, _ref3;
+
+[...(expr === void 0 ? [] : expr)];
+
+[...[arrayExpr]];
+
+[...(_ref = test ? someFunction() : void 0, _ref === void 0 ? [] : _ref)];
+
+[...(test ? [explicitValue] : [])];
+
+[...(_ref2 = test ? consequent : alternate, _ref2 === void 0 ? [] : _ref2)];
+
+[...(_ref3 = test ? [consequent] : [alternate], _ref3 === void 0 ? [] : _ref3)];