Add missing lowering of Spicy unit ctor to HILTI struct ctor.

Closes #1275.

spicy/toolchain/src/compiler/codegen/codegen.cc

@@ -244,6 +244,12 @@ struct VisitorPass2 : public hilti::visitor::PreOrder<void, VisitorPass2> {
         replaceNode(&p, std::move(x));
     }
 
+    result_t operator()(const spicy::ctor::Unit& c, position_t p) {
+        // Replace unit ctor with an equivalent struct ctor.
+        auto n = hilti::ctor::Struct(c.fields().copy(), c.meta());
+        replaceNode(&p, n);
+    }
+
     result_t operator()(const operator_::unit::ConnectFilter& n, position_t p) {
         auto x = builder::call("spicy_rt::filter_connect", {n.op0(), argument(n.op2(), 0)});
         replaceNode(&p, std::move(x));

tests/Baseline/spicy.types.unit.var-struct-init/output

@@ -0,0 +1,7 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+foo::Y {
+  x: foo::X {
+    x: 4
+  }
+  y: 2
+}

tests/spicy/types/unit/var-struct-init.spicy

@@ -0,0 +1,15 @@
+# @TEST-DOC: Validates that unit vars can be assigned to with struct initialization. Regression test for #1275.
+#
+# @TEST-EXEC: spicyc -dj %INPUT -o foo.hlto
+# @TEST-EXEC: printf '\x02' | spicy-dump foo.hlto >output 2>&1
+# @TEST-EXEC: btest-diff output
+module foo;
+
+type X = unit {
+    x: uint8;
+};
+
+public type Y = unit {
+    var x: X;
+    y: uint8 { self.x = [$x = $$ * 2]; }
+};