d2ir: spread vars in place

ci/release/changelogs/next.md

@@ -17,3 +17,4 @@
 - Globs: An edge case was fixed where globs used in edges were creating nodes when it shouldn't have [#2051](https://github.com/terrastruct/d2/pull/2051)
 - Render: Multi-line class labels/headers are rendered correctly [#2057](https://github.com/terrastruct/d2/pull/2057)
 - CLI: Watch mode uses correct backlinks (`_` usages) [#2058](https://github.com/terrastruct/d2/pull/2058)
+- Vars: Spread variables are inserted in place instead of appending to end of scope [#2062](https://github.com/terrastruct/d2/pull/2062)

d2ir/compile.go

@@ -261,11 +261,11 @@ func (c *compiler) resolveSubstitutions(varsStack []*Map, node Node) (removedFie
 							}
 						}
 					case *Field:
+						m := ParentMap(n)
 						if resolvedField.Map() != nil {
-							OverlayMap(ParentMap(n), resolvedField.Map())
+							ExpandSubstitution(m, resolvedField.Map(), n)
 						}
 						// Remove the placeholder field
-						m := n.parent.(*Map)
 						for i, f2 := range m.Fields {
 							if n == f2 {
 								m.Fields = append(m.Fields[:i], m.Fields[i+1:]...)

d2ir/compile_test.go

@@ -28,6 +28,7 @@ func TestCompile(t *testing.T) {
 	t.Run("imports", testCompileImports)
 	t.Run("patterns", testCompilePatterns)
 	t.Run("filters", testCompileFilters)
+	t.Run("vars", testCompileVars)
 }
 
 type testCase struct {
@@ -698,3 +699,32 @@ layers: {
 	}
 	runa(t, tca)
 }
+
+func testCompileVars(t *testing.T) {
+	t.Parallel()
+	tca := []testCase{
+		{
+			name: "spread-in-place",
+			run: func(t testing.TB) {
+				m, err := compile(t, `vars: {
+  person-shape: {
+    grid-columns: 1
+    grid-rows: 2
+    grid-gap: 0
+    head
+    body
+  }
+}
+
+dora: {
+  ...${person-shape}
+  body
+}
+`)
+				assert.Success(t, err)
+				assert.Equal(t, "grid-columns", m.Fields[1].Map().Fields[0].Name)
+			},
+		},
+	}
+	runa(t, tca)
+}

d2ir/merge.go

@@ -21,6 +21,39 @@ func OverlayMap(base, overlay *Map) {
 	}
 }
 
+func ExpandSubstitution(m, resolved *Map, placeholder *Field) {
+	fi := -1
+	for i := 0; i < len(m.Fields); i++ {
+		if m.Fields[i] == placeholder {
+			fi = i
+			break
+		}
+	}
+
+	for _, of := range resolved.Fields {
+		bf := m.GetField(of.Name)
+		if bf == nil {
+			m.Fields = append(m.Fields[:fi], append([]*Field{of.Copy(m).(*Field)}, m.Fields[fi:]...)...)
+			fi++
+			continue
+		}
+		OverlayField(bf, of)
+	}
+
+	// NOTE this doesn't expand edges in place, and just appends
+	// I suppose to do this, there needs to be an edge placeholder too on top of the field placeholder
+	// Will wait to see if a problem
+	for _, oe := range resolved.Edges {
+		bea := m.GetEdges(oe.ID, nil, nil)
+		if len(bea) == 0 {
+			m.Edges = append(m.Edges, oe.Copy(m).(*Edge))
+			continue
+		}
+		be := bea[0]
+		OverlayEdge(be, oe)
+	}
+}
+
 func OverlayField(bf, of *Field) {
 	if of.Primary_ != nil {
 		bf.Primary_ = of.Primary_.Copy(bf).(*Scalar)