compiler: fix vars crashing. Change scoping behavior

ci/release/changelogs/next.md

@@ -4,6 +4,7 @@
 
 - Grid cells can now contain nested edges [#1629](https://github.com/terrastruct/d2/pull/1629)
 - Edges can now go across constant nears, sequence diagrams, and grids including nested ones. [#1631](https://github.com/terrastruct/d2/pull/1631)
+- All vars defined in a scope are accessible everywhere in that scope, i.e., an object can use a var defined after itself. [#1695](https://github.com/terrastruct/d2/pull/1695)
 
 #### Bugfixes ⛑️
 
@@ -15,3 +16,4 @@
 - Fixes elk growing shapes with width/height set [#1679](https://github.com/terrastruct/d2/pull/1679)
 - Adds a compiler error when accidentally using an arrowhead on a shape [#1686](https://github.com/terrastruct/d2/pull/1686)
 - Correctly reports errors from invalid values set by globs. [#1691](https://github.com/terrastruct/d2/pull/1691)
+- Fixes panic when spread substitution referenced a nonexistant var. [#1695](https://github.com/terrastruct/d2/pull/1695)

d2compiler/compile_test.go

@@ -4160,6 +4160,30 @@ mybox: {
 `, "")
 				},
 			},
+			{
+				name: "undeclared-var-usage",
+				run: func(t *testing.T) {
+					assertCompile(t, `
+x: { ...${v} }
+`, `d2/testdata/d2compiler/TestCompile2/vars/errors/undeclared-var-usage.d2:2:4: could not resolve variable "v"`)
+				},
+			},
+			{
+				name: "split-var-usage",
+				run: func(t *testing.T) {
+					assertCompile(t, `
+x1
+
+vars: {
+  v: {
+    style.fill: green
+  }
+}
+
+x1: { ...${v} }
+`, ``)
+				},
+			},
 		}
 
 		for _, tc := range tca {

d2ir/compile.go

@@ -125,6 +125,8 @@ func (c *compiler) compileSubstitutions(m *Map, varsStack []*Map) {
 		if f.Name == "vars" && f.Map() != nil {
 			varsStack = append([]*Map{f.Map()}, varsStack...)
 		}
+	}
+	for _, f := range m.Fields {
 		if f.Primary() != nil {
 			c.resolveSubstitutions(varsStack, f)
 		}
@@ -459,6 +461,12 @@ func (c *compiler) compileMap(dst *Map, ast, scopeAST *d2ast.Map) {
 						Value: []d2ast.InterpolationBox{{Substitution: n.Substitution}},
 					},
 				},
+				References: []*FieldReference{{
+					Context_: &RefContext{
+						Scope:    ast,
+						ScopeMap: dst,
+					},
+				}},
 			}
 			dst.Fields = append(dst.Fields, f)
 		case n.Import != nil: