d2ir: fix link panic

ci/release/changelogs/next.md

@@ -18,3 +18,4 @@
 - Board links imported with spread imports work [#1972](https://github.com/terrastruct/d2/pull/1972)
 - Fix importing a file with nested boards [#1998](https://github.com/terrastruct/d2/pull/1998)
 - Fix importing a file with underscores in links [#1999](https://github.com/terrastruct/d2/pull/1999)
+- Replace a panic with an error message resulting from invalid `link` usage [#2011](https://github.com/terrastruct/d2/pull/2011)

d2compiler/compile_test.go

@@ -2986,6 +2986,38 @@ layers: {
 				tassert.Equal(t, "root.layers.x.layers.k", g.Layers[0].Layers[0].Objects[1].Link.Value)
 			},
 		},
+		{
+			name: "invalid-link-1",
+			text: `k
+
+layers: {
+  x: {...@x}
+}`,
+			files: map[string]string{
+				"x.d2": `k
+layers: {
+  y.link: @n
+}`,
+				"n.d2": `n`,
+			},
+			expErr: `d2/testdata/d2compiler/TestCompile/x.d2:3:5: a board itself cannot be linked; only objects within a board can be linked`,
+		},
+		{
+			name: "invalid-link-2",
+			text: `k
+
+layers: {
+  x: @x
+}`,
+			files: map[string]string{
+				"x.d2": `k
+layers: {
+  y.link: @n
+}`,
+				"n.d2": `n`,
+			},
+			expErr: `d2/testdata/d2compiler/TestCompile/x.d2:3:5: a board itself cannot be linked; only objects within a board can be linked`,
+		},
 		{
 			name: "import-nested-layers",
 			text: `k

d2ir/compile.go

@@ -854,8 +854,13 @@ func (c *compiler) ignoreLazyGlob(n Node) bool {
 
 // When importing a file, all of its board links need to be extended to reflect their new path
 func (c *compiler) extendLinks(m *Map, importF *Field) {
+	nodeBoardKind := NodeBoardKind(m)
 	for _, f := range m.Fields {
 		if f.Name == "link" {
+			if nodeBoardKind != "" {
+				c.errorf(f.LastRef().AST(), "a board itself cannot be linked; only objects within a board can be linked")
+				continue
+			}
 			val := f.Primary().Value.ScalarString()
 			link, err := d2parser.ParseKey(val)
 			if err != nil {
@@ -879,8 +884,13 @@ func (c *compiler) extendLinks(m *Map, importF *Field) {
 }
 
 func (c *compiler) updateLinks(m *Map) {
+	nodeBoardKind := NodeBoardKind(m)
 	for _, f := range m.Fields {
 		if f.Name == "link" {
+			if nodeBoardKind != "" {
+				c.errorf(f.LastRef().AST(), "a board itself cannot be linked; only objects within a board can be linked")
+				continue
+			}
 			val := f.Primary().Value.ScalarString()
 			link, err := d2parser.ParseKey(val)
 			if err != nil {

testdata/d2compiler/TestCompile/invalid-link-1.exp.json

@@ -0,0 +1,11 @@
+{
+  "graph": null,
+  "err": {
+    "errs": [
+      {
+        "range": "d2/testdata/d2compiler/TestCompile/x.d2,2:4:16-2:8:20",
+        "errmsg": "d2/testdata/d2compiler/TestCompile/x.d2:3:5: a board itself cannot be linked; only objects within a board can be linked"
+      }
+    ]
+  }
+}

testdata/d2compiler/TestCompile/invalid-link-2.exp.json

@@ -0,0 +1,11 @@
+{
+  "graph": null,
+  "err": {
+    "errs": [
+      {
+        "range": "d2/testdata/d2compiler/TestCompile/x.d2,2:4:16-2:8:20",
+        "errmsg": "d2/testdata/d2compiler/TestCompile/x.d2:3:5: a board itself cannot be linked; only objects within a board can be linked"
+      }
+    ]
+  }
+}

testdata/d2compiler/TestCompile/invalid-link.exp.json

@@ -0,0 +1,11 @@
+{
+  "graph": null,
+  "err": {
+    "errs": [
+      {
+        "range": "d2/testdata/d2compiler/TestCompile/x.d2,2:4:16-2:8:20",
+        "errmsg": "d2/testdata/d2compiler/TestCompile/x.d2:3:5: a board itself cannot be linked; only objects within a board can be linked"
+      }
+    ]
+  }
+}