d2parser: Error on leading @ in non import key strings

d2parser/parse.go

@@ -633,7 +633,7 @@ func (p *parser) parseMapKey() (mk *d2ast.Key) {
 		}
 	}()
 
-	// Check for ampersand.
+	// Check for ampersand/@.
 	r, eof := p.peek()
 	if eof {
 		return mk
@@ -967,6 +967,9 @@ func (p *parser) parseKey() (k *d2ast.KeyPath) {
 		if s == nil {
 			return k
 		}
+		if strings.HasPrefix(s.ScalarString(), "@") {
+			p.errorf(s.GetRange().Start, s.GetRange().End, "%s is not a valid import, did you mean ...%[2]s?", s.ScalarString())
+		}
 
 		if len(k.Path) == 0 {
 			k.Range.Start = s.GetRange().Start

d2parser/parse_test.go

@@ -460,7 +460,7 @@ func testImport(t *testing.T) {
 		{
 			text: "@file",
 			assert: func(t testing.TB, ast *d2ast.Map, err error) {
-				assert.ErrorString(t, err, "d2/testdata/d2parser/TestParse/import/#06.d2:1:5: unexpected text after import")
+				assert.ErrorString(t, err, "d2/testdata/d2parser/TestParse/import/#07.d2:1:1: @file is not a valid import, did you mean ...@file?")
 			},
 		},
 	}

testdata/d2parser/TestParse/import/#07.exp.json

@@ -0,0 +1,38 @@
+{
+  "ast": {
+    "range": "d2/testdata/d2parser/TestParse/import/#07.d2,0:0:0-0:5:5",
+    "nodes": [
+      {
+        "map_key": {
+          "range": "d2/testdata/d2parser/TestParse/import/#07.d2,0:0:0-0:5:5",
+          "key": {
+            "range": "d2/testdata/d2parser/TestParse/import/#07.d2,0:0:0-0:5:5",
+            "path": [
+              {
+                "unquoted_string": {
+                  "range": "d2/testdata/d2parser/TestParse/import/#07.d2,0:0:0-0:5:5",
+                  "value": [
+                    {
+                      "string": "@file",
+                      "raw_string": "@file"
+                    }
+                  ]
+                }
+              }
+            ]
+          },
+          "primary": {},
+          "value": {}
+        }
+      }
+    ]
+  },
+  "err": {
+    "errs": [
+      {
+        "range": "d2/testdata/d2parser/TestParse/import/#07.d2,0:0:0-0:5:5",
+        "errmsg": "d2/testdata/d2parser/TestParse/import/#07.d2:1:1: @file is not a valid import, did you mean ...@file?"
+      }
+    ]
+  }
+}