give err msg for invalid edge style

ci/release/changelogs/next.md

@@ -3,6 +3,7 @@
 #### Improvements 🧹
 
 - ELK nodes with > 1 connection grow to ensure padding around ports [#981](https://github.com/terrastruct/d2/pull/981)
+- Using a style keyword incorrectly in connections returns clear error message [#989](https://github.com/terrastruct/d2/pull/989)
 
 #### Bugfixes ⛑️
 

d2compiler/compile.go

@@ -450,6 +450,11 @@ func (c *compiler) compileEdge(obj *d2graph.Object, e *d2ir.Edge) {
 
 func (c *compiler) compileEdgeField(edge *d2graph.Edge, f *d2ir.Field) {
 	keyword := strings.ToLower(f.Name)
+	_, isStyleReserved := d2graph.StyleKeywords[keyword]
+	if isStyleReserved {
+		c.errorf(f.LastRef().AST(), "%v must be style.%v", f.Name, f.Name)
+		return
+	}
 	_, isReserved := d2graph.SimpleReservedKeywords[keyword]
 	if isReserved {
 		c.compileReserved(edge.Attributes, f)

d2compiler/compile_test.go

@@ -1117,6 +1117,24 @@ x: {
 `,
 			expErr: `d2/testdata/d2compiler/TestCompile/shape_edge_style.d2:3:2: key "animated" can only be applied to edges`,
 		},
+		{
+			name: "edge_invalid_style",
+
+			text: `x -> y: {
+  opacity: 0.5
+}
+`,
+			expErr: `d2/testdata/d2compiler/TestCompile/edge_invalid_style.d2:2:3: opacity must be style.opacity`,
+		},
+		{
+			name: "obj_invalid_style",
+
+			text: `x: {
+  opacity: 0.5
+}
+`,
+			expErr: `d2/testdata/d2compiler/TestCompile/obj_invalid_style.d2:2:3: opacity must be style.opacity`,
+		},
 		{
 			name: "edge_chain_map",
 

testdata/d2compiler/TestCompile/edge_invalid_style.exp.json

@@ -0,0 +1,12 @@
+{
+  "graph": null,
+  "err": {
+    "ioerr": null,
+    "errs": [
+      {
+        "range": "d2/testdata/d2compiler/TestCompile/edge_invalid_style.d2,1:2:12-1:9:19",
+        "errmsg": "d2/testdata/d2compiler/TestCompile/edge_invalid_style.d2:2:3: opacity must be style.opacity"
+      }
+    ]
+  }
+}

testdata/d2compiler/TestCompile/obj_invalid_style.exp.json

@@ -0,0 +1,12 @@
+{
+  "graph": null,
+  "err": {
+    "ioerr": null,
+    "errs": [
+      {
+        "range": "d2/testdata/d2compiler/TestCompile/obj_invalid_style.d2,1:2:7-1:9:14",
+        "errmsg": "d2/testdata/d2compiler/TestCompile/obj_invalid_style.d2:2:3: opacity must be style.opacity"
+      }
+    ]
+  }
+}