[display] don't warn about unused `_` variables

closes #8147

src/context/display/diagnostics.ml

@@ -20,7 +20,7 @@ let find_unused_variables com e =
 	let vars = Hashtbl.create 0 in
 	let pmin_map = Hashtbl.create 0 in
 	let rec loop e = match e.eexpr with
-		| TVar({v_kind = VUser _} as v,eo) ->
+		| TVar({v_kind = VUser _} as v,eo) when v.v_name <> "_" ->
 			Hashtbl.add pmin_map e.epos.pmin v;
 			let p = match eo with
 				| None -> e.epos