show struct dependency cycles in error & update module cycle error (#2517)

vlib/compiler/cgen.v

@@ -361,7 +361,9 @@ fn sort_structs(types []Type) []Type {
 	// sort graph
 	dep_graph_sorted := dep_graph.resolve()
 	if !dep_graph_sorted.acyclic {
-		verror('error: cgen.sort_structs() DGNAC.\nplease create a new issue here: https://github.com/vlang/v/issues and tag @joe-conigliaro')
+		verror('cgen.sort_structs(): the following structs form a dependancy cycle:\n' +
+			dep_graph_sorted.display_cycles() +
+			'\nif you feel this is an error, please create a new issue here: https://github.com/vlang/v/issues and tag @joe-conigliaro')
 	}
 	// sort types
 	mut types_sorted := []Type

vlib/compiler/depgraph.v

@@ -11,7 +11,6 @@ struct DepGraphNode  {
 mut:
 	name string
 	deps []string
-	last_cycle string
 }
 
 struct DepGraph {
@@ -84,13 +83,8 @@ pub fn(graph &DepGraph) resolve() &DepGraph {
 		if ready_set.size() == 0 {
 			mut g := new_dep_graph()
 			g.acyclic = false
-			ndk := node_deps.keys()
-			for name, _ in node_deps {				
-				mut node := node_names[name]
-				if name == ndk[node_deps.size-1] {
-					node.last_cycle = node_deps[name].items[node_deps[name].items.len-1]
-				}
-				g.nodes << node
+			for name, _ in node_deps {
+				g.nodes << node_names[name]
 			}
 			return g
 		}
@@ -112,19 +106,31 @@ pub fn(graph &DepGraph) last_node() DepGraphNode {
 	return graph.nodes[graph.nodes.len-1]
 }
 
-pub fn(graph &DepGraph) last_cycle() string {
-	return graph.last_node().last_cycle
-}
-
-pub fn(graph &DepGraph) display() {
-	for i:=0; i<graph.nodes.len; i++ {
-		node := graph.nodes[i]
+pub fn(graph &DepGraph) display() string {
+	mut out := '\n'
+	for node in graph.nodes {
 		for dep in node.deps {
-			mut out := ' * $node.name -> $dep'
-			if !graph.acyclic && i == graph.nodes.len-1 && dep == node.last_cycle {
-				out += ' <-- last cycle'
-			}
-			println(out)
+			out += ' * $node.name -> $dep\n'
 		}
 	}
+	return out
 }
+
+pub fn(graph &DepGraph) display_cycles() string {
+	mut node_names := map[string]DepGraphNode
+	for node in graph.nodes {
+		node_names[node.name] = node
+	}
+	mut out := '\n'
+	for node in graph.nodes {
+		for dep in node.deps {
+			if !(dep in node_names) { continue }
+			dn := node_names[dep]
+			if node.name in dn.deps {
+				out += ' * $node.name -> $dep\n'
+			}
+		}
+	}
+	return out
+}
+

vlib/compiler/main.v

@@ -733,8 +733,7 @@ pub fn (v &V) resolve_deps() &DepGraph {
 	dep_graph.from_import_tables(v.table.file_imports)
 	deps_resolved := dep_graph.resolve()
 	if !deps_resolved.acyclic {
-		deps_resolved.display()
-		verror('import cycle detected')
+		verror('import cycle detected between the following modules: \n' + deps_resolved.display_cycles())
 	}
 	return deps_resolved
 }