Performance: string builder speedup for Module.String()

internal/addrs/module.go

@@ -33,11 +33,22 @@ func (m Module) String() string {
 	if len(m) == 0 {
 		return ""
 	}
-	var steps []string
-	for _, s := range m {
-		steps = append(steps, "module", s)
+	// Calculate necessary space.
+	l := 0
+	for _, step := range m {
+		l += len(step)
 	}
-	return strings.Join(steps, ".")
+	buf := strings.Builder{}
+	// 8 is len(".module.") which separates entries.
+	buf.Grow(l + len(m)*8)
+	sep := ""
+	for _, step := range m {
+		buf.WriteString(sep)
+		buf.WriteString("module.")
+		buf.WriteString(step)
+		sep = "."
+	}
+	return buf.String()
 }
 
 func (m Module) Equal(other Module) bool {

internal/addrs/module_test.go

@@ -55,3 +55,42 @@ func TestModuleEqual_false(t *testing.T) {
 		})
 	}
 }
+
+func TestModuleString(t *testing.T) {
+	testCases := map[string]Module{
+		"": {},
+		"module.alpha": {
+			"alpha",
+		},
+		"module.alpha.module.beta": {
+			"alpha",
+			"beta",
+		},
+		"module.alpha.module.beta.module.charlie": {
+			"alpha",
+			"beta",
+			"charlie",
+		},
+	}
+	for str, module := range testCases {
+		t.Run(str, func(t *testing.T) {
+			if got, want := module.String(), str; got != want {
+				t.Errorf("wrong result: got %q, want %q", got, want)
+			}
+		})
+	}
+}
+
+func BenchmarkModuleStringShort(b *testing.B) {
+	module := Module{"a", "b"}
+	for n := 0; n < b.N; n++ {
+		module.String()
+	}
+}
+
+func BenchmarkModuleStringLong(b *testing.B) {
+	module := Module{"southamerica-brazil-region", "user-regional-desktop", "user-name"}
+	for n := 0; n < b.N; n++ {
+		module.String()
+	}
+}