cmd/stringer: omit negative bounds check from String for unsigned integers

cmd/stringer/golden_test.go

@@ -38,6 +38,8 @@ var golden = []Golden{
 	{"prefix", "Type", false, prefix_in, prefix_out},
 	{"tokens", "", true, tokens_in, tokens_out},
 	{"overflow8", "", false, overflow8_in, overflow8_out},
+	{"ubounds0", "", false, ubounds0_in, ubounds0_out},
+	{"ubounds1", "", false, ubounds1_in, ubounds1_out},
 }
 
 // Each example starts with "type XXX [u]int", with a single space separating them.
@@ -988,6 +990,57 @@ func (i Overflow8) String() string {
 }
 `
 
+const ubounds0_in = `type UBounds uint8
+const (
+	B0 UBounds = 0
+)
+`
+
+const ubounds0_out = `func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[B0-0]
+}
+
+const _UBounds_name = "B0"
+
+var _UBounds_index = [...]uint8{0, 2}
+
+func (i UBounds) String() string {
+	idx := int(i) - 0
+	if idx >= len(_UBounds_index)-1 {
+		return "UBounds(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _UBounds_name[_UBounds_index[idx]:_UBounds_index[idx+1]]
+}
+`
+
+const ubounds1_in = `type UBounds uint8
+const (
+	B1 UBounds = 1
+)
+`
+const ubounds1_out = `func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[B1-1]
+}
+
+const _UBounds_name = "B1"
+
+var _UBounds_index = [...]uint8{0, 2}
+
+func (i UBounds) String() string {
+	idx := int(i) - 1
+	if i < 1 || idx >= len(_UBounds_index)-1 {
+		return "UBounds(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _UBounds_name[_UBounds_index[idx]:_UBounds_index[idx+1]]
+}
+`
+
 func TestGolden(t *testing.T) {
 	testenv.NeedsTool(t, "go")
 

cmd/stringer/stringer.go

@@ -641,7 +641,12 @@ func (g *Generator) buildOneRun(runs [][]Value, typeName string) {
 	values := runs[0]
 	g.Printf("\n")
 	g.declareIndexAndNameVar(values, typeName)
-	g.Printf(stringOneRun, typeName, values[0].String())
+
+	if values[0].signed || values[0].String() != "0" {
+		g.Printf(stringOneRun, typeName, values[0].String())
+	} else {
+		g.Printf(stringOneRunUnsigned, typeName, values[0].String())
+	}
 }
 
 // Arguments to format are:
@@ -657,6 +662,22 @@ const stringOneRun = `func (i %[1]s) String() string {
 }
 `
 
+// Arguments to format are:
+//
+//	[1]: type name
+//	[2]: lowest defined value for type, as a string
+//
+// This is the unsigned version of [stringOneRun] with the lower bound check
+// removed.
+const stringOneRunUnsigned = `func (i %[1]s) String() string {
+	idx := int(i) - %[2]s
+	if idx >= len(_%[1]s_index)-1 {
+		return "%[1]s(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _%[1]s_name[_%[1]s_index[idx] : _%[1]s_index[idx+1]]
+}
+`
+
 // buildMultipleRuns generates the variables and String method for multiple runs of contiguous values.
 // For this pattern, a single Printf format won't do.
 func (g *Generator) buildMultipleRuns(runs [][]Value, typeName string) {