cache parsed runtime Go minor during package loading

Result: {"status":"keep","warm_internal_build_wall":28.574,"go_reported_s":27.996,"baseline_s":30.326,"patched_s":28.574,"delta_s":-1.752,"base_go_reported_s":29.768,"patched_go_reported_s":27.996,"wall_s":28.574}

Author: cpunion

internal/packages/load.go

@@ -67,6 +67,24 @@ const (
 	DebugPackagesLoad = false
 )
 
+var runtimeGoMinor = parseRuntimeGoMinor(runtime.Version())
+
+func parseRuntimeGoMinor(version string) int {
+	const prefix = "go1."
+	if !strings.HasPrefix(version, prefix) {
+		return 0
+	}
+	minor := 0
+	for i := len(prefix); i < len(version); i++ {
+		c := version[i]
+		if c < '0' || c > '9' {
+			break
+		}
+		minor = minor*10 + int(c-'0')
+	}
+	return minor
+}
+
 // A Config specifies details about how packages should be loaded.
 // The zero value is a valid configuration.
 // Calls to Load do not modify this struct.
@@ -303,8 +321,7 @@ func loadPackageEx(dedup Deduper, ld *loader, lpkg *loaderPackage) {
 	// - golang.org/issue/55883 (go/packages confusing error)
 	//
 	// Should we assert a hard minimum of (currently) go1.16 here?
-	var runtimeVersion int
-	if _, err := fmt.Sscanf(runtime.Version(), "go1.%d", &runtimeVersion); err == nil && runtimeVersion < lpkg.goVersion {
+	if runtimeVersion := runtimeGoMinor; runtimeVersion != 0 && runtimeVersion < lpkg.goVersion {
 		defer func() {
 			if len(lpkg.Errors) > 0 {
 				appendError(packages.Error{

internal/packages/load_test.go

@@ -0,0 +1,22 @@
+package packages
+
+import "testing"
+
+func TestParseRuntimeGoMinor(t *testing.T) {
+	tests := []struct {
+		version string
+		want    int
+	}{
+		{"go1.21.13", 21},
+		{"go1.26.0", 26},
+		{"go1.26rc1", 26},
+		{"devel go1.27", 0},
+		{"go2.0", 0},
+		{"", 0},
+	}
+	for _, tt := range tests {
+		if got := parseRuntimeGoMinor(tt.version); got != tt.want {
+			t.Fatalf("parseRuntimeGoMinor(%q) = %d, want %d", tt.version, got, tt.want)
+		}
+	}
+}