Skip analyzers that don't emit facts when ignoring diagnostics (#4402)

**What type of PR is this?**

Performance improvement

**What does this PR do? Why is it needed?**

Avoid unnecessary work by skipping analyzers that don't emit facts when
reported diagnostics are known to be ignored.

**Which issues(s) does this PR fix?**

Work towards #4327

**Other notes for review**

Author: fmeum

go/private/actions/compilepkg.bzl

@@ -249,6 +249,9 @@ def _run_nogo(
         nogo_args.add_all([cgo_go_srcs], before_each = "-ignore_src")
 
     nogo_args.add_all(archives, before_each = "-facts", map_each = _facts)
+    if not out_validation:
+        # Since diagnostics are ignored, all analyzers that don't generated facts can be skipped.
+        nogo_args.add("-facts_only")
     nogo_args.add("-out_facts", out_facts)
     nogo_args.add_all("-out", [out_diagnostics], expand_directories = False)
     nogo_args.add("-nogo", nogo.executable)

go/tools/builders/nogo.go

@@ -26,10 +26,12 @@ func nogo(args []string) error {
 	var testFilter string
 	var outFactsPath, outPath string
 	var coverMode string
+	var factsOnly bool
 	fs.Var(&unfilteredSrcs, "src", ".go, .c, .cc, .m, .mm, .s, or .S file to be filtered and checked")
 	fs.Var(&ignoreSrcs, "ignore_src", ".go, .c, .cc, .m, .mm, .s, or .S file to be filtered and checked, but with its diagnostics ignored")
 	fs.Var(&deps, "arc", "Import path, package path, and file name of a direct dependency, separated by '='")
 	fs.Var(&facts, "facts", "Import path, package path, and file name of a direct dependency's nogo facts file, separated by '='")
+	fs.BoolVar(&factsOnly, "facts_only", false, "If true, only nogo facts are emitted, no nogo checks are run")
 	fs.StringVar(&importPath, "importpath", "", "The import path of the package being compiled. Not passed to the compiler, but may be displayed in debug data.")
 	fs.StringVar(&packagePath, "p", "", "The package path (importmap) of the package being compiled")
 	fs.StringVar(&packageListPath, "package_list", "", "The file containing the list of standard library packages")
@@ -83,10 +85,10 @@ func nogo(args []string) error {
 		return err
 	}
 
-	return runNogo(workDir, nogoPath, goSrcs, ignoreSrcs, facts, importPath, importcfgPath, outFactsPath, outPath)
+	return runNogo(workDir, nogoPath, goSrcs, ignoreSrcs, facts, factsOnly, importPath, importcfgPath, outFactsPath, outPath)
 }
 
-func runNogo(workDir string, nogoPath string, srcs, ignores []string, facts []archive, packagePath, importcfgPath, outFactsPath, outDirPath string) error {
+func runNogo(workDir string, nogoPath string, srcs, ignores []string, facts []archive, factsOnly bool, packagePath, importcfgPath, outFactsPath, outDirPath string) error {
 	if len(srcs) == 0 {
 		// emit_compilepkg expects a nogo facts file, even if it's empty.
 		err := os.WriteFile(outFactsPath, nil, 0o666)
@@ -103,6 +105,9 @@ func runNogo(workDir string, nogoPath string, srcs, ignores []string, facts []ar
 	for _, fact := range facts {
 		args = append(args, "-fact", fmt.Sprintf("%s=%s", fact.importPath, fact.file))
 	}
+	if factsOnly {
+		args = append(args, "-facts_only")
+	}
 	args = append(args, "-x", outFactsPath)
 	for _, ignore := range ignores {
 		args = append(args, "-ignore", ignore)

go/tools/builders/nogo_main.go

@@ -74,6 +74,7 @@ func run(args []string) (error, int) {
 	factMap := factMultiFlag{}
 	flags := flag.NewFlagSet("nogo", flag.ExitOnError)
 	flags.Var(&factMap, "fact", "Import path and file containing facts for that library, separated by '=' (may be repeated)'")
+	factsOnly := flags.Bool("facts_only", false, "If true, only facts are emitted, no analyzers are run")
 	importcfg := flags.String("importcfg", "", "The import configuration file")
 	packagePath := flags.String("p", "", "The package path (importmap) of the package being compiled")
 	xPath := flags.String("x", "", "The archive file where serialized facts should be written")
@@ -88,7 +89,7 @@ func run(args []string) (error, int) {
 		return fmt.Errorf("error parsing importcfg: %v", err), nogoError
 	}
 
-	diagnostics, pkg, err := checkPackage(analyzers, *packagePath, packageFile, importMap, factMap, srcs, ignores)
+	diagnostics, pkg, err := checkPackage(analyzers, *packagePath, packageFile, importMap, factMap, *factsOnly, srcs, ignores)
 	if err != nil {
 		return fmt.Errorf("error running analyzers: %v", err), nogoError
 	}
@@ -214,7 +215,7 @@ func setAnalyzerFlags(a *analysis.Analyzer, flags map[string]string) error {
 // It returns an empty string if no source code diagnostics need to be printed.
 //
 // This implementation was adapted from that of golang.org/x/tools/go/checker/internal/checker.
-func checkPackage(analyzers []*analysis.Analyzer, packagePath string, packageFile, importMap, factMap map[string]string, filenames, ignoreFiles []string) ([]diagnosticEntry, *goPackage, error) {
+func checkPackage(analyzers []*analysis.Analyzer, packagePath string, packageFile, importMap, factMap map[string]string, factsOnly bool, filenames, ignoreFiles []string) ([]diagnosticEntry, *goPackage, error) {
 	// Register fact types and establish dependencies between analyzers.
 	actions := make(map[*analysis.Analyzer]*action)
 	var visit func(a *analysis.Analyzer) *action
@@ -258,7 +259,13 @@ func checkPackage(analyzers []*analysis.Analyzer, packagePath string, packageFil
 
 	roots := make([]*action, 0, len(analyzers))
 	for _, a := range analyzers {
-		roots = append(roots, visit(a))
+		if !factsOnly || len(a.FactTypes) > 0 {
+			roots = append(roots, visit(a))
+		}
+	}
+	if len(roots) == 0 {
+		// No analyzers to run, return early.
+		return nil, nil, nil
 	}
 
 	// Load the package, including AST, types, and facts.