Speed up linting: use deduplicated packages (#667)

Use deduplicated packages for all linters except megacheck.
See https://github.com/golangci/golangci-lint/pull/585 for details.

pkg/golinters/megacheck.go

@@ -177,7 +177,9 @@ func (m MegacheckMetalinter) isValidChild(name string) bool {
 }
 
 func (m megacheck) Run(ctx context.Context, lintCtx *linter.Context) ([]result.Issue, error) {
-	issues, err := m.runMegacheck(lintCtx.Packages, lintCtx.Settings().Unused.CheckExported)
+	// Use OriginalPackages not Packages because `unused` doesn't work properly
+	// when we deduplicate normal and test packages.
+	issues, err := m.runMegacheck(lintCtx.OriginalPackages, lintCtx.Settings().Unused.CheckExported)
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to run megacheck")
 	}
@@ -231,7 +233,7 @@ func (m megacheck) runMegacheck(workingPkgs []*packages.Package, checkExportedUn
 	opts := &lintutil.Options{
 		// TODO: get current go version, but now it doesn't matter,
 		// may be needed after next updates of megacheck
-		GoVersion: 11,
+		GoVersion: 12,
 
 		Config: cfg,
 		// TODO: support Ignores option

pkg/lint/linter/context.go

@@ -13,7 +13,13 @@ import (
 )
 
 type Context struct {
+	// Packages are deduplicated (test and normal packages) packages
 	Packages []*packages.Package
+
+	// OriginalPackages aren't deduplicated: they contain both normal and test
+	// version for each of packages
+	OriginalPackages []*packages.Package
+
 	NotCompilingPackages []*packages.Package
 
 	LoaderConfig *loader.Config  // deprecated, don't use for new linters

pkg/lint/load.go

@@ -381,7 +381,12 @@ func (cl ContextLoader) Load(ctx context.Context, linters []*linter.Config) (*li
 	}
 
 	ret := &linter.Context{
-		Packages:   pkgs,
+		Packages: deduplicatedPkgs,
+
+		// At least `unused` linters works properly only on original (not deduplicated) packages,
+		// see https://github.com/golangci/golangci-lint/pull/585.
+		OriginalPackages: pkgs,
+
 		Program:    prog,
 		SSAProgram: ssaProg,
 		LoaderConfig: &loader.Config{
@@ -402,6 +407,7 @@ func (cl ContextLoader) Load(ctx context.Context, linters []*linter.Config) (*li
 // separateNotCompilingPackages moves not compiling packages into separate slice:
 // a lot of linters crash on such packages
 func separateNotCompilingPackages(lintCtx *linter.Context) {
+	// Separate deduplicated packages
 	goodPkgs := make([]*packages.Package, 0, len(lintCtx.Packages))
 	for _, pkg := range lintCtx.Packages {
 		if pkg.IllTyped {
@@ -415,4 +421,13 @@ func separateNotCompilingPackages(lintCtx *linter.Context) {
 	if len(lintCtx.NotCompilingPackages) != 0 {
 		lintCtx.Log.Infof("Packages that do not compile: %+v", lintCtx.NotCompilingPackages)
 	}
+
+	// Separate original (not deduplicated) packages
+	goodOriginalPkgs := make([]*packages.Package, 0, len(lintCtx.OriginalPackages))
+	for _, pkg := range lintCtx.OriginalPackages {
+		if !pkg.IllTyped {
+			goodOriginalPkgs = append(goodOriginalPkgs, pkg)
+		}
+	}
+	lintCtx.OriginalPackages = goodOriginalPkgs
 }