cmd/go/internal/modget: consolidate Load entrypoints

This change replaces ImportPaths, ImportPathsQuiet, LoadALL, and
LoadVendor with a single LoadPackages function, with a LoadOpts struct
that more clearly documents the variations in behavior.

It also eliminates the cmd/go/internal/load.ImportPaths function,
which was undocumented and had only one call site (within its own
package).

The modload.LoadTests global variable is subsumed by a field in the
new LoadOpts struct, and is no longer needed for callers that invoke
LoadPackages directly. It has been (temporarily) replaced with a
similar global variable, load.ModResolveTests, which can itself be
converted to an explicit, local argument.

For #37438
For #36460
Updates #40775
Fixes #26977

Change-Id: I4fb6086c01b04de829d98875db19cf0118d40f8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/255938
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/list/list.go

@@ -325,7 +325,7 @@ var (
 var nl = []byte{'\n'}
 
 func runList(ctx context.Context, cmd *base.Command, args []string) {
-	modload.LoadTests = *listTest
+	load.ModResolveTests = *listTest
 	work.BuildInit()
 	out := newTrackingWriter(os.Stdout)
 	defer out.w.Flush()

src/cmd/go/internal/load/pkg.go

@@ -742,7 +742,7 @@ func loadPackageData(path, parentPath, parentDir, parentRoot string, parentIsStd
 	// For vendored imports, it is the expanded form.
 	//
 	// Note that when modules are enabled, local import paths are normally
-	// canonicalized by modload.ImportPaths before now. However, if there's an
+	// canonicalized by modload.LoadPackages before now. However, if there's an
 	// error resolving a local path, it will be returned untransformed
 	// so that 'go list -e' reports something useful.
 	importKey := importSpec{
@@ -885,7 +885,7 @@ var preloadWorkerCount = runtime.GOMAXPROCS(0)
 // because of global mutable state that cannot safely be read and written
 // concurrently. In particular, packageDataCache may be cleared by "go get"
 // in GOPATH mode, and modload.loaded (accessed via modload.Lookup) may be
-// modified by modload.ImportPaths.
+// modified by modload.LoadPackages.
 type preload struct {
 	cancel chan struct{}
 	sema   chan struct{}
@@ -2106,6 +2106,18 @@ func LoadImportWithFlags(path, srcDir string, parent *Package, stk *ImportStack,
 	return p
 }
 
+// ModResolveTests indicates whether calls to the module loader should also
+// resolve test dependencies of the requested packages.
+//
+// If ModResolveTests is true, then the module loader needs to resolve test
+// dependencies at the same time as packages; otherwise, the test dependencies
+// of those packages could be missing, and resolving those missing dependencies
+// could change the selected versions of modules that provide other packages.
+//
+// TODO(#40775): Change this from a global variable to an explicit function
+// argument where needed.
+var ModResolveTests bool
+
 // Packages returns the packages named by the
 // command line arguments 'args'. If a named package
 // cannot be loaded at all (for example, if the directory does not exist),
@@ -2147,7 +2159,18 @@ func PackagesAndErrors(ctx context.Context, patterns []string) []*Package {
 		}
 	}
 
-	matches := ImportPaths(ctx, patterns)
+	var matches []*search.Match
+	if modload.Init(); cfg.ModulesEnabled {
+		loadOpts := modload.PackageOpts{
+			ResolveMissingImports: true,
+			LoadTests:             ModResolveTests,
+			AllowErrors:           true,
+		}
+		matches, _ = modload.LoadPackages(ctx, loadOpts, patterns...)
+	} else {
+		matches = search.ImportPaths(patterns)
+	}
+
 	var (
 		pkgs    []*Package
 		stk     ImportStack
@@ -2217,13 +2240,6 @@ func setToolFlags(pkgs ...*Package) {
 	}
 }
 
-func ImportPaths(ctx context.Context, args []string) []*search.Match {
-	if modload.Init(); cfg.ModulesEnabled {
-		return modload.ImportPaths(ctx, args)
-	}
-	return search.ImportPaths(args)
-}
-
 // PackagesForBuild is like Packages but exits
 // if any of the packages or their dependencies have errors
 // (cannot be built).

src/cmd/go/internal/modcmd/tidy.go

@@ -9,6 +9,7 @@ package modcmd
 import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 	"context"
 )
@@ -49,11 +50,15 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	// those packages. In order to make 'go test' reproducible for the packages
 	// that are in 'all' but outside of the main module, we must explicitly
 	// request that their test dependencies be included.
-	modload.LoadTests = true
 	modload.ForceUseModules = true
 	modload.RootMode = modload.NeedRoot
 
-	modload.LoadALL(ctx)
+	modload.LoadPackages(ctx, modload.PackageOpts{
+		Tags:                  imports.AnyTags(),
+		ResolveMissingImports: true,
+		LoadTests:             true,
+		AllowErrors:           false, // TODO(#26603): Make this a flag.
+	}, "all")
 	modload.TidyBuildList()
 	modload.TrimGoSum()
 	modload.WriteGoMod()

src/cmd/go/internal/modcmd/vendor.go

@@ -49,7 +49,13 @@ func runVendor(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	modload.ForceUseModules = true
 	modload.RootMode = modload.NeedRoot
-	pkgs := modload.LoadVendor(ctx)
+
+	loadOpts := modload.PackageOpts{
+		Tags:                  imports.AnyTags(),
+		ResolveMissingImports: true,
+		UseVendorAll:          true,
+	}
+	_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
 
 	vdir := filepath.Join(modload.ModRoot(), "vendor")
 	if err := os.RemoveAll(vdir); err != nil {

src/cmd/go/internal/modcmd/why.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 
 	"golang.org/x/mod/module"
@@ -63,12 +64,14 @@ func init() {
 func runWhy(ctx context.Context, cmd *base.Command, args []string) {
 	modload.ForceUseModules = true
 	modload.RootMode = modload.NeedRoot
-	loadALL := modload.LoadALL
+
-	if *whyVendor {
+	loadOpts := modload.PackageOpts{
-		loadALL = modload.LoadVendor
+		Tags:         imports.AnyTags(),
-	} else {
+		LoadTests:    !*whyVendor,
-		modload.LoadTests = true
+		AllowErrors:  true,
+		UseVendorAll: *whyVendor,
 	}
+
 	if *whyM {
 		listU := false
 		listVersions := false
@@ -80,7 +83,8 @@ func runWhy(ctx context.Context, cmd *base.Command, args []string) {
 		}
 		mods := modload.ListModules(ctx, args, listU, listVersions, listRetractions)
 		byModule := make(map[module.Version][]string)
-		for _, path := range loadALL(ctx) {
+		_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
+		for _, path := range pkgs {
 			m := modload.PackageModule(path)
 			if m.Path != "" {
 				byModule[m] = append(byModule[m], path)
@@ -109,8 +113,11 @@ func runWhy(ctx context.Context, cmd *base.Command, args []string) {
 			sep = "\n"
 		}
 	} else {
-		matches := modload.ImportPaths(ctx, args) // resolve to packages
+		// Resolve to packages.
-		loadALL(ctx)                              // rebuild graph, from main module (not from named packages)
+		matches, _ := modload.LoadPackages(ctx, loadOpts, args...)
+
+		modload.LoadPackages(ctx, loadOpts, "all") // rebuild graph, from main module (not from named packages)
+
 		sep := ""
 		for _, m := range matches {
 			for _, path := range m.Pkgs {

src/cmd/go/internal/modget/get.go

@@ -284,7 +284,7 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	if cfg.Insecure {
 		fmt.Fprintf(os.Stderr, "go get: -insecure flag is deprecated; see 'go help get' for details\n")
 	}
-	modload.LoadTests = *getT
+	load.ModResolveTests = *getT
 
 	// Do not allow any updating of go.mod until we've applied
 	// all the requested changes and checked that the result matches
@@ -314,7 +314,7 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 
 	// Add missing modules to the build list.
 	// We call SetBuildList here and elsewhere, since newUpgrader,
-	// ImportPathsQuiet, and other functions read the global build list.
+	// LoadPackages, and other functions read the global build list.
 	for _, q := range queries {
 		if _, ok := selectedVersion[q.m.Path]; !ok && q.m.Version != "none" {
 			buildList = append(buildList, q.m)
@@ -400,9 +400,16 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 
 		if len(pkgPatterns) > 0 {
 			// Don't load packages if pkgPatterns is empty. Both
-			// modload.ImportPathsQuiet and ModulePackages convert an empty list
+			// modload.LoadPackages and ModulePackages convert an empty list
 			// of patterns to []string{"."}, which is not what we want.
-			matches = modload.ImportPathsQuiet(ctx, pkgPatterns, imports.AnyTags())
+			loadOpts := modload.PackageOpts{
+				Tags:                     imports.AnyTags(),
+				ResolveMissingImports:    true, // dubious; see https://golang.org/issue/32567
+				LoadTests:                *getT,
+				AllowErrors:              true, // Errors may be fixed by subsequent upgrades or downgrades.
+				SilenceUnmatchedWarnings: true, // We will warn after iterating below.
+			}
+			matches, _ = modload.LoadPackages(ctx, loadOpts, pkgPatterns...)
 			seenPkgs = make(map[string]bool)
 			for i, match := range matches {
 				arg := pkgGets[i]

src/cmd/go/internal/modload/build.go

@@ -50,7 +50,7 @@ func findStandardImportPath(path string) string {
 // PackageModuleInfo returns information about the module that provides
 // a given package. If modules are not enabled or if the package is in the
 // standard library or if the package was not successfully loaded with
-// ImportPaths or a similar loading function, nil is returned.
+// LoadPackages or ImportFromFiles, nil is returned.
 func PackageModuleInfo(pkgpath string) *modinfo.ModulePublic {
 	if isStandardImportPath(pkgpath) || !Enabled() {
 		return nil
@@ -250,8 +250,7 @@ func moduleInfo(ctx context.Context, m module.Version, fromBuildList, listRetrac
 
 // PackageBuildInfo returns a string containing module version information
 // for modules providing packages named by path and deps. path and deps must
-// name packages that were resolved successfully with ImportPaths or one of
+// name packages that were resolved successfully with LoadPackages.
-// the Load functions.
 func PackageBuildInfo(path string, deps []string) string {
 	if isStandardImportPath(path) || !Enabled() {
 		return ""
@@ -321,9 +320,8 @@ func mustFindModule(target, path string) module.Version {
 }
 
 // findModule searches for the module that contains the package at path.
-// If the package was loaded with ImportPaths or one of the other loading
+// If the package was loaded, its containing module and true are returned.
-// functions, its containing module and true are returned. Otherwise,
+// Otherwise, module.Version{} and false are returend.
-// module.Version{} and false are returend.
 func findModule(path string) (module.Version, bool) {
 	if pkg, ok := loaded.pkgCache.Get(path).(*loadPkg); ok {
 		return pkg.mod, pkg.mod != module.Version{}

src/cmd/go/internal/modload/buildlist.go

@@ -17,8 +17,8 @@ import (
 )
 
 // buildList is the list of modules to use for building packages.
-// It is initialized by calling ImportPaths, ImportFromFiles,
+// It is initialized by calling LoadPackages or ImportFromFiles,
-// LoadALL, or LoadBuildList, each of which uses loaded.load.
+// each of which uses loaded.load.
 //
 // Ideally, exactly ONE of those functions would be called,
 // and exactly once. Most of the time, that's true.
@@ -31,8 +31,8 @@ var buildList []module.Version
 // module pattern, starting with the Target module and in a deterministic
 // (stable) order, without loading any packages.
 //
-// Modules are loaded automatically (and lazily) in ImportPaths:
+// Modules are loaded automatically (and lazily) in LoadPackages:
-// LoadAllModules need only be called if ImportPaths is not,
+// LoadAllModules need only be called if LoadPackages is not,
 // typically in commands that care about modules but no particular package.
 //
 // The caller must not modify the returned list.
@@ -44,7 +44,7 @@ func LoadAllModules(ctx context.Context) []module.Version {
 }
 
 // LoadedModules returns the list of module requirements loaded or set by a
-// previous call (typically LoadAllModules or ImportPaths), starting with the
+// previous call (typically LoadAllModules or LoadPackages), starting with the
 // Target module and in a deterministic (stable) order.
 //
 // The caller must not modify the returned list.
@@ -71,8 +71,8 @@ func ReloadBuildList() []module.Version {
 }
 
 // TidyBuildList trims the build list to the minimal requirements needed to
-// retain the same versions of all packages from the preceding Load* or
+// retain the same versions of all packages from the preceding call to
-// ImportPaths* call.
+// LoadPackages.
 func TidyBuildList() {
 	used := map[module.Version]bool{Target: true}
 	for _, pkg := range loaded.pkgs {

src/cmd/go/internal/modload/init.go

@@ -83,12 +83,12 @@ const (
 
 // ModFile returns the parsed go.mod file.
 //
-// Note that after calling ImportPaths or LoadBuildList,
+// Note that after calling LoadPackages or LoadAllModules,
 // the require statements in the modfile.File are no longer
 // the source of truth and will be ignored: edits made directly
 // will be lost at the next call to WriteGoMod.
 // To make permanent changes to the require statements
-// in go.mod, edit it before calling ImportPaths or LoadBuildList.
+// in go.mod, edit it before loading.
 func ModFile() *modfile.File {
 	Init()
 	if modFile == nil {
@@ -943,9 +943,9 @@ func WriteGoMod() {
 
 // keepSums returns a set of module sums to preserve in go.sum. The set
 // includes entries for all modules used to load packages (according to
-// the last load function like ImportPaths, LoadALL, etc.). It also contains
+// the last load function such as LoadPackages or ImportFromFiles).
-// entries for go.mod files needed for MVS (the version of these entries
+// It also contains entries for go.mod files needed for MVS (the version
-// ends with "/go.mod").
+// of these entries ends with "/go.mod").
 //
 // If addDirect is true, the set also includes sums for modules directly
 // required by go.mod, as represented by the index, with replacements applied.
@@ -977,8 +977,7 @@ func keepSums(addDirect bool) map[module.Version]bool {
 	}
 	walk(Target)
 
-	// Add entries for modules that provided packages loaded with ImportPaths,
+	// Add entries for modules from which packages were loaded.
-	// LoadALL, or similar functions.
 	if loaded != nil {
 		for _, pkg := range loaded.pkgs {
 			m := pkg.mod

src/cmd/go/internal/modload/load.go

@@ -7,9 +7,9 @@ package modload
 // This file contains the module-mode package loader, as well as some accessory
 // functions pertaining to the package import graph.
 //
-// There are several exported entry points into package loading (such as
+// There are two exported entry points into package loading — LoadPackages and
-// ImportPathsQuiet and LoadALL), but they are all implemented in terms of
+// ImportFromFiles — both implemented in terms of loadFromRoots, which itself
-// loadFromRoots, which itself manipulates an instance of the loader struct.
+// manipulates an instance of the loader struct.
 //
 // Although most of the loading state is maintained in the loader struct,
 // one key piece - the build list - is a global, so that it can be modified
@@ -24,13 +24,12 @@ package modload
 //
 // The first step of each iteration identifies a set of “root” packages.
 // Normally the root packages are exactly those matching the named pattern
-// arguments. However, for the "all" meta-pattern and related functions
+// arguments. However, for the "all" meta-pattern, the final set of packages is
-// (LoadALL, LoadVendor), the final set of packages is computed from the package
+// computed from the package import graph, and therefore cannot be an initial
-// import graph, and therefore cannot be an initial input to loading that graph.
+// input to loading that graph. Instead, the root packages for the "all" pattern
-// Instead, the root packages for the "all" pattern are those contained in the
+// are those contained in the main module, and allPatternIsRoot parameter to the
-// main module, and allPatternIsRoot parameter to the loader instructs it to
+// loader instructs it to dynamically expand those roots to the full "all"
-// dynamically expand those roots to the full "all" pattern as loading
+// pattern as loading progresses.
-// progresses.
 //
 // The pkgInAll flag on each loadPkg instance tracks whether that
 // package is known to match the "all" meta-pattern.
@@ -126,25 +125,54 @@ import (
 // It holds details about individual packages.
 var loaded *loader
 
-// ImportPaths returns the set of packages matching the args (patterns),
+// PackageOpts control the behavior of the LoadPackages function.
-// on the target platform. Modules may be added to the build list
+type PackageOpts struct {
-// to satisfy new imports.
+	// Tags are the build tags in effect (as interpreted by the
-func ImportPaths(ctx context.Context, patterns []string) []*search.Match {
+	// cmd/go/internal/imports package).
-	matches := ImportPathsQuiet(ctx, patterns, imports.Tags())
+	// If nil, treated as equivalent to imports.Tags().
-	search.WarnUnmatched(matches)
+	Tags map[string]bool
-	return matches
+
+	// ResolveMissingImports indicates that we should attempt to add module
+	// dependencies as needed to resolve imports of packages that are not found.
+	//
+	// For commands that support the -mod flag, resolving imports may still fail
+	// if the flag is set to "readonly" (the default) or "vendor".
+	ResolveMissingImports bool
+
+	// LoadTests loads the test dependencies of each package matching a requested
+	// pattern. If ResolveMissingImports is also true, test dependencies will be
+	// resolved if missing.
+	LoadTests bool
+
+	// UseVendorAll causes the "all" package pattern to be interpreted as if
+	// running "go mod vendor" (or building with "-mod=vendor").
+	//
+	// Once lazy loading is implemented, this will be a no-op for modules that
+	// declare 'go 1.16' or higher.
+	UseVendorAll bool
+
+	// AllowErrors indicates that LoadPackages should not log errors in resolving
+	// patterns or imports, and should not terminate the process if such an error
+	// occurs.
+	AllowErrors bool
+
+	// SilenceUnmatchedWarnings suppresses the warnings normally emitted for
+	// patterns that did not match any packages.
+	SilenceUnmatchedWarnings bool
 }
 
-// ImportPathsQuiet is like ImportPaths but does not warn about patterns with
+// LoadPackages identifies the set of packages matching the given patterns and
-// no matches. It also lets the caller specify a set of build tags to match
+// loads the packages in the import graph rooted at that set.
-// packages. The build tags should typically be imports.Tags() or
+func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (matches []*search.Match, loadedPackages []string) {
-// imports.AnyTags(); a nil map has no special meaning.
-func ImportPathsQuiet(ctx context.Context, patterns []string, tags map[string]bool) []*search.Match {
 	InitMod(ctx)
+	if opts.Tags == nil {
+		opts.Tags = imports.Tags()
+	}
 
+	patterns = search.CleanPatterns(patterns)
+	matches = make([]*search.Match, 0, len(patterns))
 	allPatternIsRoot := false
-	var matches []*search.Match
+	for _, pattern := range patterns {
-	for _, pattern := range search.CleanPatterns(patterns) {
 		matches = append(matches, search.NewMatch(pattern))
 		if pattern == "all" {
 			allPatternIsRoot = true
@@ -191,14 +219,14 @@ func ImportPathsQuiet(ctx context.Context, patterns []string, tags map[string]bo
 
 			case strings.Contains(m.Pattern(), "..."):
 				m.Errs = m.Errs[:0]
-				matchPackages(ctx, m, tags, includeStd, buildList)
+				matchPackages(ctx, m, opts.Tags, includeStd, buildList)
 
 			case m.Pattern() == "all":
 				if ld == nil {
 					// The initial roots are the packages in the main module.
 					// loadFromRoots will expand that to "all".
 					m.Errs = m.Errs[:0]
-					matchPackages(ctx, m, tags, omitStd, []module.Version{Target})
+					matchPackages(ctx, m, opts.Tags, omitStd, []module.Version{Target})
 				} else {
 					// Starting with the packages in the main module,
 					// enumerate the full list of "all".
@@ -217,9 +245,12 @@ func ImportPathsQuiet(ctx context.Context, patterns []string, tags map[string]bo
 	}
 
 	loaded = loadFromRoots(loaderParams{
-		tags:               tags,
+		tags:           opts.Tags,
+		loadTests:      opts.LoadTests,
+		resolveMissing: opts.ResolveMissingImports,
+
+		allClosesOverTests: index.allPatternClosesOverTests() && !opts.UseVendorAll,
 		allPatternIsRoot:   allPatternIsRoot,
-		allClosesOverTests: index.allPatternClosesOverTests(),
 
 		listRoots: func() (roots []string) {
 			updateMatches(nil)
@@ -235,7 +266,31 @@ func ImportPathsQuiet(ctx context.Context, patterns []string, tags map[string]bo
 	checkMultiplePaths()
 	WriteGoMod()
 
-	return matches
+	for _, pkg := range loaded.pkgs {
+		if pkg.err != nil && !opts.AllowErrors {
+			base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+		}
+		if !pkg.isTest() {
+			loadedPackages = append(loadedPackages, pkg.path)
+		}
+	}
+	if !opts.AllowErrors {
+		// Also list errors in matching patterns (such as directory permission
+		// errors for wildcard patterns).
+		for _, match := range matches {
+			for _, err := range match.Errs {
+				base.Errorf("%v", err)
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	if !opts.SilenceUnmatchedWarnings {
+		search.WarnUnmatched(matches)
+	}
+
+	sort.Strings(loadedPackages)
+	return matches, loadedPackages
 }
 
 // matchLocalDirs is like m.MatchDirs, but tries to avoid scanning directories
@@ -425,13 +480,14 @@ func ImportFromFiles(ctx context.Context, gofiles []string) {
 	}
 
 	loaded = loadFromRoots(loaderParams{
-		tags: tags,
+		tags:               tags,
+		resolveMissing:     true,
+		allClosesOverTests: index.allPatternClosesOverTests(),
 		listRoots: func() (roots []string) {
 			roots = append(roots, imports...)
 			roots = append(roots, testImports...)
 			return roots
 		},
-		allClosesOverTests: index.allPatternClosesOverTests(),
 	})
 	WriteGoMod()
 }
@@ -462,58 +518,6 @@ func DirImportPath(dir string) string {
 	return "."
 }
 
-// LoadALL returns the set of all packages in the current module
-// and their dependencies in any other modules, without filtering
-// due to build tags, except "+build ignore".
-// It adds modules to the build list as needed to satisfy new imports.
-// This set is useful for deciding whether a particular import is needed
-// anywhere in a module.
-//
-// In modules that specify "go 1.16" or higher, ALL follows only one layer of
-// test dependencies. In "go 1.15" or lower, ALL follows the imports of tests of
-// dependencies of tests.
-func LoadALL(ctx context.Context) []string {
-	InitMod(ctx)
-	return loadAll(ctx, index.allPatternClosesOverTests())
-}
-
-// LoadVendor is like LoadALL but only follows test dependencies
-// for tests in the main module. Tests in dependency modules are
-// ignored completely.
-// This set is useful for identifying the which packages to include in a vendor directory.
-func LoadVendor(ctx context.Context) []string {
-	InitMod(ctx)
-	// 'go mod vendor' has never followed test dependencies since Go 1.11.
-	const closeOverTests = false
-	return loadAll(ctx, closeOverTests)
-}
-
-func loadAll(ctx context.Context, closeOverTests bool) []string {
-	inTarget := TargetPackages(ctx, "...")
-	loaded = loadFromRoots(loaderParams{
-		tags:               imports.AnyTags(),
-		listRoots:          func() []string { return inTarget.Pkgs },
-		allPatternIsRoot:   true,
-		allClosesOverTests: closeOverTests,
-	})
-	checkMultiplePaths()
-	WriteGoMod()
-
-	var paths []string
-	for _, pkg := range loaded.pkgs {
-		if pkg.err != nil {
-			base.Errorf("%s: %v", pkg.stackText(), pkg.err)
-			continue
-		}
-		paths = append(paths, pkg.path)
-	}
-	for _, err := range inTarget.Errs {
-		base.Errorf("%v", err)
-	}
-	base.ExitIfErrors()
-	return paths
-}
-
 // TargetPackages returns the list of packages in the target (top-level) module
 // matching pattern, which may be relative to the working directory, under all
 // build tag settings.
@@ -631,14 +635,15 @@ type loader struct {
 }
 
 type loaderParams struct {
-	tags               map[string]bool // tags for scanDir
+	tags           map[string]bool // tags for scanDir
-	listRoots          func() []string
+	loadTests      bool
-	allPatternIsRoot   bool // Is the "all" pattern an additional root?
+	resolveMissing bool
-	allClosesOverTests bool // Does the "all" pattern include the transitive closure of tests of packages in "all"?
-}
 
-// LoadTests controls whether the loaders load tests of the root packages.
+	allClosesOverTests bool // Does the "all" pattern include the transitive closure of tests of packages in "all"?
-var LoadTests bool
+	allPatternIsRoot   bool // Is the "all" pattern an additional root?
+
+	listRoots func() []string
+}
 
 func (ld *loader) reset() {
 	select {
@@ -791,6 +796,10 @@ func loadFromRoots(params loaderParams) *loader {
 
 		ld.buildStacks()
 
+		if !ld.resolveMissing {
+			// We've loaded as much as we can without resolving missing imports.
+			break
+		}
 		modAddedBy := ld.resolveMissingImports(addedModuleFor)
 		if len(modAddedBy) == 0 {
 			break
@@ -958,7 +967,7 @@ func (ld *loader) applyPkgFlags(pkg *loadPkg, flags loadPkgFlags) {
 			// also in "all" (as above).
 			wantTest = true
 
-		case LoadTests && new.has(pkgIsRoot):
+		case ld.loadTests && new.has(pkgIsRoot):
 			// LoadTest explicitly requests tests of “the root packages”.
 			wantTest = true
 		}
@@ -1252,7 +1261,7 @@ func (pkg *loadPkg) why() string {
 
 // Why returns the "go mod why" output stanza for the given package,
 // without the leading # comment.
-// The package graph must have been loaded already, usually by LoadALL.
+// The package graph must have been loaded already, usually by LoadPackages.
 // If there is no reason for the package to be in the current build,
 // Why returns an empty string.
 func Why(path string) string {

src/cmd/go/internal/test/test.go

@@ -29,7 +29,6 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
 	"cmd/go/internal/lockedfile"
-	"cmd/go/internal/modload"
 	"cmd/go/internal/str"
 	"cmd/go/internal/trace"
 	"cmd/go/internal/work"
@@ -568,7 +567,7 @@ var defaultVetFlags = []string{
 }
 
 func runTest(ctx context.Context, cmd *base.Command, args []string) {
-	modload.LoadTests = true
+	load.ModResolveTests = true
 
 	pkgArgs, testArgs = testFlags(args)
 

src/cmd/go/internal/vet/vet.go

@@ -13,7 +13,6 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
-	"cmd/go/internal/modload"
 	"cmd/go/internal/trace"
 	"cmd/go/internal/work"
 )
@@ -54,7 +53,7 @@ See also: go fmt, go fix.
 }
 
 func runVet(ctx context.Context, cmd *base.Command, args []string) {
-	modload.LoadTests = true
+	load.ModResolveTests = true
 
 	vetFlags, pkgArgs := vetFlags(args)
 

src/cmd/go/testdata/script/mod_why.txt

@@ -3,6 +3,7 @@ env GO111MODULE=on
 
 # Populate go.sum.
 go mod tidy
+cp go.mod go.mod.orig
 
 go list -test all
 stdout rsc.io/quote
@@ -20,7 +21,7 @@ cmp stdout why-text-module.txt
 go mod why rsc.io/testonly
 cmp stdout why-testonly.txt
 
-# why a module used only in tests?
+# why a module used only in a test of a dependency?
 go mod why -m rsc.io/testonly
 cmp stdout why-testonly.txt
 
@@ -44,6 +45,14 @@ cmp stdout why-both.txt
 go mod why -m rsc.io/quote rsc.io/sampler
 cmp stdout why-both-module.txt
 
+# package in a module that isn't even in the module graph
+# (https://golang.org/issue/26977)
+go mod why rsc.io/fortune
+cmp stdout why-missing.txt
+
+# None of these command should have changed the go.mod file.
+cmp go.mod go.mod.orig
+
 -- go.mod --
 module mymodule
 require rsc.io/quote v1.5.2
@@ -116,3 +125,6 @@ mymodule/y
 mymodule/y.test
 rsc.io/quote
 rsc.io/sampler
+-- why-missing.txt --
+# rsc.io/fortune
+(main module does not need package rsc.io/fortune)