cmd/go: adjust ImportMissingError when module lookup is disabled

Previously, ImportMissingError said
"cannot find module providing package …"
even when we didn't even attempt to find such a module.

Now, we write "no module requirement provides package …"
when we did not attempt to identify a suitable module,
and suggest either 'go mod tidy' or 'go get -d' as appropriate.

Fixes #41576

Change-Id: I979bb999da4066828c54d99a310ea66bb31032ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/258298
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>

12 files changed, 110 insertions, 42 deletions

diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index 3642de851a..76fe6745d9 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -26,13 +26,15 @@ import (
 	"golang.org/x/mod/semver"
 )
 
-var errImportMissing = errors.New("import missing")
-
 type ImportMissingError struct {
 	Path     string
 	Module   module.Version
 	QueryErr error
 
+	// inAll indicates whether Path is in the "all" package pattern,
+	// and thus would be added by 'go mod tidy'.
+	inAll bool
+
 	// newMissingVersion is set to a newer version of Module if one is present
 	// in the build list. When set, we can't automatically upgrade.
 	newMissingVersion string
@@ -46,7 +48,19 @@ func (e *ImportMissingError) Error() string {
 		if e.QueryErr != nil {
 			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
 		}
-		return "cannot find module providing package " + e.Path
+		if cfg.BuildMod == "mod" {
+			return "cannot find module providing package " + e.Path
+		}
+
+		suggestion := ""
+		if !HasModRoot() {
+			suggestion = ": working directory is not part of a module"
+		} else if e.inAll {
+			suggestion = "; try 'go mod tidy' to add it"
+		} else {
+			suggestion = fmt.Sprintf("; try 'go get -d %s' to add it", e.Path)
+		}
+		return fmt.Sprintf("no required module provides package %s%s", e.Path, suggestion)
 	}
 
 	if e.newMissingVersion != "" {
@@ -132,7 +146,7 @@ func (e *invalidImportError) Unwrap() error {
 // like "C" and "unsafe".
 //
 // If the package cannot be found in the current build list,
-// importFromBuildList returns errImportMissing as the error.
+// importFromBuildList returns an *ImportMissingError.
 func importFromBuildList(ctx context.Context, path string) (m module.Version, dir string, err error) {
 	if strings.Contains(path, "@") {
 		return module.Version{}, "", fmt.Errorf("import path should not have @version")
@@ -144,6 +158,10 @@ func importFromBuildList(ctx context.Context, path string) (m module.Version, di
 		// There's no directory for import "C" or import "unsafe".
 		return module.Version{}, "", nil
 	}
+	// Before any further lookup, check that the path is valid.
+	if err := module.CheckImportPath(path); err != nil {
+		return module.Version{}, "", &invalidImportError{importPath: path, err: err}
+	}
 
 	// Is the package in the standard library?
 	pathIsStd := search.IsStandardImportPath(path)
@@ -212,7 +230,7 @@ func importFromBuildList(ctx context.Context, path string) (m module.Version, di
 		return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
 	}
 
-	return module.Version{}, "", errImportMissing
+	return module.Version{}, "", &ImportMissingError{Path: path}
 }
 
 // queryImport attempts to locate a module that can be added to the current
@@ -220,13 +238,6 @@ func importFromBuildList(ctx context.Context, path string) (m module.Version, di
 func queryImport(ctx context.Context, path string) (module.Version, error) {
 	pathIsStd := search.IsStandardImportPath(path)
 
-	if modRoot == "" && !allowMissingModuleImports {
-		return module.Version{}, &ImportMissingError{
-			Path:     path,
-			QueryErr: errors.New("working directory is not part of a module"),
-		}
-	}
-
 	// Not on build list.
 	// To avoid spurious remote fetches, next try the latest replacement for each
 	// module (golang.org/issue/26241). This should give a useful message
@@ -291,11 +302,6 @@ func queryImport(ctx context.Context, path string) (module.Version, error) {
 		}
 	}
 
-	// Before any further lookup, check that the path is valid.
-	if err := module.CheckImportPath(path); err != nil {
-		return module.Version{}, &invalidImportError{importPath: path, err: err}
-	}
-
 	if pathIsStd {
 		// This package isn't in the standard library, isn't in any module already
 		// in the build list, and isn't in any other module that the user has
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 9194f9cc7c..4ddb817cf1 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -270,11 +270,19 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 	// Report errors, if any.
 	checkMultiplePaths()
 	for _, pkg := range loaded.pkgs {
-		if pkg.err != nil && !opts.SilenceErrors {
-			if opts.AllowErrors {
-				fmt.Fprintf(os.Stderr, "%s: %v\n", pkg.stackText(), pkg.err)
-			} else {
-				base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+		if pkg.err != nil {
+			if pkg.flags.has(pkgInAll) {
+				if imErr := (*ImportMissingError)(nil); errors.As(pkg.err, &imErr) {
+					imErr.inAll = true
+				}
+			}
+
+			if !opts.SilenceErrors {
+				if opts.AllowErrors {
+					fmt.Fprintf(os.Stderr, "%s: %v\n", pkg.stackText(), pkg.err)
+				} else {
+					base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+				}
 			}
 		}
 		if !pkg.isTest() {
@@ -809,7 +817,7 @@ func loadFromRoots(params loaderParams) *loader {
 
 		ld.buildStacks()
 
-		if !ld.resolveMissing {
+		if !ld.resolveMissing || (!HasModRoot() && !allowMissingModuleImports) {
 			// We've loaded as much as we can without resolving missing imports.
 			break
 		}
@@ -872,12 +880,15 @@ func loadFromRoots(params loaderParams) *loader {
 func (ld *loader) resolveMissingImports(addedModuleFor map[string]bool) (modAddedBy map[module.Version]*loadPkg) {
 	var needPkgs []*loadPkg
 	for _, pkg := range ld.pkgs {
+		if pkg.err == nil {
+			continue
+		}
 		if pkg.isTest() {
 			// If we are missing a test, we are also missing its non-test version, and
 			// we should only add the missing import once.
 			continue
 		}
-		if pkg.err != errImportMissing {
+		if !errors.As(pkg.err, new(*ImportMissingError)) {
 			// Leave other errors for Import or load.Packages to report.
 			continue
 		}
diff --git a/src/cmd/go/testdata/script/mod_bad_domain.txt b/src/cmd/go/testdata/script/mod_bad_domain.txt
index ec0d474382..868a8d43d6 100644
--- a/src/cmd/go/testdata/script/mod_bad_domain.txt
+++ b/src/cmd/go/testdata/script/mod_bad_domain.txt
@@ -6,18 +6,30 @@ stderr '^go get appengine: package appengine is not in GOROOT \(.*\)$'
 ! go get x/y.z
 stderr 'malformed module path "x/y.z": missing dot in first path element'
 
+
 # 'go list -m' should report errors about module names, never GOROOT.
 ! go list -m -versions appengine
 stderr 'malformed module path "appengine": missing dot in first path element'
 ! go list -m -versions x/y.z
 stderr 'malformed module path "x/y.z": missing dot in first path element'
 
+
 # build should report all unsatisfied imports,
 # but should be more definitive about non-module import paths
 ! go build ./useappengine
-stderr 'cannot find package'
+stderr '^useappengine[/\\]x.go:2:8: cannot find package$'
 ! go build ./usenonexistent
-stderr 'cannot find module providing package nonexistent.rsc.io'
+stderr '^usenonexistent[/\\]x.go:2:8: no required module provides package nonexistent.rsc.io; try ''go mod tidy'' to add it$'
+
+
+# 'get -d' should be similarly definitive
+
+go get -d ./useappengine  # TODO(#41315): This should fail.
+ # stderr '^useappengine[/\\]x.go:2:8: cannot find package$'
+
+! go get -d  ./usenonexistent
+stderr '^x/usenonexistent imports\n\tnonexistent.rsc.io: cannot find module providing package nonexistent.rsc.io$'
+
 
 # go mod vendor and go mod tidy should ignore appengine imports.
 rm usenonexistent/x.go
diff --git a/src/cmd/go/testdata/script/mod_build_info_err.txt b/src/cmd/go/testdata/script/mod_build_info_err.txt
index 4a6ee9e8bb..08e2a8a3c8 100644
--- a/src/cmd/go/testdata/script/mod_build_info_err.txt
+++ b/src/cmd/go/testdata/script/mod_build_info_err.txt
@@ -12,7 +12,7 @@ stderr '^bad[/\\]bad.go:3:8: malformed import path "🐧.example.com/string": in
 # TODO(#41688): This should include a file and line, and report the reason for the error..
 # (Today it includes only an import stack, and does not indicate the actual problem.)
 ! go get -d ./main
-stderr '^m/main imports\n\tm/bad imports\n\t🐧.example.com/string: import missing$'
+stderr '^m/main imports\n\tm/bad imports\n\t🐧.example.com/string: malformed import path "🐧.example.com/string": invalid char ''🐧''$'
 
 
 -- go.mod --
diff --git a/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt b/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt
index 53b789ecc5..49e17e6507 100644
--- a/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt
+++ b/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt
@@ -14,7 +14,7 @@ cmp go.mod.orig go.mod
 
 ! go get example.net/pkgadded@v1.0.0 .
 stderr -count=1 '^go: found example.net/pkgadded/subpkg in example.net/pkgadded v1\.2\.0$'  # TODO: We shouldn't even try v1.2.0.
-stderr '^example.com/m imports\n\texample.net/pkgadded/subpkg: import missing'  # TODO: better error message
+stderr '^example.com/m imports\n\texample.net/pkgadded/subpkg: cannot find module providing package example.net/pkgadded/subpkg$'
 cmp go.mod.orig go.mod
 
 go get example.net/pkgadded@v1.0.0
diff --git a/src/cmd/go/testdata/script/mod_get_errors.txt b/src/cmd/go/testdata/script/mod_get_errors.txt
index 7ce045ff82..5c37058d1c 100644
--- a/src/cmd/go/testdata/script/mod_get_errors.txt
+++ b/src/cmd/go/testdata/script/mod_get_errors.txt
@@ -6,11 +6,11 @@ cp go.mod go.mod.orig
 # the package in the current directory) cannot be resolved.
 
 ! go get
-stderr '^example.com/m imports\n\texample.com/badimport imports\n\texample.net/oops: import missing$'  # TODO: better error message
+stderr '^example.com/m imports\n\texample.com/badimport imports\n\texample.net/oops: cannot find module providing package example.net/oops$'
 cmp go.mod.orig go.mod
 
 ! go get -d
-stderr '^example.com/m imports\n\texample.com/badimport imports\n\texample.net/oops: import missing$'  # TODO: better error message
+stderr '^example.com/m imports\n\texample.com/badimport imports\n\texample.net/oops: cannot find module providing package example.net/oops$'
 cmp go.mod.orig go.mod
 
 cd importsyntax
diff --git a/src/cmd/go/testdata/script/mod_gobuild_import.txt b/src/cmd/go/testdata/script/mod_gobuild_import.txt
index 948496241e..3a133663ec 100644
--- a/src/cmd/go/testdata/script/mod_gobuild_import.txt
+++ b/src/cmd/go/testdata/script/mod_gobuild_import.txt
@@ -19,7 +19,7 @@ exec $WORK/testimport$GOEXE other/x/y/z/w .
 stdout w2.go
 
 ! exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w .
-stderr 'cannot find module providing package gobuild.example.com/x/y/z/w'
+stderr 'no required module provides package gobuild.example.com/x/y/z/w; try ''go get -d gobuild.example.com/x/y/z/w'' to add it'
 
 cd z
 exec $WORK/testimport$GOEXE other/x/y/z/w .
diff --git a/src/cmd/go/testdata/script/mod_indirect.txt b/src/cmd/go/testdata/script/mod_indirect.txt
index 87a3f0b10f..6ea1cae98b 100644
--- a/src/cmd/go/testdata/script/mod_indirect.txt
+++ b/src/cmd/go/testdata/script/mod_indirect.txt
@@ -1,6 +1,6 @@
 env GO111MODULE=on
 
-# golang.org/issue/31248: module requirements imposed by dependency versions
+# golang.org/issue/31248: required modules imposed by dependency versions
 # older than the selected version must still be taken into account.
 
 env GOFLAGS=-mod=readonly
diff --git a/src/cmd/go/testdata/script/mod_list_bad_import.txt b/src/cmd/go/testdata/script/mod_list_bad_import.txt
index b3e2fff67d..3cd50b0de2 100644
--- a/src/cmd/go/testdata/script/mod_list_bad_import.txt
+++ b/src/cmd/go/testdata/script/mod_list_bad_import.txt
@@ -39,7 +39,7 @@ stdout example.com/notfound
 
 # Listing the missing dependency directly should fail outright...
 ! go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}}' example.com/notfound
-stderr 'cannot find module providing package example.com/notfound'
+stderr 'no required module provides package example.com/notfound; try ''go get -d example.com/notfound'' to add it'
 ! stdout error
 ! stdout incomplete
 
diff --git a/src/cmd/go/testdata/script/mod_outside.txt b/src/cmd/go/testdata/script/mod_outside.txt
index e398f7bc40..d969fce145 100644
--- a/src/cmd/go/testdata/script/mod_outside.txt
+++ b/src/cmd/go/testdata/script/mod_outside.txt
@@ -39,6 +39,11 @@ stdout '^fmt$'
 go list ./needmod/needmod.go
 stdout 'command-line-arguments'
 
+# 'go list' on a package from a module should fail.
+! go list example.com/printversion
+stderr '^no required module provides package example.com/printversion: working directory is not part of a module$'
+
+
 # 'go list -m' with an explicit version should resolve that version.
 go list -m example.com/version@latest
 stdout 'example.com/version v1.1.0'
@@ -151,7 +156,7 @@ stderr 'cannot find main module'
 
 # 'go build' of source files should fail if they import anything outside std.
 ! go build -n ./needmod/needmod.go
-stderr 'needmod[/\\]needmod.go:10:2: cannot find module providing package example.com/version: working directory is not part of a module'
+stderr '^needmod[/\\]needmod.go:10:2: no required module provides package example.com/version: working directory is not part of a module$'
 
 # 'go build' of source files should succeed if they do not import anything outside std.
 go build -n -o ignore ./stdonly/stdonly.go
@@ -174,7 +179,7 @@ go doc fmt
 
 # 'go doc' should fail for a package path outside a module.
 ! go doc example.com/version
-stderr 'doc: cannot find module providing package example.com/version: working directory is not part of a module'
+stderr 'doc: no required module provides package example.com/version: working directory is not part of a module'
 
 # 'go install' with a version should succeed if all constraints are met.
 # See mod_install_pkg_version.
@@ -184,12 +189,12 @@ exists $GOPATH/bin/printversion$GOEXE
 
 # 'go install' should fail if a package argument must be resolved to a module.
 ! go install example.com/printversion
-stderr 'cannot find module providing package example.com/printversion: working directory is not part of a module'
+stderr 'no required module provides package example.com/printversion: working directory is not part of a module'
 
 # 'go install' should fail if a source file imports a package that must be
 # resolved to a module.
 ! go install ./needmod/needmod.go
-stderr 'needmod[/\\]needmod.go:10:2: cannot find module providing package example.com/version: working directory is not part of a module'
+stderr 'needmod[/\\]needmod.go:10:2: no required module provides package example.com/version: working directory is not part of a module'
 
 
 # 'go run' with a verison should fail due to syntax.
@@ -198,12 +203,12 @@ stderr 'can only use path@version syntax with'
 
 # 'go run' should fail if a package argument must be resolved to a module.
 ! go run example.com/printversion
-stderr 'cannot find module providing package example.com/printversion: working directory is not part of a module'
+stderr '^no required module provides package example.com/printversion: working directory is not part of a module$'
 
 # 'go run' should fail if a source file imports a package that must be
 # resolved to a module.
 ! go run ./needmod/needmod.go
-stderr 'needmod[/\\]needmod.go:10:2: cannot find module providing package example.com/version: working directory is not part of a module'
+stderr '^needmod[/\\]needmod.go:10:2: no required module provides package example.com/version: working directory is not part of a module$'
 
 
 # 'go fmt' should be able to format files outside of a module.
@@ -221,7 +226,7 @@ stdout 'main is example.com/printversion v0.1.0'
 stdout 'using example.com/version v1.1.0'
 
 # 'go get' of a versioned binary should build and install the latest version
-# using its minimal module requirements, ignoring replacements and exclusions.
+# using its minimal required modules, ignoring replacements and exclusions.
 go get example.com/printversion
 exec ../bin/printversion
 stdout 'path is example.com/printversion'
diff --git a/src/cmd/go/testdata/script/mod_readonly.txt b/src/cmd/go/testdata/script/mod_readonly.txt
index a8458fdea3..c2ee3ff97b 100644
--- a/src/cmd/go/testdata/script/mod_readonly.txt
+++ b/src/cmd/go/testdata/script/mod_readonly.txt
@@ -13,7 +13,7 @@ cmp go.mod go.mod.empty
 # -mod=readonly should be set by default.
 env GOFLAGS=
 ! go list all
-stderr '^x.go:2:8: cannot find module providing package rsc\.io/quote$'
+stderr '^x.go:2:8: no required module provides package rsc\.io/quote; try ''go mod tidy'' to add it$'
 cmp go.mod go.mod.empty
 
 env GOFLAGS=-mod=readonly
@@ -68,6 +68,23 @@ cp go.mod.indirect go.mod
 go list all
 cmp go.mod go.mod.indirect
 
+
+# If we identify a missing package as a dependency of some other package in the
+# main module, we should suggest 'go mod tidy' instead of resolving it.
+
+cp go.mod.untidy go.mod
+! go list all
+stderr '^x.go:2:8: no required module provides package rsc.io/quote; try ''go mod tidy'' to add it$'
+
+! go list -deps .
+stderr '^x.go:2:8: no required module provides package rsc.io/quote; try ''go mod tidy'' to add it$'
+
+# However, if we didn't see an import from the main module, we should suggest
+# 'go get -d' instead, because we don't know whether 'go mod tidy' would add it.
+! go list rsc.io/quote
+stderr '^no required module provides package rsc.io/quote; try ''go get -d rsc.io/quote'' to add it$'
+
+
 -- go.mod --
 module m
 
@@ -103,3 +120,11 @@ require (
 	rsc.io/sampler v1.3.0 // indirect
 	rsc.io/testonly v1.0.0 // indirect
 )
+-- go.mod.untidy --
+module m
+
+go 1.20
+
+require (
+	rsc.io/sampler v1.3.0 // indirect
+)
diff --git a/src/go/build/build_test.go b/src/go/build/build_test.go
index 3a4ad22f46..5a4a2d62f5 100644
--- a/src/go/build/build_test.go
+++ b/src/go/build/build_test.go
@@ -612,11 +612,13 @@ func TestImportPackageOutsideModule(t *testing.T) {
 	ctxt.GOPATH = gopath
 	ctxt.Dir = filepath.Join(gopath, "src/example.com/p")
 
-	want := "cannot find module providing package"
+	want := "working directory is not part of a module"
 	if _, err := ctxt.Import("example.com/p", gopath, FindOnly); err == nil {
 		t.Fatal("importing package when no go.mod is present succeeded unexpectedly")
 	} else if errStr := err.Error(); !strings.Contains(errStr, want) {
 		t.Fatalf("error when importing package when no go.mod is present: got %q; want %q", errStr, want)
+	} else {
+		t.Logf(`ctxt.Import("example.com/p", _, FindOnly): %v`, err)
 	}
 }
 
@@ -677,9 +679,16 @@ func TestMissingImportErrorRepetition(t *testing.T) {
 	if err == nil {
 		t.Fatal("unexpected success")
 	}
+
 	// Don't count the package path with a URL like https://...?go-get=1.
 	// See golang.org/issue/35986.
 	errStr := strings.ReplaceAll(err.Error(), "://"+pkgPath+"?go-get=1", "://...?go-get=1")
+
+	// Also don't count instances in suggested "go get" or similar commands
+	// (see https://golang.org/issue/41576). The suggested command typically
+	// follows a semicolon.
+	errStr = strings.SplitN(errStr, ";", 2)[0]
+
 	if n := strings.Count(errStr, pkgPath); n != 1 {
 		t.Fatalf("package path %q appears in error %d times; should appear once\nerror: %v", pkgPath, n, err)
 	}