cmd/go: make 'go get' fail with an error when outside a module

There's no go.mod file for 'go get' to update, so it has no effect,
other than checking arguments and filling the module cache. That might
be useul in some cases, but it seems better to fail loudly in case the
user hasn't seen the deprecation warning, for example, inside a
script.

For #43684

Change-Id: I6e67c782e3a1cb7046eac5c9df17eda7a31c7bce
Reviewed-on: https://go-review.googlesource.com/c/go/+/352149
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -273,6 +273,8 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 		base.Fatalf("go: -insecure flag is no longer supported; use GOINSECURE instead")
 	}
 
+	modload.ForceUseModules = true
+
 	// Do not allow any updating of go.mod until we've applied
 	// all the requested changes and checked that the result matches
 	// what was requested.
@@ -282,6 +284,20 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	// 'go get' is expected to do this, unlike other commands.
 	modload.AllowMissingModuleImports()
 
+	// 'go get' no longer builds or installs packages, so there's nothing to do
+	// if there's no go.mod file.
+	// TODO(#40775): make modload.Init return ErrNoModRoot instead of exiting.
+	// We could handle that here by printing a different message.
+	modload.Init()
+	if !modload.HasModRoot() {
+		base.Fatalf("go: go.mod file not found in current directory or any parent directory.\n" +
+			"\t'go get' is no longer supported outside a module.\n" +
+			"\tTo build and install a command, use 'go install' with a version,\n" +
+			"\tlike 'go install example.com/cmd@latest'\n" +
+			"\tFor more information, see https://golang.org/doc/go-get-install-deprecation\n" +
+			"\tor run 'go help get' or 'go help install'.")
+	}
+
 	queries := parseArgs(ctx, args)
 
 	r := newResolver(ctx, queries)
@@ -351,10 +367,6 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	r.checkPackageProblems(ctx, pkgPatterns)
 
-	if !modload.HasModRoot() {
-		return
-	}
-
 	// Everything succeeded. Update go.mod.
 	oldReqs := reqsFromGoMod(modload.ModFile())
 

src/cmd/go/testdata/script/get_404_meta.txt

@@ -10,3 +10,9 @@ go get -d bazil.org/fuse/fs/fstestutil
 env GO111MODULE=on
 env GOPROXY=direct
 go get -d bazil.org/fuse/fs/fstestutil
+
+
+-- go.mod --
+module m
+
+go 1.18

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

@@ -3,9 +3,9 @@ env GO111MODULE=on
 # Go should reject relative paths in GOMODCACHE environment.
 
 env GOMODCACHE="~/test"
-! go get example.com/tools/cmd/hello
+! go install example.com/tools/cmd/hello@latest
 stderr 'must be absolute path'
 
 env GOMODCACHE="./test"
-! go get example.com/tools/cmd/hello
+! go install example.com/tools/cmd/hello@latest
 stderr 'must be absolute path'

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

@@ -7,26 +7,26 @@ env GO111MODULE=on
 env GOPROXY=direct
 env GOSUMDB=off
 
-! go get -d vcs-test.golang.org/insecure/go/insecure
+! go mod download vcs-test.golang.org/insecure/go/insecure@latest
 stderr 'redirected .* to insecure URL'
 
 # insecure host
 env GOINSECURE=vcs-test.golang.org
 go clean -modcache
-go get -d vcs-test.golang.org/insecure/go/insecure
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
 
 # insecure glob host
 env GOINSECURE=*.golang.org
 go clean -modcache
-go get -d vcs-test.golang.org/insecure/go/insecure
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
 
 # insecure multiple host
 env GOINSECURE=somewhere-else.com,*.golang.org
 go clean -modcache
-go get -d vcs-test.golang.org/insecure/go/insecure
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
 
 # different insecure host does not fetch
 env GOINSECURE=somewhere-else.com
 go clean -modcache
-! go get -d vcs-test.golang.org/insecure/go/insecure
+! go mod download vcs-test.golang.org/insecure/go/insecure@latest
 stderr 'redirected .* to insecure URL'

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

@@ -5,18 +5,18 @@ env GO111MODULE=on
 [!exec:git] skip
 env GOPROXY=direct
 
-! go get github.com/golang/nonexist
+! go mod download github.com/golang/nonexist@latest
 stderr 'Confirm the import path was entered correctly.'
 stderr 'If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.'
 ! stdout .
 
 # Fetching a nonexistent commit should return an "unknown revision"
 # error message.
-! go get github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b
+! go mod download github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b
 stderr '^go: github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b: invalid version: unknown revision 86186f3aba07ed0212cfb944f3398997d2d07c6b$'
 ! stdout .
 
-! go get github.com/golang/nonexist@master
+! go mod download github.com/golang/nonexist@master
 stderr '^Confirm the import path was entered correctly.$'
 stderr '^If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.$'
 ! stderr 'unknown revision'

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

@@ -3,8 +3,8 @@ env GOPROXYBASE=$GOPROXY
 env GOPROXY=$GOPROXYBASE/redirect/11
 env GOSUMDB=off
 
-! go get -d rsc.io/quote@v1.2.0
+! go mod download rsc.io/quote@v1.2.0
 stderr 'stopped after 10 redirects'
 
 env GOPROXY=$GOPROXYBASE/redirect/9
-go get -d rsc.io/quote@v1.2.0
+go mod download rsc.io/quote@v1.2.0

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

@@ -2,7 +2,10 @@
 
 env GO111MODULE=on
 
-# TODO(#43684): test message outside module.
+# 'go get' outside a module prints an error.
+! go get example.com/cmd/a
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 
 cp go.mod.orig go.mod
 

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

@@ -8,3 +8,8 @@ env GOSUMDB=off
 go get -x -v -d golang.org/x/tools/cmd/goimports
 stderr '# get https://proxy.golang.org/golang.org/x/tools/@v/list'
 ! stderr '# get https://golang.org'
+
+-- go.mod --
+module m
+
+go 1.18

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

@@ -58,6 +58,11 @@ mkdir test/test_dir.go
 rm test/test_dir.go
 
 
+-- go.mod --
+module m
+
+go 1.18
+
 -- test.go --
 package main
 func main() {println("test")}

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

@@ -12,3 +12,8 @@ go get -x -d golang.org/x/text@v0.1.0
 stderr '^# get https://golang.org/x/text\?go-get=1$'
 stderr '^# get https://golang.org/x/text\?go-get=1: 200 OK \([0-9.]+s\)$'
 ! stderr '^# get //.*'
+
+-- go.mod --
+module m
+
+go 1.18

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

@@ -9,7 +9,7 @@ env GO111MODULE=on
 env GOPROXY=direct
 env GOSUMDB=off
 
-! go get -d vcs-test.golang.org/go/missingrepo/missingrepo-git
+! go mod download vcs-test.golang.org/go/missingrepo/missingrepo-git@latest
 stderr 'vcs-test.golang.org/go/missingrepo/missingrepo-git: git ls-remote .*: exit status .*'
 
-go get -d vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing
+go mod download vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing@latest

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

@@ -123,30 +123,30 @@ stderr '^go: go.mod file not found in current directory or any parent directory;
 stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
 
 
-# 'go get' without arguments implicitly operates on the main module, and thus
-# should fail.
+# 'go get' has no go.mod file to update outside a module and should fail.
 ! go get
-stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -u
-stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -u ./needmod
-stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
-
-# 'go get -u all' upgrades the transitive import graph of the main module,
-# which is empty.
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -u all
-stderr '^go: cannot match "all": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
-
-# 'go get' should check the proposed module graph for consistency,
-# even though we won't write it anywhere.
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -d example.com/printversion@v1.0.0 example.com/version@none
-stderr '^go: example.com/printversion@v1.0.0 requires example.com/version@v1.0.0, not example.com/version@none$'
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 
-# 'go get -d' should download and extract the source code needed to build the requested version.
-rm -r $GOPATH/pkg/mod/example.com
-go get -d example.com/printversion@v1.0.0
-exists $GOPATH/pkg/mod/example.com/printversion@v1.0.0
-exists $GOPATH/pkg/mod/example.com/version@v1.0.0
+# 'go get -d' should not download anything.
+go clean -modcache
+! go get -d example.com/printversion@v1.0.0
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! exists $GOPATH/pkg/mod/example.com/printversion@v1.0.0
+! exists $GOPATH/pkg/mod/example.com/version@v1.0.0
 
 
 # 'go build' without arguments implicitly operates on the current directory, and should fail.