cmd/go/internal/modget: clarify error for 'go get' without arguments

If the current directory doesn't contain a package, 'go get' will say
that without additional detail.

If there were no arguments, errors will start with "go get:" instead
of "go get .:".

Fixes #39080

Change-Id: I47366f2a27bce17bd8b79344ad15b8b934a888c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/234681
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>

3 files changed, 19 insertions, 2 deletions

diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 5b8eebf7cb..0b7f6bf1d5 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -505,6 +505,12 @@ func parseArgs(ctx context.Context, rawArgs []string) []*query {
 			continue
 		}
 
+		// If there were no arguments, CleanPatterns returns ".". Set the raw
+		// string back to "" for better errors.
+		if len(rawArgs) == 0 {
+			q.raw = ""
+		}
+
 		// Guard against 'go get x.go', a common mistake.
 		// Note that package and module paths may end with '.go', so only print an error
 		// if the argument has no version and either has no slash or refers to an existing file.
@@ -820,6 +826,9 @@ func (r *resolver) performLocalQueries(ctx context.Context) {
 			}
 
 			if len(match.Pkgs) == 0 {
+				if q.raw == "" || q.raw == "." {
+					return errSet(fmt.Errorf("no package in current directory"))
+				}
 				if !q.isWildcard() {
 					return errSet(fmt.Errorf("%s%s is not a package in module rooted at %s", q.pattern, absDetail, modload.ModRoot()))
 				}
diff --git a/src/cmd/go/internal/modget/query.go b/src/cmd/go/internal/modget/query.go
index 53b60cc71a..20eb0b6364 100644
--- a/src/cmd/go/internal/modget/query.go
+++ b/src/cmd/go/internal/modget/query.go
@@ -295,7 +295,11 @@ func reportError(q *query, err error) {
 		}
 	}
 
-	base.Errorf("go get %s: %s", q, errStr)
+	if qs := q.String(); qs != "" {
+		base.Errorf("go get %s: %s", qs, errStr)
+	} else {
+		base.Errorf("go get: %s", errStr)
+	}
 }
 
 func reportConflict(pq *query, m module.Version, conflict versionReason) {
diff --git a/src/cmd/go/testdata/script/mod_dot.txt b/src/cmd/go/testdata/script/mod_dot.txt
index 72be612799..ca8d5c6cc2 100644
--- a/src/cmd/go/testdata/script/mod_dot.txt
+++ b/src/cmd/go/testdata/script/mod_dot.txt
@@ -4,8 +4,12 @@ env GO111MODULE=on
 # in an empty directory should refer to the path '.' and should not attempt
 # to resolve an external module.
 cd dir
+! go get
+stderr '^go get: no package in current directory$'
 ! go get .
-stderr 'go get: \. \(.*[/\\]dir\) is not a package in module rooted at .*[/\\]dir$'
+stderr '^go get \.: no package in current directory$'
+! go get ./subdir
+stderr '^go get: \.[/\\]subdir \('$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]subdir\) is not a package in module rooted at '$WORK'[/\\]gopath[/\\]src[/\\]dir$'
 ! go list
 ! stderr 'cannot find module providing package'
 stderr '^no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir$'