cmd/dist: emphasize when all tests are excluded

As observed in https://go.dev/issue/61666#issuecomment-1739476954,
if a -run flag value matches no tests, dist test output doesn't do much
to help users notice that was what happened. It is valid and sometimes
intended¹ to match no tests, so I want to reserve failed status with
exit code 1 to the actionable outcome where at least 1 test failed.
But it seems reasonable to extend the existing "some were excluded"
mechanism of reporting partial testing to be more helpful.

In non-JSON mode, which is more likely to be used manually by humans,
print a special² last line that will hopefully be easier to notice when
matching no tests wasn't intended. Change nothing for -json mode since
that's likely used by machines and they can make sense of 0 JSON events.

The go test command already has this behavior, so this brings dist test
closer³ to it. (Slightly unfortunate duplicate maintenance for us, and
the need for the rare dist test users to learn its CLI quirks; oh well.)

¹ It might seem counter-intuitive at first: what's the point of calling
  dist test and asking it to run no tests? One possible answer is that
  it permits writing code capable of running N intended tests, where N
  is 0 or higher. That is, it allows for 0 to not be a special case that
  the caller would have no choice but handle differently.
² I initially considered making it say something like "N of M tests were
  excluded", but decided to leave it alone since the current coordinator
  code still has that text hardcoded and I don't want to break it. Hence
  the new status that I expect only humans will see. And it seems better
  this way anyway.
³ In particular, the "matched no tests" and "no tests to run" phrases
  were selected precisely because they're already used in cmd/go output.

Change-Id: I6768d9932587195ae6dbc6e2c4742479e265733b
Reviewed-on: https://go-review.googlesource.com/c/go/+/532115
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/cmd/dist/test.go b/src/cmd/dist/test.go
index 5e57c0c427..9635c4fb61 100644
--- a/src/cmd/dist/test.go
+++ b/src/cmd/dist/test.go
@@ -71,7 +71,6 @@ type tester struct {
 
 	short      bool
 	cgoEnabled bool
-	partial    bool
 	json       bool
 
 	tests        []distTest // use addTest to extend
@@ -235,11 +234,13 @@ func (t *tester) run() {
 		}
 	}
 
+	var anyIncluded, someExcluded bool
 	for _, dt := range t.tests {
 		if !t.shouldRunTest(dt.name) {
-			t.partial = true
+			someExcluded = true
 			continue
 		}
+		anyIncluded = true
 		dt := dt // dt used in background after this iteration
 		if err := dt.fn(&dt); err != nil {
 			t.runPending(&dt) // in case that hasn't been done yet
@@ -257,7 +258,11 @@ func (t *tester) run() {
 	if !t.json {
 		if t.failed {
 			fmt.Println("\nFAILED")
-		} else if t.partial {
+		} else if !anyIncluded {
+			fmt.Println()
+			errprintf("go tool dist: warning: %q matched no tests; use the -list flag to list available tests\n", t.runRxStr)
+			fmt.Println("NO TESTS TO RUN")
+		} else if someExcluded {
 			fmt.Println("\nALL TESTS PASSED (some were excluded)")
 		} else {
 			fmt.Println("\nALL TESTS PASSED")