cmd/internal/objabi,test: use correct GOEXPERIMENT build tags in test/run.go

Currently, run.go sets GOEXPERIMENT build tags based on the
*difference* from the baseline experiment configuration, rather than
the absolute experiment configuration. This differs from cmd/go. As a
result, if we set a baseline configuration and don't override it with
a GOEXPERIMENT setting, run.go won't set any GOEXPERIMENT build tags,
instead of setting the tags corresponding to the baseline
configuration.

Fix this by making compile -V=goexperiment produce the full
GOEXPERIMENT configuration, which run.go can then use to set exactly
the right set of build tags.

For #40724.

Change-Id: Ieda6ea62f1a1fabbe8d749d6d09c198fd5ca8377
Reviewed-on: https://go-review.googlesource.com/c/go/+/310171
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

2 files changed, 16 insertions, 9 deletions

diff --git a/src/cmd/internal/objabi/exp.go b/src/cmd/internal/objabi/exp.go
index 9eb3e431b3..3371c6c8f8 100644
--- a/src/cmd/internal/objabi/exp.go
+++ b/src/cmd/internal/objabi/exp.go
@@ -108,8 +108,9 @@ func parseExperiments() goexperiment.Flags {
 
 // expList returns the list of lower-cased experiment names for
 // experiments that differ from base. base may be nil to indicate no
-// experiments.
-func expList(exp, base *goexperiment.Flags) []string {
+// experiments. If all is true, then include all experiment flags,
+// regardless of base.
+func expList(exp, base *goexperiment.Flags, all bool) []string {
 	var list []string
 	rv := reflect.ValueOf(exp).Elem()
 	var rBase reflect.Value
@@ -124,7 +125,7 @@ func expList(exp, base *goexperiment.Flags) []string {
 		if base != nil {
 			baseVal = rBase.Field(i).Bool()
 		}
-		if val != baseVal {
+		if all || val != baseVal {
 			if val {
 				list = append(list, name)
 			} else {
@@ -140,11 +141,11 @@ func expList(exp, base *goexperiment.Flags) []string {
 // GOEXPERIMENT is exactly what a user would set on the command line
 // to get the set of enabled experiments.
 func GOEXPERIMENT() string {
-	return strings.Join(expList(&Experiment, &experimentBaseline), ",")
+	return strings.Join(expList(&Experiment, &experimentBaseline, false), ",")
 }
 
 // EnabledExperiments returns a list of enabled experiments, as
 // lower-cased experiment names.
 func EnabledExperiments() []string {
-	return expList(&Experiment, nil)
+	return expList(&Experiment, nil, false)
 }
diff --git a/src/cmd/internal/objabi/flag.go b/src/cmd/internal/objabi/flag.go
index 6a8a69116d..9fcab4cc85 100644
--- a/src/cmd/internal/objabi/flag.go
+++ b/src/cmd/internal/objabi/flag.go
@@ -93,10 +93,16 @@ func (versionFlag) Set(s string) error {
 
 	p := ""
 
-	// If the enabled experiments differ from the defaults,
-	// include that difference.
-	if goexperiment := GOEXPERIMENT(); goexperiment != "" {
-		p = " X:" + goexperiment
+	if s == "goexperiment" {
+		// test/run.go uses this to discover the full set of
+		// experiment tags. Report everything.
+		p = " X:" + strings.Join(expList(&Experiment, nil, true), ",")
+	} else {
+		// If the enabled experiments differ from the defaults,
+		// include that difference.
+		if goexperiment := GOEXPERIMENT(); goexperiment != "" {
+			p = " X:" + goexperiment
+		}
 	}
 
 	// The go command invokes -V=full to get a unique identifier