internal/buildcfg: change GOEXPERIMENT to always return non-empty string

Rather than returning "", we now return "," (which is a no-op). This
ensures that the returned string always overrides DefaultGOEXPERIMENT.

This fixes a bootstrapping issue where GOROOT_BOOTSTRAP was built with
"GOEXPERIMENT=fieldtrack ./make.bash". cmd/dist sets GOEXPERIMENT=none
during bootstrapping, which was causing cmd/go to set GOEXPERIMENT=""
when executing cmd/compile; but then cmd/compile ignores the
environment variable (because it's empty) and instead uses
DefaultGOEXPERIMENT.

Fixes #47921.

Change-Id: I657ff6cdfb294a94f6a2f58c306ceed7f104416b
Reviewed-on: https://go-review.googlesource.com/c/go/+/344511
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Matthew Dempsky <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: mdempsky

src/internal/buildcfg/exp.go

@@ -158,7 +158,11 @@ func expList(exp, base *goexperiment.Flags, all bool) []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, false), ",")
+	goexp := strings.Join(expList(&Experiment, &experimentBaseline, false), ",")
+	if goexp == "" && DefaultGOEXPERIMENT != "" {
+		goexp = "," // non-empty to override DefaultGOEXPERIMENT
+	}
+	return goexp
 }
 
 // EnabledExperiments returns a list of enabled experiments, as