cmd/go: add hook to check for GOEXPERIMENT in script tests

Add a new hook to allow script tests to check whether a specific
GOEXPERIMENT is enabled.

Updates #51430.

Change-Id: Icdf39f845ff2c8b10c634d49e9c27bc90e7984f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/402174
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/script_test.go

@@ -14,6 +14,7 @@ import (
 	"flag"
 	"fmt"
 	"go/build"
+	"internal/buildcfg"
 	"internal/testenv"
 	"internal/txtar"
 	"io/fs"
@@ -246,6 +247,24 @@ func goVersion() (string, error) {
 
 var execCache par.Cache
 
+func goExperimentIsValid(expname string) bool {
+	for _, exp := range buildcfg.Experiment.All() {
+		if expname == exp || expname == "no"+exp || "no"+expname == exp {
+			return true
+		}
+	}
+	return false
+}
+
+func goExperimentIsEnabled(expname string) bool {
+	for _, exp := range buildcfg.Experiment.Enabled() {
+		if exp == expname {
+			return true
+		}
+	}
+	return false
+}
+
 // run runs the test script.
 func (ts *testScript) run() {
 	// Truncate log at end of last phase marker,
@@ -444,6 +463,15 @@ Script:
 					ok = sys.BuildModeSupported(runtime.Compiler, value, runtime.GOOS, runtime.GOARCH)
 					break
 				}
+				if strings.HasPrefix(cond.tag, "GOEXPERIMENT:") {
+					rawval := strings.TrimPrefix(cond.tag, "GOEXPERIMENT:")
+					value := strings.TrimSpace(rawval)
+					if !goExperimentIsValid(value) {
+						ts.fatalf("unknown/unrecognized GOEXPERIMENT %q", value)
+					}
+					ok = goExperimentIsEnabled(value)
+					break
+				}
 				if !imports.KnownArch[cond.tag] && !imports.KnownOS[cond.tag] && cond.tag != "gc" && cond.tag != "gccgo" {
 					ts.fatalf("unknown condition %q", cond.tag)
 				}

src/cmd/go/testdata/script/README

@@ -99,6 +99,7 @@ should only run when the condition is satisfied. The available conditions are:
    - [buildmode:value] for whether -buildmode=value is supported
    - [trimpath] for whether the 'go' binary was built with -trimpath
    - [mismatched-goroot] for whether the test's GOROOT_FINAL does not match the real GOROOT
+   - [GOEXPERIMENT:expname] for whether the GOEXPERIMENT 'expname' is enabled
 
 A condition can be negated: [!short] means to run the rest of the line
 when testing.Short() is false. Multiple conditions may be given for a single

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

@@ -0,0 +1,10 @@
+# Test that [GOEXPERIMENT:x] is accepted.
+# Here fieldtrack is picked arbitrarily.
+
+[GOEXPERIMENT:nofieldtrack] env
+
+[GOEXPERIMENT:fieldtrack] env
+
+#[GOEXPERIMENT:crashme] env
+
+