libtest: Improve error when missing `-Zunstable-options`

"only accepted on the nightly compiler" is misleading when this *is* nightly.

1 files changed, 2 insertions, 2 deletions

diff --git a/library/test/src/cli.rs b/library/test/src/cli.rs
index 9d22ebbee87..6ac3b3eaa79 100644
--- a/library/test/src/cli.rs
+++ b/library/test/src/cli.rs
@@ -404,13 +404,13 @@ fn get_format(
         Some("terse") => OutputFormat::Terse,
         Some("json") => {
             if !allow_unstable {
-                return Err("The \"json\" format is only accepted on the nightly compiler".into());
+                return Err("The \"json\" format is only accepted on the nightly compiler with -Z unstable-options".into());
             }
             OutputFormat::Json
         }
         Some("junit") => {
             if !allow_unstable {
-                return Err("The \"junit\" format is only accepted on the nightly compiler".into());
+                return Err("The \"junit\" format is only accepted on the nightly compiler with -Z unstable-options".into());
             }
             OutputFormat::Junit
         }