Un-stabilize --extern-private option in rustdoc

src/librustdoc/config.rs

@@ -626,7 +626,12 @@ fn parse_extern_html_roots(
 // FIXME(eddyb) This shouldn't be duplicated with `rustc::session`.
 fn parse_externs(matches: &getopts::Matches) -> Result<Externs, String> {
     let mut externs: BTreeMap<_, ExternEntry> = BTreeMap::new();
-    for arg in matches.opt_strs("extern").iter().chain(matches.opt_strs("extern-private").iter()) {
+    let extern_private = if nightly_options::is_nightly_build() {
+        matches.opt_strs("extern-private")
+    } else {
+        Vec::new()
+    };
+    for arg in matches.opt_strs("extern").iter().chain(extern_private.iter()) {
         let mut parts = arg.splitn(2, '=');
         let name = parts.next().ok_or("--extern value must not be empty".to_string())?;
         let location = parts.next().map(|s| s.to_string());

src/librustdoc/lib.rs

@@ -144,7 +144,7 @@ fn opts() -> Vec<RustcOptGroup> {
         stable("extern", |o| {
             o.optmulti("", "extern", "pass an --extern to rustc", "NAME[=PATH]")
         }),
-        stable("extern-private", |o| {
+        unstable("extern-private", |o| {
             o.optmulti("", "extern-private",
                        "pass an --extern to rustc (compatibility only)", "NAME=PATH")
         }),

src/test/rustdoc/issue-66159.rs

@@ -1,4 +1,5 @@
 // aux-build:issue-66159-1.rs
+// compile-flags:-Z unstable-options
 // extern-private:issue_66159_1
 
 // The issue was an ICE which meant that we never actually generated the docs