Also emit missing_docs lint with --test to fulfill expectations

compiler/rustc_lint/src/builtin.rs

@@ -426,12 +426,6 @@ impl MissingDoc {
         article: &'static str,
         desc: &'static str,
     ) {
-        // If we're building a test harness, then warning about
-        // documentation is probably not really relevant right now.
-        if cx.sess().opts.test {
-            return;
-        }
-
         // Only check publicly-visible items, using the result from the privacy pass.
         // It's an option so the crate root can also use this function (it doesn't
         // have a `NodeId`).

tests/ui/lint/lint-missing-doc-expect.rs

@@ -0,0 +1,13 @@
+// Make sure that `#[expect(missing_docs)]` is always correctly fulfilled.
+
+//@ check-pass
+//@ revisions: lib bin test
+//@ [lib]compile-flags: --crate-type lib
+//@ [bin]compile-flags: --crate-type bin
+//@ [test]compile-flags: --test
+
+#[expect(missing_docs)]
+pub fn foo() {}
+
+#[cfg(bin)]
+fn main() {}