add test

compiler/rustc_lint/src/internal.rs

@@ -191,6 +191,12 @@ fn is_ty_or_ty_ctxt(cx: &LateContext<'_>, ty: &Ty<'_>) -> Option<String> {
                 Res::SelfTy(None, Some((did, _))) => {
                     if let ty::Adt(adt, substs) = cx.tcx.type_of(did).kind() {
                         if cx.tcx.is_diagnostic_item(sym::Ty, adt.did) {
+                            // NOTE: This path is currently unreachable as `Ty<'tcx>` is
+                            // defined as a type alias meaning that `impl<'tcx> Ty<'tcx>`
+                            // is not actually allowed.
+                            //
+                            // I(@lcnr) still kept this branch in so we don't miss this
+                            // if we ever change it in the future.
                             return Some(format!("Ty<{}>", substs[0]));
                         } else if cx.tcx.is_diagnostic_item(sym::TyCtxt, adt.did) {
                             return Some(format!("TyCtxt<{}>", substs[0]));

src/test/ui-fulldeps/internal-lints/pass_ty_by_ref_self.rs

@@ -0,0 +1,33 @@
+// NOTE: This test doesn't actually require `fulldeps`
+// so we could instead use it as an `ui` test.
+//
+// Considering that all other `internal-lints` are tested here
+// this seems like the cleaner solution though.
+#![feature(rustc_attrs)]
+#![deny(rustc::ty_pass_by_reference)]
+#![allow(unused)]
+
+#[rustc_diagnostic_item = "TyCtxt"]
+struct TyCtxt<'tcx> {
+    inner: &'tcx (),
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    fn by_value(self) {} // OK
+    fn by_ref(&self) {} //~ ERROR passing `TyCtxt<'tcx>` by reference
+}
+
+
+struct TyS<'tcx> {
+    inner: &'tcx (),
+}
+
+#[rustc_diagnostic_item = "Ty"]
+type Ty<'tcx> = &'tcx TyS<'tcx>;
+
+impl<'tcx> TyS<'tcx> {
+    fn by_value(self: Ty<'tcx>) {}
+    fn by_ref(self: &Ty<'tcx>) {} //~ ERROR passing `Ty<'tcx>` by reference
+}
+
+fn main() {}

src/test/ui-fulldeps/internal-lints/pass_ty_by_ref_self.stderr

@@ -0,0 +1,20 @@
+error: passing `TyCtxt<'tcx>` by reference
+  --> $DIR/pass_ty_by_ref_self.rs:17:15
+   |
+LL |     fn by_ref(&self) {}
+   |               ^^^^^ help: try passing by value: `TyCtxt<'tcx>`
+   |
+note: the lint level is defined here
+  --> $DIR/pass_ty_by_ref_self.rs:7:9
+   |
+LL | #![deny(rustc::ty_pass_by_reference)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: passing `Ty<'tcx>` by reference
+  --> $DIR/pass_ty_by_ref_self.rs:30:21
+   |
+LL |     fn by_ref(self: &Ty<'tcx>) {}
+   |                     ^^^^^^^^^ help: try passing by value: `Ty<'tcx>`
+
+error: aborting due to 2 previous errors
+