Properly synthesize fn sig value during cycle

compiler/rustc_middle/src/values.rs

@@ -32,13 +32,23 @@ impl<'tcx> Value<TyCtxt<'tcx>> for ty::SymbolName<'_> {
 }
 
 impl<'tcx> Value<TyCtxt<'tcx>> for ty::Binder<'_, ty::FnSig<'_>> {
-    fn from_cycle_error(tcx: TyCtxt<'tcx>, _: &[QueryInfo]) -> Self {
+    fn from_cycle_error(tcx: TyCtxt<'tcx>, stack: &[QueryInfo]) -> Self {
         let err = tcx.ty_error();
-        // FIXME(compiler-errors): It would be nice if we could get the
-        // query key, so we could at least generate a fn signature that
-        // has the right arity.
+
+        let arity = if let Some(frame) = stack.get(0)
+            && frame.query.name == "fn_sig"
+            && let Some(def_id) = frame.query.def_id
+            && let Some(node) = tcx.hir().get_if_local(def_id)
+            && let Some(sig) = node.fn_sig()
+        {
+            sig.decl.inputs.len() + sig.decl.implicit_self.has_implicit_self() as usize
+        } else {
+            tcx.sess.abort_if_errors();
+            unreachable!()
+        };
+
         let fn_sig = ty::Binder::dummy(tcx.mk_fn_sig(
-            [].into_iter(),
+            std::iter::repeat(err).take(arity),
             err,
             false,
             rustc_hir::Unsafety::Normal,

src/test/ui/query-system/fn-sig-cycle-arity.rs

@@ -0,0 +1,8 @@
+trait Dancer {
+    fn dance(&self) -> _ {
+        //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+        self.dance()
+    }
+}
+
+fn main() {}

src/test/ui/query-system/fn-sig-cycle-arity.stderr

@@ -0,0 +1,9 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/fn-sig-cycle-arity.rs:2:24
+   |
+LL |     fn dance(&self) -> _ {
+   |                        ^ not allowed in type signatures
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.