Do not report cycle error when inferring return type for suggestion

Cargo.lock

@@ -4359,6 +4359,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_session",
  "rustc_span",
+ "rustc_target",
  "tracing",
 ]
 

compiler/rustc_middle/src/query/mod.rs

@@ -770,6 +770,7 @@
         desc { |tcx| "computing function signature of `{}`", tcx.def_path_str(key) }
         cache_on_disk_if { key.is_local() }
         separate_provide_extern
+        cycle_delay_bug
     }
 
     /// Performs lint checking for the module.

compiler/rustc_query_impl/Cargo.toml

@@ -20,6 +20,7 @@ rustc_query_system = { path = "../rustc_query_system" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
+rustc_target = { path = "../rustc_target" }
 tracing = "0.1"
 
 [features]

compiler/rustc_query_impl/src/values.rs

@@ -43,3 +43,23 @@ fn from_cycle_error(tcx: QueryCtxt<'tcx>) -> Self {
         }
     }
 }
+
+impl<'tcx> Value<'tcx> for ty::Binder<'_, ty::FnSig<'_>> {
+    fn from_cycle_error(tcx: QueryCtxt<'tcx>) -> 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 fn_sig = ty::Binder::dummy(tcx.mk_fn_sig(
+            [].into_iter(),
+            err,
+            false,
+            rustc_hir::Unsafety::Normal,
+            rustc_target::spec::abi::Abi::Rust,
+        ));
+
+        // SAFETY: This is never called when `Self` is not `ty::Binder<'tcx, ty::FnSig<'tcx>>`.
+        // FIXME: Represent the above fact in the trait system somehow.
+        unsafe { std::mem::transmute::<ty::PolyFnSig<'tcx>, ty::Binder<'_, ty::FnSig<'_>>>(fn_sig) }
+    }
+}

src/test/ui/suggestions/return-cycle-2.rs

@@ -0,0 +1,14 @@
+use std::marker::PhantomData;
+
+struct Token<T>(PhantomData<T>);
+
+impl<T> Token<T> {
+    fn as_ref(_: i32, _: i32) -> _ {
+        //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+        //~| NOTE not allowed in type signatures
+        //~| HELP replace with the correct return type
+        Token(PhantomData::<&T>)
+    }
+}
+
+fn main() {}

src/test/ui/suggestions/return-cycle-2.stderr

@@ -0,0 +1,12 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/return-cycle-2.rs:6:34
+   |
+LL |     fn as_ref(_: i32, _: i32) -> _ {
+   |                                  ^
+   |                                  |
+   |                                  not allowed in type signatures
+   |                                  help: replace with the correct return type: `Token<&'static T>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.

src/test/ui/suggestions/return-cycle.rs

@@ -0,0 +1,14 @@
+use std::marker::PhantomData;
+
+struct Token<T>(PhantomData<T>);
+
+impl<T> Token<T> {
+    fn new() -> _ {
+        //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+        //~| NOTE not allowed in type signatures
+        //~| HELP replace with the correct return type
+        Token(PhantomData::<()>)
+    }
+}
+
+fn main() {}

src/test/ui/suggestions/return-cycle.stderr

@@ -0,0 +1,12 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/return-cycle.rs:6:17
+   |
+LL |     fn new() -> _ {
+   |                 ^
+   |                 |
+   |                 not allowed in type signatures
+   |                 help: replace with the correct return type: `Token<()>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.