Make nested impl Trait a hard error

src/test/compile-fail/impl-trait/where-allowed.rs

@@ -10,7 +10,7 @@
 
 //! A simple test for testing many permutations of allowedness of
 //! impl Trait
-#![feature(conservative_impl_trait, nested_impl_trait, universal_impl_trait, dyn_trait)]
+#![feature(conservative_impl_trait, universal_impl_trait, dyn_trait)]
 use std::fmt::Debug;
 
 // Allowed
@@ -60,6 +60,7 @@ fn in_dyn_Fn_return_in_return() -> &'static dyn Fn() -> impl Debug { panic!() }
 // Disallowed
 fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and inherent method return types
+//~^^ ERROR nested `impl Trait` is not allowed
 
 // Disallowed
 fn in_impl_Fn_return_in_parameters(_: &impl Fn() -> impl Debug) { panic!() }
@@ -68,6 +69,7 @@ fn in_impl_Fn_return_in_parameters(_: &impl Fn() -> impl Debug) { panic!() }
 // Disallowed
 fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and inherent method return types
+//~^^ ERROR nested `impl Trait` is not allowed
 
 // Disallowed
 fn in_impl_Fn_return_in_return() -> &'static impl Fn() -> impl Debug { panic!() }

src/test/run-pass/impl-trait/lifetimes.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(conservative_impl_trait, underscore_lifetimes, universal_impl_trait, nested_impl_trait)]
+#![feature(conservative_impl_trait, underscore_lifetimes, universal_impl_trait)]
 #![allow(warnings)]
 
 use std::fmt::Debug;
@@ -55,12 +55,11 @@ fn pass_through_elision_with_fn_ptr(x: &fn(&u32) -> &u32) -> impl Into<&fn(&u32)
 
 fn pass_through_elision_with_fn_path<T: Fn(&u32) -> &u32>(
     x: &T
-) -> impl Into<&impl Fn(&u32) -> &u32> { x }
+) -> &impl Fn(&u32) -> &u32 { x }
 
-fn foo(x: &impl Debug) -> impl Into<&impl Debug> { x }
-fn foo_explicit_lifetime<'a>(x: &'a impl Debug) -> impl Into<&'a impl Debug> { x }
-fn foo_no_outer_impl(x: &impl Debug) -> &impl Debug { x }
-fn foo_explicit_arg<T: Debug>(x: &T) -> impl Into<&impl Debug> { x }
+fn foo(x: &impl Debug) -> &impl Debug { x }
+fn foo_explicit_lifetime<'a>(x: &'a impl Debug) -> &'a impl Debug { x }
+fn foo_explicit_arg<T: Debug>(x: &T) -> &impl Debug { x }
 
 fn mixed_lifetimes<'a>() -> impl for<'b: 'a> Fn(&'b u32) { |_| () }
 fn mixed_as_static() -> impl Fn(&'static u32) { mixed_lifetimes() }

src/test/ui/error-codes/E0657.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 #![allow(warnings)]
-#![feature(conservative_impl_trait, nested_impl_trait)]
+#![feature(conservative_impl_trait)]
 
 trait Id<T> {}
 trait Lt<'a> {}
@@ -17,7 +17,7 @@ impl<'a> Lt<'a> for () {}
 impl<T> Id<T> for T {}
 
 fn free_fn_capture_hrtb_in_impl_trait()
-    -> impl for<'a> Id<impl Lt<'a>>
+    -> Box<for<'a> Id<impl Lt<'a>>>
         //~^ ERROR `impl Trait` can only capture lifetimes bound at the fn or impl level [E0657]
 {
     ()
@@ -26,7 +26,7 @@ fn free_fn_capture_hrtb_in_impl_trait()
 struct Foo;
 impl Foo {
     fn impl_fn_capture_hrtb_in_impl_trait()
-        -> impl for<'a> Id<impl Lt<'a>>
+        -> Box<for<'a> Id<impl Lt<'a>>>
             //~^ ERROR `impl Trait` can only capture lifetimes bound at the fn or impl level
     {
         ()

src/test/ui/nested_impl_trait.rs

@@ -14,18 +14,19 @@ use std::fmt::Debug;
 fn fine(x: impl Into<u32>) -> impl Into<u32> { x }
 
 fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-//~^ ERROR nested `impl Trait` is experimental
+//~^ ERROR nested `impl Trait` is not allowed
 
 fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
-//~^ ERROR nested `impl Trait` is experimental
+//~^ ERROR nested `impl Trait` is not allowed
+//~^^ `impl Trait` not allowed
 
 fn bad_in_arg_position(_: impl Into<impl Debug>) { }
-//~^ ERROR nested `impl Trait` is experimental
+//~^ ERROR nested `impl Trait` is not allowed
 
 struct X;
 impl X {
     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-    //~^ ERROR nested `impl Trait` is experimental
+    //~^ ERROR nested `impl Trait` is not allowed
 }
 
 fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
@@ -33,6 +34,7 @@ fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
 }
 
 fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
+//~^ `impl Trait` not allowed
     || 5
 }
 

src/test/ui/nested_impl_trait.stderr

@@ -0,0 +1,50 @@
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/nested_impl_trait.rs:16:56
+   |
+16 | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
+   |                                              ----------^^^^^^^^^^-
+   |                                              |         |
+   |                                              |         devilishly nested `impl Trait` here
+   |                                              outer `impl Trait`
+
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/nested_impl_trait.rs:19:42
+   |
+19 | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
+   |                                ----------^^^^^^^^^^-
+   |                                |         |
+   |                                |         devilishly nested `impl Trait` here
+   |                                outer `impl Trait`
+
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/nested_impl_trait.rs:23:37
+   |
+23 | fn bad_in_arg_position(_: impl Into<impl Debug>) { }
+   |                           ----------^^^^^^^^^^-
+   |                           |         |
+   |                           |         devilishly nested `impl Trait` here
+   |                           outer `impl Trait`
+
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/nested_impl_trait.rs:28:44
+   |
+28 |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
+   |                                  ----------^^^^^^^^^^-
+   |                                  |         |
+   |                                  |         devilishly nested `impl Trait` here
+   |                                  outer `impl Trait`
+
+error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
+  --> $DIR/nested_impl_trait.rs:19:32
+   |
+19 | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
+   |                                ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
+  --> $DIR/nested_impl_trait.rs:36:42
+   |
+36 | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
+   |                                          ^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+