Warn if the user tries to use GATs

src/libsyntax/feature_gate.rs

@@ -1920,6 +1920,11 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
         err.emit();
     }
 
+    // Some features are known to be incomplete and using them is likely to have
+    // unanticipated results, such as compiler crashes. We warn the user about these
+    // to alert them.
+    let incomplete_features = ["generic_associated_types"];
+
     let mut features = Features::new();
     let mut edition_enabled_features = FxHashMap();
 
@@ -1955,6 +1960,16 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
                 continue
             };
 
+            if incomplete_features.iter().any(|f| *f == name.as_str()) {
+                span_handler.struct_span_warn(
+                    mi.span,
+                    &format!(
+                        "the feature `{}` is incomplete and may cause the compiler to crash",
+                        name
+                    )
+                ).emit();
+            }
+
             if let Some(edition) = ALL_EDITIONS.iter().find(|e| name == e.feature_name()) {
                 if *edition <= crate_edition {
                     continue

src/test/ui/rfc1598-generic-associated-types/collections.rs

@@ -9,10 +9,11 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 #![feature(associated_type_defaults)]
 
-//FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
-//follow-up PR
+// FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
+// follow-up PR.
 
 // A Collection trait and collection families. Based on
 // http://smallcultfollowing.com/babysteps/blog/2016/11/03/

src/test/ui/rfc1598-generic-associated-types/collections.stderr

@@ -1,29 +1,35 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/collections.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0109]: type parameters are not allowed on this type
-  --> $DIR/collections.rs:65:90
+  --> $DIR/collections.rs:66:90
    |
 LL | fn floatify<C>(ints: &C) -> <<C as Collection<i32>>::Family as CollectionFamily>::Member<f32>
    |                                                                                          ^^^ type parameter not allowed
 
 error[E0109]: type parameters are not allowed on this type
-  --> $DIR/collections.rs:77:69
+  --> $DIR/collections.rs:78:69
    |
 LL | fn floatify_sibling<C>(ints: &C) -> <C as Collection<i32>>::Sibling<f32>
    |                                                                     ^^^ type parameter not allowed
 
 error[E0109]: type parameters are not allowed on this type
-  --> $DIR/collections.rs:26:71
+  --> $DIR/collections.rs:27:71
    |
 LL |         <<Self as Collection<T>>::Family as CollectionFamily>::Member<U>;
    |                                                                       ^ type parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/collections.rs:33:50
+  --> $DIR/collections.rs:34:50
    |
 LL |     fn iterate<'iter>(&'iter self) -> Self::Iter<'iter>;
    |                                                  ^^^^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/collections.rs:59:50
+  --> $DIR/collections.rs:60:50
    |
 LL |     fn iterate<'iter>(&'iter self) -> Self::Iter<'iter> {
    |                                                  ^^^^^ lifetime parameter not allowed

src/test/ui/rfc1598-generic-associated-types/construct_with_other_type.rs

@@ -9,11 +9,12 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 
 use std::ops::Deref;
 
-//FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
-//follow-up PR
+// FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
+// follow-up PR.
 
 trait Foo {
     type Bar<'a, 'b>;

src/test/ui/rfc1598-generic-associated-types/construct_with_other_type.stderr

@@ -1,17 +1,23 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/construct_with_other_type.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/construct_with_other_type.rs:26:46
+  --> $DIR/construct_with_other_type.rs:27:46
    |
 LL |     type Baa<'a>: Deref<Target = <Self::Quux<'a> as Foo>::Bar<'a, 'static>>;
    |                                              ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/construct_with_other_type.rs:26:63
+  --> $DIR/construct_with_other_type.rs:27:63
    |
 LL |     type Baa<'a>: Deref<Target = <Self::Quux<'a> as Foo>::Bar<'a, 'static>>;
    |                                                               ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/construct_with_other_type.rs:34:40
+  --> $DIR/construct_with_other_type.rs:35:40
    |
 LL |     type Baa<'a> = &'a <T as Foo>::Bar<'a, 'static>;
    |                                        ^^ lifetime parameter not allowed

src/test/ui/rfc1598-generic-associated-types/empty_generics.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 
 trait Foo {
     type Bar<,>;

src/test/ui/rfc1598-generic-associated-types/empty_generics.stderr

@@ -1,8 +1,14 @@
 error: expected one of `>`, identifier, or lifetime, found `,`
-  --> $DIR/empty_generics.rs:14:14
+  --> $DIR/empty_generics.rs:15:14
    |
 LL |     type Bar<,>;
    |              ^ expected one of `>`, identifier, or lifetime here
 
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/empty_generics.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
 error: aborting due to previous error
 

src/test/ui/rfc1598-generic-associated-types/gat-incomplete-warning.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-pass
+
+#![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
+
+fn main() {}

src/test/ui/rfc1598-generic-associated-types/gat-incomplete-warning.stderr

@@ -0,0 +1,6 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/gat-incomplete-warning.rs:13:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+

src/test/ui/rfc1598-generic-associated-types/generic-associated-types-where.stderr

@@ -0,0 +1,6 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/generic-associated-types-where.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+

src/test/ui/rfc1598-generic-associated-types/generic_associated_type_undeclared_lifetimes.rs

@@ -9,11 +9,12 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 
 use std::ops::Deref;
 
-//FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
-//follow-up PR
+// FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
+// follow-up PR.
 
 trait Iterable {
     type Item<'a>;

src/test/ui/rfc1598-generic-associated-types/generic_associated_type_undeclared_lifetimes.stderr

@@ -1,29 +1,35 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0261]: use of undeclared lifetime name `'b`
-  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:22:37
+  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:23:37
    |
 LL |         + Deref<Target = Self::Item<'b>>;
    |                                     ^^ undeclared lifetime
 
 error[E0261]: use of undeclared lifetime name `'undeclared`
-  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:26:41
+  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:27:41
    |
 LL |     fn iter<'a>(&'a self) -> Self::Iter<'undeclared>;
    |                                         ^^^^^^^^^^^ undeclared lifetime
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:20:47
+  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:21:47
    |
 LL |     type Iter<'a>: Iterator<Item = Self::Item<'a>>
    |                                               ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:22:37
+  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:23:37
    |
 LL |         + Deref<Target = Self::Item<'b>>;
    |                                     ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:26:41
+  --> $DIR/generic_associated_type_undeclared_lifetimes.rs:27:41
    |
 LL |     fn iter<'a>(&'a self) -> Self::Iter<'undeclared>;
    |                                         ^^^^^^^^^^^ lifetime parameter not allowed

src/test/ui/rfc1598-generic-associated-types/iterable.rs

@@ -9,11 +9,12 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 
 use std::ops::Deref;
 
-//FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
-//follow-up PR
+// FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
+// follow-up PR.
 
 trait Iterable {
     type Item<'a>;

src/test/ui/rfc1598-generic-associated-types/iterable.stderr

@@ -1,35 +1,41 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/iterable.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/iterable.rs:20:47
+  --> $DIR/iterable.rs:21:47
    |
 LL |     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
    |                                               ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/iterable.rs:49:53
+  --> $DIR/iterable.rs:50:53
    |
 LL | fn make_iter<'a, I: Iterable>(it: &'a I) -> I::Iter<'a> {
    |                                                     ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/iterable.rs:54:60
+  --> $DIR/iterable.rs:55:60
    |
 LL | fn get_first<'a, I: Iterable>(it: &'a I) -> Option<I::Item<'a>> {
    |                                                            ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/iterable.rs:23:41
+  --> $DIR/iterable.rs:24:41
    |
 LL |     fn iter<'a>(&'a self) -> Self::Iter<'a>;
    |                                         ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/iterable.rs:32:41
+  --> $DIR/iterable.rs:33:41
    |
 LL |     fn iter<'a>(&'a self) -> Self::Iter<'a> {
    |                                         ^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/iterable.rs:43:41
+  --> $DIR/iterable.rs:44:41
    |
 LL |     fn iter<'a>(&'a self) -> Self::Iter<'a> {
    |                                         ^^ lifetime parameter not allowed

src/test/ui/rfc1598-generic-associated-types/parameter_number_and_kind.rs

@@ -9,12 +9,13 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 #![feature(associated_type_defaults)]
 
-//FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
-//follow-up PR
+// FIXME(#44265): "lifetime parameters are not allowed on this type" errors will be addressed in a
+// follow-up PR.
 
-//FIXME(#44265): Update expected errors once E110 is resolved, now does not get past `trait Foo`
+// FIXME(#44265): Update expected errors once E110 is resolved, now does not get past `trait Foo`.
 
 trait Foo {
     type A<'a>;

src/test/ui/rfc1598-generic-associated-types/parameter_number_and_kind.stderr

@@ -1,29 +1,35 @@
+warning: the feature `generic_associated_types` is incomplete and may cause the compiler to crash
+  --> $DIR/parameter_number_and_kind.rs:11:12
+   |
+LL | #![feature(generic_associated_types)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/parameter_number_and_kind.rs:26:27
+  --> $DIR/parameter_number_and_kind.rs:27:27
    |
 LL |     type FOk<T> = Self::E<'static, T>;
    |                           ^^^^^^^ lifetime parameter not allowed
 
 error[E0109]: type parameters are not allowed on this type
-  --> $DIR/parameter_number_and_kind.rs:26:36
+  --> $DIR/parameter_number_and_kind.rs:27:36
    |
 LL |     type FOk<T> = Self::E<'static, T>;
    |                                    ^ type parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/parameter_number_and_kind.rs:29:26
+  --> $DIR/parameter_number_and_kind.rs:30:26
    |
 LL |     type FErr1 = Self::E<'static, 'static>; // Error
    |                          ^^^^^^^ lifetime parameter not allowed
 
 error[E0110]: lifetime parameters are not allowed on this type
-  --> $DIR/parameter_number_and_kind.rs:31:29
+  --> $DIR/parameter_number_and_kind.rs:32:29
    |
 LL |     type FErr2<T> = Self::E<'static, T, u32>; // Error
    |                             ^^^^^^^ lifetime parameter not allowed
 
 error[E0109]: type parameters are not allowed on this type
-  --> $DIR/parameter_number_and_kind.rs:31:38
+  --> $DIR/parameter_number_and_kind.rs:32:38
    |
 LL |     type FErr2<T> = Self::E<'static, T, u32>; // Error
    |                                      ^ type parameter not allowed

src/test/ui/rfc1598-generic-associated-types/pointer_family.rs

@@ -9,8 +9,9 @@
 // except according to those terms.
 
 #![feature(generic_associated_types)]
+//~^ WARNING the feature `generic_associated_types` is incomplete
 
-//FIXME(#44265): "type parameter not allowed" errors will be addressed in a follow-up PR
+// FIXME(#44265): "type parameter not allowed" errors will be addressed in a follow-up PR.
 
 use std::rc::Rc;
 use std::sync::Arc;