Warn when generic_associated_types feature gate is enabled

Author: varkor

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/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)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+