Add a stub feature so we can still test E0705

Author: scottmcm

src/libsyntax/diagnostic_list.rs

@@ -375,15 +375,15 @@ and likely to change in the future.
 
 E0705: r##"
 A `#![feature]` attribute was declared for a feature that is stable in
-the current edition.
+the current edition, but not in all editions.
 
 Erroneous code example:
 
 ```ignore (limited to a warning during 2018 edition development)
 #![feature(rust_2018_preview)]
-#![feature(impl_header_lifetime_elision)] // error: the feature
-                                          // `impl_header_lifetime_elision` is
-                                          // included in the Rust 2018 edition
+#![feature(test_2018_feature)] // error: the feature
+                               // `test_2018_feature` is
+                               // included in the Rust 2018 edition
 ```
 
 "##,

src/libsyntax/feature_gate.rs

@@ -462,6 +462,9 @@ declare_features! (
 
     (active, abi_amdgpu_kernel, "1.29.0", Some(51575), None),
 
+    // Perma-unstable; added for testing E0705
+    (active, test_2018_feature, "1.31.0", Some(0), Some(Edition::Edition2018)),
+
     // Support for arbitrary delimited token streams in non-macro attributes
     (active, unrestricted_attribute_tokens, "1.30.0", Some(44690), None),
 

src/test/ui/E0705.rs

@@ -10,8 +10,11 @@
 
 // compile-pass
 
-#![feature(impl_header_lifetime_elision)]
-//~^ WARN the feature `impl_header_lifetime_elision` is included in the Rust 2018 edition
+// This is a stub feature that doesn't control anything, so to make tidy happy,
+// gate-test-test_2018_feature
+
+#![feature(test_2018_feature)]
+//~^ WARN the feature `test_2018_feature` is included in the Rust 2018 edition
 #![feature(rust_2018_preview)]
 
 fn main() {}

src/test/ui/E0705.stderr

@@ -1,6 +1,6 @@
-warning[E0705]: the feature `impl_header_lifetime_elision` is included in the Rust 2018 edition
-  --> $DIR/E0705.rs:13:12
+warning[E0705]: the feature `test_2018_feature` is included in the Rust 2018 edition
+  --> $DIR/E0705.rs:16:12
    |
-LL | #![feature(impl_header_lifetime_elision)]
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #![feature(test_2018_feature)]
+   |            ^^^^^^^^^^^^^^^^^