Change feature edition error into a warning

src/libsyntax/feature_gate.rs

@@ -1956,7 +1956,7 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
 
             if let Some((.., set)) = ACTIVE_FEATURES.iter().find(|f| name == f.0) {
                 if let Some(edition) = edition_enabled_features.get(&name) {
-                    struct_span_err!(
+                    struct_span_warn!(
                         span_handler,
                         mi.span,
                         E0705,

src/test/ui/E0705.rs

@@ -8,9 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-pass
+
 #![feature(rust_2018_preview)]
 #![feature(raw_identifiers)]
-//~^ ERROR the feature `raw_identifiers` is included in the Rust 2018 edition
+//~^ WARN the feature `raw_identifiers` is included in the Rust 2018 edition
 
 fn main() {
     let foo = 0;

src/test/ui/E0705.stderr

@@ -1,9 +1,6 @@
-error[E0705]: the feature `raw_identifiers` is included in the Rust 2018 edition
-  --> $DIR/E0705.rs:12:12
+warning[E0705]: the feature `raw_identifiers` is included in the Rust 2018 edition
+  --> $DIR/E0705.rs:14:12
    |
 LL | #![feature(raw_identifiers)]
    |            ^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0705`.