Revise logic to match `rustc::session::config::get_unstable_features_setting`

src/libsyntax/feature_gate.rs

@@ -411,10 +411,7 @@ pub fn emit_feature_err(diag: &SpanHandler, feature: &str, span: Span, explain:
     diag.span_err(span, explain);
 
     // #23973: do not suggest `#![feature(...)]` if we are in beta/stable
-    match option_env!("CFG_RELEASE_CHANNEL") {
-        Some("stable") | Some("beta") => return,
-        _ => {}
-    }
+    if option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some() { return; }
     diag.fileline_help(span, &format!("add #![feature({})] to the \
                                    crate attributes to enable",
                                   feature));
@@ -424,10 +421,7 @@ pub fn emit_feature_warn(diag: &SpanHandler, feature: &str, span: Span, explain:
     diag.span_warn(span, explain);
 
     // #23973: do not suggest `#![feature(...)]` if we are in beta/stable
-    match option_env!("CFG_RELEASE_CHANNEL") {
-        Some("stable") | Some("beta") => return,
-        _ => {}
-    }
+    if option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some() { return; }
     if diag.handler.can_emit_warnings {
         diag.fileline_help(span, &format!("add #![feature({})] to the \
                                        crate attributes to silence this warning",