Fix a FIXME.

Contrary to what the comment says, `gate_feature_post` can be used with
multiple spans, as is seen in the `gate_all!` macro. (The treatment is
simplistic, but it works.)

Author: nnethercote

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -144,15 +144,11 @@ impl<'a> PostExpansionVisitor<'a> {
                 _ => Some(param.ident.span),
             })
             .collect();
-        // FIXME: gate_feature_post doesn't really handle multispans...
-        if !non_lt_param_spans.is_empty() && !self.features.non_lifetime_binders {
-            feature_err(
-                &self.sess.parse_sess,
-                sym::non_lifetime_binders,
-                non_lt_param_spans,
-                crate::fluent_generated::ast_passes_forbidden_non_lifetime_param,
-            )
-            .emit();
+        if !non_lt_param_spans.is_empty() {
+            for span in non_lt_param_spans {
+                let msg = crate::fluent_generated::ast_passes_forbidden_non_lifetime_param;
+                gate_feature_post!(self, non_lifetime_binders, span, msg);
+            }
         }
         for param in params {
             if !param.bounds.is_empty() {