Use 'a' article for &Option.

Author: zachs18

compiler/rustc_lint/src/for_loops_over_fallibles.rs

@@ -62,7 +62,8 @@ impl<'tcx> LateLintPass<'tcx> for ForLoopsOverFallibles {
         };
 
         let (article, ty, var) = match adt.did() {
-            did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("an", "Option", "Some"),
+            did if cx.tcx.is_diagnostic_item(sym::Option, did) && ref_mutability.is_some() => ("an", "Option", "Some"),
+            did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("a", "Option", "Some"),
             did if cx.tcx.is_diagnostic_item(sym::Result, did) => ("a", "Result", "Ok"),
             _ => return,
         };