map_identity checks for unneeded .map_err

Author: dswij

clippy_lints/src/methods/map_identity.rs

@@ -13,6 +13,7 @@ pub(super) fn check(
     expr: &hir::Expr<'_>,
     caller: &hir::Expr<'_>,
     map_arg: &hir::Expr<'_>,
+    name: &str,
     _map_span: Span,
 ) {
     let caller_ty = cx.typeck_results().expr_ty(caller);
@@ -29,7 +30,7 @@ pub(super) fn check(
                 MAP_IDENTITY,
                 sugg_span,
                 "unnecessary map of the identity function",
-                "remove the call to `map`",
+                &format!("remove the call to `{}`", name),
                 String::new(),
                 Applicability::MachineApplicable,
             )

clippy_lints/src/methods/mod.rs

@@ -2334,7 +2334,7 @@ fn check_methods<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, msrv: Optio
                     }
                 }
             },
-            ("map", [m_arg]) => {
+            (name @ ("map" | "map_err"), [m_arg]) => {
                 if let Some((name, [recv2, args @ ..], span2)) = method_call(recv) {
                     match (name, args) {
                         ("as_mut", []) => option_as_ref_deref::check(cx, expr, recv2, m_arg, true, msrv),
@@ -2346,7 +2346,7 @@ fn check_methods<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, msrv: Optio
                         _ => {},
                     }
                 }
-                map_identity::check(cx, expr, recv, m_arg, span);
+                map_identity::check(cx, expr, recv, m_arg, name, span);
             },
             ("map_or", [def, map]) => option_map_or_none::check(cx, expr, recv, def, map),
             (name @ "next", args @ []) => {