Auto merge of rust-lang#8487 - dswij:8478, r=giraffate

[`map_identity`] checks for needless `map_err`

Closes rust-lang#8478

changelog: [`map_identity`] checks for needless `map_err`

Author: bors

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

@@ -2472,7 +2472,7 @@ 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),
@@ -2484,7 +2484,7 @@ 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 @ []) => {

tests/ui/map_identity.fixed

@@ -16,6 +16,8 @@ fn main() {
     let _: Result<i8, f32> = Err(2.3).map(|x: i8| {
         return x + 3;
     });
+    let _: Result<u32, u32> = Ok(1);
+    let _: Result<u32, u32> = Ok(1).map_err(|a: u32| a * 42);
 }
 
 fn not_identity(x: &u16) -> u16 {

tests/ui/map_identity.rs

@@ -18,6 +18,8 @@ fn main() {
     let _: Result<i8, f32> = Err(2.3).map(|x: i8| {
         return x + 3;
     });
+    let _: Result<u32, u32> = Ok(1).map_err(|a| a);
+    let _: Result<u32, u32> = Ok(1).map_err(|a: u32| a * 42);
 }
 
 fn not_identity(x: &u16) -> u16 {

tests/ui/map_identity.stderr

@@ -33,5 +33,11 @@ LL | |         return x;
 LL | |     });
    | |______^ help: remove the call to `map`
 
-error: aborting due to 5 previous errors
+error: unnecessary map of the identity function
+  --> $DIR/map_identity.rs:21:36
+   |
+LL |     let _: Result<u32, u32> = Ok(1).map_err(|a| a);
+   |                                    ^^^^^^^^^^^^^^^ help: remove the call to `map_err`
+
+error: aborting due to 6 previous errors