Auto merge of rust-lang#6416 - deg4uss3r:map_err_restricted, r=ebroto

Moved map_err_ignore to restriction and updated help message

This MR moves map_err_ignore lint from `pedantic` to the `restriction` category of lints and updates the help message to give the user an option to ignore the lint by naming the closure variable e.g. `.map_err(|_ignored| ...`

---

changelog: move map_err_ignore to restriction category

clippy_lints/src/lib.rs

@@ -1217,6 +1217,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&integer_division::INTEGER_DIVISION),
         LintId::of(&let_underscore::LET_UNDERSCORE_MUST_USE),
         LintId::of(&literal_representation::DECIMAL_LITERAL_REPRESENTATION),
+        LintId::of(&map_err_ignore::MAP_ERR_IGNORE),
         LintId::of(&matches::REST_PAT_IN_FULLY_BOUND_STRUCTS),
         LintId::of(&matches::WILDCARD_ENUM_MATCH_ARM),
         LintId::of(&mem_forget::MEM_FORGET),
@@ -1283,7 +1284,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::EXPLICIT_ITER_LOOP),
         LintId::of(&macro_use::MACRO_USE_IMPORTS),
         LintId::of(&manual_ok_or::MANUAL_OK_OR),
-        LintId::of(&map_err_ignore::MAP_ERR_IGNORE),
         LintId::of(&match_on_vec_items::MATCH_ON_VEC_ITEMS),
         LintId::of(&matches::MATCH_BOOL),
         LintId::of(&matches::MATCH_SAME_ARMS),

clippy_lints/src/map_err_ignore.rs

@@ -99,7 +99,7 @@ declare_clippy_lint! {
     /// }
     /// ```
     pub MAP_ERR_IGNORE,
-    pedantic,
+    restriction,
     "`map_err` should not ignore the original error"
 }
 
@@ -133,9 +133,9 @@ impl<'tcx> LateLintPass<'tcx> for MapErrIgnore {
                                     cx,
                                     MAP_ERR_IGNORE,
                                     body_span,
-                                    "`map_err(|_|...` ignores the original error",
+                                    "`map_err(|_|...` wildcard pattern discards the original error",
                                     None,
-                                    "Consider wrapping the error in an enum variant",
+                                    "Consider storing the original error as a source in the new error, or silence this warning using an ignored identifier (`.map_err(|_foo| ...`)",
                                 );
                             }
                         }

tests/ui/map_err.rs

@@ -22,5 +22,9 @@ fn main() -> Result<(), Errors> {
 
     println!("{:?}", x.map_err(|_| Errors::Ignored));
 
+    // Should not warn you because you explicitly ignore the parameter
+    // using a named wildcard value
+    println!("{:?}", x.map_err(|_foo| Errors::Ignored));
+
     Ok(())
 }

tests/ui/map_err.stderr

@@ -1,11 +1,11 @@
-error: `map_err(|_|...` ignores the original error
+error: `map_err(|_|...` wildcard pattern discards the original error
   --> $DIR/map_err.rs:23:32
    |
 LL |     println!("{:?}", x.map_err(|_| Errors::Ignored));
    |                                ^^^
    |
    = note: `-D clippy::map-err-ignore` implied by `-D warnings`
-   = help: Consider wrapping the error in an enum variant
+   = help: Consider storing the original error as a source in the new error, or silence this warning using an ignored identifier (`.map_err(|_foo| ...`)
 
 error: aborting due to previous error