Split redundant_closure lint

Move the method checking into a new lint called
`redundant_closures_for_method_calls` and put it in the pedantic group.

This aspect of the lint seems more controversial than the rest.

cc #3942

clippy_lints/src/eta_reduction.rs

@@ -32,7 +32,26 @@ declare_clippy_lint! {
     "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
 }
 
-declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE]);
+declare_clippy_lint! {
+    /// **What it does:** Checks for closures which only invoke a method on the closure
+    /// argument and can be replaced by referencing the method directly.
+    ///
+    /// **Why is this bad?** It's unnecessary to create the closure.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// Some('a').map(|s| s.to_uppercase());
+    /// ```
+    /// may be rewritten as
+    /// ```rust,ignore
+    /// Some('a').map(char::to_uppercase);
+    /// ```
+    pub REDUNDANT_CLOSURES_FOR_METHOD_CALLS,
+    pedantic,
+    "redundant closures for method calls"
+}
+
+declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURES_FOR_METHOD_CALLS]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EtaReduction {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
@@ -104,7 +123,7 @@ fn check_closure(cx: &LateContext<'_, '_>, expr: &Expr) {
             if let Some(name) = get_ufcs_type_name(cx, method_def_id, &args[0]);
 
             then {
-                span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure found", |db| {
+                span_lint_and_then(cx, REDUNDANT_CLOSURES_FOR_METHOD_CALLS, expr.span, "redundant closure found", |db| {
                     db.span_suggestion(
                         expr.span,
                         "remove closure as shown",

clippy_lints/src/lib.rs

@@ -614,6 +614,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         enum_glob_use::ENUM_GLOB_USE,
         enum_variants::MODULE_NAME_REPETITIONS,
         enum_variants::PUB_ENUM_VARIANT_NAMES,
+        eta_reduction::REDUNDANT_CLOSURES_FOR_METHOD_CALLS,
         functions::TOO_MANY_LINES,
         if_not_else::IF_NOT_ELSE,
         infinite_iter::MAYBE_INFINITE_ITER,