Auto merge of #4101 - mikerite:redundant_closures_for_method_calls, r=Manishearth

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

changelog: Move method checking from `redundant_closure` to a new `pedantic` lint called `redundant_closure_for_method_calls`.

9 files changed, 56 insertions, 21 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 397916fe409..44787f4f281 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1035,6 +1035,7 @@ All notable changes to this project will be documented in this file.
 [`redundant_clone`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
 [`redundant_closure`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
 [`redundant_closure_call`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_call
+[`redundant_closure_for_method_calls`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_for_method_calls
 [`redundant_field_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_field_names
 [`redundant_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern
 [`redundant_pattern_matching`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern_matching
diff --git a/README.md b/README.md
index 4ed388db176..e670924de2b 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 301 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 302 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 
diff --git a/clippy_lints/src/eta_reduction.rs b/clippy_lints/src/eta_reduction.rs
index 60e2ab8b9b9..4a93101d7cb 100644
--- a/clippy_lints/src/eta_reduction.rs
+++ b/clippy_lints/src/eta_reduction.rs
@@ -32,7 +32,30 @@ 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.
+    ///
+    /// **Known problems:** rust-lang/rust-clippy#3071, rust-lang/rust-clippy#4002,
+    /// rust-lang/rust-clippy#3942
+    ///
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// Some('a').map(|s| s.to_uppercase());
+    /// ```
+    /// may be rewritten as
+    /// ```rust,ignore
+    /// Some('a').map(char::to_uppercase);
+    /// ```
+    pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
+    pedantic,
+    "redundant closures for method calls"
+}
+
+declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_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 +127,7 @@ 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_CLOSURE_FOR_METHOD_CALLS, expr.span, "redundant closure found", |db| {
                     db.span_suggestion(
                         expr.span,
                         "remove closure as shown",
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index da413cd12ce..400abb5a498 100644
--- a/clippy_lints/src/lib.rs
+++ b/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_CLOSURE_FOR_METHOD_CALLS,
         functions::TOO_MANY_LINES,
         if_not_else::IF_NOT_ELSE,
         infinite_iter::MAYBE_INFINITE_ITER,
diff --git a/tests/ui/eta.fixed b/tests/ui/eta.fixed
index cef4bda24b6..a3e57956f35 100644
--- a/tests/ui/eta.fixed
+++ b/tests/ui/eta.fixed
@@ -9,7 +9,11 @@
     clippy::option_map_unit_fn,
     clippy::trivially_copy_pass_by_ref
 )]
-#![warn(clippy::redundant_closure, clippy::needless_borrow)]
+#![warn(
+    clippy::redundant_closure,
+    clippy::redundant_closure_for_method_calls,
+    clippy::needless_borrow
+)]
 
 use std::path::PathBuf;
 
diff --git a/tests/ui/eta.rs b/tests/ui/eta.rs
index 5f029eac4c1..58adf21b1d5 100644
--- a/tests/ui/eta.rs
+++ b/tests/ui/eta.rs
@@ -9,7 +9,11 @@
     clippy::option_map_unit_fn,
     clippy::trivially_copy_pass_by_ref
 )]
-#![warn(clippy::redundant_closure, clippy::needless_borrow)]
+#![warn(
+    clippy::redundant_closure,
+    clippy::redundant_closure_for_method_calls,
+    clippy::needless_borrow
+)]
 
 use std::path::PathBuf;
 
diff --git a/tests/ui/eta.stderr b/tests/ui/eta.stderr
index afcafc46b3e..ff402a1850e 100644
--- a/tests/ui/eta.stderr
+++ b/tests/ui/eta.stderr
@@ -1,5 +1,5 @@
 error: redundant closure found
-  --> $DIR/eta.rs:17:27
+  --> $DIR/eta.rs:21:27
    |
 LL |     let a = Some(1u8).map(|a| foo(a));
    |                           ^^^^^^^^^^ help: remove closure as shown: `foo`
@@ -7,19 +7,19 @@ LL |     let a = Some(1u8).map(|a| foo(a));
    = note: `-D clippy::redundant-closure` implied by `-D warnings`
 
 error: redundant closure found
-  --> $DIR/eta.rs:18:10
+  --> $DIR/eta.rs:22:10
    |
 LL |     meta(|a| foo(a));
    |          ^^^^^^^^^^ help: remove closure as shown: `foo`
 
 error: redundant closure found
-  --> $DIR/eta.rs:19:27
+  --> $DIR/eta.rs:23:27
    |
 LL |     let c = Some(1u8).map(|a| {1+2; foo}(a));
    |                           ^^^^^^^^^^^^^^^^^ help: remove closure as shown: `{1+2; foo}`
 
 error: this expression borrows a reference that is immediately dereferenced by the compiler
-  --> $DIR/eta.rs:21:21
+  --> $DIR/eta.rs:25:21
    |
 LL |     all(&[1, 2, 3], &&2, |x, y| below(x, y)); //is adjusted
    |                     ^^^ help: change this to: `&2`
@@ -27,61 +27,63 @@ LL |     all(&[1, 2, 3], &&2, |x, y| below(x, y)); //is adjusted
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
 
 error: redundant closure found
-  --> $DIR/eta.rs:28:27
+  --> $DIR/eta.rs:32:27
    |
 LL |     let e = Some(1u8).map(|a| generic(a));
    |                           ^^^^^^^^^^^^^^ help: remove closure as shown: `generic`
 
 error: redundant closure found
-  --> $DIR/eta.rs:71:51
+  --> $DIR/eta.rs:75:51
    |
 LL |     let e = Some(TestStruct { some_ref: &i }).map(|a| a.foo());
    |                                                   ^^^^^^^^^^^ help: remove closure as shown: `TestStruct::foo`
+   |
+   = note: `-D clippy::redundant-closure-for-method-calls` implied by `-D warnings`
 
 error: redundant closure found
-  --> $DIR/eta.rs:73:51
+  --> $DIR/eta.rs:77:51
    |
 LL |     let e = Some(TestStruct { some_ref: &i }).map(|a| a.trait_foo());
    |                                                   ^^^^^^^^^^^^^^^^^ help: remove closure as shown: `TestTrait::trait_foo`
 
 error: redundant closure found
-  --> $DIR/eta.rs:76:42
+  --> $DIR/eta.rs:80:42
    |
 LL |     let e = Some(&mut vec![1, 2, 3]).map(|v| v.clear());
    |                                          ^^^^^^^^^^^^^ help: remove closure as shown: `std::vec::Vec::clear`
 
 error: redundant closure found
-  --> $DIR/eta.rs:81:29
+  --> $DIR/eta.rs:85:29
    |
 LL |     let e = Some("str").map(|s| s.to_string());
    |                             ^^^^^^^^^^^^^^^^^ help: remove closure as shown: `std::string::ToString::to_string`
 
 error: redundant closure found
-  --> $DIR/eta.rs:83:27
+  --> $DIR/eta.rs:87:27
    |
 LL |     let e = Some('a').map(|s| s.to_uppercase());
    |                           ^^^^^^^^^^^^^^^^^^^^ help: remove closure as shown: `char::to_uppercase`
 
 error: redundant closure found
-  --> $DIR/eta.rs:86:65
+  --> $DIR/eta.rs:90:65
    |
 LL |     let e: std::vec::Vec<char> = vec!['a', 'b', 'c'].iter().map(|c| c.to_ascii_uppercase()).collect();
    |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove closure as shown: `char::to_ascii_uppercase`
 
 error: redundant closure found
-  --> $DIR/eta.rs:104:50
+  --> $DIR/eta.rs:108:50
    |
 LL |     let _: Vec<_> = arr.iter().map(|x| x.map_err(|e| some.take().unwrap()(e))).collect();
    |                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove closure as shown: `some.take().unwrap()`
 
 error: redundant closure found
-  --> $DIR/eta.rs:169:27
+  --> $DIR/eta.rs:173:27
    |
 LL |     let a = Some(1u8).map(|a| foo_ptr(a));
    |                           ^^^^^^^^^^^^^^ help: remove closure as shown: `foo_ptr`
 
 error: redundant closure found
-  --> $DIR/eta.rs:174:27
+  --> $DIR/eta.rs:178:27
    |
 LL |     let a = Some(1u8).map(|a| closure(a));
    |                           ^^^^^^^^^^^^^^ help: remove closure as shown: `closure`
diff --git a/tests/ui/map_clone.fixed b/tests/ui/map_clone.fixed
index 228671ca209..c8b9bc04944 100644
--- a/tests/ui/map_clone.fixed
+++ b/tests/ui/map_clone.fixed
@@ -3,7 +3,7 @@
 #![allow(clippy::iter_cloned_collect)]
 #![allow(clippy::clone_on_copy)]
 #![allow(clippy::missing_docs_in_private_items)]
-#![allow(clippy::redundant_closure)]
+#![allow(clippy::redundant_closure_for_method_calls)]
 
 fn main() {
     let _: Vec<i8> = vec![5_i8; 6].iter().copied().collect();
diff --git a/tests/ui/map_clone.rs b/tests/ui/map_clone.rs
index 9a7be3da206..5f216823eb4 100644
--- a/tests/ui/map_clone.rs
+++ b/tests/ui/map_clone.rs
@@ -3,7 +3,7 @@
 #![allow(clippy::iter_cloned_collect)]
 #![allow(clippy::clone_on_copy)]
 #![allow(clippy::missing_docs_in_private_items)]
-#![allow(clippy::redundant_closure)]
+#![allow(clippy::redundant_closure_for_method_calls)]
 
 fn main() {
     let _: Vec<i8> = vec![5_i8; 6].iter().map(|x| *x).collect();