Improve redundant_slicing lint

* Lint when slicing triggers auto-deref
* Lint when slicing returns the same type as dereferencing

tests/ui/redundant_slicing.fixed

@@ -0,0 +1,40 @@
+// run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::redundant_slicing)]
+
+fn main() {
+    let slice: &[u32] = &[0];
+    let _ = slice; // Redundant slice
+
+    let v = vec![0];
+    let _ = &*v; // Deref instead of slice
+    let _ = (&*v); // Outer borrow is redundant
+
+    static S: &[u8] = &[0, 1, 2];
+    let err = &mut &*S; // Should reborrow instead of slice
+
+    let mut vec = vec![0];
+    let mut_slice = &mut *vec; // Deref instead of slice
+    let _ = &mut *mut_slice; // Should reborrow instead of slice
+
+    let ref_vec = &vec;
+    let _ = &**ref_vec; // Deref instead of slice
+
+    macro_rules! m {
+        ($e:expr) => {
+            $e
+        };
+    }
+    let _ = slice;
+
+    macro_rules! m2 {
+        ($e:expr) => {
+            &$e[..]
+        };
+    }
+    let _ = m2!(slice); // Don't lint in a macro
+
+    let slice_ref = &slice;
+    let _ = *slice_ref; // Deref instead of slice
+}

tests/ui/redundant_slicing.rs

@@ -1,21 +1,26 @@
+// run-rustfix
+
 #![allow(unused)]
 #![warn(clippy::redundant_slicing)]
 
 fn main() {
     let slice: &[u32] = &[0];
-    let _ = &slice[..];
+    let _ = &slice[..]; // Redundant slice
 
     let v = vec![0];
-    let _ = &v[..]; // Changes the type
-    let _ = &(&v[..])[..]; // Outer borrow is redundant
+    let _ = &v[..]; // Deref instead of slice
+    let _ = &(&*v)[..]; // Outer borrow is redundant
 
     static S: &[u8] = &[0, 1, 2];
     let err = &mut &S[..]; // Should reborrow instead of slice
 
     let mut vec = vec![0];
-    let mut_slice = &mut *vec;
+    let mut_slice = &mut vec[..]; // Deref instead of slice
     let _ = &mut mut_slice[..]; // Should reborrow instead of slice
 
+    let ref_vec = &vec;
+    let _ = &ref_vec[..]; // Deref instead of slice
+
     macro_rules! m {
         ($e:expr) => {
             $e
@@ -29,4 +34,7 @@ fn main() {
         };
     }
     let _ = m2!(slice); // Don't lint in a macro
+
+    let slice_ref = &slice;
+    let _ = &slice_ref[..]; // Deref instead of slice
 }

tests/ui/redundant_slicing.stderr

@@ -1,34 +1,58 @@
 error: redundant slicing of the whole range
-  --> $DIR/redundant_slicing.rs:6:13
+  --> $DIR/redundant_slicing.rs:8:13
    |
-LL |     let _ = &slice[..];
+LL |     let _ = &slice[..]; // Redundant slice
    |             ^^^^^^^^^^ help: use the original value instead: `slice`
    |
    = note: `-D clippy::redundant-slicing` implied by `-D warnings`
 
 error: redundant slicing of the whole range
-  --> $DIR/redundant_slicing.rs:10:13
+  --> $DIR/redundant_slicing.rs:11:13
    |
-LL |     let _ = &(&v[..])[..]; // Outer borrow is redundant
-   |             ^^^^^^^^^^^^^ help: use the original value instead: `(&v[..])`
+LL |     let _ = &v[..]; // Deref instead of slice
+   |             ^^^^^^ help: dereference the original value instead: `&*v`
 
 error: redundant slicing of the whole range
-  --> $DIR/redundant_slicing.rs:13:20
+  --> $DIR/redundant_slicing.rs:12:13
+   |
+LL |     let _ = &(&*v)[..]; // Outer borrow is redundant
+   |             ^^^^^^^^^^ help: use the original value instead: `(&*v)`
+
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:15:20
    |
 LL |     let err = &mut &S[..]; // Should reborrow instead of slice
    |                    ^^^^^^ help: reborrow the original value instead: `&*S`
 
 error: redundant slicing of the whole range
-  --> $DIR/redundant_slicing.rs:17:13
+  --> $DIR/redundant_slicing.rs:18:21
+   |
+LL |     let mut_slice = &mut vec[..]; // Deref instead of slice
+   |                     ^^^^^^^^^^^^ help: dereference the original value instead: `&mut *vec`
+
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:19:13
    |
 LL |     let _ = &mut mut_slice[..]; // Should reborrow instead of slice
    |             ^^^^^^^^^^^^^^^^^^ help: reborrow the original value instead: `&mut *mut_slice`
 
 error: redundant slicing of the whole range
-  --> $DIR/redundant_slicing.rs:24:13
+  --> $DIR/redundant_slicing.rs:22:13
+   |
+LL |     let _ = &ref_vec[..]; // Deref instead of slice
+   |             ^^^^^^^^^^^^ help: dereference the original value instead: `&**ref_vec`
+
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:29:13
    |
 LL |     let _ = &m!(slice)[..];
    |             ^^^^^^^^^^^^^^ help: use the original value instead: `slice`
 
-error: aborting due to 5 previous errors
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:39:13
+   |
+LL |     let _ = &slice_ref[..]; // Deref instead of slice
+   |             ^^^^^^^^^^^^^^ help: dereference the original value instead: `*slice_ref`
+
+error: aborting due to 9 previous errors