Fix [redundant_slicing] when the slice is behind a mutable reference

Fixes rust-lang#12751

changelog: Fix [`redundant_slicing`] when the slice is behind a mutable reference

Author: apoisternex

clippy_lints/src/redundant_slicing.rs

@@ -113,8 +113,11 @@ impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
                         a.kind,
                         Adjust::Borrow(AutoBorrow::Ref(_, AutoBorrowMutability::Mut { .. }))
                     )
-                }) {
-                    // The slice was used to make a temporary reference.
+                }) || (matches!(
+                    cx.typeck_results().expr_ty(indexed).ref_mutability(),
+                    Some(Mutability::Mut)
+                ) && mutability == Mutability::Not)
+                {
                     (DEREF_BY_SLICING_LINT, "&*", "reborrow the original value instead")
                 } else if deref_count != 0 {
                     (DEREF_BY_SLICING_LINT, "", "dereference the original value instead")

tests/ui/deref_by_slicing.fixed

@@ -25,4 +25,8 @@ fn main() {
 
     let bytes: &[u8] = &[];
     let _ = (&*bytes).read_to_end(&mut vec![]).unwrap(); // Err, re-borrows slice
+
+    // issue 12751
+    let a = &mut [1, 2, 3][..];
+    let _ = &*a;
 }

tests/ui/deref_by_slicing.rs

@@ -25,4 +25,8 @@ fn main() {
 
     let bytes: &[u8] = &[];
     let _ = (&bytes[..]).read_to_end(&mut vec![]).unwrap(); // Err, re-borrows slice
+
+    // issue 12751
+    let a = &mut [1, 2, 3][..];
+    let _ = &a[..];
 }

tests/ui/deref_by_slicing.stderr

@@ -55,5 +55,11 @@ error: slicing when dereferencing would work
 LL |     let _ = (&bytes[..]).read_to_end(&mut vec![]).unwrap(); // Err, re-borrows slice
    |             ^^^^^^^^^^^^ help: reborrow the original value instead: `(&*bytes)`
 
-error: aborting due to 9 previous errors
+error: slicing when dereferencing would work
+  --> tests/ui/deref_by_slicing.rs:31:13
+   |
+LL |     let _ = &a[..];
+   |             ^^^^^^ help: reborrow the original value instead: `&*a`
+
+error: aborting due to 10 previous errors