Fix size computation when element is a slice

In this case, a dereference is needed before using `mem::size_of_val()`.

clippy_lints/src/manual_slice_size_calculation.rs

@@ -49,7 +49,11 @@ impl<'tcx> LateLintPass<'tcx> for ManualSliceSizeCalculation {
         {
             let ctxt = expr.span.ctxt();
             let mut app = Applicability::MachineApplicable;
-            let deref = "*".repeat(refs_count - 1);
+            let deref = if refs_count > 0 {
+                "*".repeat(refs_count - 1)
+            } else {
+                "&".into()
+            };
             let val_name = snippet_with_context(cx, receiver.span, ctxt, "slice", &mut app).0;
             let Some(sugg) = std_or_core(cx) else { return };
 

tests/ui/manual_slice_size_calculation.fixed

@@ -64,3 +64,16 @@ const fn _const(s_i32: &[i32]) {
     // True negative:
     let _ = s_i32.len() * size_of::<i32>(); // Ok, can't use size_of_val in const
 }
+
+fn issue_14802() {
+    struct IcedSlice {
+        dst: [u8],
+    }
+
+    impl IcedSlice {
+        fn get_len(&self) -> usize {
+            std::mem::size_of_val(&self.dst)
+            //~^ manual_slice_size_calculation
+        }
+    }
+}

tests/ui/manual_slice_size_calculation.rs

@@ -64,3 +64,16 @@ const fn _const(s_i32: &[i32]) {
     // True negative:
     let _ = s_i32.len() * size_of::<i32>(); // Ok, can't use size_of_val in const
 }
+
+fn issue_14802() {
+    struct IcedSlice {
+        dst: [u8],
+    }
+
+    impl IcedSlice {
+        fn get_len(&self) -> usize {
+            self.dst.len() * size_of::<u8>()
+            //~^ manual_slice_size_calculation
+        }
+    }
+}

tests/ui/manual_slice_size_calculation.stderr

@@ -55,5 +55,11 @@ error: manual slice size calculation
 LL |     let _ = external!(&[1u64][..]).len() * size_of::<u64>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::mem::size_of_val(external!(&[1u64][..]))`
 
-error: aborting due to 9 previous errors
+error: manual slice size calculation
+  --> tests/ui/manual_slice_size_calculation.rs:75:13
+   |
+LL |             self.dst.len() * size_of::<u8>()
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::mem::size_of_val(&self.dst)`
+
+error: aborting due to 10 previous errors