Auto merge of #10601 - schubart:manual_slice_size_calculation, r=llogiq

Add [`manual_slice_size_calculation`]

Fixes: #10518

---

changelog: new lint [`manual_slice_size_calculation`]

tests/ui/manual_slice_size_calculation.rs

@@ -0,0 +1,36 @@
+#![allow(unused)]
+#![warn(clippy::manual_slice_size_calculation)]
+
+use core::mem::{align_of, size_of};
+
+fn main() {
+    let v_i32 = Vec::<i32>::new();
+    let s_i32 = v_i32.as_slice();
+
+    // True positives:
+    let _ = s_i32.len() * size_of::<i32>(); // WARNING
+    let _ = size_of::<i32>() * s_i32.len(); // WARNING
+    let _ = size_of::<i32>() * s_i32.len() * 5; // WARNING
+
+    let len = s_i32.len();
+    let size = size_of::<i32>();
+    let _ = len * size_of::<i32>(); // WARNING
+    let _ = s_i32.len() * size; // WARNING
+    let _ = len * size; // WARNING
+
+    // True negatives:
+    let _ = size_of::<i32>() + s_i32.len(); // Ok, not a multiplication
+    let _ = size_of::<i32>() * s_i32.partition_point(|_| true); // Ok, not len()
+    let _ = size_of::<i32>() * v_i32.len(); // Ok, not a slice
+    let _ = align_of::<i32>() * s_i32.len(); // Ok, not size_of()
+    let _ = size_of::<u32>() * s_i32.len(); // Ok, different types
+
+    // False negatives:
+    let _ = 5 * size_of::<i32>() * s_i32.len(); // Ok (MISSED OPPORTUNITY)
+    let _ = size_of::<i32>() * 5 * s_i32.len(); // Ok (MISSED OPPORTUNITY)
+}
+
+const fn _const(s_i32: &[i32]) {
+    // True negative:
+    let _ = s_i32.len() * size_of::<i32>(); // Ok, can't use size_of_val in const
+}

tests/ui/manual_slice_size_calculation.stderr

@@ -0,0 +1,51 @@
+error: manual slice size calculation
+  --> $DIR/manual_slice_size_calculation.rs:11:13
+   |
+LL |     let _ = s_i32.len() * size_of::<i32>(); // WARNING
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using std::mem::size_of_value instead
+   = note: `-D clippy::manual-slice-size-calculation` implied by `-D warnings`
+
+error: manual slice size calculation
+  --> $DIR/manual_slice_size_calculation.rs:12:13
+   |
+LL |     let _ = size_of::<i32>() * s_i32.len(); // WARNING
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using std::mem::size_of_value instead
+
+error: manual slice size calculation
+  --> $DIR/manual_slice_size_calculation.rs:13:13
+   |
+LL |     let _ = size_of::<i32>() * s_i32.len() * 5; // WARNING
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using std::mem::size_of_value instead
+
+error: manual slice size calculation
+  --> $DIR/manual_slice_size_calculation.rs:17:13
+   |
+LL |     let _ = len * size_of::<i32>(); // WARNING
+   |             ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using std::mem::size_of_value instead
+
+error: manual slice size calculation
+  --> $DIR/manual_slice_size_calculation.rs:18:13
+   |
+LL |     let _ = s_i32.len() * size; // WARNING
+   |             ^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using std::mem::size_of_value instead
+
+error: manual slice size calculation
+  --> $DIR/manual_slice_size_calculation.rs:19:13
+   |
+LL |     let _ = len * size; // WARNING
+   |             ^^^^^^^^^^
+   |
+   = help: consider using std::mem::size_of_value instead
+
+error: aborting due to 6 previous errors
+