From 4ec219393a48eff84c8c08b854d50a454b8f422c Mon Sep 17 00:00:00 2001
From: Samuel Tardieu <sam@rfc1149.net>
Date: Wed, 14 May 2025 23:17:56 +0200
Subject: [PATCH] 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 |  6 +++++-
 tests/ui/manual_slice_size_calculation.fixed      | 13 +++++++++++++
 tests/ui/manual_slice_size_calculation.rs         | 13 +++++++++++++
 tests/ui/manual_slice_size_calculation.stderr     |  8 +++++++-
 4 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/clippy_lints/src/manual_slice_size_calculation.rs b/clippy_lints/src/manual_slice_size_calculation.rs
index 2dad0fa4925e..b6a4f44b0e49 100644
--- a/clippy_lints/src/manual_slice_size_calculation.rs
+++ b/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 };
 
diff --git a/tests/ui/manual_slice_size_calculation.fixed b/tests/ui/manual_slice_size_calculation.fixed
index 294d1e1506de..9df5a2ac5cd5 100644
--- a/tests/ui/manual_slice_size_calculation.fixed
+++ b/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
+        }
+    }
+}
diff --git a/tests/ui/manual_slice_size_calculation.rs b/tests/ui/manual_slice_size_calculation.rs
index ae5225663139..db14d891c80b 100644
--- a/tests/ui/manual_slice_size_calculation.rs
+++ b/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
+        }
+    }
+}
diff --git a/tests/ui/manual_slice_size_calculation.stderr b/tests/ui/manual_slice_size_calculation.stderr
index f07e97a1c863..00eb32b3e1c3 100644
--- a/tests/ui/manual_slice_size_calculation.stderr
+++ b/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