Fixes manual_slice_size_computation ICE and triggers in const context (rust-lang#14804)

The first commit fixes rust-lang/rust-clippy#14802: when a slice is
directly present, it must be dereferenced (instead of referenced -1
times) before being passed to `mem::size_of_val()`.

The second commit triggers the lint in a `const` contact when MSRV ≥
1.85.

changelog: [`manual_slice_size_computation`]: fix ICE in suggestion to
efficiently compute the size of a slice, and trigger the lint in `const`
context as well

Author: y21

book/src/lint_configuration.md

@@ -836,6 +836,7 @@ The minimum rust version that the project supports. Defaults to the `rust-versio
 * [`manual_repeat_n`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_repeat_n)
 * [`manual_retain`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_retain)
 * [`manual_slice_fill`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_fill)
+* [`manual_slice_size_calculation`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_size_calculation)
 * [`manual_split_once`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_split_once)
 * [`manual_str_repeat`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_str_repeat)
 * [`manual_strip`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip)

clippy_config/src/conf.rs

@@ -739,6 +739,7 @@ define_Conf! {
         manual_repeat_n,
         manual_retain,
         manual_slice_fill,
+        manual_slice_size_calculation,
         manual_split_once,
         manual_str_repeat,
         manual_strip,

clippy_lints/src/lib.rs

@@ -867,7 +867,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| Box::new(unnecessary_box_returns::UnnecessaryBoxReturns::new(conf)));
     store.register_late_pass(move |_| Box::new(lines_filter_map_ok::LinesFilterMapOk::new(conf)));
     store.register_late_pass(|_| Box::new(tests_outside_test_module::TestsOutsideTestModule));
-    store.register_late_pass(|_| Box::new(manual_slice_size_calculation::ManualSliceSizeCalculation));
+    store.register_late_pass(|_| Box::new(manual_slice_size_calculation::ManualSliceSizeCalculation::new(conf)));
     store.register_early_pass(move || Box::new(excessive_nesting::ExcessiveNesting::new(conf)));
     store.register_late_pass(|_| Box::new(items_after_test_module::ItemsAfterTestModule));
     store.register_early_pass(|| Box::new(ref_patterns::RefPatterns));

clippy_lints/src/manual_slice_size_calculation.rs

@@ -1,11 +1,13 @@
+use clippy_config::Conf;
 use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::source::snippet_with_context;
 use clippy_utils::{expr_or_init, is_in_const_context, std_or_core};
 use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
-use rustc_session::declare_lint_pass;
+use rustc_session::impl_lint_pass;
 use rustc_span::symbol::sym;
 
 declare_clippy_lint! {
@@ -36,20 +38,33 @@ declare_clippy_lint! {
     complexity,
     "manual slice size calculation"
 }
-declare_lint_pass!(ManualSliceSizeCalculation => [MANUAL_SLICE_SIZE_CALCULATION]);
+impl_lint_pass!(ManualSliceSizeCalculation => [MANUAL_SLICE_SIZE_CALCULATION]);
+
+pub struct ManualSliceSizeCalculation {
+    msrv: Msrv,
+}
+
+impl ManualSliceSizeCalculation {
+    pub fn new(conf: &Conf) -> Self {
+        Self { msrv: conf.msrv }
+    }
+}
 
 impl<'tcx> LateLintPass<'tcx> for ManualSliceSizeCalculation {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
         if let ExprKind::Binary(ref op, left, right) = expr.kind
             && BinOpKind::Mul == op.node
             && !expr.span.from_expansion()
-            // Does not apply inside const because size_of_val is not cost in stable.
-            && !is_in_const_context(cx)
             && let Some((receiver, refs_count)) = simplify(cx, left, right)
+            && (!is_in_const_context(cx) || self.msrv.meets(cx, msrvs::CONST_SIZE_OF_VAL))
         {
             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 };
 

clippy_utils/src/msrvs.rs

@@ -24,7 +24,7 @@ macro_rules! msrv_aliases {
 msrv_aliases! {
     1,88,0 { LET_CHAINS }
     1,87,0 { OS_STR_DISPLAY, INT_MIDPOINT, CONST_CHAR_IS_DIGIT }
-    1,85,0 { UINT_FLOAT_MIDPOINT }
+    1,85,0 { UINT_FLOAT_MIDPOINT, CONST_SIZE_OF_VAL }
     1,84,0 { CONST_OPTION_AS_SLICE, MANUAL_DANGLING_PTR }
     1,83,0 { CONST_EXTERN_FN, CONST_FLOAT_BITS_CONV, CONST_FLOAT_CLASSIFY, CONST_MUT_REFS, CONST_UNWRAP }
     1,82,0 { IS_NONE_OR, REPEAT_N, RAW_REF_OP }

tests/ui/manual_slice_size_calculation.fixed

@@ -60,7 +60,26 @@ fn main() {
     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
+#[clippy::msrv = "1.85"]
+const fn const_ok(s_i32: &[i32]) {
+    let _ = std::mem::size_of_val(s_i32);
+    //~^ manual_slice_size_calculation
+}
+
+#[clippy::msrv = "1.84"]
+const fn const_before_msrv(s_i32: &[i32]) {
+    let _ = s_i32.len() * size_of::<i32>();
+}
+
+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

@@ -60,7 +60,26 @@ fn main() {
     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
+#[clippy::msrv = "1.85"]
+const fn const_ok(s_i32: &[i32]) {
+    let _ = s_i32.len() * size_of::<i32>();
+    //~^ manual_slice_size_calculation
+}
+
+#[clippy::msrv = "1.84"]
+const fn const_before_msrv(s_i32: &[i32]) {
+    let _ = s_i32.len() * size_of::<i32>();
+}
+
+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 +55,17 @@ 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:65:13
+   |
+LL |     let _ = s_i32.len() * size_of::<i32>();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::mem::size_of_val(s_i32)`
+
+error: manual slice size calculation
+  --> tests/ui/manual_slice_size_calculation.rs:81:13
+   |
+LL |             self.dst.len() * size_of::<u8>()
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::mem::size_of_val(&self.dst)`
+
+error: aborting due to 11 previous errors