feat: lint against transmute int to ptr

clippy_lints/src/transmute/wrong_transmute.rs

@@ -6,9 +6,18 @@ use rustc_middle::ty::{self, Ty};
 
 /// Checks for `wrong_transmute` lint.
 /// Returns `true` if it's triggered, otherwise returns `false`.
-pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, mut from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+    if let ty::Adt(def, args) = from_ty.kind() {
+        let mut fields = def.variants().iter().filter_map(|v| non_zst_inner_ty(cx, v));
+        if fields.clone().count() != 1 {
+            return false;
+        }
+        if let Some(field) = fields.next_back() {
+            from_ty = field.ty(cx.tcx, args);
+        }
+    }
     match (&from_ty.kind(), &to_ty.kind()) {
-        (ty::Float(_) | ty::Char, ty::Ref(..) | ty::RawPtr(_, _)) => {
+        (ty::Uint(_) | ty::Int(_) | ty::Float(_) | ty::Char, ty::Ref(..) | ty::RawPtr(_, _)) => {
             span_lint(
                 cx,
                 WRONG_TRANSMUTE,
@@ -20,3 +29,15 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty
         _ => false,
     }
 }
+
+fn non_zst_inner_ty<'a>(cx: &LateContext<'_>, variant: &'a ty::VariantDef) -> Option<&'a ty::FieldDef> {
+    let tcx = cx.tcx;
+    let param_env = tcx.param_env(variant.def_id);
+    variant.fields.iter().find(|field| {
+        let field_ty = tcx.type_of(field.did).instantiate_identity();
+        let is_1zst = tcx
+            .layout_of(param_env.and(field_ty))
+            .is_ok_and(|layout| layout.is_1zst());
+        !is_1zst
+    })
+}

tests/ui/transmute_int_to_ptr.rs

@@ -0,0 +1,26 @@
+#![warn(clippy::wrong_transmute)]
+
+#[repr(transparent)]
+#[allow(dead_code)]
+pub struct K1 {
+    b: usize,
+    c: (),
+}
+
+pub fn foo(k: K1) {
+    let x = unsafe { std::mem::transmute::<K1, *mut i8>(k) };
+    std::hint::black_box(unsafe { *x });
+}
+
+#[allow(dead_code)]
+pub struct K2 {
+    b: usize,
+    c: (),
+}
+
+pub fn bar(k: K2) {
+    let x = unsafe { std::mem::transmute::<K2, *mut i8>(k) };
+    std::hint::black_box(unsafe { *x });
+}
+
+fn main() {}

tests/ui/transmute_int_to_ptr.stderr

@@ -0,0 +1,17 @@
+error: transmute from a `usize` to a pointer
+  --> tests/ui/transmute_int_to_ptr.rs:11:22
+   |
+LL |     let x = unsafe { std::mem::transmute::<K1, *mut i8>(k) };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::wrong-transmute` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::wrong_transmute)]`
+
+error: transmute from a `usize` to a pointer
+  --> tests/ui/transmute_int_to_ptr.rs:22:22
+   |
+LL |     let x = unsafe { std::mem::transmute::<K2, *mut i8>(k) };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+