add lint for transmute from &T to &mut T of a ADT arguement

Author: mendelsshop

clippy_lints/src/transmute/mod.rs

@@ -1,6 +1,7 @@
 mod crosspointer_transmute;
 mod eager_transmute;
 mod missing_transmute_annotations;
+mod transmute_adt_arguement;
 mod transmute_int_to_bool;
 mod transmute_int_to_non_zero;
 mod transmute_null_to_fn;
@@ -44,6 +45,22 @@ declare_clippy_lint! {
     correctness,
     "transmutes that are confusing at best, undefined behavior at worst and always useless"
 }
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for transmutes between the same adt, where at least one of the type arguement goes from &T to &mut T.
+    /// This is an a more complicated version of https://doc.rust-lang.org/rustc/lints/listing/deny-by-default.html#mutable-transmutes.
+    /// ### Example
+    ///
+    /// ```ignore
+    /// unsafe {
+    ///    std::mem::transmute::<Option<&i32>, Option<&mut i32>>(&Some(5));
+    /// }
+    /// ```
+    #[clippy::version = "1.92.0"]
+    pub MUTABLE_ADT_ARGUEMENT_TRANSMUTE,
+    correctness,
+    "transmutes on the same adt where at least one of the type arguement goes from &T to &mut T"
+}
 
 declare_clippy_lint! {
     /// ### What it does
@@ -516,6 +533,7 @@ impl<'tcx> LateLintPass<'tcx> for Transmute {
             }
 
             let linted = wrong_transmute::check(cx, e, from_ty, to_ty)
+                | transmute_adt_arguement::check(cx, e, from_ty, to_ty)
                 | crosspointer_transmute::check(cx, e, from_ty, to_ty)
                 | transmuting_null::check(cx, e, arg, to_ty)
                 | transmute_null_to_fn::check(cx, e, arg, to_ty)

clippy_lints/src/transmute/transmute_adt_arguement.rs

@@ -0,0 +1,35 @@
+use super::MUTABLE_ADT_ARGUEMENT_TRANSMUTE;
+use clippy_utils::diagnostics::span_lint;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, GenericArgKind, Ty};
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+    from_ty
+        .walk()
+        .zip(to_ty.walk())
+        .filter_map(|(from_ty, to_ty)| {
+            if let (GenericArgKind::Type(from_ty), GenericArgKind::Type(to_ty)) = (from_ty.kind(), to_ty.kind()) {
+                Some((from_ty, to_ty))
+            } else {
+                None
+            }
+        })
+        .filter(|(from_ty_inner, to_ty_inner)| {
+            if let (ty::Ref(_, _, from_mut), ty::Ref(_, _, to_mut)) = (from_ty_inner.kind(), to_ty_inner.kind())
+                && from_mut < to_mut
+            {
+                span_lint(
+                    cx,
+                    MUTABLE_ADT_ARGUEMENT_TRANSMUTE,
+                    e.span,
+                    format!("transmute of type arguement {from_ty_inner} to {from_ty_inner}"),
+                );
+                true
+            } else {
+                false
+            }
+        })
+        .count()
+        > 0
+}