Auto merge of #8398 - Jarcho:unordered_transmute, r=llogiq

Add lint `transmute_undefined_repr`

Partially implements #3999 and #546

This doesn't consider `enum`s at all right now as those are going to be a pain to deal with. This also allows `#[repr(Rust)]` structs with only one non-zero sized fields. I think those are technically undefined when transmuted.

changelog: Add lint `transmute_undefined_repr`

20 files changed, 463 insertions, 51 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ce31c697ee1..c9adf77c0d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3475,6 +3475,7 @@ Released 2018-09-13
 [`transmute_num_to_bytes`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmute_num_to_bytes
 [`transmute_ptr_to_ptr`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmute_ptr_to_ptr
 [`transmute_ptr_to_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmute_ptr_to_ref
+[`transmute_undefined_repr`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmute_undefined_repr
 [`transmutes_expressible_as_ptr_casts`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmutes_expressible_as_ptr_casts
 [`transmuting_null`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmuting_null
 [`trivial_regex`]: https://rust-lang.github.io/rust-clippy/master/index.html#trivial_regex
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index 4721b7f2b47..d93e34e76b4 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -277,6 +277,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(transmute::TRANSMUTE_INT_TO_FLOAT),
     LintId::of(transmute::TRANSMUTE_NUM_TO_BYTES),
     LintId::of(transmute::TRANSMUTE_PTR_TO_REF),
+    LintId::of(transmute::TRANSMUTE_UNDEFINED_REPR),
     LintId::of(transmute::UNSOUND_COLLECTION_TRANSMUTE),
     LintId::of(transmute::WRONG_TRANSMUTE),
     LintId::of(transmuting_null::TRANSMUTING_NULL),
diff --git a/clippy_lints/src/lib.register_correctness.rs b/clippy_lints/src/lib.register_correctness.rs
index 4217fd3a3ea..d013daa8e08 100644
--- a/clippy_lints/src/lib.register_correctness.rs
+++ b/clippy_lints/src/lib.register_correctness.rs
@@ -58,6 +58,7 @@ store.register_group(true, "clippy::correctness", Some("clippy_correctness"), ve
     LintId::of(size_of_in_element_count::SIZE_OF_IN_ELEMENT_COUNT),
     LintId::of(swap::ALMOST_SWAPPED),
     LintId::of(to_string_in_display::TO_STRING_IN_DISPLAY),
+    LintId::of(transmute::TRANSMUTE_UNDEFINED_REPR),
     LintId::of(transmute::UNSOUND_COLLECTION_TRANSMUTE),
     LintId::of(transmute::WRONG_TRANSMUTE),
     LintId::of(transmuting_null::TRANSMUTING_NULL),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index e5e1c052c15..a80320a578f 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -473,6 +473,7 @@ store.register_lints(&[
     transmute::TRANSMUTE_NUM_TO_BYTES,
     transmute::TRANSMUTE_PTR_TO_PTR,
     transmute::TRANSMUTE_PTR_TO_REF,
+    transmute::TRANSMUTE_UNDEFINED_REPR,
     transmute::UNSOUND_COLLECTION_TRANSMUTE,
     transmute::USELESS_TRANSMUTE,
     transmute::WRONG_TRANSMUTE,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 9d42185dc0e..0b07726519e 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -5,6 +5,7 @@
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]
 #![feature(iter_intersperse)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(once_cell)]
 #![feature(rustc_private)]
diff --git a/clippy_lints/src/transmute/mod.rs b/clippy_lints/src/transmute/mod.rs
index 3ad4ec74bf5..4c320deecc2 100644
--- a/clippy_lints/src/transmute/mod.rs
+++ b/clippy_lints/src/transmute/mod.rs
@@ -7,6 +7,7 @@ mod transmute_num_to_bytes;
 mod transmute_ptr_to_ptr;
 mod transmute_ptr_to_ref;
 mod transmute_ref_to_ref;
+mod transmute_undefined_repr;
 mod transmutes_expressible_as_ptr_casts;
 mod unsound_collection_transmute;
 mod useless_transmute;
@@ -355,6 +356,30 @@ declare_clippy_lint! {
     "transmute between collections of layout-incompatible types"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for transmutes either to or from a type which does not have a defined representation.
+    ///
+    /// ### Why is this bad?
+    /// The results of such a transmute are not defined.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct Foo<T>(u32, T);
+    /// let _ = unsafe { core::mem::transmute::<Foo<u32>, Foo<i32>>(Foo(0u32, 0u32)) };
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// #[repr(C)]
+    /// struct Foo<T>(u32, T);
+    /// let _ = unsafe { core::mem::transmute::<Foo<u32>, Foo<i32>>(Foo(0u32, 0u32)) };
+    /// ```
+    #[clippy::version = "1.60.0"]
+    pub TRANSMUTE_UNDEFINED_REPR,
+    correctness,
+    "transmute to or from a type with an undefined representation"
+}
+
 declare_lint_pass!(Transmute => [
     CROSSPOINTER_TRANSMUTE,
     TRANSMUTE_PTR_TO_REF,
@@ -369,13 +394,13 @@ declare_lint_pass!(Transmute => [
     TRANSMUTE_NUM_TO_BYTES,
     UNSOUND_COLLECTION_TRANSMUTE,
     TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
+    TRANSMUTE_UNDEFINED_REPR,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Transmute {
-    #[allow(clippy::similar_names, clippy::too_many_lines)]
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
         if_chain! {
-            if let ExprKind::Call(path_expr, args) = e.kind;
+            if let ExprKind::Call(path_expr, [arg]) = e.kind;
             if let ExprKind::Path(ref qpath) = path_expr.kind;
             if let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id();
             if cx.tcx.is_diagnostic_item(sym::transmute, def_id);
@@ -385,28 +410,31 @@ impl<'tcx> LateLintPass<'tcx> for Transmute {
                 // And see https://github.com/rust-lang/rust/issues/51911 for dereferencing raw pointers.
                 let const_context = in_constant(cx, e.hir_id);
 
-                let from_ty = cx.typeck_results().expr_ty(&args[0]);
+                let from_ty = cx.typeck_results().expr_ty(arg);
                 let to_ty = cx.typeck_results().expr_ty(e);
 
                 // If useless_transmute is triggered, the other lints can be skipped.
-                if useless_transmute::check(cx, e, from_ty, to_ty, args) {
+                if useless_transmute::check(cx, e, from_ty, to_ty, arg) {
                     return;
                 }
 
-                let mut linted = wrong_transmute::check(cx, e, from_ty, to_ty);
-                linted |= crosspointer_transmute::check(cx, e, from_ty, to_ty);
-                linted |= transmute_ptr_to_ref::check(cx, e, from_ty, to_ty, args, qpath);
-                linted |= transmute_int_to_char::check(cx, e, from_ty, to_ty, args);
-                linted |= transmute_ref_to_ref::check(cx, e, from_ty, to_ty, args, const_context);
-                linted |= transmute_ptr_to_ptr::check(cx, e, from_ty, to_ty, args);
-                linted |= transmute_int_to_bool::check(cx, e, from_ty, to_ty, args);
-                linted |= transmute_int_to_float::check(cx, e, from_ty, to_ty, args, const_context);
-                linted |= transmute_float_to_int::check(cx, e, from_ty, to_ty, args, const_context);
-                linted |= transmute_num_to_bytes::check(cx, e, from_ty, to_ty, args, const_context);
-                linted |= unsound_collection_transmute::check(cx, e, from_ty, to_ty);
+                let linted = wrong_transmute::check(cx, e, from_ty, to_ty)
+                    | crosspointer_transmute::check(cx, e, from_ty, to_ty)
+                    | transmute_ptr_to_ref::check(cx, e, from_ty, to_ty, arg, qpath)
+                    | transmute_int_to_char::check(cx, e, from_ty, to_ty, arg)
+                    | transmute_ref_to_ref::check(cx, e, from_ty, to_ty, arg, const_context)
+                    | transmute_ptr_to_ptr::check(cx, e, from_ty, to_ty, arg)
+                    | transmute_int_to_bool::check(cx, e, from_ty, to_ty, arg)
+                    | transmute_int_to_float::check(cx, e, from_ty, to_ty, arg, const_context)
+                    | transmute_float_to_int::check(cx, e, from_ty, to_ty, arg, const_context)
+                    | transmute_num_to_bytes::check(cx, e, from_ty, to_ty, arg, const_context)
+                    | (
+                        unsound_collection_transmute::check(cx, e, from_ty, to_ty)
+                        || transmute_undefined_repr::check(cx, e, from_ty, to_ty)
+                    );
 
                 if !linted {
-                    transmutes_expressible_as_ptr_casts::check(cx, e, from_ty, to_ty, args);
+                    transmutes_expressible_as_ptr_casts::check(cx, e, from_ty, to_ty, arg);
                 }
             }
         }
diff --git a/clippy_lints/src/transmute/transmute_float_to_int.rs b/clippy_lints/src/transmute/transmute_float_to_int.rs
index 3aa3c393ba5..d5ef86dc4e5 100644
--- a/clippy_lints/src/transmute/transmute_float_to_int.rs
+++ b/clippy_lints/src/transmute/transmute_float_to_int.rs
@@ -15,7 +15,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    mut arg: &'tcx Expr<'_>,
     const_context: bool,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
@@ -26,37 +26,36 @@ pub(super) fn check<'tcx>(
                 e.span,
                 &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
                 |diag| {
-                    let mut expr = &args[0];
-                    let mut arg = sugg::Sugg::hir(cx, expr, "..");
+                    let mut sugg = sugg::Sugg::hir(cx, arg, "..");
 
-                    if let ExprKind::Unary(UnOp::Neg, inner_expr) = &expr.kind {
-                        expr = inner_expr;
+                    if let ExprKind::Unary(UnOp::Neg, inner_expr) = &arg.kind {
+                        arg = inner_expr;
                     }
 
                     if_chain! {
                         // if the expression is a float literal and it is unsuffixed then
                         // add a suffix so the suggestion is valid and unambiguous
-                        if let ExprKind::Lit(lit) = &expr.kind;
+                        if let ExprKind::Lit(lit) = &arg.kind;
                         if let ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) = lit.node;
                         then {
-                            let op = format!("{}{}", arg, float_ty.name_str()).into();
-                            match arg {
-                                sugg::Sugg::MaybeParen(_) => arg = sugg::Sugg::MaybeParen(op),
-                                _ => arg = sugg::Sugg::NonParen(op)
+                            let op = format!("{}{}", sugg, float_ty.name_str()).into();
+                            match sugg {
+                                sugg::Sugg::MaybeParen(_) => sugg = sugg::Sugg::MaybeParen(op),
+                                _ => sugg = sugg::Sugg::NonParen(op)
                             }
                         }
                     }
 
-                    arg = sugg::Sugg::NonParen(format!("{}.to_bits()", arg.maybe_par()).into());
+                    sugg = sugg::Sugg::NonParen(format!("{}.to_bits()", sugg.maybe_par()).into());
 
                     // cast the result of `to_bits` if `to_ty` is signed
-                    arg = if let ty::Int(int_ty) = to_ty.kind() {
-                        arg.as_ty(int_ty.name_str().to_string())
+                    sugg = if let ty::Int(int_ty) = to_ty.kind() {
+                        sugg.as_ty(int_ty.name_str().to_string())
                     } else {
-                        arg
+                        sugg
                     };
 
-                    diag.span_suggestion(e.span, "consider using", arg.to_string(), Applicability::Unspecified);
+                    diag.span_suggestion(e.span, "consider using", sugg.to_string(), Applicability::Unspecified);
                 },
             );
             true
diff --git a/clippy_lints/src/transmute/transmute_int_to_bool.rs b/clippy_lints/src/transmute/transmute_int_to_bool.rs
index cc0a5643e2a..8c50b58ca4b 100644
--- a/clippy_lints/src/transmute/transmute_int_to_bool.rs
+++ b/clippy_lints/src/transmute/transmute_int_to_bool.rs
@@ -15,7 +15,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
         (ty::Int(ty::IntTy::I8) | ty::Uint(ty::UintTy::U8), ty::Bool) => {
@@ -25,7 +25,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 &format!("transmute from a `{}` to a `bool`", from_ty),
                 |diag| {
-                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let arg = sugg::Sugg::hir(cx, arg, "..");
                     let zero = sugg::Sugg::NonParen(Cow::from("0"));
                     diag.span_suggestion(
                         e.span,
diff --git a/clippy_lints/src/transmute/transmute_int_to_char.rs b/clippy_lints/src/transmute/transmute_int_to_char.rs
index e83d2e06b9a..3eb07b68992 100644
--- a/clippy_lints/src/transmute/transmute_int_to_char.rs
+++ b/clippy_lints/src/transmute/transmute_int_to_char.rs
@@ -14,7 +14,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
         (ty::Int(ty::IntTy::I32) | ty::Uint(ty::UintTy::U32), &ty::Char) => {
@@ -24,7 +24,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 &format!("transmute from a `{}` to a `char`", from_ty),
                 |diag| {
-                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let arg = sugg::Sugg::hir(cx, arg, "..");
                     let arg = if let ty::Int(_) = from_ty.kind() {
                         arg.as_ty(ast::UintTy::U32.name_str())
                     } else {
diff --git a/clippy_lints/src/transmute/transmute_int_to_float.rs b/clippy_lints/src/transmute/transmute_int_to_float.rs
index 05eee380d6f..b8703052e6c 100644
--- a/clippy_lints/src/transmute/transmute_int_to_float.rs
+++ b/clippy_lints/src/transmute/transmute_int_to_float.rs
@@ -13,7 +13,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
     const_context: bool,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
@@ -24,7 +24,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
                 |diag| {
-                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let arg = sugg::Sugg::hir(cx, arg, "..");
                     let arg = if let ty::Int(int_ty) = from_ty.kind() {
                         arg.as_ty(format!(
                             "u{}",
diff --git a/clippy_lints/src/transmute/transmute_num_to_bytes.rs b/clippy_lints/src/transmute/transmute_num_to_bytes.rs
index 5ba58a76494..52d193d11e1 100644
--- a/clippy_lints/src/transmute/transmute_num_to_bytes.rs
+++ b/clippy_lints/src/transmute/transmute_num_to_bytes.rs
@@ -13,7 +13,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
     const_context: bool,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
@@ -33,7 +33,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
                 |diag| {
-                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let arg = sugg::Sugg::hir(cx, arg, "..");
                     diag.span_suggestion(
                         e.span,
                         "consider using `to_ne_bytes()`",
diff --git a/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs b/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs
index 7b646bfc0c6..d712b33de9e 100644
--- a/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs
+++ b/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs
@@ -13,7 +13,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
         (ty::RawPtr(_), ty::RawPtr(to_ty)) => {
@@ -23,7 +23,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 "transmute from a pointer to a pointer",
                 |diag| {
-                    if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                    if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
                         let sugg = arg.as_ty(cx.tcx.mk_ptr(*to_ty));
                         diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
                     }
diff --git a/clippy_lints/src/transmute/transmute_ptr_to_ref.rs b/clippy_lints/src/transmute/transmute_ptr_to_ref.rs
index f14eef93645..5699f8e92cf 100644
--- a/clippy_lints/src/transmute/transmute_ptr_to_ref.rs
+++ b/clippy_lints/src/transmute/transmute_ptr_to_ref.rs
@@ -14,7 +14,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
     qpath: &'tcx QPath<'_>,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
@@ -28,7 +28,7 @@ pub(super) fn check<'tcx>(
                     from_ty, to_ty
                 ),
                 |diag| {
-                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let arg = sugg::Sugg::hir(cx, arg, "..");
                     let (deref, cast) = if *mutbl == Mutability::Mut {
                         ("&mut *", "*mut")
                     } else {
diff --git a/clippy_lints/src/transmute/transmute_ref_to_ref.rs b/clippy_lints/src/transmute/transmute_ref_to_ref.rs
index d105e37abf9..fdef8bac7f9 100644
--- a/clippy_lints/src/transmute/transmute_ref_to_ref.rs
+++ b/clippy_lints/src/transmute/transmute_ref_to_ref.rs
@@ -15,7 +15,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
     const_context: bool,
 ) -> bool {
     let mut triggered = false;
@@ -41,7 +41,7 @@ pub(super) fn check<'tcx>(
                     format!(
                         "std::str::from_utf8{}({}).unwrap()",
                         postfix,
-                        snippet(cx, args[0].span, ".."),
+                        snippet(cx, arg.span, ".."),
                     ),
                     Applicability::Unspecified,
                 );
@@ -54,7 +54,7 @@ pub(super) fn check<'tcx>(
                         TRANSMUTE_PTR_TO_PTR,
                         e.span,
                         "transmute from a reference to a reference",
-                        |diag| if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                        |diag| if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
                             let ty_from_and_mut = ty::TypeAndMut {
                                 ty: ty_from,
                                 mutbl: *from_mutbl
diff --git a/clippy_lints/src/transmute/transmute_undefined_repr.rs b/clippy_lints/src/transmute/transmute_undefined_repr.rs
new file mode 100644
index 00000000000..4b7c493278f
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_undefined_repr.rs
@@ -0,0 +1,291 @@
+use super::TRANSMUTE_UNDEFINED_REPR;
+use clippy_utils::diagnostics::span_lint_and_then;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::subst::{GenericArg, Subst};
+use rustc_middle::ty::{self, Ty, TyS, TypeAndMut};
+use rustc_span::Span;
+
+#[allow(clippy::too_many_lines)]
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty_orig: Ty<'tcx>,
+    to_ty_orig: Ty<'tcx>,
+) -> bool {
+    let mut from_ty = cx.tcx.erase_regions(from_ty_orig);
+    let mut to_ty = cx.tcx.erase_regions(to_ty_orig);
+
+    while !TyS::same_type(from_ty, to_ty) {
+        match reduce_refs(cx, e.span, from_ty, to_ty) {
+            ReducedTys::FromFatPtr { unsized_ty, .. } => {
+                span_lint_and_then(
+                    cx,
+                    TRANSMUTE_UNDEFINED_REPR,
+                    e.span,
+                    &format!("transmute from `{}` which has an undefined layout", from_ty_orig),
+                    |diag| {
+                        if !TyS::same_type(from_ty_orig.peel_refs(), unsized_ty) {
+                            diag.note(&format!("the contained type `&{}` has an undefined layout", unsized_ty));
+                        }
+                    },
+                );
+                return true;
+            },
+            ReducedTys::ToFatPtr { unsized_ty, .. } => {
+                span_lint_and_then(
+                    cx,
+                    TRANSMUTE_UNDEFINED_REPR,
+                    e.span,
+                    &format!("transmute to `{}` which has an undefined layout", to_ty_orig),
+                    |diag| {
+                        if !TyS::same_type(to_ty_orig.peel_refs(), unsized_ty) {
+                            diag.note(&format!("the contained type `&{}` has an undefined layout", unsized_ty));
+                        }
+                    },
+                );
+                return true;
+            },
+            ReducedTys::ToPtr {
+                from_ty: from_sub_ty,
+                to_ty: to_sub_ty,
+            } => match reduce_ty(cx, from_sub_ty) {
+                ReducedTy::UnorderedFields(from_ty) => {
+                    span_lint_and_then(
+                        cx,
+                        TRANSMUTE_UNDEFINED_REPR,
+                        e.span,
+                        &format!("transmute from `{}` which has an undefined layout", from_ty_orig),
+                        |diag| {
+                            if !TyS::same_type(from_ty_orig.peel_refs(), from_ty) {
+                                diag.note(&format!("the contained type `{}` has an undefined layout", from_ty));
+                            }
+                        },
+                    );
+                    return true;
+                },
+                ReducedTy::Ref(from_sub_ty) => {
+                    from_ty = from_sub_ty;
+                    to_ty = to_sub_ty;
+                    continue;
+                },
+                _ => break,
+            },
+            ReducedTys::FromPtr {
+                from_ty: from_sub_ty,
+                to_ty: to_sub_ty,
+            } => match reduce_ty(cx, to_sub_ty) {
+                ReducedTy::UnorderedFields(to_ty) => {
+                    span_lint_and_then(
+                        cx,
+                        TRANSMUTE_UNDEFINED_REPR,
+                        e.span,
+                        &format!("transmute to `{}` which has an undefined layout", to_ty_orig),
+                        |diag| {
+                            if !TyS::same_type(to_ty_orig.peel_refs(), to_ty) {
+                                diag.note(&format!("the contained type `{}` has an undefined layout", to_ty));
+                            }
+                        },
+                    );
+                    return true;
+                },
+                ReducedTy::Ref(to_sub_ty) => {
+                    from_ty = from_sub_ty;
+                    to_ty = to_sub_ty;
+                    continue;
+                },
+                _ => break,
+            },
+            ReducedTys::Other {
+                from_ty: from_sub_ty,
+                to_ty: to_sub_ty,
+            } => match (reduce_ty(cx, from_sub_ty), reduce_ty(cx, to_sub_ty)) {
+                (ReducedTy::IntArray, _) | (_, ReducedTy::IntArray) => return false,
+                (ReducedTy::UnorderedFields(from_ty), ReducedTy::UnorderedFields(to_ty))
+                    if !TyS::same_type(from_ty, to_ty) =>
+                {
+                    span_lint_and_then(
+                        cx,
+                        TRANSMUTE_UNDEFINED_REPR,
+                        e.span,
+                        &format!(
+                            "transmute from `{}` to `{}`, both of which have an undefined layout",
+                            from_ty_orig, to_ty_orig
+                        ),
+                        |diag| {
+                            if let (Some(from_def), Some(to_def)) = (from_ty.ty_adt_def(), to_ty.ty_adt_def())
+                                && from_def == to_def
+                            {
+                                diag.note(&format!(
+                                    "two instances of the same generic type (`{}`) may have different layouts",
+                                    cx.tcx.item_name(from_def.did)
+                                ));
+                            } else {
+                                if !TyS::same_type(from_ty_orig.peel_refs(), from_ty) {
+                                    diag.note(&format!("the contained type `{}` has an undefined layout", from_ty));
+                                }
+                                if !TyS::same_type(to_ty_orig.peel_refs(), to_ty) {
+                                    diag.note(&format!("the contained type `{}` has an undefined layout", to_ty));
+                                }
+                            }
+                        },
+                    );
+                    return true;
+                },
+                (
+                    ReducedTy::UnorderedFields(from_ty),
+                    ReducedTy::Other(_) | ReducedTy::OrderedFields(_) | ReducedTy::Ref(_),
+                ) => {
+                    span_lint_and_then(
+                        cx,
+                        TRANSMUTE_UNDEFINED_REPR,
+                        e.span,
+                        &format!("transmute from `{}` which has an undefined layout", from_ty_orig),
+                        |diag| {
+                            if !TyS::same_type(from_ty_orig.peel_refs(), from_ty) {
+                                diag.note(&format!("the contained type `{}` has an undefined layout", from_ty));
+                            }
+                        },
+                    );
+                    return true;
+                },
+                (
+                    ReducedTy::Other(_) | ReducedTy::OrderedFields(_) | ReducedTy::Ref(_),
+                    ReducedTy::UnorderedFields(to_ty),
+                ) => {
+                    span_lint_and_then(
+                        cx,
+                        TRANSMUTE_UNDEFINED_REPR,
+                        e.span,
+                        &format!("transmute into `{}` which has an undefined layout", to_ty_orig),
+                        |diag| {
+                            if !TyS::same_type(to_ty_orig.peel_refs(), to_ty) {
+                                diag.note(&format!("the contained type `{}` has an undefined layout", to_ty));
+                            }
+                        },
+                    );
+                    return true;
+                },
+                (ReducedTy::Ref(from_sub_ty), ReducedTy::Ref(to_sub_ty)) => {
+                    from_ty = from_sub_ty;
+                    to_ty = to_sub_ty;
+                    continue;
+                },
+                (
+                    ReducedTy::OrderedFields(_) | ReducedTy::Ref(_) | ReducedTy::Other(_),
+                    ReducedTy::OrderedFields(_) | ReducedTy::Ref(_) | ReducedTy::Other(_),
+                )
+                | (ReducedTy::UnorderedFields(_), ReducedTy::UnorderedFields(_)) => break,
+            },
+        }
+    }
+
+    false
+}
+
+enum ReducedTys<'tcx> {
+    FromFatPtr { unsized_ty: Ty<'tcx> },
+    ToFatPtr { unsized_ty: Ty<'tcx> },
+    ToPtr { from_ty: Ty<'tcx>, to_ty: Ty<'tcx> },
+    FromPtr { from_ty: Ty<'tcx>, to_ty: Ty<'tcx> },
+    Other { from_ty: Ty<'tcx>, to_ty: Ty<'tcx> },
+}
+
+fn reduce_refs<'tcx>(
+    cx: &LateContext<'tcx>,
+    span: Span,
+    mut from_ty: Ty<'tcx>,
+    mut to_ty: Ty<'tcx>,
+) -> ReducedTys<'tcx> {
+    loop {
+        return match (from_ty.kind(), to_ty.kind()) {
+            (
+                ty::Ref(_, from_sub_ty, _) | ty::RawPtr(TypeAndMut { ty: from_sub_ty, .. }),
+                ty::Ref(_, to_sub_ty, _) | ty::RawPtr(TypeAndMut { ty: to_sub_ty, .. }),
+            ) => {
+                from_ty = from_sub_ty;
+                to_ty = to_sub_ty;
+                continue;
+            },
+            (ty::Ref(_, unsized_ty, _) | ty::RawPtr(TypeAndMut { ty: unsized_ty, .. }), _)
+                if !unsized_ty.is_sized(cx.tcx.at(span), cx.param_env) =>
+            {
+                ReducedTys::FromFatPtr { unsized_ty }
+            },
+            (_, ty::Ref(_, unsized_ty, _) | ty::RawPtr(TypeAndMut { ty: unsized_ty, .. }))
+                if !unsized_ty.is_sized(cx.tcx.at(span), cx.param_env) =>
+            {
+                ReducedTys::ToFatPtr { unsized_ty }
+            },
+            (ty::Ref(_, from_ty, _) | ty::RawPtr(TypeAndMut { ty: from_ty, .. }), _) => {
+                ReducedTys::FromPtr { from_ty, to_ty }
+            },
+            (_, ty::Ref(_, to_ty, _) | ty::RawPtr(TypeAndMut { ty: to_ty, .. })) => {
+                ReducedTys::ToPtr { from_ty, to_ty }
+            },
+            _ => ReducedTys::Other { from_ty, to_ty },
+        };
+    }
+}
+
+enum ReducedTy<'tcx> {
+    OrderedFields(Ty<'tcx>),
+    UnorderedFields(Ty<'tcx>),
+    Ref(Ty<'tcx>),
+    Other(Ty<'tcx>),
+    IntArray,
+}
+
+fn reduce_ty<'tcx>(cx: &LateContext<'tcx>, mut ty: Ty<'tcx>) -> ReducedTy<'tcx> {
+    loop {
+        ty = cx.tcx.try_normalize_erasing_regions(cx.param_env, ty).unwrap_or(ty);
+        return match *ty.kind() {
+            ty::Array(sub_ty, _) if matches!(sub_ty.kind(), ty::Int(_) | ty::Uint(_)) => ReducedTy::IntArray,
+            ty::Array(sub_ty, _) | ty::Slice(sub_ty) => {
+                ty = sub_ty;
+                continue;
+            },
+            ty::Tuple(args) => {
+                let mut iter = args.iter().map(GenericArg::expect_ty);
+                let Some(sized_ty) = iter.find(|ty| !is_zero_sized_ty(cx, ty)) else {
+                    return ReducedTy::OrderedFields(ty);
+                };
+                if iter.all(|ty| is_zero_sized_ty(cx, ty)) {
+                    ty = sized_ty;
+                    continue;
+                }
+                ReducedTy::UnorderedFields(ty)
+            },
+            ty::Adt(def, substs) if def.is_struct() => {
+                if def.repr.inhibit_struct_field_reordering_opt() {
+                    return ReducedTy::OrderedFields(ty);
+                }
+                let mut iter = def
+                    .non_enum_variant()
+                    .fields
+                    .iter()
+                    .map(|f| cx.tcx.type_of(f.did).subst(cx.tcx, substs));
+                let Some(sized_ty) = iter.find(|ty| !is_zero_sized_ty(cx, ty)) else {
+                    return ReducedTy::OrderedFields(ty);
+                };
+                if iter.all(|ty| is_zero_sized_ty(cx, ty)) {
+                    ty = sized_ty;
+                    continue;
+                }
+                ReducedTy::UnorderedFields(ty)
+            },
+            ty::Ref(..) | ty::RawPtr(_) => ReducedTy::Ref(ty),
+            _ => ReducedTy::Other(ty),
+        };
+    }
+}
+
+fn is_zero_sized_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+    if let Ok(ty) = cx.tcx.try_normalize_erasing_regions(cx.param_env, ty)
+        && let Ok(layout) = cx.tcx.layout_of(cx.param_env.and(ty))
+    {
+        layout.layout.size.bytes() == 0
+    } else {
+        false
+    }
+}
diff --git a/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs b/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
index e2c6d130f3c..626d7cd46fc 100644
--- a/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
+++ b/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
@@ -14,7 +14,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
 ) -> bool {
     if can_be_expressed_as_pointer_cast(cx, e, from_ty, to_ty) {
         span_lint_and_then(
@@ -26,7 +26,7 @@ pub(super) fn check<'tcx>(
                 from_ty, to_ty
             ),
             |diag| {
-                if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
                     let sugg = arg.as_ty(&to_ty.to_string()).to_string();
                     diag.span_suggestion(e.span, "try", sugg, Applicability::MachineApplicable);
                 }
diff --git a/clippy_lints/src/transmute/useless_transmute.rs b/clippy_lints/src/transmute/useless_transmute.rs
index 445bcf60fa7..998f97eb5d8 100644
--- a/clippy_lints/src/transmute/useless_transmute.rs
+++ b/clippy_lints/src/transmute/useless_transmute.rs
@@ -13,7 +13,7 @@ pub(super) fn check<'tcx>(
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
     to_ty: Ty<'tcx>,
-    args: &'tcx [Expr<'_>],
+    arg: &'tcx Expr<'_>,
 ) -> bool {
     match (&from_ty.kind(), &to_ty.kind()) {
         _ if from_ty == to_ty => {
@@ -32,7 +32,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 "transmute from a reference to a pointer",
                 |diag| {
-                    if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                    if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
                         let rty_and_mut = ty::TypeAndMut {
                             ty: rty,
                             mutbl: *rty_mutbl,
@@ -57,7 +57,7 @@ pub(super) fn check<'tcx>(
                 e.span,
                 "transmute from an integer to a pointer",
                 |diag| {
-                    if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                    if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
                         diag.span_suggestion(
                             e.span,
                             "try",
diff --git a/tests/ui/crashes/ice-4968.rs b/tests/ui/crashes/ice-4968.rs
index 3822f174598..e0510d942c2 100644
--- a/tests/ui/crashes/ice-4968.rs
+++ b/tests/ui/crashes/ice-4968.rs
@@ -3,6 +3,7 @@
 // Test for https://github.com/rust-lang/rust-clippy/issues/4968
 
 #![warn(clippy::unsound_collection_transmute)]
+#![allow(clippy::transmute_undefined_repr)]
 
 trait Trait {
     type Assoc;
diff --git a/tests/ui/transmute_undefined_repr.rs b/tests/ui/transmute_undefined_repr.rs
new file mode 100644
index 00000000000..71539940fbf
--- /dev/null
+++ b/tests/ui/transmute_undefined_repr.rs
@@ -0,0 +1,44 @@
+#![warn(clippy::transmute_undefined_repr)]
+#![allow(clippy::unit_arg)]
+
+fn value<T>() -> T {
+    unimplemented!()
+}
+
+struct Empty;
+struct Ty<T>(T);
+struct Ty2<T, U>(T, U);
+
+#[repr(C)]
+struct Ty2C<T, U>(T, U);
+
+fn main() {
+    unsafe {
+        let _: () = core::mem::transmute(value::<Empty>());
+        let _: Empty = core::mem::transmute(value::<()>());
+
+        let _: Ty<u32> = core::mem::transmute(value::<u32>());
+        let _: Ty<u32> = core::mem::transmute(value::<u32>());
+
+        let _: Ty2C<u32, i32> = core::mem::transmute(value::<Ty2<u32, i32>>()); // Lint, Ty2 is unordered
+        let _: Ty2<u32, i32> = core::mem::transmute(value::<Ty2C<u32, i32>>()); // Lint, Ty2 is unordered
+
+        let _: Ty2<u32, i32> = core::mem::transmute(value::<Ty<Ty2<u32, i32>>>()); // Ok, Ty2 types are the same
+        let _: Ty<Ty2<u32, i32>> = core::mem::transmute(value::<Ty2<u32, i32>>()); // Ok, Ty2 types are the same
+
+        let _: Ty2<u32, f32> = core::mem::transmute(value::<Ty<Ty2<u32, i32>>>()); // Lint, different Ty2 instances
+        let _: Ty<Ty2<u32, i32>> = core::mem::transmute(value::<Ty2<u32, f32>>()); // Lint, different Ty2 instances
+
+        let _: Ty<&()> = core::mem::transmute(value::<&()>());
+        let _: &() = core::mem::transmute(value::<Ty<&()>>());
+
+        let _: &Ty2<u32, f32> = core::mem::transmute(value::<Ty<&Ty2<u32, i32>>>()); // Lint, different Ty2 instances
+        let _: Ty<&Ty2<u32, i32>> = core::mem::transmute(value::<&Ty2<u32, f32>>()); // Lint, different Ty2 instances
+
+        let _: Ty<usize> = core::mem::transmute(value::<&Ty2<u32, i32>>()); // Ok, pointer to usize conversion
+        let _: &Ty2<u32, i32> = core::mem::transmute(value::<Ty<usize>>()); // Ok, pointer to usize conversion
+
+        let _: Ty<[u8; 8]> = core::mem::transmute(value::<Ty2<u32, i32>>()); // Ok, transmute to byte array
+        let _: Ty2<u32, i32> = core::mem::transmute(value::<Ty<[u8; 8]>>()); // Ok, transmute from byte array
+    }
+}
diff --git a/tests/ui/transmute_undefined_repr.stderr b/tests/ui/transmute_undefined_repr.stderr
new file mode 100644
index 00000000000..040c63c7afa
--- /dev/null
+++ b/tests/ui/transmute_undefined_repr.stderr
@@ -0,0 +1,44 @@
+error: transmute from `Ty2<u32, i32>` which has an undefined layout
+  --> $DIR/transmute_undefined_repr.rs:23:33
+   |
+LL |         let _: Ty2C<u32, i32> = core::mem::transmute(value::<Ty2<u32, i32>>()); // Lint, Ty2 is unordered
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::transmute-undefined-repr` implied by `-D warnings`
+
+error: transmute into `Ty2<u32, i32>` which has an undefined layout
+  --> $DIR/transmute_undefined_repr.rs:24:32
+   |
+LL |         let _: Ty2<u32, i32> = core::mem::transmute(value::<Ty2C<u32, i32>>()); // Lint, Ty2 is unordered
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `Ty<Ty2<u32, i32>>` to `Ty2<u32, f32>`, both of which have an undefined layout
+  --> $DIR/transmute_undefined_repr.rs:29:32
+   |
+LL |         let _: Ty2<u32, f32> = core::mem::transmute(value::<Ty<Ty2<u32, i32>>>()); // Lint, different Ty2 instances
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: two instances of the same generic type (`Ty2`) may have different layouts
+
+error: transmute from `Ty2<u32, f32>` to `Ty<Ty2<u32, i32>>`, both of which have an undefined layout
+  --> $DIR/transmute_undefined_repr.rs:30:36
+   |
+LL |         let _: Ty<Ty2<u32, i32>> = core::mem::transmute(value::<Ty2<u32, f32>>()); // Lint, different Ty2 instances
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: two instances of the same generic type (`Ty2`) may have different layouts
+
+error: transmute to `&Ty2<u32, f32>` which has an undefined layout
+  --> $DIR/transmute_undefined_repr.rs:35:33
+   |
+LL |         let _: &Ty2<u32, f32> = core::mem::transmute(value::<Ty<&Ty2<u32, i32>>>()); // Lint, different Ty2 instances
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `&Ty2<u32, f32>` which has an undefined layout
+  --> $DIR/transmute_undefined_repr.rs:36:37
+   |
+LL |         let _: Ty<&Ty2<u32, i32>> = core::mem::transmute(value::<&Ty2<u32, f32>>()); // Lint, different Ty2 instances
+   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+