From 442d4ce1c36011a8b12fb3ce9a4d70a7031fc4d5 Mon Sep 17 00:00:00 2001
From: Jason Newcomb <jsnewcomb@pm.me>
Date: Tue, 15 Mar 2022 22:31:07 -0400
Subject: [PATCH] Don't lint fat pointer to `(usize, usize)` conversion in
 `transmute_undefined_repr`

---
 .../src/transmute/transmute_undefined_repr.rs     | 15 ++++++++++++++-
 tests/ui/transmute_undefined_repr.rs              |  7 +++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/clippy_lints/src/transmute/transmute_undefined_repr.rs b/clippy_lints/src/transmute/transmute_undefined_repr.rs
index 3cd9d208943..30059e5ba2b 100644
--- a/clippy_lints/src/transmute/transmute_undefined_repr.rs
+++ b/clippy_lints/src/transmute/transmute_undefined_repr.rs
@@ -4,7 +4,7 @@ use clippy_utils::ty::is_c_void;
 use rustc_hir::Expr;
 use rustc_lint::LateContext;
 use rustc_middle::ty::subst::Subst;
-use rustc_middle::ty::{self, Ty, TypeAndMut};
+use rustc_middle::ty::{self, IntTy, Ty, TypeAndMut, UintTy};
 use rustc_span::Span;
 
 #[allow(clippy::too_many_lines)]
@@ -24,6 +24,7 @@ pub(super) fn check<'tcx>(
                 to_ty: to_sub_ty,
             } => match reduce_ty(cx, to_sub_ty) {
                 ReducedTy::IntArray | ReducedTy::TypeErasure => break,
+                ReducedTy::UnorderedFields(ty) if is_size_pair(ty) => break,
                 ReducedTy::Ref(to_sub_ty) => {
                     from_ty = unsized_ty;
                     to_ty = to_sub_ty;
@@ -49,6 +50,7 @@ pub(super) fn check<'tcx>(
                 from_ty: from_sub_ty,
             } => match reduce_ty(cx, from_sub_ty) {
                 ReducedTy::IntArray | ReducedTy::TypeErasure => break,
+                ReducedTy::UnorderedFields(ty) if is_size_pair(ty) => break,
                 ReducedTy::Ref(from_sub_ty) => {
                     from_ty = from_sub_ty;
                     to_ty = unsized_ty;
@@ -333,3 +335,14 @@ fn is_zero_sized_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
         }
     }
 }
+
+fn is_size_pair(ty: Ty<'_>) -> bool {
+    if let ty::Tuple(tys) = *ty.kind()
+        && let [ty1, ty2] = &**tys
+    {
+        matches!(ty1.kind(), ty::Int(IntTy::Isize) | ty::Uint(UintTy::Usize))
+            && matches!(ty2.kind(), ty::Int(IntTy::Isize) | ty::Uint(UintTy::Usize))
+    } else {
+        false
+    }
+}
diff --git a/tests/ui/transmute_undefined_repr.rs b/tests/ui/transmute_undefined_repr.rs
index e7a59dbed95..13d8cb10e98 100644
--- a/tests/ui/transmute_undefined_repr.rs
+++ b/tests/ui/transmute_undefined_repr.rs
@@ -96,5 +96,12 @@ fn main() {
 
         let _: Ty2<u32, u32> = transmute(value::<((), Ty2<u32, u32>)>()); // Ok
         let _: ((), Ty2<u32, u32>) = transmute(value::<Ty2<u32, u32>>()); // Ok
+
+        let _: (usize, usize) = transmute(value::<&[u8]>()); // Ok
+        let _: &[u8] = transmute(value::<(usize, usize)>()); // Ok
+
+        trait Trait {}
+        let _: (isize, isize) = transmute(value::<&dyn Trait>()); // Ok
+        let _: &dyn Trait = transmute(value::<(isize, isize)>()); // Ok
     }
 }