Auto merge of #8547 - Jarcho:transmute_8501, r=flip1995

More `transmute_undefined_repr` fixes

fixes: #8498
fixes: #8501
fixes: #8503

changelog: Allow `transumte_undefined_repr` between fat pointers and `(usize, usize)`
changelog: Allow `transumte_undefined_repr` when one side is a union
changelog: Fix `transumte_undefined_repr` on tuples with one non-zero-sized type.

4 files changed, 49 insertions, 15 deletions

diff --git a/clippy_lints/src/transmute/mod.rs b/clippy_lints/src/transmute/mod.rs
index 23cb9d40dfd..02569bd3a47 100644
--- a/clippy_lints/src/transmute/mod.rs
+++ b/clippy_lints/src/transmute/mod.rs
@@ -415,7 +415,8 @@ 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(arg);
+                let from_ty = cx.typeck_results().expr_ty_adjusted(arg);
+                // Adjustments for `to_ty` happen after the call to `transmute`, so don't use them.
                 let to_ty = cx.typeck_results().expr_ty(e);
 
                 // If useless_transmute is triggered, the other lints can be skipped.
diff --git a/clippy_lints/src/transmute/transmute_undefined_repr.rs b/clippy_lints/src/transmute/transmute_undefined_repr.rs
index 6edff224092..4922c40bcef 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)]
@@ -23,7 +23,8 @@ pub(super) fn check<'tcx>(
                 unsized_ty,
                 to_ty: to_sub_ty,
             } => match reduce_ty(cx, to_sub_ty) {
-                ReducedTy::IntArray | ReducedTy::TypeErasure => break,
+                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;
@@ -48,7 +49,8 @@ pub(super) fn check<'tcx>(
                 unsized_ty,
                 from_ty: from_sub_ty,
             } => match reduce_ty(cx, from_sub_ty) {
-                ReducedTy::IntArray | ReducedTy::TypeErasure => break,
+                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;
@@ -123,8 +125,7 @@ pub(super) fn check<'tcx>(
                 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::TypeErasure, _)
-                | (_, ReducedTy::IntArray | ReducedTy::TypeErasure) => return false,
+                (ReducedTy::TypeErasure, _) | (_, ReducedTy::TypeErasure) => return false,
                 (ReducedTy::UnorderedFields(from_ty), ReducedTy::UnorderedFields(to_ty)) if from_ty != to_ty => {
                     span_lint_and_then(
                         cx,
@@ -263,9 +264,6 @@ enum ReducedTy<'tcx> {
     UnorderedFields(Ty<'tcx>),
     /// The type is a reference to the contained type.
     Ref(Ty<'tcx>),
-    /// The type is an array of a primitive integer type. These can be used as storage for a value
-    /// of another type.
-    IntArray,
     /// Any other type.
     Other(Ty<'tcx>),
 }
@@ -275,17 +273,18 @@ 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, _) if matches!(sub_ty.kind(), ty::Int(_) | ty::Uint(_)) => ReducedTy::TypeErasure,
             ty::Array(sub_ty, _) | ty::Slice(sub_ty) => {
                 ty = sub_ty;
                 continue;
             },
             ty::Tuple(args) if args.is_empty() => ReducedTy::TypeErasure,
             ty::Tuple(args) => {
-                let Some(sized_ty) = args.iter().find(|&ty| !is_zero_sized_ty(cx, ty)) else {
+                let mut iter = args.iter();
+                let Some(sized_ty) = iter.find(|&ty| !is_zero_sized_ty(cx, ty)) else {
                     return ReducedTy::OrderedFields(ty);
                 };
-                if args.iter().all(|ty| is_zero_sized_ty(cx, ty)) {
+                if iter.all(|ty| is_zero_sized_ty(cx, ty)) {
                     ty = sized_ty;
                     continue;
                 }
@@ -313,6 +312,8 @@ fn reduce_ty<'tcx>(cx: &LateContext<'tcx>, mut ty: Ty<'tcx>) -> ReducedTy<'tcx>
             ty::Adt(def, _) if def.is_enum() && (def.variants().is_empty() || is_c_void(cx, ty)) => {
                 ReducedTy::TypeErasure
             },
+            // TODO: Check if the conversion to or from at least one of a union's fields is valid.
+            ty::Adt(def, _) if def.is_union() => ReducedTy::TypeErasure,
             ty::Foreign(_) => ReducedTy::TypeErasure,
             ty::Ref(_, ty, _) => ReducedTy::Ref(ty),
             ty::RawPtr(ty) => ReducedTy::Ref(ty.ty),
@@ -332,3 +333,14 @@ 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 b163d605634..7cc03b89fe0 100644
--- a/tests/ui/transmute_undefined_repr.rs
+++ b/tests/ui/transmute_undefined_repr.rs
@@ -2,7 +2,7 @@
 #![allow(clippy::unit_arg, clippy::transmute_ptr_to_ref)]
 
 use core::ffi::c_void;
-use core::mem::{size_of, transmute};
+use core::mem::{size_of, transmute, MaybeUninit};
 
 fn value<T>() -> T {
     unimplemented!()
@@ -87,5 +87,26 @@ fn main() {
 
         let _: *const [u8] = transmute(value::<Box<[u8]>>()); // Ok
         let _: Box<[u8]> = transmute(value::<*mut [u8]>()); // Ok
+
+        let _: Ty2<u32, u32> = transmute(value::<(Ty2<u32, u32>,)>()); // Ok
+        let _: (Ty2<u32, u32>,) = transmute(value::<Ty2<u32, u32>>()); // Ok
+
+        let _: Ty2<u32, u32> = transmute(value::<(Ty2<u32, u32>, ())>()); // Ok
+        let _: (Ty2<u32, u32>, ()) = transmute(value::<Ty2<u32, u32>>()); // Ok
+
+        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
+
+        let _: MaybeUninit<Ty2<u32, u32>> = transmute(value::<Ty2<u32, u32>>()); // Ok
+        let _: Ty2<u32, u32> = transmute(value::<MaybeUninit<Ty2<u32, u32>>>()); // Ok
+
+        let _: Ty<&[u32]> = transmute::<&[u32], _>(value::<&Vec<u32>>()); // Ok
     }
 }
diff --git a/tests/ui/transmutes_expressible_as_ptr_casts.stderr b/tests/ui/transmutes_expressible_as_ptr_casts.stderr
index d9b64a0ed7b..de9418c8d1a 100644
--- a/tests/ui/transmutes_expressible_as_ptr_casts.stderr
+++ b/tests/ui/transmutes_expressible_as_ptr_casts.stderr
@@ -34,13 +34,13 @@ error: transmute from a reference to a pointer
 LL |     let _array_ptr_transmute = unsafe { transmute::<&[i32; 4], *const [i32; 4]>(array_ref) };
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `array_ref as *const [i32; 4]`
 
-error: transmute from `fn(usize) -> u8 {main::foo}` to `*const usize` which could be expressed as a pointer cast instead
+error: transmute from `fn(usize) -> u8` to `*const usize` which could be expressed as a pointer cast instead
   --> $DIR/transmutes_expressible_as_ptr_casts.rs:48:41
    |
 LL |     let _usize_ptr_transmute = unsafe { transmute::<fn(usize) -> u8, *const usize>(foo) };
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `foo as *const usize`
 
-error: transmute from `fn(usize) -> u8 {main::foo}` to `usize` which could be expressed as a pointer cast instead
+error: transmute from `fn(usize) -> u8` to `usize` which could be expressed as a pointer cast instead
   --> $DIR/transmutes_expressible_as_ptr_casts.rs:52:49
    |
 LL |     let _usize_from_fn_ptr_transmute = unsafe { transmute::<fn(usize) -> u8, usize>(foo) };