Auto merge of #8425 - Jarcho:transmute_8417, r=giraffate

Fix `transmute_undefined_repr` with single field `#[repr(C)]` structs

Fixes: #8417

The description has also been made more precise.

changelog: Fix `transmute_undefined_repr` with single field `#[repr(C)]` structs
changelog: Move `transmute_undefined_repr` back to `correctness`

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),

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),

clippy_lints/src/lib.register_nursery.rs

@@ -26,7 +26,6 @@ store.register_group(true, "clippy::nursery", Some("clippy_nursery"), vec![
     LintId::of(strings::STRING_LIT_AS_BYTES),
     LintId::of(suspicious_operation_groupings::SUSPICIOUS_OPERATION_GROUPINGS),
     LintId::of(trailing_empty_array::TRAILING_EMPTY_ARRAY),
-    LintId::of(transmute::TRANSMUTE_UNDEFINED_REPR),
     LintId::of(transmute::USELESS_TRANSMUTE),
     LintId::of(use_self::USE_SELF),
 ])

clippy_lints/src/transmute/mod.rs

@@ -358,7 +358,8 @@ declare_clippy_lint! {
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for transmutes either to or from a type which does not have a defined representation.
+    /// Checks for transmutes between types which do not have a representation defined relative to
+    /// each other.
     ///
     /// ### Why is this bad?
     /// The results of such a transmute are not defined.
@@ -376,7 +377,7 @@ declare_clippy_lint! {
     /// ```
     #[clippy::version = "1.60.0"]
     pub TRANSMUTE_UNDEFINED_REPR,
-    nursery,
+    correctness,
     "transmute to or from a type with an undefined representation"
 }
 

clippy_lints/src/transmute/transmute_undefined_repr.rs

@@ -255,9 +255,6 @@ fn reduce_ty<'tcx>(cx: &LateContext<'tcx>, mut ty: Ty<'tcx>) -> ReducedTy<'tcx>
                 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
@@ -270,7 +267,11 @@ fn reduce_ty<'tcx>(cx: &LateContext<'tcx>, mut ty: Ty<'tcx>) -> ReducedTy<'tcx>
                     ty = sized_ty;
                     continue;
                 }
-                ReducedTy::UnorderedFields(ty)
+                if def.repr.inhibit_struct_field_reordering_opt() {
+                    ReducedTy::OrderedFields(ty)
+                } else {
+                    ReducedTy::UnorderedFields(ty)
+                }
             },
             ty::Ref(..) | ty::RawPtr(_) => ReducedTy::Ref(ty),
             _ => ReducedTy::Other(ty),

tests/ui/transmute_undefined_repr.rs

@@ -40,5 +40,9 @@ fn main() {
 
         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
+
+        // issue #8417
+        let _: Ty2C<Ty2<u32, i32>, ()> = core::mem::transmute(value::<Ty2<u32, i32>>()); // Ok, Ty2 types are the same
+        let _: Ty2<u32, i32> = core::mem::transmute(value::<Ty2C<Ty2<u32, i32>, ()>>()); // Ok, Ty2 types are the same
     }
 }