Auto merge of #128703 - compiler-errors:normalizing-tails, r=lcnr

Miscellaneous improvements to struct tail normalization

1. Make checks for foreign tails more accurate by normalizing the struct tail. I didn't write a test for this one.
2. Normalize when computing struct tail for `offset_of` for slice/str. This fixes the new solver only.
3. Normalizing when computing tails for disaligned reference check. This fixes both solvers.

r? lcnr

compiler/rustc_codegen_cranelift/src/common.rs

@@ -107,7 +107,7 @@ pub(crate) fn has_ptr_meta<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
         return false;
     }
 
-    let tail = tcx.struct_tail_erasing_lifetimes(ty, ParamEnv::reveal_all());
+    let tail = tcx.struct_tail_for_codegen(ty, ParamEnv::reveal_all());
     match tail.kind() {
         ty::Foreign(..) => false,
         ty::Str | ty::Slice(..) | ty::Dynamic(..) => true,

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -22,7 +22,7 @@ pub(crate) fn unsized_info<'tcx>(
     old_info: Option<Value>,
 ) -> Value {
     let (source, target) =
-        fx.tcx.struct_lockstep_tails_erasing_lifetimes(source, target, ParamEnv::reveal_all());
+        fx.tcx.struct_lockstep_tails_for_codegen(source, target, ParamEnv::reveal_all());
     match (&source.kind(), &target.kind()) {
         (&ty::Array(_, len), &ty::Slice(_)) => fx
             .bcx

compiler/rustc_codegen_llvm/src/builder.rs

@@ -12,7 +12,8 @@ use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::ty::layout::{
-    FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOfHelpers, TyAndLayout,
+    FnAbiError, FnAbiOfHelpers, FnAbiRequest, HasParamEnv, LayoutError, LayoutOfHelpers,
+    TyAndLayout,
 };
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_sanitizers::{cfi, kcfi};
@@ -531,7 +532,7 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
     #[instrument(level = "trace", skip(self))]
     fn load_operand(&mut self, place: PlaceRef<'tcx, &'ll Value>) -> OperandRef<'tcx, &'ll Value> {
         if place.layout.is_unsized() {
-            let tail = self.tcx.struct_tail_with_normalize(place.layout.ty, |ty| ty, || {});
+            let tail = self.tcx.struct_tail_for_codegen(place.layout.ty, self.param_env());
             if matches!(tail.kind(), ty::Foreign(..)) {
                 // Unsized locals and, at least conceptually, even unsized arguments must be copied
                 // around, which requires dynamically determining their size. Therefore, we cannot

compiler/rustc_codegen_llvm/src/debuginfo/utils.rs

@@ -62,7 +62,7 @@ pub(crate) fn fat_pointer_kind<'ll, 'tcx>(
     cx: &CodegenCx<'ll, 'tcx>,
     pointee_ty: Ty<'tcx>,
 ) -> Option<FatPtrKind> {
-    let pointee_tail_ty = cx.tcx.struct_tail_erasing_lifetimes(pointee_ty, cx.param_env());
+    let pointee_tail_ty = cx.tcx.struct_tail_for_codegen(pointee_ty, cx.param_env());
     let layout = cx.layout_of(pointee_tail_ty);
     trace!(
         "fat_pointer_kind: {:?} has layout {:?} (is_unsized? {})",

compiler/rustc_codegen_ssa/src/base.rs

@@ -143,7 +143,7 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 ) -> Bx::Value {
     let cx = bx.cx();
     let (source, target) =
-        cx.tcx().struct_lockstep_tails_erasing_lifetimes(source, target, bx.param_env());
+        cx.tcx().struct_lockstep_tails_for_codegen(source, target, bx.param_env());
     match (source.kind(), target.kind()) {
         (&ty::Array(_, len), &ty::Slice(_)) => {
             cx.const_usize(len.eval_target_usize(cx.tcx(), ty::ParamEnv::reveal_all()))

compiler/rustc_codegen_ssa/src/traits/type_.rs

@@ -91,7 +91,7 @@ pub trait DerivedTypeMethods<'tcx>: BaseTypeMethods<'tcx> + MiscMethods<'tcx> {
             return false;
         }
 
-        let tail = self.tcx().struct_tail_erasing_lifetimes(ty, param_env);
+        let tail = self.tcx().struct_tail_for_codegen(ty, param_env);
         match tail.kind() {
             ty::Foreign(..) => false,
             ty::Str | ty::Slice(..) | ty::Dynamic(..) => true,

compiler/rustc_const_eval/src/interpret/call.rs

@@ -678,9 +678,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 } else {
                     // Doesn't have to be a `dyn Trait`, but the unsized tail must be `dyn Trait`.
                     // (For that reason we also cannot use `unpack_dyn_trait`.)
-                    let receiver_tail = self
+                    let receiver_tail =
-                        .tcx
+                        self.tcx.struct_tail_for_codegen(receiver_place.layout.ty, self.param_env);
-                        .struct_tail_erasing_lifetimes(receiver_place.layout.ty, self.param_env);
                     let ty::Dynamic(receiver_trait, _, ty::Dyn) = receiver_tail.kind() else {
                         span_bug!(
                             self.cur_span(),

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -386,7 +386,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     ) -> InterpResult<'tcx> {
         // A<Struct> -> A<Trait> conversion
         let (src_pointee_ty, dest_pointee_ty) =
-            self.tcx.struct_lockstep_tails_erasing_lifetimes(source_ty, cast_ty, self.param_env);
+            self.tcx.struct_lockstep_tails_for_codegen(source_ty, cast_ty, self.param_env);
 
         match (&src_pointee_ty.kind(), &dest_pointee_ty.kind()) {
             (&ty::Array(_, length), &ty::Slice(_)) => {

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -343,7 +343,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
         meta: MemPlaceMeta<M::Provenance>,
         pointee: TyAndLayout<'tcx>,
     ) -> InterpResult<'tcx> {
-        let tail = self.ecx.tcx.struct_tail_erasing_lifetimes(pointee.ty, self.ecx.param_env);
+        let tail = self.ecx.tcx.struct_tail_for_codegen(pointee.ty, self.ecx.param_env);
         match tail.kind() {
             ty::Dynamic(data, _, ty::Dyn) => {
                 let vtable = meta.unwrap_meta().to_pointer(self.ecx)?;

compiler/rustc_const_eval/src/util/alignment.rs

@@ -22,7 +22,7 @@ where
     };
 
     let ty = place.ty(local_decls, tcx).ty;
-    let unsized_tail = || tcx.struct_tail_with_normalize(ty, |ty| ty, || {});
+    let unsized_tail = || tcx.struct_tail_for_codegen(ty, param_env);
     match tcx.layout_of(param_env.and(ty)) {
         Ok(layout)
             if layout.align.abi <= pack

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1276,7 +1276,7 @@ fn check_item_type(
             UnsizedHandling::Forbid => true,
             UnsizedHandling::Allow => false,
             UnsizedHandling::AllowIfForeignTail => {
-                let tail = tcx.struct_tail_erasing_lifetimes(item_ty, wfcx.param_env);
+                let tail = tcx.struct_tail_for_codegen(item_ty, wfcx.param_env);
                 !matches!(tail.kind(), ty::Foreign(_))
             }
         };

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -404,8 +404,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         code: traits::ObligationCauseCode<'tcx>,
     ) {
         if !ty.references_error() {
-            let tail =
+            let tail = self.tcx.struct_tail_with_normalize(
-                self.tcx.struct_tail_with_normalize(ty, |ty| self.normalize(span, ty), || {});
+                ty,
+                |ty| {
+                    if self.next_trait_solver() {
+                        self.try_structurally_resolve_type(span, ty)
+                    } else {
+                        self.normalize(span, ty)
+                    }
+                },
+                || {},
+            );
             // Sized types have static alignment, and so do slices.
             if tail.is_trivially_sized(self.tcx) || matches!(tail.kind(), ty::Slice(..)) {
                 // Nothing else is required here.

compiler/rustc_middle/src/ty/layout.rs

@@ -869,7 +869,7 @@ where
                             metadata
                         }
                     } else {
-                        match tcx.struct_tail_erasing_lifetimes(pointee, cx.param_env()).kind() {
+                        match tcx.struct_tail_for_codegen(pointee, cx.param_env()).kind() {
                             ty::Slice(_) | ty::Str => tcx.types.usize,
                             ty::Dynamic(data, _, ty::Dyn) => mk_dyn_vtable(data.principal()),
                             _ => bug!("TyAndLayout::field({:?}): not applicable", this),
@@ -1348,7 +1348,7 @@ impl<'tcx> TyCtxt<'tcx> {
             layout = layout.field(&cx, index);
             if !layout.is_sized() {
                 // If it is not sized, then the tail must still have at least a known static alignment.
-                let tail = self.struct_tail_erasing_lifetimes(layout.ty, param_env);
+                let tail = self.struct_tail_for_codegen(layout.ty, param_env);
                 if !matches!(tail.kind(), ty::Slice(..)) {
                     bug!(
                         "offset of not-statically-aligned field (type {:?}) cannot be computed statically",

compiler/rustc_middle/src/ty/util.rs

@@ -186,11 +186,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Should only be called if `ty` has no inference variables and does not
     /// need its lifetimes preserved (e.g. as part of codegen); otherwise
     /// normalization attempt may cause compiler bugs.
-    pub fn struct_tail_erasing_lifetimes(
+    pub fn struct_tail_for_codegen(self, ty: Ty<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Ty<'tcx> {
-        self,
-        ty: Ty<'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-    ) -> Ty<'tcx> {
         let tcx = self;
         tcx.struct_tail_with_normalize(ty, |ty| tcx.normalize_erasing_regions(param_env, ty), || {})
     }
@@ -203,7 +199,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// handle `<T as Trait>::Assoc` and `impl Trait`); pass the identity
     /// function to indicate no normalization should take place.
     ///
-    /// See also `struct_tail_erasing_lifetimes`, which is suitable for use
+    /// See also `struct_tail_for_codegen`, which is suitable for use
     /// during codegen.
     pub fn struct_tail_with_normalize(
         self,
@@ -272,13 +268,13 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Same as applying `struct_tail` on `source` and `target`, but only
     /// keeps going as long as the two types are instances of the same
     /// structure definitions.
-    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
+    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, dyn Trait)`,
     /// whereas struct_tail produces `T`, and `Trait`, respectively.
     ///
     /// Should only be called if the types have no inference variables and do
     /// not need their lifetimes preserved (e.g., as part of codegen); otherwise,
     /// normalization attempt may cause compiler bugs.
-    pub fn struct_lockstep_tails_erasing_lifetimes(
+    pub fn struct_lockstep_tails_for_codegen(
         self,
         source: Ty<'tcx>,
         target: Ty<'tcx>,
@@ -296,7 +292,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
     /// whereas struct_tail produces `T`, and `Trait`, respectively.
     ///
-    /// See also `struct_lockstep_tails_erasing_lifetimes`, which is suitable for use
+    /// See also `struct_lockstep_tails_for_codegen`, which is suitable for use
     /// during codegen.
     pub fn struct_lockstep_tails_with_normalize(
         self,

compiler/rustc_monomorphize/src/collector.rs

@@ -1019,7 +1019,7 @@ fn find_vtable_types_for_unsizing<'tcx>(
             if ty.is_sized(tcx.tcx, param_env) {
                 return false;
             }
-            let tail = tcx.struct_tail_erasing_lifetimes(ty, param_env);
+            let tail = tcx.struct_tail_for_codegen(ty, param_env);
             match tail.kind() {
                 ty::Foreign(..) => false,
                 ty::Str | ty::Slice(..) | ty::Dynamic(..) => true,
@@ -1029,7 +1029,7 @@ fn find_vtable_types_for_unsizing<'tcx>(
         if type_has_metadata(inner_source) {
             (inner_source, inner_target)
         } else {
-            tcx.struct_lockstep_tails_erasing_lifetimes(inner_source, inner_target, param_env)
+            tcx.struct_lockstep_tails_for_codegen(inner_source, inner_target, param_env)
         }
     };
 

compiler/rustc_ty_utils/src/abi.rs

@@ -549,7 +549,7 @@ fn fn_abi_sanity_check<'tcx>(
                 // With metadata. Must be unsized and not on the stack.
                 assert!(arg.layout.is_unsized() && !on_stack);
                 // Also, must not be `extern` type.
-                let tail = cx.tcx.struct_tail_with_normalize(arg.layout.ty, |ty| ty, || {});
+                let tail = cx.tcx.struct_tail_for_codegen(arg.layout.ty, cx.param_env());
                 if matches!(tail.kind(), ty::Foreign(..)) {
                     // These types do not have metadata, so having `meta_attrs` is bogus.
                     // Conceptually, unsized arguments must be copied around, which requires dynamically

compiler/rustc_ty_utils/src/layout.rs

@@ -244,7 +244,7 @@ fn layout_of_uncached<'tcx>(
 
                 metadata
             } else {
-                let unsized_part = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
+                let unsized_part = tcx.struct_tail_for_codegen(pointee, param_env);
 
                 match unsized_part.kind() {
                     ty::Foreign(..) => {

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -142,7 +142,7 @@ fn check_rvalue<'tcx>(
                 // We cannot allow this for now.
                 return Err((span, "unsizing casts are only allowed for references right now".into()));
             };
-            let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
+            let unsized_ty = tcx.struct_tail_for_codegen(pointee_ty, tcx.param_env(def_id));
             if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
                 check_operand(tcx, op, span, body, msrv)?;
                 // Casting/coercing things to slices is fine.

tests/ui/lint/unaligned_references.current.stderr

@@ -1,5 +1,5 @@
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:28:13
+  --> $DIR/unaligned_references.rs:32:13
    |
 LL |             &self.x;
    |             ^^^^^^^
@@ -9,7 +9,7 @@ LL |             &self.x;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:40:24
+  --> $DIR/unaligned_references.rs:44:24
    |
 LL |     println!("{:?}", &*foo.0);
    |                        ^^^^^
@@ -19,7 +19,7 @@ LL |     println!("{:?}", &*foo.0);
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:42:24
+  --> $DIR/unaligned_references.rs:46:24
    |
 LL |     println!("{:?}", &*foo.0);
    |                        ^^^^^
@@ -29,7 +29,7 @@ LL |     println!("{:?}", &*foo.0);
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:47:24
+  --> $DIR/unaligned_references.rs:51:24
    |
 LL |     println!("{:?}", &*foo.0);
    |                        ^^^^^
@@ -39,7 +39,7 @@ LL |     println!("{:?}", &*foo.0);
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:57:17
+  --> $DIR/unaligned_references.rs:81:17
    |
 LL |         let _ = &good.ptr;
    |                 ^^^^^^^^^
@@ -49,7 +49,7 @@ LL |         let _ = &good.ptr;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:58:17
+  --> $DIR/unaligned_references.rs:82:17
    |
 LL |         let _ = &good.data;
    |                 ^^^^^^^^^^
@@ -59,7 +59,7 @@ LL |         let _ = &good.data;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:60:17
+  --> $DIR/unaligned_references.rs:84:17
    |
 LL |         let _ = &good.data as *const _;
    |                 ^^^^^^^^^^
@@ -69,7 +69,7 @@ LL |         let _ = &good.data as *const _;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:61:27
+  --> $DIR/unaligned_references.rs:85:27
    |
 LL |         let _: *const _ = &good.data;
    |                           ^^^^^^^^^^
@@ -79,7 +79,7 @@ LL |         let _: *const _ = &good.data;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:63:17
+  --> $DIR/unaligned_references.rs:87:17
    |
 LL |         let _ = good.data.clone();
    |                 ^^^^^^^^^
@@ -89,7 +89,7 @@ LL |         let _ = good.data.clone();
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:65:17
+  --> $DIR/unaligned_references.rs:89:17
    |
 LL |         let _ = &good.data2[0];
    |                 ^^^^^^^^^^^^^^
@@ -99,7 +99,7 @@ LL |         let _ = &good.data2[0];
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:74:17
+  --> $DIR/unaligned_references.rs:98:17
    |
 LL |         let _ = &packed2.x;
    |                 ^^^^^^^^^^
@@ -109,7 +109,7 @@ LL |         let _ = &packed2.x;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:113:20
+  --> $DIR/unaligned_references.rs:137:20
    |
 LL |         let _ref = &m1.1.a;
    |                    ^^^^^^^
@@ -119,7 +119,7 @@ LL |         let _ref = &m1.1.a;
    = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
 
 error[E0793]: reference to packed field is unaligned
-  --> $DIR/unaligned_references.rs:116:20
+  --> $DIR/unaligned_references.rs:140:20
    |
 LL |         let _ref = &m2.1.a;
    |                    ^^^^^^^

tests/ui/lint/unaligned_references.next.stderr

@@ -0,0 +1,133 @@
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:32:13
+   |
+LL |             &self.x;
+   |             ^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:44:24
+   |
+LL |     println!("{:?}", &*foo.0);
+   |                        ^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:46:24
+   |
+LL |     println!("{:?}", &*foo.0);
+   |                        ^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:51:24
+   |
+LL |     println!("{:?}", &*foo.0);
+   |                        ^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:81:17
+   |
+LL |         let _ = &good.ptr;
+   |                 ^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:82:17
+   |
+LL |         let _ = &good.data;
+   |                 ^^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:84:17
+   |
+LL |         let _ = &good.data as *const _;
+   |                 ^^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:85:27
+   |
+LL |         let _: *const _ = &good.data;
+   |                           ^^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:87:17
+   |
+LL |         let _ = good.data.clone();
+   |                 ^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:89:17
+   |
+LL |         let _ = &good.data2[0];
+   |                 ^^^^^^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:98:17
+   |
+LL |         let _ = &packed2.x;
+   |                 ^^^^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:137:20
+   |
+LL |         let _ref = &m1.1.a;
+   |                    ^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error[E0793]: reference to packed field is unaligned
+  --> $DIR/unaligned_references.rs:140:20
+   |
+LL |         let _ref = &m2.1.a;
+   |                    ^^^^^^^
+   |
+   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+
+error: aborting due to 13 previous errors
+
+For more information about this error, try `rustc --explain E0793`.

tests/ui/lint/unaligned_references.rs

@@ -1,5 +1,9 @@
-use std::mem::ManuallyDrop;
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
 use std::fmt::Debug;
+use std::mem::ManuallyDrop;
 
 #[repr(packed)]
 pub struct Good {
@@ -50,6 +54,26 @@ fn packed_dyn() {
     println!("{:?}", &*foo.0); // no error!
 }
 
+// Test for #115396
+fn packed_slice_behind_alias() {
+    trait Mirror {
+        type Assoc: ?Sized;
+    }
+    impl<T: ?Sized> Mirror for T {
+        type Assoc = T;
+    }
+
+    struct W<T: ?Sized>(<T as Mirror>::Assoc);
+
+    #[repr(packed)]
+    struct Unaligned<T: ?Sized>(ManuallyDrop<W<T>>);
+
+    // Even if the actual alignment is 1, we cannot know that when looking at `dyn Debug.`
+    let ref local: Unaligned<[_; 3]> = Unaligned(ManuallyDrop::new(W([3, 5, 8u8])));
+    let foo: &Unaligned<[u8]> = local;
+    let x = &foo.0; // Fine, since the tail of `foo` is `[_]`
+}
+
 fn main() {
     unsafe {
         let good = Good { data: 0, ptr: &0, data2: [0, 0], aligned: [0; 32] };

tests/ui/offset-of/offset-of-slice-normalized.rs

@@ -0,0 +1,37 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ run-pass
+
+#![feature(offset_of_slice)]
+
+use std::mem::offset_of;
+
+trait Mirror {
+    type Assoc: ?Sized;
+}
+impl<T: ?Sized> Mirror for T {
+    type Assoc = T;
+}
+
+#[repr(C)]
+struct S {
+    a: u8,
+    b: (u8, u8),
+    c: <[i32] as Mirror>::Assoc,
+}
+
+#[repr(C)]
+struct T {
+    x: i8,
+    y: S,
+}
+
+type Tup = (i16, <[i32] as Mirror>::Assoc);
+
+fn main() {
+    assert_eq!(offset_of!(S, c), 4);
+    assert_eq!(offset_of!(T, y), 4);
+    assert_eq!(offset_of!(T, y.c), 8);
+    assert_eq!(offset_of!(Tup, 1), 4);
+}