do not impl Field for FRTs of repr(packed) types

Author: BennoLossin

compiler/rustc_middle/src/ty/sty.rs

@@ -1157,6 +1157,10 @@ impl<'tcx> rustc_type_ir::inherent::Ty<TyCtxt<'tcx>> for Ty<'tcx> {
     fn has_unsafe_fields(self) -> bool {
         Ty::has_unsafe_fields(self)
     }
+
+    fn is_packed(self) -> bool {
+        Ty::is_packed(self)
+    }
 }
 
 /// Type utilities
@@ -1211,6 +1215,11 @@ impl<'tcx> Ty<'tcx> {
         matches!(self.kind(), Adt(..))
     }
 
+    #[inline]
+    pub fn is_packed(self) -> bool {
+        matches!(self.kind(), Adt(def, _) if def.repr().packed())
+    }
+
     #[inline]
     pub fn is_ref(self) -> bool {
         matches!(self.kind(), Ref(..))

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -850,7 +850,9 @@ where
         if goal.predicate.polarity != ty::PredicatePolarity::Positive {
             return Err(NoSolution);
         }
-        if let ty::FRT(..) = goal.predicate.self_ty().kind() {
+        if let ty::FRT(ty, _) = goal.predicate.self_ty().kind()
+            && !ty.is_packed()
+        {
             ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc)
                 .enter(|ecx| ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes))
         } else {

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -1457,7 +1457,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PolyTraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        if let ty::FRT(..) = obligation.predicate.self_ty().skip_binder().kind() {
+        if let ty::FRT(ty, _) = obligation.predicate.self_ty().skip_binder().kind()
+            && !ty.is_packed()
+        {
             candidates.vec.push(BuiltinCandidate);
         }
     }

compiler/rustc_type_ir/src/inherent.rs

@@ -154,6 +154,9 @@ pub trait Ty<I: Interner<Ty = Self>>:
     /// Checks whether this type is an ADT that has unsafe fields.
     fn has_unsafe_fields(self) -> bool;
 
+    /// Checks whether this type is an ADT that is `repr(packed)`.
+    fn is_packed(self) -> bool;
+
     fn fn_sig(self, interner: I) -> ty::Binder<I, ty::FnSig<I>> {
         self.kind().fn_sig(interner)
     }

tests/ui/field_representing_types/not-field-if-packed.next.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `field_of!(MyStruct, 0): Field` is not satisfied
+  --> $DIR/not-field-if-packed.rs:14:20
+   |
+LL |     assert_field::<field_of!(MyStruct, 0)>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ the trait `Field` is not implemented for `field_of!(MyStruct, 0)`
+   |
+note: required by a bound in `assert_field`
+  --> $DIR/not-field-if-packed.rs:11:20
+   |
+LL | fn assert_field<F: Field>() {}
+   |                    ^^^^^ required by this bound in `assert_field`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/field_representing_types/not-field-if-packed.old.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `field_of!(MyStruct, 0): Field` is not satisfied
+  --> $DIR/not-field-if-packed.rs:14:20
+   |
+LL |     assert_field::<field_of!(MyStruct, 0)>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ the trait `Field` is not implemented for `field_of!(MyStruct, 0)`
+   |
+note: required by a bound in `assert_field`
+  --> $DIR/not-field-if-packed.rs:11:20
+   |
+LL | fn assert_field<F: Field>() {}
+   |                    ^^^^^ required by this bound in `assert_field`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/field_representing_types/not-field-if-packed.rs

@@ -0,0 +1,16 @@
+//@ revisions: old next
+//@ [next] compile-flags: -Znext-solver
+#![allow(incomplete_features)]
+#![feature(field_projections)]
+
+use std::field::{Field, field_of};
+
+#[repr(packed)]
+pub struct MyStruct(usize);
+
+fn assert_field<F: Field>() {}
+
+fn main() {
+    assert_field::<field_of!(MyStruct, 0)>();
+    //~^ ERROR: the trait bound `field_of!(MyStruct, 0): Field` is not satisfied [E0277]
+}