Remove `BackendRepr::Uninhabited`, replaced with an `uninhabited: bool` field in `LayoutData`.

Also update comments that refered to BackendRepr::Uninhabited.

26 files changed, 93 insertions, 113 deletions

diff --git a/compiler/rustc_abi/src/callconv.rs b/compiler/rustc_abi/src/callconv.rs
index 9fb70b80c9e..4529ab8058e 100644
--- a/compiler/rustc_abi/src/callconv.rs
+++ b/compiler/rustc_abi/src/callconv.rs
@@ -65,8 +65,6 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
         Ty: TyAbiInterface<'a, C> + Copy,
     {
         match self.backend_repr {
-            BackendRepr::Uninhabited => Err(Heterogeneous),
-
             // The primitive for this algorithm.
             BackendRepr::Scalar(scalar) => {
                 let kind = match scalar.primitive() {
diff --git a/compiler/rustc_abi/src/layout.rs b/compiler/rustc_abi/src/layout.rs
index 45cd0b517f6..53243266f99 100644
--- a/compiler/rustc_abi/src/layout.rs
+++ b/compiler/rustc_abi/src/layout.rs
@@ -130,6 +130,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             },
             backend_repr: BackendRepr::ScalarPair(a, b),
             largest_niche,
+            uninhabited: false,
             align,
             size,
             max_repr_align: None,
@@ -221,8 +222,9 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         LayoutData {
             variants: Variants::Empty,
             fields: FieldsShape::Primitive,
-            backend_repr: BackendRepr::Uninhabited,
+            backend_repr: BackendRepr::Memory { sized: true },
             largest_niche: None,
+            uninhabited: true,
             align: dl.i8_align,
             size: Size::ZERO,
             max_repr_align: None,
@@ -400,6 +402,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             fields: FieldsShape::Union(union_field_count),
             backend_repr: abi,
             largest_niche: None,
+            uninhabited: false,
             align,
             size: size.align_to(align.abi),
             max_repr_align,
@@ -447,7 +450,6 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 Scalar::Union { .. } => {}
             };
             match &mut st.backend_repr {
-                BackendRepr::Uninhabited => {}
                 BackendRepr::Scalar(scalar) => hide_niches(scalar),
                 BackendRepr::ScalarPair(a, b) => {
                     hide_niches(a);
@@ -639,9 +641,8 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             let same_size = size == variant_layouts[largest_variant_index].size;
             let same_align = align == variant_layouts[largest_variant_index].align;
 
-            let abi = if variant_layouts.iter().all(|v| v.is_uninhabited()) {
-                BackendRepr::Uninhabited
-            } else if same_size && same_align && others_zst {
+            let uninhabited = variant_layouts.iter().all(|v| v.is_uninhabited());
+            let abi = if same_size && same_align && others_zst {
                 match variant_layouts[largest_variant_index].backend_repr {
                     // When the total alignment and size match, we can use the
                     // same ABI as the scalar variant with the reserved niche.
@@ -683,6 +684,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 },
                 backend_repr: abi,
                 largest_niche,
+                uninhabited,
                 size,
                 align,
                 max_repr_align,
@@ -853,9 +855,8 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         };
         let mut abi = BackendRepr::Memory { sized: true };
 
-        if layout_variants.iter().all(|v| v.is_uninhabited()) {
-            abi = BackendRepr::Uninhabited;
-        } else if tag.size(dl) == size {
+        let uninhabited = layout_variants.iter().all(|v| v.is_uninhabited());
+        if tag.size(dl) == size {
             // Make sure we only use scalar layout when the enum is entirely its
             // own tag (i.e. it has no padding nor any non-ZST variant fields).
             abi = BackendRepr::Scalar(tag);
@@ -995,6 +996,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 memory_index: [0].into(),
             },
             largest_niche,
+            uninhabited,
             backend_repr: abi,
             align,
             size,
@@ -1355,9 +1357,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 _ => {}
             }
         }
-        if fields.iter().any(|f| f.is_uninhabited()) {
-            abi = BackendRepr::Uninhabited;
-        }
+        let uninhabited = fields.iter().any(|f| f.is_uninhabited());
 
         let unadjusted_abi_align = if repr.transparent() {
             match layout_of_single_non_zst_field {
@@ -1378,6 +1378,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             fields: FieldsShape::Arbitrary { offsets, memory_index },
             backend_repr: abi,
             largest_niche,
+            uninhabited,
             align,
             size,
             max_repr_align,
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index dbb4bed5cdd..14a8a579200 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -1404,7 +1404,6 @@ impl AddressSpace {
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
 #[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
 pub enum BackendRepr {
-    Uninhabited,
     Scalar(Scalar),
     ScalarPair(Scalar, Scalar),
     Vector {
@@ -1423,10 +1422,9 @@ impl BackendRepr {
     #[inline]
     pub fn is_unsized(&self) -> bool {
         match *self {
-            BackendRepr::Uninhabited
-            | BackendRepr::Scalar(_)
-            | BackendRepr::ScalarPair(..)
-            | BackendRepr::Vector { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) | BackendRepr::Vector { .. } => {
+                false
+            }
             BackendRepr::Memory { sized } => !sized,
         }
     }
@@ -1445,12 +1443,6 @@ impl BackendRepr {
         }
     }
 
-    /// Returns `true` if this is an uninhabited type
-    #[inline]
-    pub fn is_uninhabited(&self) -> bool {
-        matches!(*self, BackendRepr::Uninhabited)
-    }
-
     /// Returns `true` if this is a scalar type
     #[inline]
     pub fn is_scalar(&self) -> bool {
@@ -1471,7 +1463,7 @@ impl BackendRepr {
             BackendRepr::Vector { element, count } => {
                 cx.data_layout().vector_align(element.size(cx) * count)
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => return None,
+            BackendRepr::Memory { .. } => return None,
         })
     }
 
@@ -1492,7 +1484,7 @@ impl BackendRepr {
                 // to make the size a multiple of align (e.g. for vectors of size 3).
                 (element.size(cx) * count).align_to(self.inherent_align(cx)?.abi)
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => return None,
+            BackendRepr::Memory { .. } => return None,
         })
     }
 
@@ -1506,9 +1498,7 @@ impl BackendRepr {
             BackendRepr::Vector { element, count } => {
                 BackendRepr::Vector { element: element.to_union(), count }
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {
-                BackendRepr::Memory { sized: true }
-            }
+            BackendRepr::Memory { .. } => BackendRepr::Memory { sized: true },
         }
     }
 
@@ -1704,6 +1694,11 @@ pub struct LayoutData<FieldIdx: Idx, VariantIdx: Idx> {
     /// The leaf scalar with the largest number of invalid values
     /// (i.e. outside of its `valid_range`), if it exists.
     pub largest_niche: Option<Niche>,
+    /// Is this type known to be uninhabted?
+    ///
+    /// This is separate from BackendRepr, because an uninhabited return type may require special
+    /// consideration based on its size or other attributes.
+    pub uninhabited: bool,
 
     pub align: AbiAndPrefAlign,
     pub size: Size,
@@ -1735,14 +1730,14 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
     /// Returns `true` if this is an aggregate type (including a ScalarPair!)
     pub fn is_aggregate(&self) -> bool {
         match self.backend_repr {
-            BackendRepr::Uninhabited | BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => false,
             BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => true,
         }
     }
 
     /// Returns `true` if this is an uninhabited type
     pub fn is_uninhabited(&self) -> bool {
-        self.backend_repr.is_uninhabited()
+        self.uninhabited
     }
 
     pub fn scalar<C: HasDataLayout>(cx: &C, scalar: Scalar) -> Self {
@@ -1778,6 +1773,7 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
             fields: FieldsShape::Primitive,
             backend_repr: BackendRepr::Scalar(scalar),
             largest_niche,
+            uninhabited: false,
             size,
             align,
             max_repr_align: None,
@@ -1802,6 +1798,7 @@ where
             backend_repr,
             fields,
             largest_niche,
+            uninhabited,
             variants,
             max_repr_align,
             unadjusted_abi_align,
@@ -1813,6 +1810,7 @@ where
             .field("abi", backend_repr)
             .field("fields", fields)
             .field("largest_niche", largest_niche)
+            .field("uninhabited", uninhabited)
             .field("variants", variants)
             .field("max_repr_align", max_repr_align)
             .field("unadjusted_abi_align", unadjusted_abi_align)
@@ -1877,7 +1875,6 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
             BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) | BackendRepr::Vector { .. } => {
                 false
             }
-            BackendRepr::Uninhabited => self.size.bytes() == 0,
             BackendRepr::Memory { sized } => sized && self.size.bytes() == 0,
         }
     }
diff --git a/compiler/rustc_codegen_cranelift/src/value_and_place.rs b/compiler/rustc_codegen_cranelift/src/value_and_place.rs
index a9b8e1bd393..1b3f86c8405 100644
--- a/compiler/rustc_codegen_cranelift/src/value_and_place.rs
+++ b/compiler/rustc_codegen_cranelift/src/value_and_place.rs
@@ -638,9 +638,7 @@ impl<'tcx> CPlace<'tcx> {
             }
             CPlaceInner::Addr(_, Some(_)) => bug!("Can't write value to unsized place {:?}", self),
             CPlaceInner::Addr(to_ptr, None) => {
-                if dst_layout.size == Size::ZERO
-                    || dst_layout.backend_repr == BackendRepr::Uninhabited
-                {
+                if dst_layout.size == Size::ZERO {
                     return;
                 }
 
diff --git a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
index 5322b731d8b..433868e238a 100644
--- a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
+++ b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
@@ -315,7 +315,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tc
                 let layout = self.layout_of(tp_ty).layout;
                 let _use_integer_compare = match layout.backend_repr() {
                     Scalar(_) | ScalarPair(_, _) => true,
-                    Uninhabited | Vector { .. } => false,
+                    Vector { .. } => false,
                     Memory { .. } => {
                         // For rusty ABIs, small aggregates are actually passed
                         // as `RegKind::Integer` (see `FnAbi::adjust_for_abi`),
diff --git a/compiler/rustc_codegen_gcc/src/type_of.rs b/compiler/rustc_codegen_gcc/src/type_of.rs
index 8b8b54753e7..bac4fc51300 100644
--- a/compiler/rustc_codegen_gcc/src/type_of.rs
+++ b/compiler/rustc_codegen_gcc/src/type_of.rs
@@ -84,7 +84,7 @@ fn uncached_gcc_type<'gcc, 'tcx>(
                 false,
             );
         }
-        BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {}
+        BackendRepr::Memory { .. } => {}
     }
 
     let name = match *layout.ty.kind() {
@@ -179,19 +179,16 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
     fn is_gcc_immediate(&self) -> bool {
         match self.backend_repr {
             BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => true,
-            BackendRepr::ScalarPair(..) | BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {
-                false
-            }
+            BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => false,
         }
     }
 
     fn is_gcc_scalar_pair(&self) -> bool {
         match self.backend_repr {
             BackendRepr::ScalarPair(..) => true,
-            BackendRepr::Uninhabited
-            | BackendRepr::Scalar(_)
-            | BackendRepr::Vector { .. }
-            | BackendRepr::Memory { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => {
+                false
+            }
         }
     }
 
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 7e1a9d361e6..f68365f6c69 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -476,7 +476,7 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 let layout = self.layout_of(tp_ty).layout;
                 let use_integer_compare = match layout.backend_repr() {
                     Scalar(_) | ScalarPair(_, _) => true,
-                    Uninhabited | Vector { .. } => false,
+                    Vector { .. } => false,
                     Memory { .. } => {
                         // For rusty ABIs, small aggregates are actually passed
                         // as `RegKind::Integer` (see `FnAbi::adjust_for_abi`),
diff --git a/compiler/rustc_codegen_llvm/src/type_of.rs b/compiler/rustc_codegen_llvm/src/type_of.rs
index b0b6da869da..ba01fbff385 100644
--- a/compiler/rustc_codegen_llvm/src/type_of.rs
+++ b/compiler/rustc_codegen_llvm/src/type_of.rs
@@ -23,7 +23,7 @@ fn uncached_llvm_type<'a, 'tcx>(
             let element = layout.scalar_llvm_type_at(cx, element);
             return cx.type_vector(element, count);
         }
-        BackendRepr::Uninhabited | BackendRepr::Memory { .. } | BackendRepr::ScalarPair(..) => {}
+        BackendRepr::Memory { .. } | BackendRepr::ScalarPair(..) => {}
     }
 
     let name = match layout.ty.kind() {
@@ -172,19 +172,16 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
     fn is_llvm_immediate(&self) -> bool {
         match self.backend_repr {
             BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => true,
-            BackendRepr::ScalarPair(..) | BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {
-                false
-            }
+            BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => false,
         }
     }
 
     fn is_llvm_scalar_pair(&self) -> bool {
         match self.backend_repr {
             BackendRepr::ScalarPair(..) => true,
-            BackendRepr::Uninhabited
-            | BackendRepr::Scalar(_)
-            | BackendRepr::Vector { .. }
-            | BackendRepr::Memory { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => {
+                false
+            }
         }
     }
 
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
index 958a52a2cb1..faf1300e473 100644
--- a/compiler/rustc_codegen_ssa/src/mir/operand.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -422,9 +422,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                          See <https://github.com/rust-lang/rust/issues/137108>",
                     );
                 }
-                BackendRepr::Uninhabited
-                | BackendRepr::ScalarPair(_, _)
-                | BackendRepr::Memory { sized: false } => bug!(),
+                BackendRepr::ScalarPair(_, _) | BackendRepr::Memory { sized: false } => bug!(),
             })
         };
 
diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index 5d905cff1f2..36da9037e43 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -385,7 +385,7 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
             (Immediate::Uninit, _) => Immediate::Uninit,
             // If the field is uninhabited, we can forget the data (can happen in ConstProp).
             // `enum S { A(!), B, C }` is an example of an enum with Scalar layout that
-            // has an `Uninhabited` variant, which means this case is possible.
+            // has an uninhabited variant, which means this case is possible.
             _ if layout.is_uninhabited() => Immediate::Uninit,
             // the field contains no information, can be left uninit
             // (Scalar/ScalarPair can contain even aligned ZST, not just 1-ZST)
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index 0ac34f4633b..3667cc84d70 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -1274,11 +1274,11 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
         // FIXME: We could avoid some redundant checks here. For newtypes wrapping
         // scalars, we do the same check on every "level" (e.g., first we check
         // MyNewtype and then the scalar in there).
+        if val.layout.is_uninhabited() {
+            let ty = val.layout.ty;
+            throw_validation_failure!(self.path, UninhabitedVal { ty });
+        }
         match val.layout.backend_repr {
-            BackendRepr::Uninhabited => {
-                let ty = val.layout.ty;
-                throw_validation_failure!(self.path, UninhabitedVal { ty });
-            }
             BackendRepr::Scalar(scalar_layout) => {
                 if !scalar_layout.is_uninit_valid() {
                     // There is something to check here.
diff --git a/compiler/rustc_const_eval/src/util/check_validity_requirement.rs b/compiler/rustc_const_eval/src/util/check_validity_requirement.rs
index 79baf91c3ce..6426bca2332 100644
--- a/compiler/rustc_const_eval/src/util/check_validity_requirement.rs
+++ b/compiler/rustc_const_eval/src/util/check_validity_requirement.rs
@@ -111,13 +111,15 @@ fn check_validity_requirement_lax<'tcx>(
     };
 
     // Check the ABI.
-    let valid = match this.backend_repr {
-        BackendRepr::Uninhabited => false, // definitely UB
-        BackendRepr::Scalar(s) => scalar_allows_raw_init(s),
-        BackendRepr::ScalarPair(s1, s2) => scalar_allows_raw_init(s1) && scalar_allows_raw_init(s2),
-        BackendRepr::Vector { element: s, count } => count == 0 || scalar_allows_raw_init(s),
-        BackendRepr::Memory { .. } => true, // Fields are checked below.
-    };
+    let valid = !this.is_uninhabited() // definitely UB if uninhabited
+        && match this.backend_repr {
+            BackendRepr::Scalar(s) => scalar_allows_raw_init(s),
+            BackendRepr::ScalarPair(s1, s2) => {
+                scalar_allows_raw_init(s1) && scalar_allows_raw_init(s2)
+            }
+            BackendRepr::Vector { element: s, count } => count == 0 || scalar_allows_raw_init(s),
+            BackendRepr::Memory { .. } => true, // Fields are checked below.
+        };
     if !valid {
         // This is definitely not okay.
         return Ok(false);
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 19fa8323574..eb14ed20fba 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -794,8 +794,9 @@ where
                         Some(fields) => FieldsShape::Union(fields),
                         None => FieldsShape::Arbitrary { offsets: IndexVec::new(), memory_index: IndexVec::new() },
                     },
-                    backend_repr: BackendRepr::Uninhabited,
+                    backend_repr: BackendRepr::Memory { sized: true },
                     largest_niche: None,
+                    uninhabited: true,
                     align: tcx.data_layout.i8_align,
                     size: Size::ZERO,
                     max_repr_align: None,
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 77a3854ebde..2f8a3050199 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -1558,8 +1558,11 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             return true;
         };
 
+        if layout.uninhabited {
+            return true;
+        }
+
         match layout.backend_repr {
-            BackendRepr::Uninhabited => true,
             BackendRepr::Scalar(a) => !a.is_always_valid(&self.ecx),
             BackendRepr::ScalarPair(a, b) => {
                 !a.is_always_valid(&self.ecx) || !b.is_always_valid(&self.ecx)
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/abi.rs b/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
index fb2e838cdc9..a627e0e69b6 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
@@ -202,7 +202,6 @@ impl<'tcx> Stable<'tcx> for rustc_abi::BackendRepr {
 
     fn stable(&self, tables: &mut Tables<'_>) -> Self::T {
         match *self {
-            rustc_abi::BackendRepr::Uninhabited => ValueAbi::Uninhabited,
             rustc_abi::BackendRepr::Scalar(scalar) => ValueAbi::Scalar(scalar.stable(tables)),
             rustc_abi::BackendRepr::ScalarPair(first, second) => {
                 ValueAbi::ScalarPair(first.stable(tables), second.stable(tables))
diff --git a/compiler/rustc_target/src/callconv/loongarch.rs b/compiler/rustc_target/src/callconv/loongarch.rs
index 47566bde6b4..3fa67c624a7 100644
--- a/compiler/rustc_target/src/callconv/loongarch.rs
+++ b/compiler/rustc_target/src/callconv/loongarch.rs
@@ -80,7 +80,7 @@ where
                 }
             }
         },
-        BackendRepr::Vector { .. } | BackendRepr::Uninhabited => return Err(CannotUseFpConv),
+        BackendRepr::Vector { .. } => return Err(CannotUseFpConv),
         BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => match arg_layout.fields {
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")
diff --git a/compiler/rustc_target/src/callconv/mod.rs b/compiler/rustc_target/src/callconv/mod.rs
index c2a176facdf..e5f986269dd 100644
--- a/compiler/rustc_target/src/callconv/mod.rs
+++ b/compiler/rustc_target/src/callconv/mod.rs
@@ -38,7 +38,7 @@ mod xtensa;
 pub enum PassMode {
     /// Ignore the argument.
     ///
-    /// The argument is either uninhabited or a ZST.
+    /// The argument is a ZST.
     Ignore,
     /// Pass the argument directly.
     ///
@@ -350,7 +350,6 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
         scalar_attrs: impl Fn(&TyAndLayout<'a, Ty>, Scalar, Size) -> ArgAttributes,
     ) -> Self {
         let mode = match layout.backend_repr {
-            BackendRepr::Uninhabited => PassMode::Ignore,
             BackendRepr::Scalar(scalar) => {
                 PassMode::Direct(scalar_attrs(&layout, scalar, Size::ZERO))
             }
diff --git a/compiler/rustc_target/src/callconv/riscv.rs b/compiler/rustc_target/src/callconv/riscv.rs
index 265ae20f991..785175229b0 100644
--- a/compiler/rustc_target/src/callconv/riscv.rs
+++ b/compiler/rustc_target/src/callconv/riscv.rs
@@ -86,7 +86,7 @@ where
                 }
             }
         },
-        BackendRepr::Vector { .. } | BackendRepr::Uninhabited => return Err(CannotUseFpConv),
+        BackendRepr::Vector { .. } => return Err(CannotUseFpConv),
         BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => match arg_layout.fields {
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")
diff --git a/compiler/rustc_target/src/callconv/x86.rs b/compiler/rustc_target/src/callconv/x86.rs
index 5f4f4cd57f6..7e5aab0201b 100644
--- a/compiler/rustc_target/src/callconv/x86.rs
+++ b/compiler/rustc_target/src/callconv/x86.rs
@@ -108,9 +108,7 @@ where
                 Ty: TyAbiInterface<'a, C> + Copy,
             {
                 match layout.backend_repr {
-                    BackendRepr::Uninhabited
-                    | BackendRepr::Scalar(_)
-                    | BackendRepr::ScalarPair(..) => false,
+                    BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) => false,
                     BackendRepr::Vector { .. } => true,
                     BackendRepr::Memory { .. } => {
                         for i in 0..layout.fields.count() {
diff --git a/compiler/rustc_target/src/callconv/x86_64.rs b/compiler/rustc_target/src/callconv/x86_64.rs
index b15d82c26da..ab306e20239 100644
--- a/compiler/rustc_target/src/callconv/x86_64.rs
+++ b/compiler/rustc_target/src/callconv/x86_64.rs
@@ -51,8 +51,6 @@ where
         }
 
         let mut c = match layout.backend_repr {
-            BackendRepr::Uninhabited => return Ok(()),
-
             BackendRepr::Scalar(scalar) => match scalar.primitive() {
                 Primitive::Int(..) | Primitive::Pointer(_) => Class::Int,
                 Primitive::Float(_) => Class::Sse,
diff --git a/compiler/rustc_target/src/callconv/x86_win64.rs b/compiler/rustc_target/src/callconv/x86_win64.rs
index 2ef5127de04..4d99a9f9ba0 100644
--- a/compiler/rustc_target/src/callconv/x86_win64.rs
+++ b/compiler/rustc_target/src/callconv/x86_win64.rs
@@ -8,7 +8,7 @@ use crate::spec::{HasTargetSpec, RustcAbi};
 pub(crate) fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>) {
     let fixup = |a: &mut ArgAbi<'_, Ty>, is_ret: bool| {
         match a.layout.backend_repr {
-            BackendRepr::Uninhabited | BackendRepr::Memory { sized: false } => {}
+            BackendRepr::Memory { sized: false } => {}
             BackendRepr::ScalarPair(..) | BackendRepr::Memory { sized: true } => {
                 match a.layout.size.bits() {
                     8 => a.cast_to(Reg::i8()),
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index c5d2d0afbfa..2d20ef3d690 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -472,9 +472,7 @@ fn fn_abi_sanity_check<'tcx>(
                 // `layout.backend_repr` and ignore everything else. We should just reject
                 //`Aggregate` entirely here, but some targets need to be fixed first.
                 match arg.layout.backend_repr {
-                    BackendRepr::Uninhabited
-                    | BackendRepr::Scalar(_)
-                    | BackendRepr::Vector { .. } => {}
+                    BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => {}
                     BackendRepr::ScalarPair(..) => {
                         panic!("`PassMode::Direct` used for ScalarPair type {}", arg.layout.ty)
                     }
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index 556512e0236..b6a14d147ca 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -348,19 +348,17 @@ fn layout_of_uncached<'tcx>(
                 .checked_mul(count, dl)
                 .ok_or_else(|| error(cx, LayoutError::SizeOverflow(ty)))?;
 
-            let abi = if count != 0 && ty.is_privately_uninhabited(tcx, cx.typing_env) {
-                BackendRepr::Uninhabited
-            } else {
-                BackendRepr::Memory { sized: true }
-            };
+            let abi = BackendRepr::Memory { sized: true };
 
             let largest_niche = if count != 0 { element.largest_niche } else { None };
+            let uninhabited = if count != 0 { element.uninhabited } else { false };
 
             tcx.mk_layout(LayoutData {
                 variants: Variants::Single { index: FIRST_VARIANT },
                 fields: FieldsShape::Array { stride: element.size, count },
                 backend_repr: abi,
                 largest_niche,
+                uninhabited,
                 align: element.align,
                 size,
                 max_repr_align: None,
@@ -375,6 +373,7 @@ fn layout_of_uncached<'tcx>(
                 fields: FieldsShape::Array { stride: element.size, count: 0 },
                 backend_repr: BackendRepr::Memory { sized: false },
                 largest_niche: None,
+                uninhabited: false,
                 align: element.align,
                 size: Size::ZERO,
                 max_repr_align: None,
@@ -390,6 +389,7 @@ fn layout_of_uncached<'tcx>(
             fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
             backend_repr: BackendRepr::Memory { sized: false },
             largest_niche: None,
+            uninhabited: false,
             align: dl.i8_align,
             size: Size::ZERO,
             max_repr_align: None,
@@ -555,6 +555,7 @@ fn layout_of_uncached<'tcx>(
                 fields,
                 backend_repr: abi,
                 largest_niche: e_ly.largest_niche,
+                uninhabited: false,
                 size,
                 align,
                 max_repr_align: None,
@@ -1014,13 +1015,8 @@ fn coroutine_layout<'tcx>(
 
     size = size.align_to(align.abi);
 
-    let abi = if prefix.backend_repr.is_uninhabited()
-        || variants.iter().all(|v| v.backend_repr.is_uninhabited())
-    {
-        BackendRepr::Uninhabited
-    } else {
-        BackendRepr::Memory { sized: true }
-    };
+    let uninhabited = prefix.uninhabited || variants.iter().all(|v| v.is_uninhabited());
+    let abi = BackendRepr::Memory { sized: true };
 
     // this is similar to how ReprOptions populates its field_shuffle_seed
     let def_hash = tcx.def_path_hash(def_id).0.to_smaller_hash();
@@ -1041,6 +1037,7 @@ fn coroutine_layout<'tcx>(
         // See <https://github.com/rust-lang/rust/issues/63818>, <https://github.com/rust-lang/miri/issues/3780>.
         // FIXME: Remove when <https://github.com/rust-lang/rust/issues/125735> is implemented and aliased coroutine fields are wrapped in `UnsafePinned`.
         largest_niche: None,
+        uninhabited,
         size,
         align,
         max_repr_align: None,
diff --git a/compiler/rustc_ty_utils/src/layout/invariant.rs b/compiler/rustc_ty_utils/src/layout/invariant.rs
index 8d5403ed324..5ea6716c5ca 100644
--- a/compiler/rustc_ty_utils/src/layout/invariant.rs
+++ b/compiler/rustc_ty_utils/src/layout/invariant.rs
@@ -10,7 +10,11 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
 
     // Type-level uninhabitedness should always imply ABI uninhabitedness.
     if layout.ty.is_privately_uninhabited(tcx, cx.typing_env) {
-        assert!(layout.is_uninhabited());
+        assert!(
+            layout.is_uninhabited(),
+            "{:?} is type-level uninhabited but not ABI-uninhabited?",
+            layout.ty
+        );
     }
 
     if layout.size.bytes() % layout.align.abi.bytes() != 0 {
@@ -71,7 +75,7 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
         let Some((align, size)) = align.zip(size) else {
             assert_matches!(
                 layout.layout.backend_repr(),
-                BackendRepr::Uninhabited | BackendRepr::Memory { .. },
+                BackendRepr::Memory { .. },
                 "ABI unexpectedly missing alignment and/or size in {layout:#?}"
             );
             return;
@@ -235,7 +239,7 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                 assert!(align >= element.align(cx).abi); // just sanity-checking `vector_align`.
                 // FIXME: Do some kind of check of the inner type, like for Scalar and ScalarPair.
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {} // Nothing to check.
+            BackendRepr::Memory { .. } => {} // Nothing to check.
         }
     }
 
@@ -291,8 +295,8 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                     || variant.is_uninhabited()
                 {
                     // These are never actually accessed anyway, so we can skip the coherence check
-                    // for them. They also fail that check, since they have
-                    // `Aggregate`/`Uninhabited` ABI even when the main type is
+                    // for them. They also fail that check, since they may have
+                    // a different ABI even when the main type is
                     // `Scalar`/`ScalarPair`. (Note that sometimes, variants with fields have size
                     // 0, and sometimes, variants without fields have non-0 size.)
                     continue;
@@ -306,7 +310,6 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                     (BackendRepr::ScalarPair(a1, b1), BackendRepr::ScalarPair(a2, b2)) => {
                         scalar_coherent(a1, a2) && scalar_coherent(b1, b2)
                     }
-                    (BackendRepr::Uninhabited, _) => true,
                     (BackendRepr::Memory { .. }, _) => true,
                     _ => false,
                 };
diff --git a/compiler/stable_mir/src/abi.rs b/compiler/stable_mir/src/abi.rs
index 861b6692b53..091f3e1a95e 100644
--- a/compiler/stable_mir/src/abi.rs
+++ b/compiler/stable_mir/src/abi.rs
@@ -227,7 +227,6 @@ pub enum TagEncoding {
 /// in terms of categories of C types there are ABI rules for.
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Serialize)]
 pub enum ValueAbi {
-    Uninhabited,
     Scalar(Scalar),
     ScalarPair(Scalar, Scalar),
     Vector {
@@ -244,10 +243,7 @@ impl ValueAbi {
     /// Returns `true` if the layout corresponds to an unsized type.
     pub fn is_unsized(&self) -> bool {
         match *self {
-            ValueAbi::Uninhabited
-            | ValueAbi::Scalar(_)
-            | ValueAbi::ScalarPair(..)
-            | ValueAbi::Vector { .. } => false,
+            ValueAbi::Scalar(_) | ValueAbi::ScalarPair(..) | ValueAbi::Vector { .. } => false,
             ValueAbi::Aggregate { sized } => !sized,
         }
     }
diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs b/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs
index b6f7c44c2ae..f5a7b658123 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs
@@ -194,6 +194,7 @@ fn layout_of_simd_ty(
         fields,
         backend_repr: BackendRepr::Vector { element: e_abi, count: e_len },
         largest_niche: e_ly.largest_niche,
+        uninhabited: false,
         size,
         align,
         max_repr_align: None,
@@ -297,20 +298,17 @@ pub fn layout_of_ty_query(
                 .checked_mul(count, dl)
                 .ok_or(LayoutError::BadCalc(LayoutCalculatorError::SizeOverflow))?;
 
-            let backend_repr =
-                if count != 0 && matches!(element.backend_repr, BackendRepr::Uninhabited) {
-                    BackendRepr::Uninhabited
-                } else {
-                    BackendRepr::Memory { sized: true }
-                };
+            let backend_repr = BackendRepr::Memory { sized: true };
 
             let largest_niche = if count != 0 { element.largest_niche } else { None };
+            let uninhabited = if count != 0 { element.uninhabited } else { false };
 
             Layout {
                 variants: Variants::Single { index: struct_variant_idx() },
                 fields: FieldsShape::Array { stride: element.size, count },
                 backend_repr,
                 largest_niche,
+                uninhabited,
                 align: element.align,
                 size,
                 max_repr_align: None,
@@ -325,6 +323,7 @@ pub fn layout_of_ty_query(
                 fields: FieldsShape::Array { stride: element.size, count: 0 },
                 backend_repr: BackendRepr::Memory { sized: false },
                 largest_niche: None,
+                uninhabited: false,
                 align: element.align,
                 size: Size::ZERO,
                 max_repr_align: None,
@@ -337,6 +336,7 @@ pub fn layout_of_ty_query(
             fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
             backend_repr: BackendRepr::Memory { sized: false },
             largest_niche: None,
+            uninhabited: false,
             align: dl.i8_align,
             size: Size::ZERO,
             max_repr_align: None,