Merging upstream version 1.66.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1803 insertions, 0 deletions

diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
new file mode 100644
index 000000000..52ba0eee9
--- /dev/null
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -0,0 +1,1803 @@
+use rustc_hir as hir;
+use rustc_index::bit_set::BitSet;
+use rustc_index::vec::{Idx, IndexVec};
+use rustc_middle::mir::{GeneratorLayout, GeneratorSavedLocal};
+use rustc_middle::ty::layout::{
+    IntegerExt, LayoutCx, LayoutError, LayoutOf, TyAndLayout, MAX_SIMD_LANES,
+};
+use rustc_middle::ty::{
+    self, subst::SubstsRef, EarlyBinder, ReprOptions, Ty, TyCtxt, TypeVisitable,
+};
+use rustc_session::{DataTypeKind, FieldInfo, SizeKind, VariantInfo};
+use rustc_span::symbol::Symbol;
+use rustc_span::DUMMY_SP;
+use rustc_target::abi::*;
+
+use std::cmp::{self, Ordering};
+use std::iter;
+use std::num::NonZeroUsize;
+use std::ops::Bound;
+
+use rand::{seq::SliceRandom, SeedableRng};
+use rand_xoshiro::Xoshiro128StarStar;
+
+use crate::layout_sanity_check::sanity_check_layout;
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    *providers = ty::query::Providers { layout_of, ..*providers };
+}
+
+#[instrument(skip(tcx, query), level = "debug")]
+fn layout_of<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
+) -> Result<TyAndLayout<'tcx>, LayoutError<'tcx>> {
+    let (param_env, ty) = query.into_parts();
+    debug!(?ty);
+
+    let param_env = param_env.with_reveal_all_normalized(tcx);
+    let unnormalized_ty = ty;
+
+    // FIXME: We might want to have two different versions of `layout_of`:
+    // One that can be called after typecheck has completed and can use
+    // `normalize_erasing_regions` here and another one that can be called
+    // before typecheck has completed and uses `try_normalize_erasing_regions`.
+    let ty = match tcx.try_normalize_erasing_regions(param_env, ty) {
+        Ok(t) => t,
+        Err(normalization_error) => {
+            return Err(LayoutError::NormalizationFailure(ty, normalization_error));
+        }
+    };
+
+    if ty != unnormalized_ty {
+        // Ensure this layout is also cached for the normalized type.
+        return tcx.layout_of(param_env.and(ty));
+    }
+
+    let cx = LayoutCx { tcx, param_env };
+
+    let layout = layout_of_uncached(&cx, ty)?;
+    let layout = TyAndLayout { ty, layout };
+
+    record_layout_for_printing(&cx, layout);
+
+    sanity_check_layout(&cx, &layout);
+
+    Ok(layout)
+}
+
+#[derive(Copy, Clone, Debug)]
+enum StructKind {
+    /// A tuple, closure, or univariant which cannot be coerced to unsized.
+    AlwaysSized,
+    /// A univariant, the last field of which may be coerced to unsized.
+    MaybeUnsized,
+    /// A univariant, but with a prefix of an arbitrary size & alignment (e.g., enum tag).
+    Prefixed(Size, Align),
+}
+
+// Invert a bijective mapping, i.e. `invert(map)[y] = x` if `map[x] = y`.
+// This is used to go between `memory_index` (source field order to memory order)
+// and `inverse_memory_index` (memory order to source field order).
+// See also `FieldsShape::Arbitrary::memory_index` for more details.
+// FIXME(eddyb) build a better abstraction for permutations, if possible.
+fn invert_mapping(map: &[u32]) -> Vec<u32> {
+    let mut inverse = vec![0; map.len()];
+    for i in 0..map.len() {
+        inverse[map[i] as usize] = i as u32;
+    }
+    inverse
+}
+
+fn scalar_pair<'tcx>(cx: &LayoutCx<'tcx, TyCtxt<'tcx>>, a: Scalar, b: Scalar) -> LayoutS<'tcx> {
+    let dl = cx.data_layout();
+    let b_align = b.align(dl);
+    let align = a.align(dl).max(b_align).max(dl.aggregate_align);
+    let b_offset = a.size(dl).align_to(b_align.abi);
+    let size = (b_offset + b.size(dl)).align_to(align.abi);
+
+    // HACK(nox): We iter on `b` and then `a` because `max_by_key`
+    // returns the last maximum.
+    let largest_niche = Niche::from_scalar(dl, b_offset, b)
+        .into_iter()
+        .chain(Niche::from_scalar(dl, Size::ZERO, a))
+        .max_by_key(|niche| niche.available(dl));
+
+    LayoutS {
+        variants: Variants::Single { index: VariantIdx::new(0) },
+        fields: FieldsShape::Arbitrary {
+            offsets: vec![Size::ZERO, b_offset],
+            memory_index: vec![0, 1],
+        },
+        abi: Abi::ScalarPair(a, b),
+        largest_niche,
+        align,
+        size,
+    }
+}
+
+fn univariant_uninterned<'tcx>(
+    cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
+    ty: Ty<'tcx>,
+    fields: &[TyAndLayout<'_>],
+    repr: &ReprOptions,
+    kind: StructKind,
+) -> Result<LayoutS<'tcx>, LayoutError<'tcx>> {
+    let dl = cx.data_layout();
+    let pack = repr.pack;
+    if pack.is_some() && repr.align.is_some() {
+        cx.tcx.sess.delay_span_bug(DUMMY_SP, "struct cannot be packed and aligned");
+        return Err(LayoutError::Unknown(ty));
+    }
+
+    let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };
+
+    let mut inverse_memory_index: Vec<u32> = (0..fields.len() as u32).collect();
+
+    let optimize = !repr.inhibit_struct_field_reordering_opt();
+    if optimize {
+        let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
+        let optimizing = &mut inverse_memory_index[..end];
+        let field_align = |f: &TyAndLayout<'_>| {
+            if let Some(pack) = pack { f.align.abi.min(pack) } else { f.align.abi }
+        };
+
+        // If `-Z randomize-layout` was enabled for the type definition we can shuffle
+        // the field ordering to try and catch some code making assumptions about layouts
+        // we don't guarantee
+        if repr.can_randomize_type_layout() {
+            // `ReprOptions.layout_seed` is a deterministic seed that we can use to
+            // randomize field ordering with
+            let mut rng = Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);
+
+            // Shuffle the ordering of the fields
+            optimizing.shuffle(&mut rng);
+
+            // Otherwise we just leave things alone and actually optimize the type's fields
+        } else {
+            match kind {
+                StructKind::AlwaysSized | StructKind::MaybeUnsized => {
+                    optimizing.sort_by_key(|&x| {
+                        // Place ZSTs first to avoid "interesting offsets",
+                        // especially with only one or two non-ZST fields.
+                        let f = &fields[x as usize];
+                        (!f.is_zst(), cmp::Reverse(field_align(f)))
+                    });
+                }
+
+                StructKind::Prefixed(..) => {
+                    // Sort in ascending alignment so that the layout stays optimal
+                    // regardless of the prefix
+                    optimizing.sort_by_key(|&x| field_align(&fields[x as usize]));
+                }
+            }
+
+            // FIXME(Kixiron): We can always shuffle fields within a given alignment class
+            //                 regardless of the status of `-Z randomize-layout`
+        }
+    }
+
+    // inverse_memory_index holds field indices by increasing memory offset.
+    // That is, if field 5 has offset 0, the first element of inverse_memory_index is 5.
+    // We now write field offsets to the corresponding offset slot;
+    // field 5 with offset 0 puts 0 in offsets[5].
+    // At the bottom of this function, we invert `inverse_memory_index` to
+    // produce `memory_index` (see `invert_mapping`).
+
+    let mut sized = true;
+    let mut offsets = vec![Size::ZERO; fields.len()];
+    let mut offset = Size::ZERO;
+    let mut largest_niche = None;
+    let mut largest_niche_available = 0;
+
+    if let StructKind::Prefixed(prefix_size, prefix_align) = kind {
+        let prefix_align =
+            if let Some(pack) = pack { prefix_align.min(pack) } else { prefix_align };
+        align = align.max(AbiAndPrefAlign::new(prefix_align));
+        offset = prefix_size.align_to(prefix_align);
+    }
+
+    for &i in &inverse_memory_index {
+        let field = fields[i as usize];
+        if !sized {
+            cx.tcx.sess.delay_span_bug(
+                DUMMY_SP,
+                &format!(
+                    "univariant: field #{} of `{}` comes after unsized field",
+                    offsets.len(),
+                    ty
+                ),
+            );
+        }
+
+        if field.is_unsized() {
+            sized = false;
+        }
+
+        // Invariant: offset < dl.obj_size_bound() <= 1<<61
+        let field_align = if let Some(pack) = pack {
+            field.align.min(AbiAndPrefAlign::new(pack))
+        } else {
+            field.align
+        };
+        offset = offset.align_to(field_align.abi);
+        align = align.max(field_align);
+
+        debug!("univariant offset: {:?} field: {:#?}", offset, field);
+        offsets[i as usize] = offset;
+
+        if let Some(mut niche) = field.largest_niche {
+            let available = niche.available(dl);
+            if available > largest_niche_available {
+                largest_niche_available = available;
+                niche.offset += offset;
+                largest_niche = Some(niche);
+            }
+        }
+
+        offset = offset.checked_add(field.size, dl).ok_or(LayoutError::SizeOverflow(ty))?;
+    }
+
+    if let Some(repr_align) = repr.align {
+        align = align.max(AbiAndPrefAlign::new(repr_align));
+    }
+
+    debug!("univariant min_size: {:?}", offset);
+    let min_size = offset;
+
+    // As stated above, inverse_memory_index holds field indices by increasing offset.
+    // This makes it an already-sorted view of the offsets vec.
+    // To invert it, consider:
+    // If field 5 has offset 0, offsets[0] is 5, and memory_index[5] should be 0.
+    // Field 5 would be the first element, so memory_index is i:
+    // Note: if we didn't optimize, it's already right.
+
+    let memory_index =
+        if optimize { invert_mapping(&inverse_memory_index) } else { inverse_memory_index };
+
+    let size = min_size.align_to(align.abi);
+    let mut abi = Abi::Aggregate { sized };
+
+    // Unpack newtype ABIs and find scalar pairs.
+    if sized && size.bytes() > 0 {
+        // All other fields must be ZSTs.
+        let mut non_zst_fields = fields.iter().enumerate().filter(|&(_, f)| !f.is_zst());
+
+        match (non_zst_fields.next(), non_zst_fields.next(), non_zst_fields.next()) {
+            // We have exactly one non-ZST field.
+            (Some((i, field)), None, None) => {
+                // Field fills the struct and it has a scalar or scalar pair ABI.
+                if offsets[i].bytes() == 0 && align.abi == field.align.abi && size == field.size {
+                    match field.abi {
+                        // For plain scalars, or vectors of them, we can't unpack
+                        // newtypes for `#[repr(C)]`, as that affects C ABIs.
+                        Abi::Scalar(_) | Abi::Vector { .. } if optimize => {
+                            abi = field.abi;
+                        }
+                        // But scalar pairs are Rust-specific and get
+                        // treated as aggregates by C ABIs anyway.
+                        Abi::ScalarPair(..) => {
+                            abi = field.abi;
+                        }
+                        _ => {}
+                    }
+                }
+            }
+
+            // Two non-ZST fields, and they're both scalars.
+            (Some((i, a)), Some((j, b)), None) => {
+                match (a.abi, b.abi) {
+                    (Abi::Scalar(a), Abi::Scalar(b)) => {
+                        // Order by the memory placement, not source order.
+                        let ((i, a), (j, b)) = if offsets[i] < offsets[j] {
+                            ((i, a), (j, b))
+                        } else {
+                            ((j, b), (i, a))
+                        };
+                        let pair = scalar_pair(cx, a, b);
+                        let pair_offsets = match pair.fields {
+                            FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
+                                assert_eq!(memory_index, &[0, 1]);
+                                offsets
+                            }
+                            _ => bug!(),
+                        };
+                        if offsets[i] == pair_offsets[0]
+                            && offsets[j] == pair_offsets[1]
+                            && align == pair.align
+                            && size == pair.size
+                        {
+                            // We can use `ScalarPair` only when it matches our
+                            // already computed layout (including `#[repr(C)]`).
+                            abi = pair.abi;
+                        }
+                    }
+                    _ => {}
+                }
+            }
+
+            _ => {}
+        }
+    }
+
+    if fields.iter().any(|f| f.abi.is_uninhabited()) {
+        abi = Abi::Uninhabited;
+    }
+
+    Ok(LayoutS {
+        variants: Variants::Single { index: VariantIdx::new(0) },
+        fields: FieldsShape::Arbitrary { offsets, memory_index },
+        abi,
+        largest_niche,
+        align,
+        size,
+    })
+}
+
+fn layout_of_uncached<'tcx>(
+    cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
+    ty: Ty<'tcx>,
+) -> Result<Layout<'tcx>, LayoutError<'tcx>> {
+    let tcx = cx.tcx;
+    let param_env = cx.param_env;
+    let dl = cx.data_layout();
+    let scalar_unit = |value: Primitive| {
+        let size = value.size(dl);
+        assert!(size.bits() <= 128);
+        Scalar::Initialized { value, valid_range: WrappingRange::full(size) }
+    };
+    let scalar = |value: Primitive| tcx.intern_layout(LayoutS::scalar(cx, scalar_unit(value)));
+
+    let univariant = |fields: &[TyAndLayout<'_>], repr: &ReprOptions, kind| {
+        Ok(tcx.intern_layout(univariant_uninterned(cx, ty, fields, repr, kind)?))
+    };
+    debug_assert!(!ty.has_non_region_infer());
+
+    Ok(match *ty.kind() {
+        // Basic scalars.
+        ty::Bool => tcx.intern_layout(LayoutS::scalar(
+            cx,
+            Scalar::Initialized {
+                value: Int(I8, false),
+                valid_range: WrappingRange { start: 0, end: 1 },
+            },
+        )),
+        ty::Char => tcx.intern_layout(LayoutS::scalar(
+            cx,
+            Scalar::Initialized {
+                value: Int(I32, false),
+                valid_range: WrappingRange { start: 0, end: 0x10FFFF },
+            },
+        )),
+        ty::Int(ity) => scalar(Int(Integer::from_int_ty(dl, ity), true)),
+        ty::Uint(ity) => scalar(Int(Integer::from_uint_ty(dl, ity), false)),
+        ty::Float(fty) => scalar(match fty {
+            ty::FloatTy::F32 => F32,
+            ty::FloatTy::F64 => F64,
+        }),
+        ty::FnPtr(_) => {
+            let mut ptr = scalar_unit(Pointer);
+            ptr.valid_range_mut().start = 1;
+            tcx.intern_layout(LayoutS::scalar(cx, ptr))
+        }
+
+        // The never type.
+        ty::Never => tcx.intern_layout(LayoutS {
+            variants: Variants::Single { index: VariantIdx::new(0) },
+            fields: FieldsShape::Primitive,
+            abi: Abi::Uninhabited,
+            largest_niche: None,
+            align: dl.i8_align,
+            size: Size::ZERO,
+        }),
+
+        // Potentially-wide pointers.
+        ty::Ref(_, pointee, _) | ty::RawPtr(ty::TypeAndMut { ty: pointee, .. }) => {
+            let mut data_ptr = scalar_unit(Pointer);
+            if !ty.is_unsafe_ptr() {
+                data_ptr.valid_range_mut().start = 1;
+            }
+
+            let pointee = tcx.normalize_erasing_regions(param_env, pointee);
+            if pointee.is_sized(tcx, param_env) {
+                return Ok(tcx.intern_layout(LayoutS::scalar(cx, data_ptr)));
+            }
+
+            let unsized_part = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
+            let metadata = match unsized_part.kind() {
+                ty::Foreign(..) => {
+                    return Ok(tcx.intern_layout(LayoutS::scalar(cx, data_ptr)));
+                }
+                ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
+                ty::Dynamic(..) => {
+                    let mut vtable = scalar_unit(Pointer);
+                    vtable.valid_range_mut().start = 1;
+                    vtable
+                }
+                _ => return Err(LayoutError::Unknown(unsized_part)),
+            };
+
+            // Effectively a (ptr, meta) tuple.
+            tcx.intern_layout(scalar_pair(cx, data_ptr, metadata))
+        }
+
+        ty::Dynamic(_, _, ty::DynStar) => {
+            let mut data = scalar_unit(Int(dl.ptr_sized_integer(), false));
+            data.valid_range_mut().start = 0;
+            let mut vtable = scalar_unit(Pointer);
+            vtable.valid_range_mut().start = 1;
+            tcx.intern_layout(scalar_pair(cx, data, vtable))
+        }
+
+        // Arrays and slices.
+        ty::Array(element, mut count) => {
+            if count.has_projections() {
+                count = tcx.normalize_erasing_regions(param_env, count);
+                if count.has_projections() {
+                    return Err(LayoutError::Unknown(ty));
+                }
+            }
+
+            let count = count.try_eval_usize(tcx, param_env).ok_or(LayoutError::Unknown(ty))?;
+            let element = cx.layout_of(element)?;
+            let size = element.size.checked_mul(count, dl).ok_or(LayoutError::SizeOverflow(ty))?;
+
+            let abi = if count != 0 && tcx.conservative_is_privately_uninhabited(param_env.and(ty))
+            {
+                Abi::Uninhabited
+            } else {
+                Abi::Aggregate { sized: true }
+            };
+
+            let largest_niche = if count != 0 { element.largest_niche } else { None };
+
+            tcx.intern_layout(LayoutS {
+                variants: Variants::Single { index: VariantIdx::new(0) },
+                fields: FieldsShape::Array { stride: element.size, count },
+                abi,
+                largest_niche,
+                align: element.align,
+                size,
+            })
+        }
+        ty::Slice(element) => {
+            let element = cx.layout_of(element)?;
+            tcx.intern_layout(LayoutS {
+                variants: Variants::Single { index: VariantIdx::new(0) },
+                fields: FieldsShape::Array { stride: element.size, count: 0 },
+                abi: Abi::Aggregate { sized: false },
+                largest_niche: None,
+                align: element.align,
+                size: Size::ZERO,
+            })
+        }
+        ty::Str => tcx.intern_layout(LayoutS {
+            variants: Variants::Single { index: VariantIdx::new(0) },
+            fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
+            abi: Abi::Aggregate { sized: false },
+            largest_niche: None,
+            align: dl.i8_align,
+            size: Size::ZERO,
+        }),
+
+        // Odd unit types.
+        ty::FnDef(..) => univariant(&[], &ReprOptions::default(), StructKind::AlwaysSized)?,
+        ty::Dynamic(_, _, ty::Dyn) | ty::Foreign(..) => {
+            let mut unit = univariant_uninterned(
+                cx,
+                ty,
+                &[],
+                &ReprOptions::default(),
+                StructKind::AlwaysSized,
+            )?;
+            match unit.abi {
+                Abi::Aggregate { ref mut sized } => *sized = false,
+                _ => bug!(),
+            }
+            tcx.intern_layout(unit)
+        }
+
+        ty::Generator(def_id, substs, _) => generator_layout(cx, ty, def_id, substs)?,
+
+        ty::Closure(_, ref substs) => {
+            let tys = substs.as_closure().upvar_tys();
+            univariant(
+                &tys.map(|ty| cx.layout_of(ty)).collect::<Result<Vec<_>, _>>()?,
+                &ReprOptions::default(),
+                StructKind::AlwaysSized,
+            )?
+        }
+
+        ty::Tuple(tys) => {
+            let kind =
+                if tys.len() == 0 { StructKind::AlwaysSized } else { StructKind::MaybeUnsized };
+
+            univariant(
+                &tys.iter().map(|k| cx.layout_of(k)).collect::<Result<Vec<_>, _>>()?,
+                &ReprOptions::default(),
+                kind,
+            )?
+        }
+
+        // SIMD vector types.
+        ty::Adt(def, substs) if def.repr().simd() => {
+            if !def.is_struct() {
+                // Should have yielded E0517 by now.
+                tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    "#[repr(simd)] was applied to an ADT that is not a struct",
+                );
+                return Err(LayoutError::Unknown(ty));
+            }
+
+            // Supported SIMD vectors are homogeneous ADTs with at least one field:
+            //
+            // * #[repr(simd)] struct S(T, T, T, T);
+            // * #[repr(simd)] struct S { x: T, y: T, z: T, w: T }
+            // * #[repr(simd)] struct S([T; 4])
+            //
+            // where T is a primitive scalar (integer/float/pointer).
+
+            // SIMD vectors with zero fields are not supported.
+            // (should be caught by typeck)
+            if def.non_enum_variant().fields.is_empty() {
+                tcx.sess.fatal(&format!("monomorphising SIMD type `{}` of zero length", ty));
+            }
+
+            // Type of the first ADT field:
+            let f0_ty = def.non_enum_variant().fields[0].ty(tcx, substs);
+
+            // Heterogeneous SIMD vectors are not supported:
+            // (should be caught by typeck)
+            for fi in &def.non_enum_variant().fields {
+                if fi.ty(tcx, substs) != f0_ty {
+                    tcx.sess.fatal(&format!("monomorphising heterogeneous SIMD type `{}`", ty));
+                }
+            }
+
+            // The element type and number of elements of the SIMD vector
+            // are obtained from:
+            //
+            // * the element type and length of the single array field, if
+            // the first field is of array type, or
+            //
+            // * the homogeneous field type and the number of fields.
+            let (e_ty, e_len, is_array) = if let ty::Array(e_ty, _) = f0_ty.kind() {
+                // First ADT field is an array:
+
+                // SIMD vectors with multiple array fields are not supported:
+                // (should be caught by typeck)
+                if def.non_enum_variant().fields.len() != 1 {
+                    tcx.sess.fatal(&format!(
+                        "monomorphising SIMD type `{}` with more than one array field",
+                        ty
+                    ));
+                }
+
+                // Extract the number of elements from the layout of the array field:
+                let FieldsShape::Array { count, .. } = cx.layout_of(f0_ty)?.layout.fields() else {
+                        return Err(LayoutError::Unknown(ty));
+                    };
+
+                (*e_ty, *count, true)
+            } else {
+                // First ADT field is not an array:
+                (f0_ty, def.non_enum_variant().fields.len() as _, false)
+            };
+
+            // SIMD vectors of zero length are not supported.
+            // Additionally, lengths are capped at 2^16 as a fixed maximum backends must
+            // support.
+            //
+            // Can't be caught in typeck if the array length is generic.
+            if e_len == 0 {
+                tcx.sess.fatal(&format!("monomorphising SIMD type `{}` of zero length", ty));
+            } else if e_len > MAX_SIMD_LANES {
+                tcx.sess.fatal(&format!(
+                    "monomorphising SIMD type `{}` of length greater than {}",
+                    ty, MAX_SIMD_LANES,
+                ));
+            }
+
+            // Compute the ABI of the element type:
+            let e_ly = cx.layout_of(e_ty)?;
+            let Abi::Scalar(e_abi) = e_ly.abi else {
+                    // This error isn't caught in typeck, e.g., if
+                    // the element type of the vector is generic.
+                    tcx.sess.fatal(&format!(
+                        "monomorphising SIMD type `{}` with a non-primitive-scalar \
+                        (integer/float/pointer) element type `{}`",
+                        ty, e_ty
+                    ))
+                };
+
+            // Compute the size and alignment of the vector:
+            let size = e_ly.size.checked_mul(e_len, dl).ok_or(LayoutError::SizeOverflow(ty))?;
+            let align = dl.vector_align(size);
+            let size = size.align_to(align.abi);
+
+            // Compute the placement of the vector fields:
+            let fields = if is_array {
+                FieldsShape::Arbitrary { offsets: vec![Size::ZERO], memory_index: vec![0] }
+            } else {
+                FieldsShape::Array { stride: e_ly.size, count: e_len }
+            };
+
+            tcx.intern_layout(LayoutS {
+                variants: Variants::Single { index: VariantIdx::new(0) },
+                fields,
+                abi: Abi::Vector { element: e_abi, count: e_len },
+                largest_niche: e_ly.largest_niche,
+                size,
+                align,
+            })
+        }
+
+        // ADTs.
+        ty::Adt(def, substs) => {
+            // Cache the field layouts.
+            let variants = def
+                .variants()
+                .iter()
+                .map(|v| {
+                    v.fields
+                        .iter()
+                        .map(|field| cx.layout_of(field.ty(tcx, substs)))
+                        .collect::<Result<Vec<_>, _>>()
+                })
+                .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+            if def.is_union() {
+                if def.repr().pack.is_some() && def.repr().align.is_some() {
+                    cx.tcx.sess.delay_span_bug(
+                        tcx.def_span(def.did()),
+                        "union cannot be packed and aligned",
+                    );
+                    return Err(LayoutError::Unknown(ty));
+                }
+
+                let mut align =
+                    if def.repr().pack.is_some() { dl.i8_align } else { dl.aggregate_align };
+
+                if let Some(repr_align) = def.repr().align {
+                    align = align.max(AbiAndPrefAlign::new(repr_align));
+                }
+
+                let optimize = !def.repr().inhibit_union_abi_opt();
+                let mut size = Size::ZERO;
+                let mut abi = Abi::Aggregate { sized: true };
+                let index = VariantIdx::new(0);
+                for field in &variants[index] {
+                    assert!(!field.is_unsized());
+                    align = align.max(field.align);
+
+                    // If all non-ZST fields have the same ABI, forward this ABI
+                    if optimize && !field.is_zst() {
+                        // Discard valid range information and allow undef
+                        let field_abi = match field.abi {
+                            Abi::Scalar(x) => Abi::Scalar(x.to_union()),
+                            Abi::ScalarPair(x, y) => Abi::ScalarPair(x.to_union(), y.to_union()),
+                            Abi::Vector { element: x, count } => {
+                                Abi::Vector { element: x.to_union(), count }
+                            }
+                            Abi::Uninhabited | Abi::Aggregate { .. } => {
+                                Abi::Aggregate { sized: true }
+                            }
+                        };
+
+                        if size == Size::ZERO {
+                            // first non ZST: initialize 'abi'
+                            abi = field_abi;
+                        } else if abi != field_abi {
+                            // different fields have different ABI: reset to Aggregate
+                            abi = Abi::Aggregate { sized: true };
+                        }
+                    }
+
+                    size = cmp::max(size, field.size);
+                }
+
+                if let Some(pack) = def.repr().pack {
+                    align = align.min(AbiAndPrefAlign::new(pack));
+                }
+
+                return Ok(tcx.intern_layout(LayoutS {
+                    variants: Variants::Single { index },
+                    fields: FieldsShape::Union(
+                        NonZeroUsize::new(variants[index].len()).ok_or(LayoutError::Unknown(ty))?,
+                    ),
+                    abi,
+                    largest_niche: None,
+                    align,
+                    size: size.align_to(align.abi),
+                }));
+            }
+
+            // A variant is absent if it's uninhabited and only has ZST fields.
+            // Present uninhabited variants only require space for their fields,
+            // but *not* an encoding of the discriminant (e.g., a tag value).
+            // See issue #49298 for more details on the need to leave space
+            // for non-ZST uninhabited data (mostly partial initialization).
+            let absent = |fields: &[TyAndLayout<'_>]| {
+                let uninhabited = fields.iter().any(|f| f.abi.is_uninhabited());
+                let is_zst = fields.iter().all(|f| f.is_zst());
+                uninhabited && is_zst
+            };
+            let (present_first, present_second) = {
+                let mut present_variants = variants
+                    .iter_enumerated()
+                    .filter_map(|(i, v)| if absent(v) { None } else { Some(i) });
+                (present_variants.next(), present_variants.next())
+            };
+            let present_first = match present_first {
+                Some(present_first) => present_first,
+                // Uninhabited because it has no variants, or only absent ones.
+                None if def.is_enum() => {
+                    return Ok(tcx.layout_of(param_env.and(tcx.types.never))?.layout);
+                }
+                // If it's a struct, still compute a layout so that we can still compute the
+                // field offsets.
+                None => VariantIdx::new(0),
+            };
+
+            let is_struct = !def.is_enum() ||
+                    // Only one variant is present.
+                    (present_second.is_none() &&
+                        // Representation optimizations are allowed.
+                        !def.repr().inhibit_enum_layout_opt());
+            if is_struct {
+                // Struct, or univariant enum equivalent to a struct.
+                // (Typechecking will reject discriminant-sizing attrs.)
+
+                let v = present_first;
+                let kind = if def.is_enum() || variants[v].is_empty() {
+                    StructKind::AlwaysSized
+                } else {
+                    let param_env = tcx.param_env(def.did());
+                    let last_field = def.variant(v).fields.last().unwrap();
+                    let always_sized = tcx.type_of(last_field.did).is_sized(tcx, param_env);
+                    if !always_sized { StructKind::MaybeUnsized } else { StructKind::AlwaysSized }
+                };
+
+                let mut st = univariant_uninterned(cx, ty, &variants[v], &def.repr(), kind)?;
+                st.variants = Variants::Single { index: v };
+
+                if def.is_unsafe_cell() {
+                    let hide_niches = |scalar: &mut _| match scalar {
+                        Scalar::Initialized { value, valid_range } => {
+                            *valid_range = WrappingRange::full(value.size(dl))
+                        }
+                        // Already doesn't have any niches
+                        Scalar::Union { .. } => {}
+                    };
+                    match &mut st.abi {
+                        Abi::Uninhabited => {}
+                        Abi::Scalar(scalar) => hide_niches(scalar),
+                        Abi::ScalarPair(a, b) => {
+                            hide_niches(a);
+                            hide_niches(b);
+                        }
+                        Abi::Vector { element, count: _ } => hide_niches(element),
+                        Abi::Aggregate { sized: _ } => {}
+                    }
+                    st.largest_niche = None;
+                    return Ok(tcx.intern_layout(st));
+                }
+
+                let (start, end) = cx.tcx.layout_scalar_valid_range(def.did());
+                match st.abi {
+                    Abi::Scalar(ref mut scalar) | Abi::ScalarPair(ref mut scalar, _) => {
+                        // the asserts ensure that we are not using the
+                        // `#[rustc_layout_scalar_valid_range(n)]`
+                        // attribute to widen the range of anything as that would probably
+                        // result in UB somewhere
+                        // FIXME(eddyb) the asserts are probably not needed,
+                        // as larger validity ranges would result in missed
+                        // optimizations, *not* wrongly assuming the inner
+                        // value is valid. e.g. unions enlarge validity ranges,
+                        // because the values may be uninitialized.
+                        if let Bound::Included(start) = start {
+                            // FIXME(eddyb) this might be incorrect - it doesn't
+                            // account for wrap-around (end < start) ranges.
+                            let valid_range = scalar.valid_range_mut();
+                            assert!(valid_range.start <= start);
+                            valid_range.start = start;
+                        }
+                        if let Bound::Included(end) = end {
+                            // FIXME(eddyb) this might be incorrect - it doesn't
+                            // account for wrap-around (end < start) ranges.
+                            let valid_range = scalar.valid_range_mut();
+                            assert!(valid_range.end >= end);
+                            valid_range.end = end;
+                        }
+
+                        // Update `largest_niche` if we have introduced a larger niche.
+                        let niche = Niche::from_scalar(dl, Size::ZERO, *scalar);
+                        if let Some(niche) = niche {
+                            match st.largest_niche {
+                                Some(largest_niche) => {
+                                    // Replace the existing niche even if they're equal,
+                                    // because this one is at a lower offset.
+                                    if largest_niche.available(dl) <= niche.available(dl) {
+                                        st.largest_niche = Some(niche);
+                                    }
+                                }
+                                None => st.largest_niche = Some(niche),
+                            }
+                        }
+                    }
+                    _ => assert!(
+                        start == Bound::Unbounded && end == Bound::Unbounded,
+                        "nonscalar layout for layout_scalar_valid_range type {:?}: {:#?}",
+                        def,
+                        st,
+                    ),
+                }
+
+                return Ok(tcx.intern_layout(st));
+            }
+
+            // At this point, we have handled all unions and
+            // structs. (We have also handled univariant enums
+            // that allow representation optimization.)
+            assert!(def.is_enum());
+
+            // Until we've decided whether to use the tagged or
+            // niche filling LayoutS, we don't want to intern the
+            // variant layouts, so we can't store them in the
+            // overall LayoutS. Store the overall LayoutS
+            // and the variant LayoutSs here until then.
+            struct TmpLayout<'tcx> {
+                layout: LayoutS<'tcx>,
+                variants: IndexVec<VariantIdx, LayoutS<'tcx>>,
+            }
+
+            let calculate_niche_filling_layout =
+                || -> Result<Option<TmpLayout<'tcx>>, LayoutError<'tcx>> {
+                    // The current code for niche-filling relies on variant indices
+                    // instead of actual discriminants, so enums with
+                    // explicit discriminants (RFC #2363) would misbehave.
+                    if def.repr().inhibit_enum_layout_opt()
+                        || def
+                            .variants()
+                            .iter_enumerated()
+                            .any(|(i, v)| v.discr != ty::VariantDiscr::Relative(i.as_u32()))
+                    {
+                        return Ok(None);
+                    }
+
+                    if variants.len() < 2 {
+                        return Ok(None);
+                    }
+
+                    let mut align = dl.aggregate_align;
+                    let mut variant_layouts = variants
+                        .iter_enumerated()
+                        .map(|(j, v)| {
+                            let mut st = univariant_uninterned(
+                                cx,
+                                ty,
+                                v,
+                                &def.repr(),
+                                StructKind::AlwaysSized,
+                            )?;
+                            st.variants = Variants::Single { index: j };
+
+                            align = align.max(st.align);
+
+                            Ok(st)
+                        })
+                        .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+                    let largest_variant_index = match variant_layouts
+                        .iter_enumerated()
+                        .max_by_key(|(_i, layout)| layout.size.bytes())
+                        .map(|(i, _layout)| i)
+                    {
+                        None => return Ok(None),
+                        Some(i) => i,
+                    };
+
+                    let all_indices = VariantIdx::new(0)..=VariantIdx::new(variants.len() - 1);
+                    let needs_disc = |index: VariantIdx| {
+                        index != largest_variant_index && !absent(&variants[index])
+                    };
+                    let niche_variants = all_indices.clone().find(|v| needs_disc(*v)).unwrap()
+                        ..=all_indices.rev().find(|v| needs_disc(*v)).unwrap();
+
+                    let count = niche_variants.size_hint().1.unwrap() as u128;
+
+                    // Find the field with the largest niche
+                    let (field_index, niche, (niche_start, niche_scalar)) = match variants
+                        [largest_variant_index]
+                        .iter()
+                        .enumerate()
+                        .filter_map(|(j, field)| Some((j, field.largest_niche?)))
+                        .max_by_key(|(_, niche)| niche.available(dl))
+                        .and_then(|(j, niche)| Some((j, niche, niche.reserve(cx, count)?)))
+                    {
+                        None => return Ok(None),
+                        Some(x) => x,
+                    };
+
+                    let niche_offset = niche.offset
+                        + variant_layouts[largest_variant_index].fields.offset(field_index);
+                    let niche_size = niche.value.size(dl);
+                    let size = variant_layouts[largest_variant_index].size.align_to(align.abi);
+
+                    let all_variants_fit =
+                        variant_layouts.iter_enumerated_mut().all(|(i, layout)| {
+                            if i == largest_variant_index {
+                                return true;
+                            }
+
+                            layout.largest_niche = None;
+
+                            if layout.size <= niche_offset {
+                                // This variant will fit before the niche.
+                                return true;
+                            }
+
+                            // Determine if it'll fit after the niche.
+                            let this_align = layout.align.abi;
+                            let this_offset = (niche_offset + niche_size).align_to(this_align);
+
+                            if this_offset + layout.size > size {
+                                return false;
+                            }
+
+                            // It'll fit, but we need to make some adjustments.
+                            match layout.fields {
+                                FieldsShape::Arbitrary { ref mut offsets, .. } => {
+                                    for (j, offset) in offsets.iter_mut().enumerate() {
+                                        if !variants[i][j].is_zst() {
+                                            *offset += this_offset;
+                                        }
+                                    }
+                                }
+                                _ => {
+                                    panic!("Layout of fields should be Arbitrary for variants")
+                                }
+                            }
+
+                            // It can't be a Scalar or ScalarPair because the offset isn't 0.
+                            if !layout.abi.is_uninhabited() {
+                                layout.abi = Abi::Aggregate { sized: true };
+                            }
+                            layout.size += this_offset;
+
+                            true
+                        });
+
+                    if !all_variants_fit {
+                        return Ok(None);
+                    }
+
+                    let largest_niche = Niche::from_scalar(dl, niche_offset, niche_scalar);
+
+                    let others_zst = variant_layouts
+                        .iter_enumerated()
+                        .all(|(i, layout)| i == largest_variant_index || layout.size == Size::ZERO);
+                    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.abi.is_uninhabited()) {
+                        Abi::Uninhabited
+                    } else if same_size && same_align && others_zst {
+                        match variant_layouts[largest_variant_index].abi {
+                            // When the total alignment and size match, we can use the
+                            // same ABI as the scalar variant with the reserved niche.
+                            Abi::Scalar(_) => Abi::Scalar(niche_scalar),
+                            Abi::ScalarPair(first, second) => {
+                                // Only the niche is guaranteed to be initialised,
+                                // so use union layouts for the other primitive.
+                                if niche_offset == Size::ZERO {
+                                    Abi::ScalarPair(niche_scalar, second.to_union())
+                                } else {
+                                    Abi::ScalarPair(first.to_union(), niche_scalar)
+                                }
+                            }
+                            _ => Abi::Aggregate { sized: true },
+                        }
+                    } else {
+                        Abi::Aggregate { sized: true }
+                    };
+
+                    let layout = LayoutS {
+                        variants: Variants::Multiple {
+                            tag: niche_scalar,
+                            tag_encoding: TagEncoding::Niche {
+                                untagged_variant: largest_variant_index,
+                                niche_variants,
+                                niche_start,
+                            },
+                            tag_field: 0,
+                            variants: IndexVec::new(),
+                        },
+                        fields: FieldsShape::Arbitrary {
+                            offsets: vec![niche_offset],
+                            memory_index: vec![0],
+                        },
+                        abi,
+                        largest_niche,
+                        size,
+                        align,
+                    };
+
+                    Ok(Some(TmpLayout { layout, variants: variant_layouts }))
+                };
+
+            let niche_filling_layout = calculate_niche_filling_layout()?;
+
+            let (mut min, mut max) = (i128::MAX, i128::MIN);
+            let discr_type = def.repr().discr_type();
+            let bits = Integer::from_attr(cx, discr_type).size().bits();
+            for (i, discr) in def.discriminants(tcx) {
+                if variants[i].iter().any(|f| f.abi.is_uninhabited()) {
+                    continue;
+                }
+                let mut x = discr.val as i128;
+                if discr_type.is_signed() {
+                    // sign extend the raw representation to be an i128
+                    x = (x << (128 - bits)) >> (128 - bits);
+                }
+                if x < min {
+                    min = x;
+                }
+                if x > max {
+                    max = x;
+                }
+            }
+            // We might have no inhabited variants, so pretend there's at least one.
+            if (min, max) == (i128::MAX, i128::MIN) {
+                min = 0;
+                max = 0;
+            }
+            assert!(min <= max, "discriminant range is {}...{}", min, max);
+            let (min_ity, signed) = Integer::repr_discr(tcx, ty, &def.repr(), min, max);
+
+            let mut align = dl.aggregate_align;
+            let mut size = Size::ZERO;
+
+            // We're interested in the smallest alignment, so start large.
+            let mut start_align = Align::from_bytes(256).unwrap();
+            assert_eq!(Integer::for_align(dl, start_align), None);
+
+            // repr(C) on an enum tells us to make a (tag, union) layout,
+            // so we need to grow the prefix alignment to be at least
+            // the alignment of the union. (This value is used both for
+            // determining the alignment of the overall enum, and the
+            // determining the alignment of the payload after the tag.)
+            let mut prefix_align = min_ity.align(dl).abi;
+            if def.repr().c() {
+                for fields in &variants {
+                    for field in fields {
+                        prefix_align = prefix_align.max(field.align.abi);
+                    }
+                }
+            }
+
+            // Create the set of structs that represent each variant.
+            let mut layout_variants = variants
+                .iter_enumerated()
+                .map(|(i, field_layouts)| {
+                    let mut st = univariant_uninterned(
+                        cx,
+                        ty,
+                        &field_layouts,
+                        &def.repr(),
+                        StructKind::Prefixed(min_ity.size(), prefix_align),
+                    )?;
+                    st.variants = Variants::Single { index: i };
+                    // Find the first field we can't move later
+                    // to make room for a larger discriminant.
+                    for field in st.fields.index_by_increasing_offset().map(|j| field_layouts[j]) {
+                        if !field.is_zst() || field.align.abi.bytes() != 1 {
+                            start_align = start_align.min(field.align.abi);
+                            break;
+                        }
+                    }
+                    size = cmp::max(size, st.size);
+                    align = align.max(st.align);
+                    Ok(st)
+                })
+                .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+            // Align the maximum variant size to the largest alignment.
+            size = size.align_to(align.abi);
+
+            if size.bytes() >= dl.obj_size_bound() {
+                return Err(LayoutError::SizeOverflow(ty));
+            }
+
+            let typeck_ity = Integer::from_attr(dl, def.repr().discr_type());
+            if typeck_ity < min_ity {
+                // It is a bug if Layout decided on a greater discriminant size than typeck for
+                // some reason at this point (based on values discriminant can take on). Mostly
+                // because this discriminant will be loaded, and then stored into variable of
+                // type calculated by typeck. Consider such case (a bug): typeck decided on
+                // byte-sized discriminant, but layout thinks we need a 16-bit to store all
+                // discriminant values. That would be a bug, because then, in codegen, in order
+                // to store this 16-bit discriminant into 8-bit sized temporary some of the
+                // space necessary to represent would have to be discarded (or layout is wrong
+                // on thinking it needs 16 bits)
+                bug!(
+                    "layout decided on a larger discriminant type ({:?}) than typeck ({:?})",
+                    min_ity,
+                    typeck_ity
+                );
+                // However, it is fine to make discr type however large (as an optimisation)
+                // after this point – we’ll just truncate the value we load in codegen.
+            }
+
+            // Check to see if we should use a different type for the
+            // discriminant. We can safely use a type with the same size
+            // as the alignment of the first field of each variant.
+            // We increase the size of the discriminant to avoid LLVM copying
+            // padding when it doesn't need to. This normally causes unaligned
+            // load/stores and excessive memcpy/memset operations. By using a
+            // bigger integer size, LLVM can be sure about its contents and
+            // won't be so conservative.
+
+            // Use the initial field alignment
+            let mut ity = if def.repr().c() || def.repr().int.is_some() {
+                min_ity
+            } else {
+                Integer::for_align(dl, start_align).unwrap_or(min_ity)
+            };
+
+            // If the alignment is not larger than the chosen discriminant size,
+            // don't use the alignment as the final size.
+            if ity <= min_ity {
+                ity = min_ity;
+            } else {
+                // Patch up the variants' first few fields.
+                let old_ity_size = min_ity.size();
+                let new_ity_size = ity.size();
+                for variant in &mut layout_variants {
+                    match variant.fields {
+                        FieldsShape::Arbitrary { ref mut offsets, .. } => {
+                            for i in offsets {
+                                if *i <= old_ity_size {
+                                    assert_eq!(*i, old_ity_size);
+                                    *i = new_ity_size;
+                                }
+                            }
+                            // We might be making the struct larger.
+                            if variant.size <= old_ity_size {
+                                variant.size = new_ity_size;
+                            }
+                        }
+                        _ => bug!(),
+                    }
+                }
+            }
+
+            let tag_mask = ity.size().unsigned_int_max();
+            let tag = Scalar::Initialized {
+                value: Int(ity, signed),
+                valid_range: WrappingRange {
+                    start: (min as u128 & tag_mask),
+                    end: (max as u128 & tag_mask),
+                },
+            };
+            let mut abi = Abi::Aggregate { sized: true };
+
+            if layout_variants.iter().all(|v| v.abi.is_uninhabited()) {
+                abi = Abi::Uninhabited;
+            } else 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 = Abi::Scalar(tag);
+            } else {
+                // Try to use a ScalarPair for all tagged enums.
+                let mut common_prim = None;
+                let mut common_prim_initialized_in_all_variants = true;
+                for (field_layouts, layout_variant) in iter::zip(&variants, &layout_variants) {
+                    let FieldsShape::Arbitrary { ref offsets, .. } = layout_variant.fields else {
+                            bug!();
+                        };
+                    let mut fields = iter::zip(field_layouts, offsets).filter(|p| !p.0.is_zst());
+                    let (field, offset) = match (fields.next(), fields.next()) {
+                        (None, None) => {
+                            common_prim_initialized_in_all_variants = false;
+                            continue;
+                        }
+                        (Some(pair), None) => pair,
+                        _ => {
+                            common_prim = None;
+                            break;
+                        }
+                    };
+                    let prim = match field.abi {
+                        Abi::Scalar(scalar) => {
+                            common_prim_initialized_in_all_variants &=
+                                matches!(scalar, Scalar::Initialized { .. });
+                            scalar.primitive()
+                        }
+                        _ => {
+                            common_prim = None;
+                            break;
+                        }
+                    };
+                    if let Some(pair) = common_prim {
+                        // This is pretty conservative. We could go fancier
+                        // by conflating things like i32 and u32, or even
+                        // realising that (u8, u8) could just cohabit with
+                        // u16 or even u32.
+                        if pair != (prim, offset) {
+                            common_prim = None;
+                            break;
+                        }
+                    } else {
+                        common_prim = Some((prim, offset));
+                    }
+                }
+                if let Some((prim, offset)) = common_prim {
+                    let prim_scalar = if common_prim_initialized_in_all_variants {
+                        scalar_unit(prim)
+                    } else {
+                        // Common prim might be uninit.
+                        Scalar::Union { value: prim }
+                    };
+                    let pair = scalar_pair(cx, tag, prim_scalar);
+                    let pair_offsets = match pair.fields {
+                        FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
+                            assert_eq!(memory_index, &[0, 1]);
+                            offsets
+                        }
+                        _ => bug!(),
+                    };
+                    if pair_offsets[0] == Size::ZERO
+                        && pair_offsets[1] == *offset
+                        && align == pair.align
+                        && size == pair.size
+                    {
+                        // We can use `ScalarPair` only when it matches our
+                        // already computed layout (including `#[repr(C)]`).
+                        abi = pair.abi;
+                    }
+                }
+            }
+
+            // If we pick a "clever" (by-value) ABI, we might have to adjust the ABI of the
+            // variants to ensure they are consistent. This is because a downcast is
+            // semantically a NOP, and thus should not affect layout.
+            if matches!(abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
+                for variant in &mut layout_variants {
+                    // We only do this for variants with fields; the others are not accessed anyway.
+                    // Also do not overwrite any already existing "clever" ABIs.
+                    if variant.fields.count() > 0 && matches!(variant.abi, Abi::Aggregate { .. }) {
+                        variant.abi = abi;
+                        // Also need to bump up the size and alignment, so that the entire value fits in here.
+                        variant.size = cmp::max(variant.size, size);
+                        variant.align.abi = cmp::max(variant.align.abi, align.abi);
+                    }
+                }
+            }
+
+            let largest_niche = Niche::from_scalar(dl, Size::ZERO, tag);
+
+            let tagged_layout = LayoutS {
+                variants: Variants::Multiple {
+                    tag,
+                    tag_encoding: TagEncoding::Direct,
+                    tag_field: 0,
+                    variants: IndexVec::new(),
+                },
+                fields: FieldsShape::Arbitrary { offsets: vec![Size::ZERO], memory_index: vec![0] },
+                largest_niche,
+                abi,
+                align,
+                size,
+            };
+
+            let tagged_layout = TmpLayout { layout: tagged_layout, variants: layout_variants };
+
+            let mut best_layout = match (tagged_layout, niche_filling_layout) {
+                (tl, Some(nl)) => {
+                    // Pick the smaller layout; otherwise,
+                    // pick the layout with the larger niche; otherwise,
+                    // pick tagged as it has simpler codegen.
+                    use Ordering::*;
+                    let niche_size = |tmp_l: &TmpLayout<'_>| {
+                        tmp_l.layout.largest_niche.map_or(0, |n| n.available(dl))
+                    };
+                    match (
+                        tl.layout.size.cmp(&nl.layout.size),
+                        niche_size(&tl).cmp(&niche_size(&nl)),
+                    ) {
+                        (Greater, _) => nl,
+                        (Equal, Less) => nl,
+                        _ => tl,
+                    }
+                }
+                (tl, None) => tl,
+            };
+
+            // Now we can intern the variant layouts and store them in the enum layout.
+            best_layout.layout.variants = match best_layout.layout.variants {
+                Variants::Multiple { tag, tag_encoding, tag_field, .. } => Variants::Multiple {
+                    tag,
+                    tag_encoding,
+                    tag_field,
+                    variants: best_layout
+                        .variants
+                        .into_iter()
+                        .map(|layout| tcx.intern_layout(layout))
+                        .collect(),
+                },
+                _ => bug!(),
+            };
+
+            tcx.intern_layout(best_layout.layout)
+        }
+
+        // Types with no meaningful known layout.
+        ty::Projection(_) | ty::Opaque(..) => {
+            // NOTE(eddyb) `layout_of` query should've normalized these away,
+            // if that was possible, so there's no reason to try again here.
+            return Err(LayoutError::Unknown(ty));
+        }
+
+        ty::Placeholder(..) | ty::GeneratorWitness(..) | ty::Infer(_) => {
+            bug!("Layout::compute: unexpected type `{}`", ty)
+        }
+
+        ty::Bound(..) | ty::Param(_) | ty::Error(_) => {
+            return Err(LayoutError::Unknown(ty));
+        }
+    })
+}
+
+/// Overlap eligibility and variant assignment for each GeneratorSavedLocal.
+#[derive(Clone, Debug, PartialEq)]
+enum SavedLocalEligibility {
+    Unassigned,
+    Assigned(VariantIdx),
+    // FIXME: Use newtype_index so we aren't wasting bytes
+    Ineligible(Option<u32>),
+}
+
+// When laying out generators, we divide our saved local fields into two
+// categories: overlap-eligible and overlap-ineligible.
+//
+// Those fields which are ineligible for overlap go in a "prefix" at the
+// beginning of the layout, and always have space reserved for them.
+//
+// Overlap-eligible fields are only assigned to one variant, so we lay
+// those fields out for each variant and put them right after the
+// prefix.
+//
+// Finally, in the layout details, we point to the fields from the
+// variants they are assigned to. It is possible for some fields to be
+// included in multiple variants. No field ever "moves around" in the
+// layout; its offset is always the same.
+//
+// Also included in the layout are the upvars and the discriminant.
+// These are included as fields on the "outer" layout; they are not part
+// of any variant.
+
+/// Compute the eligibility and assignment of each local.
+fn generator_saved_local_eligibility<'tcx>(
+    info: &GeneratorLayout<'tcx>,
+) -> (BitSet<GeneratorSavedLocal>, IndexVec<GeneratorSavedLocal, SavedLocalEligibility>) {
+    use SavedLocalEligibility::*;
+
+    let mut assignments: IndexVec<GeneratorSavedLocal, SavedLocalEligibility> =
+        IndexVec::from_elem_n(Unassigned, info.field_tys.len());
+
+    // The saved locals not eligible for overlap. These will get
+    // "promoted" to the prefix of our generator.
+    let mut ineligible_locals = BitSet::new_empty(info.field_tys.len());
+
+    // Figure out which of our saved locals are fields in only
+    // one variant. The rest are deemed ineligible for overlap.
+    for (variant_index, fields) in info.variant_fields.iter_enumerated() {
+        for local in fields {
+            match assignments[*local] {
+                Unassigned => {
+                    assignments[*local] = Assigned(variant_index);
+                }
+                Assigned(idx) => {
+                    // We've already seen this local at another suspension
+                    // point, so it is no longer a candidate.
+                    trace!(
+                        "removing local {:?} in >1 variant ({:?}, {:?})",
+                        local,
+                        variant_index,
+                        idx
+                    );
+                    ineligible_locals.insert(*local);
+                    assignments[*local] = Ineligible(None);
+                }
+                Ineligible(_) => {}
+            }
+        }
+    }
+
+    // Next, check every pair of eligible locals to see if they
+    // conflict.
+    for local_a in info.storage_conflicts.rows() {
+        let conflicts_a = info.storage_conflicts.count(local_a);
+        if ineligible_locals.contains(local_a) {
+            continue;
+        }
+
+        for local_b in info.storage_conflicts.iter(local_a) {
+            // local_a and local_b are storage live at the same time, therefore they
+            // cannot overlap in the generator layout. The only way to guarantee
+            // this is if they are in the same variant, or one is ineligible
+            // (which means it is stored in every variant).
+            if ineligible_locals.contains(local_b) || assignments[local_a] == assignments[local_b] {
+                continue;
+            }
+
+            // If they conflict, we will choose one to make ineligible.
+            // This is not always optimal; it's just a greedy heuristic that
+            // seems to produce good results most of the time.
+            let conflicts_b = info.storage_conflicts.count(local_b);
+            let (remove, other) =
+                if conflicts_a > conflicts_b { (local_a, local_b) } else { (local_b, local_a) };
+            ineligible_locals.insert(remove);
+            assignments[remove] = Ineligible(None);
+            trace!("removing local {:?} due to conflict with {:?}", remove, other);
+        }
+    }
+
+    // Count the number of variants in use. If only one of them, then it is
+    // impossible to overlap any locals in our layout. In this case it's
+    // always better to make the remaining locals ineligible, so we can
+    // lay them out with the other locals in the prefix and eliminate
+    // unnecessary padding bytes.
+    {
+        let mut used_variants = BitSet::new_empty(info.variant_fields.len());
+        for assignment in &assignments {
+            if let Assigned(idx) = assignment {
+                used_variants.insert(*idx);
+            }
+        }
+        if used_variants.count() < 2 {
+            for assignment in assignments.iter_mut() {
+                *assignment = Ineligible(None);
+            }
+            ineligible_locals.insert_all();
+        }
+    }
+
+    // Write down the order of our locals that will be promoted to the prefix.
+    {
+        for (idx, local) in ineligible_locals.iter().enumerate() {
+            assignments[local] = Ineligible(Some(idx as u32));
+        }
+    }
+    debug!("generator saved local assignments: {:?}", assignments);
+
+    (ineligible_locals, assignments)
+}
+
+/// Compute the full generator layout.
+fn generator_layout<'tcx>(
+    cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
+    ty: Ty<'tcx>,
+    def_id: hir::def_id::DefId,
+    substs: SubstsRef<'tcx>,
+) -> Result<Layout<'tcx>, LayoutError<'tcx>> {
+    use SavedLocalEligibility::*;
+    let tcx = cx.tcx;
+    let subst_field = |ty: Ty<'tcx>| EarlyBinder(ty).subst(tcx, substs);
+
+    let Some(info) = tcx.generator_layout(def_id) else {
+            return Err(LayoutError::Unknown(ty));
+        };
+    let (ineligible_locals, assignments) = generator_saved_local_eligibility(&info);
+
+    // Build a prefix layout, including "promoting" all ineligible
+    // locals as part of the prefix. We compute the layout of all of
+    // these fields at once to get optimal packing.
+    let tag_index = substs.as_generator().prefix_tys().count();
+
+    // `info.variant_fields` already accounts for the reserved variants, so no need to add them.
+    let max_discr = (info.variant_fields.len() - 1) as u128;
+    let discr_int = Integer::fit_unsigned(max_discr);
+    let discr_int_ty = discr_int.to_ty(tcx, false);
+    let tag = Scalar::Initialized {
+        value: Primitive::Int(discr_int, false),
+        valid_range: WrappingRange { start: 0, end: max_discr },
+    };
+    let tag_layout = cx.tcx.intern_layout(LayoutS::scalar(cx, tag));
+    let tag_layout = TyAndLayout { ty: discr_int_ty, layout: tag_layout };
+
+    let promoted_layouts = ineligible_locals
+        .iter()
+        .map(|local| subst_field(info.field_tys[local]))
+        .map(|ty| tcx.mk_maybe_uninit(ty))
+        .map(|ty| cx.layout_of(ty));
+    let prefix_layouts = substs
+        .as_generator()
+        .prefix_tys()
+        .map(|ty| cx.layout_of(ty))
+        .chain(iter::once(Ok(tag_layout)))
+        .chain(promoted_layouts)
+        .collect::<Result<Vec<_>, _>>()?;
+    let prefix = univariant_uninterned(
+        cx,
+        ty,
+        &prefix_layouts,
+        &ReprOptions::default(),
+        StructKind::AlwaysSized,
+    )?;
+
+    let (prefix_size, prefix_align) = (prefix.size, prefix.align);
+
+    // Split the prefix layout into the "outer" fields (upvars and
+    // discriminant) and the "promoted" fields. Promoted fields will
+    // get included in each variant that requested them in
+    // GeneratorLayout.
+    debug!("prefix = {:#?}", prefix);
+    let (outer_fields, promoted_offsets, promoted_memory_index) = match prefix.fields {
+        FieldsShape::Arbitrary { mut offsets, memory_index } => {
+            let mut inverse_memory_index = invert_mapping(&memory_index);
+
+            // "a" (`0..b_start`) and "b" (`b_start..`) correspond to
+            // "outer" and "promoted" fields respectively.
+            let b_start = (tag_index + 1) as u32;
+            let offsets_b = offsets.split_off(b_start as usize);
+            let offsets_a = offsets;
+
+            // Disentangle the "a" and "b" components of `inverse_memory_index`
+            // by preserving the order but keeping only one disjoint "half" each.
+            // FIXME(eddyb) build a better abstraction for permutations, if possible.
+            let inverse_memory_index_b: Vec<_> =
+                inverse_memory_index.iter().filter_map(|&i| i.checked_sub(b_start)).collect();
+            inverse_memory_index.retain(|&i| i < b_start);
+            let inverse_memory_index_a = inverse_memory_index;
+
+            // Since `inverse_memory_index_{a,b}` each only refer to their
+            // respective fields, they can be safely inverted
+            let memory_index_a = invert_mapping(&inverse_memory_index_a);
+            let memory_index_b = invert_mapping(&inverse_memory_index_b);
+
+            let outer_fields =
+                FieldsShape::Arbitrary { offsets: offsets_a, memory_index: memory_index_a };
+            (outer_fields, offsets_b, memory_index_b)
+        }
+        _ => bug!(),
+    };
+
+    let mut size = prefix.size;
+    let mut align = prefix.align;
+    let variants = info
+        .variant_fields
+        .iter_enumerated()
+        .map(|(index, variant_fields)| {
+            // Only include overlap-eligible fields when we compute our variant layout.
+            let variant_only_tys = variant_fields
+                .iter()
+                .filter(|local| match assignments[**local] {
+                    Unassigned => bug!(),
+                    Assigned(v) if v == index => true,
+                    Assigned(_) => bug!("assignment does not match variant"),
+                    Ineligible(_) => false,
+                })
+                .map(|local| subst_field(info.field_tys[*local]));
+
+            let mut variant = univariant_uninterned(
+                cx,
+                ty,
+                &variant_only_tys.map(|ty| cx.layout_of(ty)).collect::<Result<Vec<_>, _>>()?,
+                &ReprOptions::default(),
+                StructKind::Prefixed(prefix_size, prefix_align.abi),
+            )?;
+            variant.variants = Variants::Single { index };
+
+            let FieldsShape::Arbitrary { offsets, memory_index } = variant.fields else {
+                    bug!();
+                };
+
+            // Now, stitch the promoted and variant-only fields back together in
+            // the order they are mentioned by our GeneratorLayout.
+            // Because we only use some subset (that can differ between variants)
+            // of the promoted fields, we can't just pick those elements of the
+            // `promoted_memory_index` (as we'd end up with gaps).
+            // So instead, we build an "inverse memory_index", as if all of the
+            // promoted fields were being used, but leave the elements not in the
+            // subset as `INVALID_FIELD_IDX`, which we can filter out later to
+            // obtain a valid (bijective) mapping.
+            const INVALID_FIELD_IDX: u32 = !0;
+            let mut combined_inverse_memory_index =
+                vec![INVALID_FIELD_IDX; promoted_memory_index.len() + memory_index.len()];
+            let mut offsets_and_memory_index = iter::zip(offsets, memory_index);
+            let combined_offsets = variant_fields
+                .iter()
+                .enumerate()
+                .map(|(i, local)| {
+                    let (offset, memory_index) = match assignments[*local] {
+                        Unassigned => bug!(),
+                        Assigned(_) => {
+                            let (offset, memory_index) = offsets_and_memory_index.next().unwrap();
+                            (offset, promoted_memory_index.len() as u32 + memory_index)
+                        }
+                        Ineligible(field_idx) => {
+                            let field_idx = field_idx.unwrap() as usize;
+                            (promoted_offsets[field_idx], promoted_memory_index[field_idx])
+                        }
+                    };
+                    combined_inverse_memory_index[memory_index as usize] = i as u32;
+                    offset
+                })
+                .collect();
+
+            // Remove the unused slots and invert the mapping to obtain the
+            // combined `memory_index` (also see previous comment).
+            combined_inverse_memory_index.retain(|&i| i != INVALID_FIELD_IDX);
+            let combined_memory_index = invert_mapping(&combined_inverse_memory_index);
+
+            variant.fields = FieldsShape::Arbitrary {
+                offsets: combined_offsets,
+                memory_index: combined_memory_index,
+            };
+
+            size = size.max(variant.size);
+            align = align.max(variant.align);
+            Ok(tcx.intern_layout(variant))
+        })
+        .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+    size = size.align_to(align.abi);
+
+    let abi = if prefix.abi.is_uninhabited() || variants.iter().all(|v| v.abi().is_uninhabited()) {
+        Abi::Uninhabited
+    } else {
+        Abi::Aggregate { sized: true }
+    };
+
+    let layout = tcx.intern_layout(LayoutS {
+        variants: Variants::Multiple {
+            tag,
+            tag_encoding: TagEncoding::Direct,
+            tag_field: tag_index,
+            variants,
+        },
+        fields: outer_fields,
+        abi,
+        largest_niche: prefix.largest_niche,
+        size,
+        align,
+    });
+    debug!("generator layout ({:?}): {:#?}", ty, layout);
+    Ok(layout)
+}
+
+/// This is invoked by the `layout_of` query to record the final
+/// layout of each type.
+#[inline(always)]
+fn record_layout_for_printing<'tcx>(cx: &LayoutCx<'tcx, TyCtxt<'tcx>>, layout: TyAndLayout<'tcx>) {
+    // If we are running with `-Zprint-type-sizes`, maybe record layouts
+    // for dumping later.
+    if cx.tcx.sess.opts.unstable_opts.print_type_sizes {
+        record_layout_for_printing_outlined(cx, layout)
+    }
+}
+
+fn record_layout_for_printing_outlined<'tcx>(
+    cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
+    layout: TyAndLayout<'tcx>,
+) {
+    // Ignore layouts that are done with non-empty environments or
+    // non-monomorphic layouts, as the user only wants to see the stuff
+    // resulting from the final codegen session.
+    if layout.ty.has_non_region_param() || !cx.param_env.caller_bounds().is_empty() {
+        return;
+    }
+
+    // (delay format until we actually need it)
+    let record = |kind, packed, opt_discr_size, variants| {
+        let type_desc = format!("{:?}", layout.ty);
+        cx.tcx.sess.code_stats.record_type_size(
+            kind,
+            type_desc,
+            layout.align.abi,
+            layout.size,
+            packed,
+            opt_discr_size,
+            variants,
+        );
+    };
+
+    let adt_def = match *layout.ty.kind() {
+        ty::Adt(ref adt_def, _) => {
+            debug!("print-type-size t: `{:?}` process adt", layout.ty);
+            adt_def
+        }
+
+        ty::Closure(..) => {
+            debug!("print-type-size t: `{:?}` record closure", layout.ty);
+            record(DataTypeKind::Closure, false, None, vec![]);
+            return;
+        }
+
+        _ => {
+            debug!("print-type-size t: `{:?}` skip non-nominal", layout.ty);
+            return;
+        }
+    };
+
+    let adt_kind = adt_def.adt_kind();
+    let adt_packed = adt_def.repr().pack.is_some();
+
+    let build_variant_info = |n: Option<Symbol>, flds: &[Symbol], layout: TyAndLayout<'tcx>| {
+        let mut min_size = Size::ZERO;
+        let field_info: Vec<_> = flds
+            .iter()
+            .enumerate()
+            .map(|(i, &name)| {
+                let field_layout = layout.field(cx, i);
+                let offset = layout.fields.offset(i);
+                let field_end = offset + field_layout.size;
+                if min_size < field_end {
+                    min_size = field_end;
+                }
+                FieldInfo {
+                    name,
+                    offset: offset.bytes(),
+                    size: field_layout.size.bytes(),
+                    align: field_layout.align.abi.bytes(),
+                }
+            })
+            .collect();
+
+        VariantInfo {
+            name: n,
+            kind: if layout.is_unsized() { SizeKind::Min } else { SizeKind::Exact },
+            align: layout.align.abi.bytes(),
+            size: if min_size.bytes() == 0 { layout.size.bytes() } else { min_size.bytes() },
+            fields: field_info,
+        }
+    };
+
+    match layout.variants {
+        Variants::Single { index } => {
+            if !adt_def.variants().is_empty() && layout.fields != FieldsShape::Primitive {
+                debug!("print-type-size `{:#?}` variant {}", layout, adt_def.variant(index).name);
+                let variant_def = &adt_def.variant(index);
+                let fields: Vec<_> = variant_def.fields.iter().map(|f| f.name).collect();
+                record(
+                    adt_kind.into(),
+                    adt_packed,
+                    None,
+                    vec![build_variant_info(Some(variant_def.name), &fields, layout)],
+                );
+            } else {
+                // (This case arises for *empty* enums; so give it
+                // zero variants.)
+                record(adt_kind.into(), adt_packed, None, vec![]);
+            }
+        }
+
+        Variants::Multiple { tag, ref tag_encoding, .. } => {
+            debug!(
+                "print-type-size `{:#?}` adt general variants def {}",
+                layout.ty,
+                adt_def.variants().len()
+            );
+            let variant_infos: Vec<_> = adt_def
+                .variants()
+                .iter_enumerated()
+                .map(|(i, variant_def)| {
+                    let fields: Vec<_> = variant_def.fields.iter().map(|f| f.name).collect();
+                    build_variant_info(Some(variant_def.name), &fields, layout.for_variant(cx, i))
+                })
+                .collect();
+            record(
+                adt_kind.into(),
+                adt_packed,
+                match tag_encoding {
+                    TagEncoding::Direct => Some(tag.size(cx)),
+                    _ => None,
+                },
+                variant_infos,
+            );
+        }
+    }
+}