layout.rs

use std::fmt::{self, Write};
use std::ops::{Bound, Deref};
use std::{cmp, iter};

use rustc_hashes::Hash64;
use rustc_index::Idx;
use tracing::debug;

use crate::{
    AbiAndPrefAlign, Align, BackendRepr, FieldsShape, HasDataLayout, IndexSlice, IndexVec, Integer,
    LayoutData, Niche, NonZeroUsize, Primitive, ReprOptions, Scalar, Size, StructKind, TagEncoding,
    Variants, WrappingRange,
};

#[cfg(feature = "nightly")]
mod ty;

#[cfg(feature = "nightly")]
pub use ty::{FIRST_VARIANT, FieldIdx, Layout, TyAbiInterface, TyAndLayout, VariantIdx};

// 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).
fn absent<'a, FieldIdx, VariantIdx, F>(fields: &IndexSlice<FieldIdx, F>) -> bool
where
    FieldIdx: Idx,
    VariantIdx: Idx,
    F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug,
{
    let uninhabited = fields.iter().any(|f| f.is_uninhabited());
    // We cannot ignore alignment; that might lead us to entirely discard a variant and
    // produce an enum that is less aligned than it should be!
    let is_1zst = fields.iter().all(|f| f.is_1zst());
    uninhabited && is_1zst
}

/// Determines towards which end of a struct layout optimizations will try to place the best niches.
enum NicheBias {
    Start,
    End,
}

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum LayoutCalculatorError<F> {
    /// An unsized type was found in a location where a sized type was expected.
    ///
    /// This is not always a compile error, for example if there is a `[T]: Sized`
    /// bound in a where clause.
    ///
    /// Contains the field that was unexpectedly unsized.
    UnexpectedUnsized(F),

    /// A type was too large for the target platform.
    SizeOverflow,

    /// A union had no fields.
    EmptyUnion,

    /// The fields or variants have irreconcilable reprs
    ReprConflict,
}

impl<F> LayoutCalculatorError<F> {
    pub fn without_payload(&self) -> LayoutCalculatorError<()> {
        match self {
            LayoutCalculatorError::UnexpectedUnsized(_) => {
                LayoutCalculatorError::UnexpectedUnsized(())
            }
            LayoutCalculatorError::SizeOverflow => LayoutCalculatorError::SizeOverflow,
            LayoutCalculatorError::EmptyUnion => LayoutCalculatorError::EmptyUnion,
            LayoutCalculatorError::ReprConflict => LayoutCalculatorError::ReprConflict,
        }
    }

    /// Format an untranslated diagnostic for this type
    ///
    /// Intended for use by rust-analyzer, as neither it nor `rustc_abi` depend on fluent infra.
    pub fn fallback_fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(match self {
            LayoutCalculatorError::UnexpectedUnsized(_) => {
                "an unsized type was found where a sized type was expected"
            }
            LayoutCalculatorError::SizeOverflow => "size overflow",
            LayoutCalculatorError::EmptyUnion => "type is a union with no fields",
            LayoutCalculatorError::ReprConflict => "type has an invalid repr",
        })
    }
}

type LayoutCalculatorResult<FieldIdx, VariantIdx, F> =
    Result<LayoutData<FieldIdx, VariantIdx>, LayoutCalculatorError<F>>;

#[derive(Clone, Copy, Debug)]
pub struct LayoutCalculator<Cx> {
    pub cx: Cx,
}

impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
    pub fn new(cx: Cx) -> Self {
        Self { cx }
    }

    pub fn scalar_pair<FieldIdx: Idx, VariantIdx: Idx>(
        &self,
        a: Scalar,
        b: Scalar,
    ) -> LayoutData<FieldIdx, VariantIdx> {
        let dl = self.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));

        let combined_seed = a.size(&self.cx).bytes().wrapping_add(b.size(&self.cx).bytes());

        LayoutData {
            variants: Variants::Single { index: VariantIdx::new(0) },
            fields: FieldsShape::Arbitrary {
                offsets: [Size::ZERO, b_offset].into(),
                memory_index: [0, 1].into(),
            },
            backend_repr: BackendRepr::ScalarPair(a, b),
            largest_niche,
            uninhabited: false,
            align,
            size,
            max_repr_align: None,
            unadjusted_abi_align: align.abi,
            randomization_seed: Hash64::new(combined_seed),
        }
    }

    pub fn univariant<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
    >(
        &self,
        fields: &IndexSlice<FieldIdx, F>,
        repr: &ReprOptions,
        kind: StructKind,
    ) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
        let dl = self.cx.data_layout();
        let layout = self.univariant_biased(fields, repr, kind, NicheBias::Start);
        // Enums prefer niches close to the beginning or the end of the variants so that other
        // (smaller) data-carrying variants can be packed into the space after/before the niche.
        // If the default field ordering does not give us a niche at the front then we do a second
        // run and bias niches to the right and then check which one is closer to one of the
        // struct's edges.
        if let Ok(layout) = &layout {
            // Don't try to calculate an end-biased layout for unsizable structs,
            // otherwise we could end up with different layouts for
            // Foo<Type> and Foo<dyn Trait> which would break unsizing.
            if !matches!(kind, StructKind::MaybeUnsized) {
                if let Some(niche) = layout.largest_niche {
                    let head_space = niche.offset.bytes();
                    let niche_len = niche.value.size(dl).bytes();
                    let tail_space = layout.size.bytes() - head_space - niche_len;

                    // This may end up doing redundant work if the niche is already in the last
                    // field (e.g. a trailing bool) and there is tail padding. But it's non-trivial
                    // to get the unpadded size so we try anyway.
                    if fields.len() > 1 && head_space != 0 && tail_space > 0 {
                        let alt_layout = self
                            .univariant_biased(fields, repr, kind, NicheBias::End)
                            .expect("alt layout should always work");
                        let alt_niche = alt_layout
                            .largest_niche
                            .expect("alt layout should have a niche like the regular one");
                        let alt_head_space = alt_niche.offset.bytes();
                        let alt_niche_len = alt_niche.value.size(dl).bytes();
                        let alt_tail_space =
                            alt_layout.size.bytes() - alt_head_space - alt_niche_len;

                        debug_assert_eq!(layout.size.bytes(), alt_layout.size.bytes());

                        let prefer_alt_layout =
                            alt_head_space > head_space && alt_head_space > tail_space;

                        debug!(
                            "sz: {}, default_niche_at: {}+{}, default_tail_space: {}, alt_niche_at/head_space: {}+{}, alt_tail: {}, num_fields: {}, better: {}\n\
                            layout: {}\n\
                            alt_layout: {}\n",
                            layout.size.bytes(),
                            head_space,
                            niche_len,
                            tail_space,
                            alt_head_space,
                            alt_niche_len,
                            alt_tail_space,
                            layout.fields.count(),
                            prefer_alt_layout,
                            self.format_field_niches(layout, fields),
                            self.format_field_niches(&alt_layout, fields),
                        );

                        if prefer_alt_layout {
                            return Ok(alt_layout);
                        }
                    }
                }
            }
        }
        layout
    }

    pub fn layout_of_never_type<FieldIdx: Idx, VariantIdx: Idx>(
        &self,
    ) -> LayoutData<FieldIdx, VariantIdx> {
        let dl = self.cx.data_layout();
        // This is also used for uninhabited enums, so we use `Variants::Empty`.
        LayoutData {
            variants: Variants::Empty,
            fields: FieldsShape::Primitive,
            backend_repr: BackendRepr::Memory { sized: true },
            largest_niche: None,
            uninhabited: true,
            align: dl.i8_align,
            size: Size::ZERO,
            max_repr_align: None,
            unadjusted_abi_align: dl.i8_align.abi,
            randomization_seed: Hash64::ZERO,
        }
    }

    pub fn layout_of_struct_or_enum<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
    >(
        &self,
        repr: &ReprOptions,
        variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
        is_enum: bool,
        is_unsafe_cell: bool,
        scalar_valid_range: (Bound<u128>, Bound<u128>),
        discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
        discriminants: impl Iterator<Item = (VariantIdx, i128)>,
        dont_niche_optimize_enum: bool,
        always_sized: bool,
    ) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
        let (present_first, present_second) = {
            let mut present_variants = variants
                .iter_enumerated()
                .filter_map(|(i, v)| if !repr.c() && 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 is_enum => {
                return Ok(self.layout_of_never_type());
            }
            // If it's a struct, still compute a layout so that we can still compute the
            // field offsets.
            None => VariantIdx::new(0),
        };

        // take the struct path if it is an actual struct
        if !is_enum ||
            // or for optimizing univariant enums
            (present_second.is_none() && !repr.inhibit_enum_layout_opt())
        {
            self.layout_of_struct(
                repr,
                variants,
                is_enum,
                is_unsafe_cell,
                scalar_valid_range,
                always_sized,
                present_first,
            )
        } else {
            // At this point, we have handled all unions and
            // structs. (We have also handled univariant enums
            // that allow representation optimization.)
            assert!(is_enum);
            self.layout_of_enum(
                repr,
                variants,
                discr_range_of_repr,
                discriminants,
                dont_niche_optimize_enum,
            )
        }
    }

    pub fn layout_of_union<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
    >(
        &self,
        repr: &ReprOptions,
        variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
    ) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
        let dl = self.cx.data_layout();
        let mut align = if repr.pack.is_some() { dl.i8_align } else { dl.aggregate_align };
        let mut max_repr_align = repr.align;

        // If all the non-ZST fields have the same repr and union repr optimizations aren't
        // disabled, we can use that common repr for the union as a whole.
        struct AbiMismatch;
        let mut common_non_zst_repr_and_align = if repr.inhibits_union_abi_opt() {
            // Can't optimize
            Err(AbiMismatch)
        } else {
            Ok(None)
        };

        let mut size = Size::ZERO;
        let only_variant_idx = VariantIdx::new(0);
        let only_variant = &variants[only_variant_idx];
        for field in only_variant {
            if field.is_unsized() {
                return Err(LayoutCalculatorError::UnexpectedUnsized(*field));
            }

            align = align.max(field.align);
            max_repr_align = max_repr_align.max(field.max_repr_align);
            size = cmp::max(size, field.size);

            if field.is_zst() {
                // Nothing more to do for ZST fields
                continue;
            }

            if let Ok(common) = common_non_zst_repr_and_align {
                // Discard valid range information and allow undef
                let field_abi = field.backend_repr.to_union();

                if let Some((common_abi, common_align)) = common {
                    if common_abi != field_abi {
                        // Different fields have different ABI: disable opt
                        common_non_zst_repr_and_align = Err(AbiMismatch);
                    } else {
                        // Fields with the same non-Aggregate ABI should also
                        // have the same alignment
                        if !matches!(common_abi, BackendRepr::Memory { .. }) {
                            assert_eq!(
                                common_align, field.align.abi,
                                "non-Aggregate field with matching ABI but differing alignment"
                            );
                        }
                    }
                } else {
                    // First non-ZST field: record its ABI and alignment
                    common_non_zst_repr_and_align = Ok(Some((field_abi, field.align.abi)));
                }
            }
        }

        if let Some(pack) = repr.pack {
            align = align.min(AbiAndPrefAlign::new(pack));
        }
        // The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
        // See documentation on `LayoutS::unadjusted_abi_align`.
        let unadjusted_abi_align = align.abi;
        if let Some(repr_align) = repr.align {
            align = align.max(AbiAndPrefAlign::new(repr_align));
        }
        // `align` must not be modified after this, or `unadjusted_abi_align` could be inaccurate.
        let align = align;

        // If all non-ZST fields have the same ABI, we may forward that ABI
        // for the union as a whole, unless otherwise inhibited.
        let backend_repr = match common_non_zst_repr_and_align {
            Err(AbiMismatch) | Ok(None) => BackendRepr::Memory { sized: true },
            Ok(Some((repr, _))) => match repr {
                // Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
                BackendRepr::Scalar(_) | BackendRepr::ScalarPair(_, _)
                    if repr.scalar_align(dl).unwrap() != align.abi =>
                {
                    BackendRepr::Memory { sized: true }
                }
                // Vectors require at least element alignment, else disable the opt
                BackendRepr::SimdVector { element, count: _ }
                    if element.align(dl).abi > align.abi =>
                {
                    BackendRepr::Memory { sized: true }
                }
                // the alignment tests passed and we can use this
                BackendRepr::Scalar(..)
                | BackendRepr::ScalarPair(..)
                | BackendRepr::SimdVector { .. }
                | BackendRepr::Memory { .. } => repr,
            },
        };

        let Some(union_field_count) = NonZeroUsize::new(only_variant.len()) else {
            return Err(LayoutCalculatorError::EmptyUnion);
        };

        let combined_seed = only_variant
            .iter()
            .map(|v| v.randomization_seed)
            .fold(repr.field_shuffle_seed, |acc, seed| acc.wrapping_add(seed));

        Ok(LayoutData {
            variants: Variants::Single { index: only_variant_idx },
            fields: FieldsShape::Union(union_field_count),
            backend_repr,
            largest_niche: None,
            uninhabited: false,
            align,
            size: size.align_to(align.abi),
            max_repr_align,
            unadjusted_abi_align,
            randomization_seed: combined_seed,
        })
    }

    /// single-variant enums are just structs, if you think about it
    fn layout_of_struct<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
    >(
        &self,
        repr: &ReprOptions,
        variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
        is_enum: bool,
        is_unsafe_cell: bool,
        scalar_valid_range: (Bound<u128>, Bound<u128>),
        always_sized: bool,
        present_first: VariantIdx,
    ) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
        // Struct, or univariant enum equivalent to a struct.
        // (Typechecking will reject discriminant-sizing attrs.)

        let dl = self.cx.data_layout();
        let v = present_first;
        let kind = if is_enum || variants[v].is_empty() || always_sized {
            StructKind::AlwaysSized
        } else {
            StructKind::MaybeUnsized
        };

        let mut st = self.univariant(&variants[v], repr, kind)?;
        st.variants = Variants::Single { index: v };

        if 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.backend_repr {
                BackendRepr::Scalar(scalar) => hide_niches(scalar),
                BackendRepr::ScalarPair(a, b) => {
                    hide_niches(a);
                    hide_niches(b);
                }
                BackendRepr::SimdVector { element, count: _ } => hide_niches(element),
                BackendRepr::Memory { sized: _ } => {}
            }
            st.largest_niche = None;
            return Ok(st);
        }

        let (start, end) = scalar_valid_range;
        match st.backend_repr {
            BackendRepr::Scalar(ref mut scalar) | BackendRepr::ScalarPair(ref mut scalar, _) => {
                // Enlarging validity ranges would result in missed
                // optimizations, *not* wrongly assuming the inner
                // value is valid. e.g. unions already enlarge validity ranges,
                // because the values may be uninitialized.
                //
                // Because of that we only check that the start and end
                // of the range is representable with this scalar type.

                let max_value = scalar.size(dl).unsigned_int_max();
                if let Bound::Included(start) = start {
                    // FIXME(eddyb) this might be incorrect - it doesn't
                    // account for wrap-around (end < start) ranges.
                    assert!(start <= max_value, "{start} > {max_value}");
                    scalar.valid_range_mut().start = start;
                }
                if let Bound::Included(end) = end {
                    // FIXME(eddyb) this might be incorrect - it doesn't
                    // account for wrap-around (end < start) ranges.
                    assert!(end <= max_value, "{end} > {max_value}");
                    scalar.valid_range_mut().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: {st:#?}",
            ),
        }

        Ok(st)
    }

    fn layout_of_enum<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
    >(
        &self,
        repr: &ReprOptions,
        variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
        discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
        discriminants: impl Iterator<Item = (VariantIdx, i128)>,
        dont_niche_optimize_enum: bool,
    ) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
        // 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<FieldIdx: Idx, VariantIdx: Idx> {
            layout: LayoutData<FieldIdx, VariantIdx>,
            variants: IndexVec<VariantIdx, LayoutData<FieldIdx, VariantIdx>>,
        }

        let dl = self.cx.data_layout();
        // bail if the enum has an incoherent repr that cannot be computed
        if repr.packed() {
            return Err(LayoutCalculatorError::ReprConflict);
        }

        let calculate_niche_filling_layout = || -> Option<TmpLayout<FieldIdx, VariantIdx>> {
            if dont_niche_optimize_enum {
                return None;
            }

            if variants.len() < 2 {
                return None;
            }

            let mut align = dl.aggregate_align;
            let mut max_repr_align = repr.align;
            let mut unadjusted_abi_align = align.abi;

            let mut variant_layouts = variants
                .iter_enumerated()
                .map(|(j, v)| {
                    let mut st = self.univariant(v, repr, StructKind::AlwaysSized).ok()?;
                    st.variants = Variants::Single { index: j };

                    align = align.max(st.align);
                    max_repr_align = max_repr_align.max(st.max_repr_align);
                    unadjusted_abi_align = unadjusted_abi_align.max(st.unadjusted_abi_align);

                    Some(st)
                })
                .collect::<Option<IndexVec<VariantIdx, _>>>()?;

            let largest_variant_index = variant_layouts
                .iter_enumerated()
                .max_by_key(|(_i, layout)| layout.size.bytes())
                .map(|(i, _layout)| i)?;

            let all_indices = variants.indices();
            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.end().index() as u128 - niche_variants.start().index() as u128) + 1;

            // Use the largest niche in the largest variant.
            let niche = variant_layouts[largest_variant_index].largest_niche?;
            let (niche_start, niche_scalar) = niche.reserve(dl, count)?;
            let niche_offset = niche.offset;
            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 offset in offsets.iter_mut() {
                            *offset += this_offset;
                        }
                    }
                    FieldsShape::Primitive | FieldsShape::Array { .. } | FieldsShape::Union(..) => {
                        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.is_uninhabited() {
                    layout.backend_repr = BackendRepr::Memory { sized: true };
                }
                layout.size += this_offset;

                true
            });

            if !all_variants_fit {
                return 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 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.
                    BackendRepr::Scalar(_) => BackendRepr::Scalar(niche_scalar),
                    BackendRepr::ScalarPair(first, second) => {
                        // Only the niche is guaranteed to be initialised,
                        // so use union layouts for the other primitive.
                        if niche_offset == Size::ZERO {
                            BackendRepr::ScalarPair(niche_scalar, second.to_union())
                        } else {
                            BackendRepr::ScalarPair(first.to_union(), niche_scalar)
                        }
                    }
                    _ => BackendRepr::Memory { sized: true },
                }
            } else {
                BackendRepr::Memory { sized: true }
            };

            let combined_seed = variant_layouts
                .iter()
                .map(|v| v.randomization_seed)
                .fold(repr.field_shuffle_seed, |acc, seed| acc.wrapping_add(seed));

            let layout = LayoutData {
                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: [niche_offset].into(),
                    memory_index: [0].into(),
                },
                backend_repr: abi,
                largest_niche,
                uninhabited,
                size,
                align,
                max_repr_align,
                unadjusted_abi_align,
                randomization_seed: combined_seed,
            };

            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 = repr.discr_type();
        let bits = Integer::from_attr(dl, discr_type).size().bits();
        for (i, mut val) in discriminants {
            if !repr.c() && variants[i].iter().any(|f| f.is_uninhabited()) {
                continue;
            }
            if discr_type.is_signed() {
                // sign extend the raw representation to be an i128
                val = (val << (128 - bits)) >> (128 - bits);
            }
            if val < min {
                min = val;
            }
            if val > max {
                max = val;
            }
        }
        // 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) = discr_range_of_repr(min, max); //Integer::repr_discr(tcx, ty, &repr, min, max);

        let mut align = dl.aggregate_align;
        let mut max_repr_align = repr.align;
        let mut unadjusted_abi_align = align.abi;

        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 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 = self.univariant(
                    field_layouts,
                    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_idx in st.fields.index_by_increasing_offset() {
                    let field = &field_layouts[FieldIdx::new(field_idx)];
                    if !field.is_1zst() {
                        start_align = start_align.min(field.align.abi);
                        break;
                    }
                }
                size = cmp::max(size, st.size);
                align = align.max(st.align);
                max_repr_align = max_repr_align.max(st.max_repr_align);
                unadjusted_abi_align = unadjusted_abi_align.max(st.unadjusted_abi_align);
                Ok(st)
            })
            .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;

        // Align the maximum variant size to the largest alignment.
        size = size.align_to(align.abi);

        // FIXME(oli-obk): deduplicate and harden these checks
        if size.bytes() >= dl.obj_size_bound() {
            return Err(LayoutCalculatorError::SizeOverflow);
        }

        let typeck_ity = Integer::from_attr(dl, 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)
            panic!(
                "layout decided on a larger discriminant type ({min_ity:?}) than typeck ({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 repr.c() || 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;
                        }
                    }
                    FieldsShape::Primitive | FieldsShape::Array { .. } | FieldsShape::Union(..) => {
                        panic!("encountered a non-arbitrary layout during enum layout")
                    }
                }
            }
        }

        let tag_mask = ity.size().unsigned_int_max();
        let tag = Scalar::Initialized {
            value: Primitive::Int(ity, signed),
            valid_range: WrappingRange {
                start: (min as u128 & tag_mask),
                end: (max as u128 & tag_mask),
            },
        };
        let mut abi = BackendRepr::Memory { sized: true };

        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);
        } else {
            // Try to use a ScalarPair for all tagged enums.
            // That's possible only if we can find a common primitive type for all variants.
            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 {
                    panic!("encountered a non-arbitrary layout during enum layout");
                };
                // We skip *all* ZST here and later check if we are good in terms of alignment.
                // This lets us handle some cases involving aligned ZST.
                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.backend_repr {
                    BackendRepr::Scalar(scalar) => {
                        common_prim_initialized_in_all_variants &=
                            matches!(scalar, Scalar::Initialized { .. });
                        scalar.primitive()
                    }
                    _ => {
                        common_prim = None;
                        break;
                    }
                };
                if let Some((old_prim, common_offset)) = common_prim {
                    // All variants must be at the same offset
                    if offset != common_offset {
                        common_prim = None;
                        break;
                    }
                    // This is pretty conservative. We could go fancier
                    // by realising that (u8, u8) could just cohabit with
                    // u16 or even u32.
                    let new_prim = match (old_prim, prim) {
                        // Allow all identical primitives.
                        (x, y) if x == y => x,
                        // Allow integers of the same size with differing signedness.
                        // We arbitrarily choose the signedness of the first variant.
                        (p @ Primitive::Int(x, _), Primitive::Int(y, _)) if x == y => p,
                        // Allow integers mixed with pointers of the same layout.
                        // We must represent this using a pointer, to avoid
                        // roundtripping pointers through ptrtoint/inttoptr.
                        (p @ Primitive::Pointer(_), i @ Primitive::Int(..))
                        | (i @ Primitive::Int(..), p @ Primitive::Pointer(_))
                            if p.size(dl) == i.size(dl) && p.align(dl) == i.align(dl) =>
                        {
                            p
                        }
                        _ => {
                            common_prim = None;
                            break;
                        }
                    };
                    // We may be updating the primitive here, for example from int->ptr.
                    common_prim = Some((new_prim, common_offset));
                } else {
                    common_prim = Some((prim, offset));
                }
            }
            if let Some((prim, offset)) = common_prim {
                let prim_scalar = if common_prim_initialized_in_all_variants {
                    let size = prim.size(dl);
                    assert!(size.bits() <= 128);
                    Scalar::Initialized { value: prim, valid_range: WrappingRange::full(size) }
                } else {
                    // Common prim might be uninit.
                    Scalar::Union { value: prim }
                };
                let pair = self.scalar_pair::<FieldIdx, VariantIdx>(tag, prim_scalar);
                let pair_offsets = match pair.fields {
                    FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
                        assert_eq!(memory_index.raw, [0, 1]);
                        offsets
                    }
                    _ => panic!("encountered a non-arbitrary layout during enum layout"),
                };
                if pair_offsets[FieldIdx::new(0)] == Size::ZERO
                    && pair_offsets[FieldIdx::new(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.backend_repr;
                }
            }
        }

        // 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, BackendRepr::Scalar(..) | BackendRepr::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.backend_repr, BackendRepr::Memory { .. })
                {
                    variant.backend_repr = 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 combined_seed = layout_variants
            .iter()
            .map(|v| v.randomization_seed)
            .fold(repr.field_shuffle_seed, |acc, seed| acc.wrapping_add(seed));

        let tagged_layout = LayoutData {
            variants: Variants::Multiple {
                tag,
                tag_encoding: TagEncoding::Direct,
                tag_field: 0,
                variants: IndexVec::new(),
            },
            fields: FieldsShape::Arbitrary {
                offsets: [Size::ZERO].into(),
                memory_index: [0].into(),
            },
            largest_niche,
            uninhabited,
            backend_repr: abi,
            align,
            size,
            max_repr_align,
            unadjusted_abi_align,
            randomization_seed: combined_seed,
        };

        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 cmp::Ordering::*;
                let niche_size = |tmp_l: &TmpLayout<FieldIdx, VariantIdx>| {
                    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 }
            }
            Variants::Single { .. } | Variants::Empty => {
                panic!("encountered a single-variant or empty enum during multi-variant layout")
            }
        };
        Ok(best_layout.layout)
    }

    fn univariant_biased<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
    >(
        &self,
        fields: &IndexSlice<FieldIdx, F>,
        repr: &ReprOptions,
        kind: StructKind,
        niche_bias: NicheBias,
    ) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
        let dl = self.cx.data_layout();
        let pack = repr.pack;
        let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };
        let mut max_repr_align = repr.align;
        let mut inverse_memory_index: IndexVec<u32, FieldIdx> = fields.indices().collect();
        let optimize_field_order = !repr.inhibit_struct_field_reordering();
        let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
        let optimizing = &mut inverse_memory_index.raw[..end];
        let fields_excluding_tail = &fields.raw[..end];
        // unsizable tail fields are excluded so that we use the same seed for the sized and unsized layouts.
        let field_seed = fields_excluding_tail
            .iter()
            .fold(Hash64::ZERO, |acc, f| acc.wrapping_add(f.randomization_seed));

        if optimize_field_order && fields.len() > 1 {
            // 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() && cfg!(feature = "randomize") {
                #[cfg(feature = "randomize")]
                {
                    use rand::SeedableRng;
                    use rand::seq::SliceRandom;
                    // `ReprOptions.field_shuffle_seed` is a deterministic seed we can use to randomize field
                    // ordering.
                    let mut rng = rand_xoshiro::Xoshiro128StarStar::seed_from_u64(
                        field_seed.wrapping_add(repr.field_shuffle_seed).as_u64(),
                    );

                    // Shuffle the ordering of the fields.
                    optimizing.shuffle(&mut rng);
                }
                // Otherwise we just leave things alone and actually optimize the type's fields
            } else {
                // To allow unsizing `&Foo<Type>` -> `&Foo<dyn Trait>`, the layout of the struct must
                // not depend on the layout of the tail.
                let max_field_align =
                    fields_excluding_tail.iter().map(|f| f.align.abi.bytes()).max().unwrap_or(1);
                let largest_niche_size = fields_excluding_tail
                    .iter()
                    .filter_map(|f| f.largest_niche)
                    .map(|n| n.available(dl))
                    .max()
                    .unwrap_or(0);

                // Calculates a sort key to group fields by their alignment or possibly some
                // size-derived pseudo-alignment.
                let alignment_group_key = |layout: &F| {
                    // The two branches here return values that cannot be meaningfully compared with
                    // each other. However, we know that consistently for all executions of
                    // `alignment_group_key`, one or the other branch will be taken, so this is okay.
                    if let Some(pack) = pack {
                        // Return the packed alignment in bytes.
                        layout.align.abi.min(pack).bytes()
                    } else {
                        // Returns `log2(effective-align)`. The calculation assumes that size is an
                        // integer multiple of align, except for ZSTs.
                        let align = layout.align.abi.bytes();
                        let size = layout.size.bytes();
                        let niche_size = layout.largest_niche.map(|n| n.available(dl)).unwrap_or(0);
                        // Group [u8; 4] with align-4 or [u8; 6] with align-2 fields.
                        let size_as_align = align.max(size).trailing_zeros();
                        let size_as_align = if largest_niche_size > 0 {
                            match niche_bias {
                                // Given `A(u8, [u8; 16])` and `B(bool, [u8; 16])` we want to bump the
                                // array to the front in the first case (for aligned loads) but keep
                                // the bool in front in the second case for its niches.
                                NicheBias::Start => {
                                    max_field_align.trailing_zeros().min(size_as_align)
                                }
                                // When moving niches towards the end of the struct then for
                                // A((u8, u8, u8, bool), (u8, bool, u8)) we want to keep the first tuple
                                // in the align-1 group because its bool can be moved closer to the end.
                                NicheBias::End if niche_size == largest_niche_size => {
                                    align.trailing_zeros()
                                }
                                NicheBias::End => size_as_align,
                            }
                        } else {
                            size_as_align
                        };
                        size_as_align as u64
                    }
                };

                match kind {
                    StructKind::AlwaysSized | StructKind::MaybeUnsized => {
                        // Currently `LayoutS` only exposes a single niche so sorting is usually
                        // sufficient to get one niche into the preferred position. If it ever
                        // supported multiple niches then a more advanced pick-and-pack approach could
                        // provide better results. But even for the single-niche cache it's not
                        // optimal. E.g. for A(u32, (bool, u8), u16) it would be possible to move the
                        // bool to the front but it would require packing the tuple together with the
                        // u16 to build a 4-byte group so that the u32 can be placed after it without
                        // padding. This kind of packing can't be achieved by sorting.
                        optimizing.sort_by_key(|&x| {
                            let f = &fields[x];
                            let field_size = f.size.bytes();
                            let niche_size = f.largest_niche.map_or(0, |n| n.available(dl));
                            let niche_size_key = match niche_bias {
                                // large niche first
                                NicheBias::Start => !niche_size,
                                // large niche last
                                NicheBias::End => niche_size,
                            };
                            let inner_niche_offset_key = match niche_bias {
                                NicheBias::Start => f.largest_niche.map_or(0, |n| n.offset.bytes()),
                                NicheBias::End => f.largest_niche.map_or(0, |n| {
                                    !(field_size - n.value.size(dl).bytes() - n.offset.bytes())
                                }),
                            };

                            (
                                // Then place largest alignments first.
                                cmp::Reverse(alignment_group_key(f)),
                                // Then prioritize niche placement within alignment group according to
                                // `niche_bias_start`.
                                niche_size_key,
                                // Then among fields with equally-sized niches prefer the ones
                                // closer to the start/end of the field.
                                inner_niche_offset_key,
                            )
                        });
                    }

                    StructKind::Prefixed(..) => {
                        // Sort in ascending alignment so that the layout stays optimal
                        // regardless of the prefix.
                        // And put the largest niche in an alignment group at the end
                        // so it can be used as discriminant in jagged enums
                        optimizing.sort_by_key(|&x| {
                            let f = &fields[x];
                            let niche_size = f.largest_niche.map_or(0, |n| n.available(dl));
                            (alignment_group_key(f), niche_size)
                        });
                    }
                }

                // 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 unsized_field = None::<&F>;
        let mut offsets = IndexVec::from_elem(Size::ZERO, fields);
        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];
            if let Some(unsized_field) = unsized_field {
                return Err(LayoutCalculatorError::UnexpectedUnsized(*unsized_field));
            }

            if field.is_unsized() {
                if let StructKind::MaybeUnsized = kind {
                    unsized_field = Some(field);
                } else {
                    return Err(LayoutCalculatorError::UnexpectedUnsized(*field));
                }
            }

            // 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);
            max_repr_align = max_repr_align.max(field.max_repr_align);

            debug!("univariant offset: {:?} field: {:#?}", offset, field);
            offsets[i] = offset;

            if let Some(mut niche) = field.largest_niche {
                let available = niche.available(dl);
                // Pick up larger niches.
                let prefer_new_niche = match niche_bias {
                    NicheBias::Start => available > largest_niche_available,
                    // if there are several niches of the same size then pick the last one
                    NicheBias::End => available >= largest_niche_available,
                };
                if prefer_new_niche {
                    largest_niche_available = available;
                    niche.offset += offset;
                    largest_niche = Some(niche);
                }
            }

            offset =
                offset.checked_add(field.size, dl).ok_or(LayoutCalculatorError::SizeOverflow)?;
        }

        // The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
        // See documentation on `LayoutS::unadjusted_abi_align`.
        let unadjusted_abi_align = align.abi;
        if let Some(repr_align) = repr.align {
            align = align.max(AbiAndPrefAlign::new(repr_align));
        }
        // `align` must not be modified after this point, or `unadjusted_abi_align` could be inaccurate.
        let align = 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_field_order {
            inverse_memory_index.invert_bijective_mapping()
        } else {
            debug_assert!(inverse_memory_index.iter().copied().eq(fields.indices()));
            inverse_memory_index.into_iter().map(|it| it.index() as u32).collect()
        };
        let size = min_size.align_to(align.abi);
        // FIXME(oli-obk): deduplicate and harden these checks
        if size.bytes() >= dl.obj_size_bound() {
            return Err(LayoutCalculatorError::SizeOverflow);
        }
        let mut layout_of_single_non_zst_field = None;
        let sized = unsized_field.is_none();
        let mut abi = BackendRepr::Memory { sized };

        let optimize_abi = !repr.inhibit_newtype_abi_optimization();

        // Try to make this a Scalar/ScalarPair.
        if sized && size.bytes() > 0 {
            // We skip *all* ZST here and later check if we are good in terms of alignment.
            // This lets us handle some cases involving aligned ZST.
            let mut non_zst_fields = fields.iter_enumerated().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) => {
                    layout_of_single_non_zst_field = Some(field);

                    // 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.backend_repr {
                            // For plain scalars, or vectors of them, we can't unpack
                            // newtypes for `#[repr(C)]`, as that affects C ABIs.
                            BackendRepr::Scalar(_) | BackendRepr::SimdVector { .. }
                                if optimize_abi =>
                            {
                                abi = field.backend_repr;
                            }
                            // But scalar pairs are Rust-specific and get
                            // treated as aggregates by C ABIs anyway.
                            BackendRepr::ScalarPair(..) => {
                                abi = field.backend_repr;
                            }
                            _ => {}
                        }
                    }
                }

                // Two non-ZST fields, and they're both scalars.
                (Some((i, a)), Some((j, b)), None) => {
                    match (a.backend_repr, b.backend_repr) {
                        (BackendRepr::Scalar(a), BackendRepr::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 = self.scalar_pair::<FieldIdx, VariantIdx>(a, b);
                            let pair_offsets = match pair.fields {
                                FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
                                    assert_eq!(memory_index.raw, [0, 1]);
                                    offsets
                                }
                                FieldsShape::Primitive
                                | FieldsShape::Array { .. }
                                | FieldsShape::Union(..) => {
                                    panic!("encountered a non-arbitrary layout during enum layout")
                                }
                            };
                            if offsets[i] == pair_offsets[FieldIdx::new(0)]
                                && offsets[j] == pair_offsets[FieldIdx::new(1)]
                                && align == pair.align
                                && size == pair.size
                            {
                                // We can use `ScalarPair` only when it matches our
                                // already computed layout (including `#[repr(C)]`).
                                abi = pair.backend_repr;
                            }
                        }
                        _ => {}
                    }
                }

                _ => {}
            }
        }
        let uninhabited = fields.iter().any(|f| f.is_uninhabited());

        let unadjusted_abi_align = if repr.transparent() {
            match layout_of_single_non_zst_field {
                Some(l) => l.unadjusted_abi_align,
                None => {
                    // `repr(transparent)` with all ZST fields.
                    align.abi
                }
            }
        } else {
            unadjusted_abi_align
        };

        let seed = field_seed.wrapping_add(repr.field_shuffle_seed);

        Ok(LayoutData {
            variants: Variants::Single { index: VariantIdx::new(0) },
            fields: FieldsShape::Arbitrary { offsets, memory_index },
            backend_repr: abi,
            largest_niche,
            uninhabited,
            align,
            size,
            max_repr_align,
            unadjusted_abi_align,
            randomization_seed: seed,
        })
    }

    fn format_field_niches<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + fmt::Debug,
    >(
        &self,
        layout: &LayoutData<FieldIdx, VariantIdx>,
        fields: &IndexSlice<FieldIdx, F>,
    ) -> String {
        let dl = self.cx.data_layout();
        let mut s = String::new();
        for i in layout.fields.index_by_increasing_offset() {
            let offset = layout.fields.offset(i);
            let f = &fields[FieldIdx::new(i)];
            write!(s, "[o{}a{}s{}", offset.bytes(), f.align.abi.bytes(), f.size.bytes()).unwrap();
            if let Some(n) = f.largest_niche {
                write!(
                    s,
                    " n{}b{}s{}",
                    n.offset.bytes(),
                    n.available(dl).ilog2(),
                    n.value.size(dl).bytes()
                )
                .unwrap();
            }
            write!(s, "] ").unwrap();
        }
        s
    }
}