rustc_transmute/layout/
tree.rs

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1use std::ops::ControlFlow;
2
3use super::{Byte, Def, Ref};
4
5#[cfg(test)]
6mod tests;
7
8/// A tree-based representation of a type layout.
9///
10/// Invariants:
11/// 1. All paths through the layout have the same length (in bytes).
12///
13/// Nice-to-haves:
14/// 1. An `Alt` is never directly nested beneath another `Alt`.
15/// 2. A `Seq` is never directly nested beneath another `Seq`.
16/// 3. `Seq`s and `Alt`s with a single member do not exist.
17#[derive(Clone, Debug, Hash, PartialEq, Eq)]
18pub(crate) enum Tree<D, R>
19where
20    D: Def,
21    R: Ref,
22{
23    /// A sequence of successive layouts.
24    Seq(Vec<Self>),
25    /// A choice between alternative layouts.
26    Alt(Vec<Self>),
27    /// A definition node.
28    Def(D),
29    /// A reference node.
30    Ref(R),
31    /// A byte node.
32    Byte(Byte),
33}
34
35impl<D, R> Tree<D, R>
36where
37    D: Def,
38    R: Ref,
39{
40    /// A `Tree` consisting only of a definition node.
41    pub(crate) fn def(def: D) -> Self {
42        Self::Def(def)
43    }
44
45    /// A `Tree` representing an uninhabited type.
46    pub(crate) fn uninhabited() -> Self {
47        Self::Alt(vec![])
48    }
49
50    /// A `Tree` representing a zero-sized type.
51    pub(crate) fn unit() -> Self {
52        Self::Seq(Vec::new())
53    }
54
55    /// A `Tree` containing a single, uninitialized byte.
56    pub(crate) fn uninit() -> Self {
57        Self::Byte(Byte::Uninit)
58    }
59
60    /// A `Tree` representing the layout of `bool`.
61    pub(crate) fn bool() -> Self {
62        Self::from_bits(0x00).or(Self::from_bits(0x01))
63    }
64
65    /// A `Tree` whose layout matches that of a `u8`.
66    pub(crate) fn u8() -> Self {
67        Self::Alt((0u8..=255).map(Self::from_bits).collect())
68    }
69
70    /// A `Tree` whose layout accepts exactly the given bit pattern.
71    pub(crate) fn from_bits(bits: u8) -> Self {
72        Self::Byte(Byte::Init(bits))
73    }
74
75    /// A `Tree` whose layout is a number of the given width.
76    pub(crate) fn number(width_in_bytes: usize) -> Self {
77        Self::Seq(vec![Self::u8(); width_in_bytes])
78    }
79
80    /// A `Tree` whose layout is entirely padding of the given width.
81    pub(crate) fn padding(width_in_bytes: usize) -> Self {
82        Self::Seq(vec![Self::uninit(); width_in_bytes])
83    }
84
85    /// Remove all `Def` nodes, and all branches of the layout for which `f`
86    /// produces `true`.
87    pub(crate) fn prune<F>(self, f: &F) -> Tree<!, R>
88    where
89        F: Fn(D) -> bool,
90    {
91        match self {
92            Self::Seq(elts) => match elts.into_iter().map(|elt| elt.prune(f)).try_fold(
93                Tree::unit(),
94                |elts, elt| {
95                    if elt == Tree::uninhabited() {
96                        ControlFlow::Break(Tree::uninhabited())
97                    } else {
98                        ControlFlow::Continue(elts.then(elt))
99                    }
100                },
101            ) {
102                ControlFlow::Break(node) | ControlFlow::Continue(node) => node,
103            },
104            Self::Alt(alts) => alts
105                .into_iter()
106                .map(|alt| alt.prune(f))
107                .fold(Tree::uninhabited(), |alts, alt| alts.or(alt)),
108            Self::Byte(b) => Tree::Byte(b),
109            Self::Ref(r) => Tree::Ref(r),
110            Self::Def(d) => {
111                if f(d) {
112                    Tree::uninhabited()
113                } else {
114                    Tree::unit()
115                }
116            }
117        }
118    }
119
120    /// Produces `true` if `Tree` is an inhabited type; otherwise false.
121    pub(crate) fn is_inhabited(&self) -> bool {
122        match self {
123            Self::Seq(elts) => elts.into_iter().all(|elt| elt.is_inhabited()),
124            Self::Alt(alts) => alts.into_iter().any(|alt| alt.is_inhabited()),
125            Self::Byte(..) | Self::Ref(..) | Self::Def(..) => true,
126        }
127    }
128}
129
130impl<D, R> Tree<D, R>
131where
132    D: Def,
133    R: Ref,
134{
135    /// Produces a new `Tree` where `other` is sequenced after `self`.
136    pub(crate) fn then(self, other: Self) -> Self {
137        match (self, other) {
138            (Self::Seq(elts), other) | (other, Self::Seq(elts)) if elts.len() == 0 => other,
139            (Self::Seq(mut lhs), Self::Seq(mut rhs)) => {
140                lhs.append(&mut rhs);
141                Self::Seq(lhs)
142            }
143            (Self::Seq(mut lhs), rhs) => {
144                lhs.push(rhs);
145                Self::Seq(lhs)
146            }
147            (lhs, Self::Seq(mut rhs)) => {
148                rhs.insert(0, lhs);
149                Self::Seq(rhs)
150            }
151            (lhs, rhs) => Self::Seq(vec![lhs, rhs]),
152        }
153    }
154
155    /// Produces a new `Tree` accepting either `self` or `other` as alternative layouts.
156    pub(crate) fn or(self, other: Self) -> Self {
157        match (self, other) {
158            (Self::Alt(alts), other) | (other, Self::Alt(alts)) if alts.len() == 0 => other,
159            (Self::Alt(mut lhs), Self::Alt(rhs)) => {
160                lhs.extend(rhs);
161                Self::Alt(lhs)
162            }
163            (Self::Alt(mut alts), alt) | (alt, Self::Alt(mut alts)) => {
164                alts.push(alt);
165                Self::Alt(alts)
166            }
167            (lhs, rhs) => Self::Alt(vec![lhs, rhs]),
168        }
169    }
170}
171
172#[cfg(feature = "rustc")]
173pub(crate) mod rustc {
174    use rustc_abi::{
175        FieldIdx, FieldsShape, Layout, Size, TagEncoding, TyAndLayout, VariantIdx, Variants,
176    };
177    use rustc_middle::ty::layout::{HasTyCtxt, LayoutCx, LayoutError};
178    use rustc_middle::ty::{self, AdtDef, AdtKind, List, ScalarInt, Ty, TyCtxt, TypeVisitableExt};
179    use rustc_span::ErrorGuaranteed;
180
181    use super::Tree;
182    use crate::layout::rustc::{Def, Ref, layout_of};
183
184    #[derive(Debug, Copy, Clone)]
185    pub(crate) enum Err {
186        /// The layout of the type is not yet supported.
187        NotYetSupported,
188        /// This error will be surfaced elsewhere by rustc, so don't surface it.
189        UnknownLayout,
190        /// Overflow size
191        SizeOverflow,
192        TypeError(ErrorGuaranteed),
193    }
194
195    impl<'tcx> From<&LayoutError<'tcx>> for Err {
196        fn from(err: &LayoutError<'tcx>) -> Self {
197            match err {
198                LayoutError::Unknown(..)
199                | LayoutError::ReferencesError(..)
200                | LayoutError::TooGeneric(..)
201                | LayoutError::NormalizationFailure(..) => Self::UnknownLayout,
202                LayoutError::SizeOverflow(..) => Self::SizeOverflow,
203                LayoutError::Cycle(err) => Self::TypeError(*err),
204            }
205        }
206    }
207
208    impl<'tcx> Tree<Def<'tcx>, Ref<'tcx>> {
209        pub(crate) fn from_ty(ty: Ty<'tcx>, cx: LayoutCx<'tcx>) -> Result<Self, Err> {
210            use rustc_abi::HasDataLayout;
211            let layout = layout_of(cx, ty)?;
212
213            if let Err(e) = ty.error_reported() {
214                return Err(Err::TypeError(e));
215            }
216
217            let target = cx.data_layout();
218            let pointer_size = target.pointer_size;
219
220            match ty.kind() {
221                ty::Bool => Ok(Self::bool()),
222
223                ty::Float(nty) => {
224                    let width = nty.bit_width() / 8;
225                    Ok(Self::number(width as _))
226                }
227
228                ty::Int(nty) => {
229                    let width = nty.normalize(pointer_size.bits() as _).bit_width().unwrap() / 8;
230                    Ok(Self::number(width as _))
231                }
232
233                ty::Uint(nty) => {
234                    let width = nty.normalize(pointer_size.bits() as _).bit_width().unwrap() / 8;
235                    Ok(Self::number(width as _))
236                }
237
238                ty::Tuple(members) => Self::from_tuple((ty, layout), members, cx),
239
240                ty::Array(inner_ty, _len) => {
241                    let FieldsShape::Array { stride, count } = &layout.fields else {
242                        return Err(Err::NotYetSupported);
243                    };
244                    let inner_layout = layout_of(cx, *inner_ty)?;
245                    assert_eq!(*stride, inner_layout.size);
246                    let elt = Tree::from_ty(*inner_ty, cx)?;
247                    Ok(std::iter::repeat(elt)
248                        .take(*count as usize)
249                        .fold(Tree::unit(), |tree, elt| tree.then(elt)))
250                }
251
252                ty::Adt(adt_def, _args_ref) if !ty.is_box() => match adt_def.adt_kind() {
253                    AdtKind::Struct => Self::from_struct((ty, layout), *adt_def, cx),
254                    AdtKind::Enum => Self::from_enum((ty, layout), *adt_def, cx),
255                    AdtKind::Union => Self::from_union((ty, layout), *adt_def, cx),
256                },
257
258                ty::Ref(lifetime, ty, mutability) => {
259                    let layout = layout_of(cx, *ty)?;
260                    let align = layout.align.abi.bytes_usize();
261                    let size = layout.size.bytes_usize();
262                    Ok(Tree::Ref(Ref {
263                        lifetime: *lifetime,
264                        ty: *ty,
265                        mutability: *mutability,
266                        align,
267                        size,
268                    }))
269                }
270
271                _ => Err(Err::NotYetSupported),
272            }
273        }
274
275        /// Constructs a `Tree` from a tuple.
276        fn from_tuple(
277            (ty, layout): (Ty<'tcx>, Layout<'tcx>),
278            members: &'tcx List<Ty<'tcx>>,
279            cx: LayoutCx<'tcx>,
280        ) -> Result<Self, Err> {
281            match &layout.fields {
282                FieldsShape::Primitive => {
283                    assert_eq!(members.len(), 1);
284                    let inner_ty = members[0];
285                    Self::from_ty(inner_ty, cx)
286                }
287                FieldsShape::Arbitrary { offsets, .. } => {
288                    assert_eq!(offsets.len(), members.len());
289                    Self::from_variant(Def::Primitive, None, (ty, layout), layout.size, cx)
290                }
291                FieldsShape::Array { .. } | FieldsShape::Union(_) => Err(Err::NotYetSupported),
292            }
293        }
294
295        /// Constructs a `Tree` from a struct.
296        ///
297        /// # Panics
298        ///
299        /// Panics if `def` is not a struct definition.
300        fn from_struct(
301            (ty, layout): (Ty<'tcx>, Layout<'tcx>),
302            def: AdtDef<'tcx>,
303            cx: LayoutCx<'tcx>,
304        ) -> Result<Self, Err> {
305            assert!(def.is_struct());
306            let def = Def::Adt(def);
307            Self::from_variant(def, None, (ty, layout), layout.size, cx)
308        }
309
310        /// Constructs a `Tree` from an enum.
311        ///
312        /// # Panics
313        ///
314        /// Panics if `def` is not an enum definition.
315        fn from_enum(
316            (ty, layout): (Ty<'tcx>, Layout<'tcx>),
317            def: AdtDef<'tcx>,
318            cx: LayoutCx<'tcx>,
319        ) -> Result<Self, Err> {
320            assert!(def.is_enum());
321
322            // Computes the layout of a variant.
323            let layout_of_variant =
324                |index, encoding: Option<TagEncoding<VariantIdx>>| -> Result<Self, Err> {
325                    let variant_layout = ty_variant(cx, (ty, layout), index);
326                    if variant_layout.is_uninhabited() {
327                        return Ok(Self::uninhabited());
328                    }
329                    let tag = cx.tcx().tag_for_variant((cx.tcx().erase_regions(ty), index));
330                    let variant_def = Def::Variant(def.variant(index));
331                    Self::from_variant(
332                        variant_def,
333                        tag.map(|tag| (tag, index, encoding.unwrap())),
334                        (ty, variant_layout),
335                        layout.size,
336                        cx,
337                    )
338                };
339
340            match layout.variants() {
341                Variants::Empty => Ok(Self::uninhabited()),
342                Variants::Single { index } => {
343                    // `Variants::Single` on enums with variants denotes that
344                    // the enum delegates its layout to the variant at `index`.
345                    layout_of_variant(*index, None)
346                }
347                Variants::Multiple { tag: _, tag_encoding, tag_field, .. } => {
348                    // `Variants::Multiple` denotes an enum with multiple
349                    // variants. The layout of such an enum is the disjunction
350                    // of the layouts of its tagged variants.
351
352                    // For enums (but not coroutines), the tag field is
353                    // currently always the first field of the layout.
354                    assert_eq!(*tag_field, 0);
355
356                    let variants = def.discriminants(cx.tcx()).try_fold(
357                        Self::uninhabited(),
358                        |variants, (idx, _discriminant)| {
359                            let variant = layout_of_variant(idx, Some(tag_encoding.clone()))?;
360                            Result::<Self, Err>::Ok(variants.or(variant))
361                        },
362                    )?;
363
364                    Ok(Self::def(Def::Adt(def)).then(variants))
365                }
366            }
367        }
368
369        /// Constructs a `Tree` from a 'variant-like' layout.
370        ///
371        /// A 'variant-like' layout includes those of structs and, of course,
372        /// enum variants. Pragmatically speaking, this method supports anything
373        /// with `FieldsShape::Arbitrary`.
374        ///
375        /// Note: This routine assumes that the optional `tag` is the first
376        /// field, and enum callers should check that `tag_field` is, in fact,
377        /// `0`.
378        fn from_variant(
379            def: Def<'tcx>,
380            tag: Option<(ScalarInt, VariantIdx, TagEncoding<VariantIdx>)>,
381            (ty, layout): (Ty<'tcx>, Layout<'tcx>),
382            total_size: Size,
383            cx: LayoutCx<'tcx>,
384        ) -> Result<Self, Err> {
385            // This constructor does not support non-`FieldsShape::Arbitrary`
386            // layouts.
387            let FieldsShape::Arbitrary { offsets, memory_index } = layout.fields() else {
388                return Err(Err::NotYetSupported);
389            };
390
391            // When this function is invoked with enum variants,
392            // `ty_and_layout.size` does not encompass the entire size of the
393            // enum. We rely on `total_size` for this.
394            assert!(layout.size <= total_size);
395
396            let mut size = Size::ZERO;
397            let mut struct_tree = Self::def(def);
398
399            // If a `tag` is provided, place it at the start of the layout.
400            if let Some((tag, index, encoding)) = &tag {
401                match encoding {
402                    TagEncoding::Direct => {
403                        size += tag.size();
404                    }
405                    TagEncoding::Niche { niche_variants, .. } => {
406                        if !niche_variants.contains(index) {
407                            size += tag.size();
408                        }
409                    }
410                }
411                struct_tree = struct_tree.then(Self::from_tag(*tag, cx.tcx()));
412            }
413
414            // Append the fields, in memory order, to the layout.
415            let inverse_memory_index = memory_index.invert_bijective_mapping();
416            for &field_idx in inverse_memory_index.iter() {
417                // Add interfield padding.
418                let padding_needed = offsets[field_idx] - size;
419                let padding = Self::padding(padding_needed.bytes_usize());
420
421                let field_ty = ty_field(cx, (ty, layout), field_idx);
422                let field_layout = layout_of(cx, field_ty)?;
423                let field_tree = Self::from_ty(field_ty, cx)?;
424
425                struct_tree = struct_tree.then(padding).then(field_tree);
426
427                size += padding_needed + field_layout.size;
428            }
429
430            // Add trailing padding.
431            let padding_needed = total_size - size;
432            let trailing_padding = Self::padding(padding_needed.bytes_usize());
433
434            Ok(struct_tree.then(trailing_padding))
435        }
436
437        /// Constructs a `Tree` representing the value of a enum tag.
438        fn from_tag(tag: ScalarInt, tcx: TyCtxt<'tcx>) -> Self {
439            use rustc_abi::Endian;
440            let size = tag.size();
441            let bits = tag.to_bits(size);
442            let bytes: [u8; 16];
443            let bytes = match tcx.data_layout.endian {
444                Endian::Little => {
445                    bytes = bits.to_le_bytes();
446                    &bytes[..size.bytes_usize()]
447                }
448                Endian::Big => {
449                    bytes = bits.to_be_bytes();
450                    &bytes[bytes.len() - size.bytes_usize()..]
451                }
452            };
453            Self::Seq(bytes.iter().map(|&b| Self::from_bits(b)).collect())
454        }
455
456        /// Constructs a `Tree` from a union.
457        ///
458        /// # Panics
459        ///
460        /// Panics if `def` is not a union definition.
461        fn from_union(
462            (ty, layout): (Ty<'tcx>, Layout<'tcx>),
463            def: AdtDef<'tcx>,
464            cx: LayoutCx<'tcx>,
465        ) -> Result<Self, Err> {
466            assert!(def.is_union());
467
468            // This constructor does not support non-`FieldsShape::Union`
469            // layouts. Fields of this shape are all placed at offset 0.
470            let FieldsShape::Union(_fields) = layout.fields() else {
471                return Err(Err::NotYetSupported);
472            };
473
474            let fields = &def.non_enum_variant().fields;
475            let fields = fields.iter_enumerated().try_fold(
476                Self::uninhabited(),
477                |fields, (idx, _field_def)| {
478                    let field_ty = ty_field(cx, (ty, layout), idx);
479                    let field_layout = layout_of(cx, field_ty)?;
480                    let field = Self::from_ty(field_ty, cx)?;
481                    let trailing_padding_needed = layout.size - field_layout.size;
482                    let trailing_padding = Self::padding(trailing_padding_needed.bytes_usize());
483                    let field_and_padding = field.then(trailing_padding);
484                    Result::<Self, Err>::Ok(fields.or(field_and_padding))
485                },
486            )?;
487
488            Ok(Self::def(Def::Adt(def)).then(fields))
489        }
490    }
491
492    fn ty_field<'tcx>(
493        cx: LayoutCx<'tcx>,
494        (ty, layout): (Ty<'tcx>, Layout<'tcx>),
495        i: FieldIdx,
496    ) -> Ty<'tcx> {
497        // We cannot use `ty_and_layout_field` to retrieve the field type, since
498        // `ty_and_layout_field` erases regions in the returned type. We must
499        // not erase regions here, since we may need to ultimately emit outlives
500        // obligations as a consequence of the transmutability analysis.
501        match ty.kind() {
502            ty::Adt(def, args) => {
503                match layout.variants {
504                    Variants::Single { index } => {
505                        let field = &def.variant(index).fields[i];
506                        field.ty(cx.tcx(), args)
507                    }
508                    Variants::Empty => panic!("there is no field in Variants::Empty types"),
509                    // Discriminant field for enums (where applicable).
510                    Variants::Multiple { tag, .. } => {
511                        assert_eq!(i.as_usize(), 0);
512                        ty::layout::PrimitiveExt::to_ty(&tag.primitive(), cx.tcx())
513                    }
514                }
515            }
516            ty::Tuple(fields) => fields[i.as_usize()],
517            kind @ _ => unimplemented!(
518                "only a subset of `Ty::ty_and_layout_field`'s functionality is implemented. implementation needed for {:?}",
519                kind
520            ),
521        }
522    }
523
524    fn ty_variant<'tcx>(
525        cx: LayoutCx<'tcx>,
526        (ty, layout): (Ty<'tcx>, Layout<'tcx>),
527        i: VariantIdx,
528    ) -> Layout<'tcx> {
529        let ty = cx.tcx().erase_regions(ty);
530        TyAndLayout { ty, layout }.for_variant(&cx, i).layout
531    }
532}
rustc_transmute/layout/tree.rs

rustc_transmute/layout/
tree.rs
