Add tag_for_variant query

This query allows for sharing code between `rustc_const_eval` and
`rustc_transmutability`.

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -3,7 +3,7 @@ use either::{Left, Right};
 use rustc_hir::def::DefKind;
 use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo};
 use rustc_middle::mir::{self, ConstAlloc, ConstValue};
-use rustc_middle::query::TyCtxtAt;
+use rustc_middle::query::{Key, TyCtxtAt};
 use rustc_middle::traits::Reveal;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -243,6 +243,24 @@ pub(crate) fn turn_into_const_value<'tcx>(
     op_to_const(&ecx, &mplace.into(), /* for diagnostics */ false)
 }
 
+/// Computes the tag (if any) for a given type and variant.
+#[instrument(skip(tcx), level = "debug")]
+pub fn tag_for_variant_provider<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    (ty, variant_index): (Ty<'tcx>, abi::VariantIdx),
+) -> Option<ty::ScalarInt> {
+    assert!(ty.is_enum());
+
+    let ecx = InterpCx::new(
+        tcx,
+        ty.default_span(tcx),
+        ty::ParamEnv::reveal_all(),
+        CompileTimeInterpreter::new(CanAccessMutGlobal::No, CheckAlignment::Error),
+    );
+
+    ecx.tag_for_variant(ty, variant_index).unwrap().map(|(tag, _tag_field)| tag)
+}
+
 #[instrument(skip(tcx), level = "debug")]
 pub fn eval_to_const_value_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,

compiler/rustc_const_eval/src/interpret/discriminant.rs

@@ -2,7 +2,7 @@
 
 use rustc_middle::mir;
 use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt};
-use rustc_middle::ty::{self, Ty};
+use rustc_middle::ty::{self, ScalarInt, Ty};
 use rustc_target::abi::{self, TagEncoding};
 use rustc_target::abi::{VariantIdx, Variants};
 
@@ -28,78 +28,27 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             throw_ub!(UninhabitedEnumVariantWritten(variant_index))
         }
 
-        match dest.layout().variants {
-            abi::Variants::Single { index } => {
-                assert_eq!(index, variant_index);
-            }
-            abi::Variants::Multiple {
-                tag_encoding: TagEncoding::Direct,
-                tag: tag_layout,
-                tag_field,
-                ..
-            } => {
+        match self.tag_for_variant(dest.layout().ty, variant_index)? {
+            Some((tag, tag_field)) => {
                 // No need to validate that the discriminant here because the
-                // `TyAndLayout::for_variant()` call earlier already checks the variant is valid.
-
-                let discr_val = dest
-                    .layout()
-                    .ty
-                    .discriminant_for_variant(*self.tcx, variant_index)
-                    .unwrap()
-                    .val;
-
-                // raw discriminants for enums are isize or bigger during
-                // their computation, but the in-memory tag is the smallest possible
-                // representation
-                let size = tag_layout.size(self);
-                let tag_val = size.truncate(discr_val);
-
+                // `TyAndLayout::for_variant()` call earlier already checks the
+                // variant is valid.
                 let tag_dest = self.project_field(dest, tag_field)?;
-                self.write_scalar(Scalar::from_uint(tag_val, size), &tag_dest)?;
+                self.write_scalar(tag, &tag_dest)
             }
-            abi::Variants::Multiple {
-                tag_encoding:
-                    TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start },
-                tag: tag_layout,
-                tag_field,
-                ..
-            } => {
-                // No need to validate that the discriminant here because the
-                // `TyAndLayout::for_variant()` call earlier already checks the variant is valid.
-
-                if variant_index != untagged_variant {
-                    let variants_start = niche_variants.start().as_u32();
-                    let variant_index_relative = variant_index
-                        .as_u32()
-                        .checked_sub(variants_start)
-                        .expect("overflow computing relative variant idx");
-                    // We need to use machine arithmetic when taking into account `niche_start`:
-                    // tag_val = variant_index_relative + niche_start_val
-                    let tag_layout = self.layout_of(tag_layout.primitive().to_int_ty(*self.tcx))?;
-                    let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
-                    let variant_index_relative_val =
-                        ImmTy::from_uint(variant_index_relative, tag_layout);
-                    let tag_val = self.wrapping_binary_op(
-                        mir::BinOp::Add,
-                        &variant_index_relative_val,
-                        &niche_start_val,
-                    )?;
-                    // Write result.
-                    let niche_dest = self.project_field(dest, tag_field)?;
-                    self.write_immediate(*tag_val, &niche_dest)?;
-                } else {
-                    // The untagged variant is implicitly encoded simply by having a value that is
-                    // outside the niche variants. But what if the data stored here does not
-                    // actually encode this variant? That would be bad! So let's double-check...
-                    let actual_variant = self.read_discriminant(&dest.to_op(self)?)?;
-                    if actual_variant != variant_index {
-                        throw_ub!(InvalidNichedEnumVariantWritten { enum_ty: dest.layout().ty });
-                    }
+            None => {
+                // No need to write the tag here, because an untagged variant is
+                // implicitly encoded. For `Niche`-optimized enums, it's by
+                // simply by having a value that is outside the niche variants.
+                // But what if the data stored here does not actually encode
+                // this variant? That would be bad! So let's double-check...
+                let actual_variant = self.read_discriminant(&dest.to_op(self)?)?;
+                if actual_variant != variant_index {
+                    throw_ub!(InvalidNichedEnumVariantWritten { enum_ty: dest.layout().ty });
                 }
+                Ok(())
             }
         }
-
-        Ok(())
     }
 
     /// Read discriminant, return the runtime value as well as the variant index.
@@ -277,4 +226,77 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         };
         Ok(ImmTy::from_scalar(discr_value, discr_layout))
     }
+
+    /// Computes the tag value and its field number (if any) of a given variant
+    /// of type `ty`.
+    pub(crate) fn tag_for_variant(
+        &self,
+        ty: Ty<'tcx>,
+        variant_index: VariantIdx,
+    ) -> InterpResult<'tcx, Option<(ScalarInt, usize)>> {
+        match self.layout_of(ty)?.variants {
+            abi::Variants::Single { index } => {
+                assert_eq!(index, variant_index);
+                Ok(None)
+            }
+
+            abi::Variants::Multiple {
+                tag_encoding: TagEncoding::Direct,
+                tag: tag_layout,
+                tag_field,
+                ..
+            } => {
+                // raw discriminants for enums are isize or bigger during
+                // their computation, but the in-memory tag is the smallest possible
+                // representation
+                let discr = self.discriminant_for_variant(ty, variant_index)?;
+                let discr_size = discr.layout.size;
+                let discr_val = discr.to_scalar().to_bits(discr_size)?;
+                let tag_size = tag_layout.size(self);
+                let tag_val = tag_size.truncate(discr_val);
+                let tag = ScalarInt::try_from_uint(tag_val, tag_size).unwrap();
+                Ok(Some((tag, tag_field)))
+            }
+
+            abi::Variants::Multiple {
+                tag_encoding: TagEncoding::Niche { untagged_variant, .. },
+                ..
+            } if untagged_variant == variant_index => {
+                // The untagged variant is implicitly encoded simply by having a
+                // value that is outside the niche variants.
+                Ok(None)
+            }
+
+            abi::Variants::Multiple {
+                tag_encoding:
+                    TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start },
+                tag: tag_layout,
+                tag_field,
+                ..
+            } => {
+                assert!(variant_index != untagged_variant);
+                let variants_start = niche_variants.start().as_u32();
+                let variant_index_relative = variant_index
+                    .as_u32()
+                    .checked_sub(variants_start)
+                    .expect("overflow computing relative variant idx");
+                // We need to use machine arithmetic when taking into account `niche_start`:
+                // tag_val = variant_index_relative + niche_start_val
+                let tag_layout = self.layout_of(tag_layout.primitive().to_int_ty(*self.tcx))?;
+                let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
+                let variant_index_relative_val =
+                    ImmTy::from_uint(variant_index_relative, tag_layout);
+                let tag = self
+                    .wrapping_binary_op(
+                        mir::BinOp::Add,
+                        &variant_index_relative_val,
+                        &niche_start_val,
+                    )?
+                    .to_scalar()
+                    .try_to_int()
+                    .unwrap();
+                Ok(Some((tag, tag_field)))
+            }
+        }
+    }
 }

compiler/rustc_const_eval/src/lib.rs

@@ -40,6 +40,7 @@ rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub fn provide(providers: &mut Providers) {
     const_eval::provide(providers);
+    providers.tag_for_variant = const_eval::tag_for_variant_provider;
     providers.eval_to_const_value_raw = const_eval::eval_to_const_value_raw_provider;
     providers.eval_to_allocation_raw = const_eval::eval_to_allocation_raw_provider;
     providers.eval_static_initializer = const_eval::eval_static_initializer_provider;

compiler/rustc_middle/src/query/erase.rs

@@ -234,6 +234,7 @@ trivial! {
     Option<rustc_middle::middle::stability::DeprecationEntry>,
     Option<rustc_middle::ty::Destructor>,
     Option<rustc_middle::ty::ImplTraitInTraitData>,
+    Option<rustc_middle::ty::ScalarInt>,
     Option<rustc_span::def_id::CrateNum>,
     Option<rustc_span::def_id::DefId>,
     Option<rustc_span::def_id::LocalDefId>,

compiler/rustc_middle/src/query/keys.rs

@@ -13,6 +13,7 @@ use rustc_query_system::query::DefIdCacheSelector;
 use rustc_query_system::query::{DefaultCacheSelector, SingleCacheSelector, VecCacheSelector};
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
+use rustc_target::abi;
 
 /// Placeholder for `CrateNum`'s "local" counterpart
 #[derive(Copy, Clone, Debug)]
@@ -502,6 +503,14 @@ impl<'tcx> Key for (DefId, Ty<'tcx>, GenericArgsRef<'tcx>, ty::ParamEnv<'tcx>) {
     }
 }
 
+impl<'tcx> Key for (Ty<'tcx>, abi::VariantIdx) {
+    type CacheSelector = DefaultCacheSelector<Self>;
+
+    fn default_span(&self, _tcx: TyCtxt<'_>) -> Span {
+        DUMMY_SP
+    }
+}
+
 impl<'tcx> Key for (ty::Predicate<'tcx>, traits::WellFormedLoc) {
     type CacheSelector = DefaultCacheSelector<Self>;
 

compiler/rustc_middle/src/query/mod.rs

@@ -1042,6 +1042,13 @@ rustc_queries! {
         }
     }
 
+    /// Computes the tag (if any) for a given type and variant.
+    query tag_for_variant(
+        key: (Ty<'tcx>, abi::VariantIdx)
+    ) -> Option<ty::ScalarInt> {
+        desc { "computing variant tag for enum" }
+    }
+
     /// Evaluates a constant and returns the computed allocation.
     ///
     /// **Do not use this** directly, use the `eval_to_const_value` or `eval_to_valtree` instead.

compiler/rustc_transmute/src/layout/tree.rs

@@ -174,10 +174,10 @@ pub(crate) mod rustc {
     use crate::layout::rustc::{Def, Ref};
 
     use rustc_middle::ty::layout::LayoutError;
-    use rustc_middle::ty::util::Discr;
     use rustc_middle::ty::AdtDef;
     use rustc_middle::ty::GenericArgsRef;
     use rustc_middle::ty::ParamEnv;
+    use rustc_middle::ty::ScalarInt;
     use rustc_middle::ty::VariantDef;
     use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
     use rustc_span::ErrorGuaranteed;
@@ -331,14 +331,15 @@ pub(crate) mod rustc {
                             trace!(?adt_def, "treeifying enum");
                             let mut tree = Tree::uninhabited();
 
-                            for (idx, discr) in adt_def.discriminants(tcx) {
+                            for (idx, variant) in adt_def.variants().iter_enumerated() {
+                                let tag = tcx.tag_for_variant((ty, idx));
                                 tree = tree.or(Self::from_repr_c_variant(
                                     ty,
                                     *adt_def,
                                     args_ref,
                                     &layout_summary,
-                                    Some(discr),
-                                    adt_def.variant(idx),
+                                    tag,
+                                    variant,
                                     tcx,
                                 )?);
                             }
@@ -393,7 +394,7 @@ pub(crate) mod rustc {
             adt_def: AdtDef<'tcx>,
             args_ref: GenericArgsRef<'tcx>,
             layout_summary: &LayoutSummary,
-            discr: Option<Discr<'tcx>>,
+            tag: Option<ScalarInt>,
             variant_def: &'tcx VariantDef,
             tcx: TyCtxt<'tcx>,
         ) -> Result<Self, Err> {
@@ -403,9 +404,6 @@ pub(crate) mod rustc {
             let min_align = repr.align.unwrap_or(Align::ONE);
             let max_align = repr.pack.unwrap_or(Align::MAX);
 
-            let clamp =
-                |align: Align| align.clamp(min_align, max_align).bytes().try_into().unwrap();
-
             let variant_span = trace_span!(
                 "treeifying variant",
                 min_align = ?min_align,
@@ -419,17 +417,12 @@ pub(crate) mod rustc {
             )
             .unwrap();
 
-            // The layout of the variant is prefixed by the discriminant, if any.
-            if let Some(discr) = discr {
-                trace!(?discr, "treeifying discriminant");
-                let discr_layout = alloc::Layout::from_size_align(
-                    layout_summary.discriminant_size,
-                    clamp(layout_summary.discriminant_align),
-                )
-                .unwrap();
-                trace!(?discr_layout, "computed discriminant layout");
-                variant_layout = variant_layout.extend(discr_layout).unwrap().0;
-                tree = tree.then(Self::from_discr(discr, tcx, layout_summary.discriminant_size));
+            // The layout of the variant is prefixed by the tag, if any.
+            if let Some(tag) = tag {
+                let tag_layout =
+                    alloc::Layout::from_size_align(tag.size().bytes_usize(), 1).unwrap();
+                tree = tree.then(Self::from_tag(tag, tcx));
+                variant_layout = variant_layout.extend(tag_layout).unwrap().0;
             }
 
             // Next come fields.
@@ -469,18 +462,19 @@ pub(crate) mod rustc {
             Ok(tree)
         }
 
-        pub fn from_discr(discr: Discr<'tcx>, tcx: TyCtxt<'tcx>, size: usize) -> Self {
+        pub fn from_tag(tag: ScalarInt, tcx: TyCtxt<'tcx>) -> Self {
             use rustc_target::abi::Endian;
-
+            let size = tag.size();
+            let bits = tag.to_bits(size).unwrap();
             let bytes: [u8; 16];
             let bytes = match tcx.data_layout.endian {
                 Endian::Little => {
-                    bytes = discr.val.to_le_bytes();
-                    &bytes[..size]
+                    bytes = bits.to_le_bytes();
+                    &bytes[..size.bytes_usize()]
                 }
                 Endian::Big => {
-                    bytes = discr.val.to_be_bytes();
-                    &bytes[bytes.len() - size..]
+                    bytes = bits.to_be_bytes();
+                    &bytes[bytes.len() - size.bytes_usize()..]
                 }
             };
             Self::Seq(bytes.iter().map(|&b| Self::from_bits(b)).collect())