Rollup merge of #109621 - scottmcm:update-variantidx, r=compiler-errors

Refactor: `VariantIdx::from_u32(0)` -> `FIRST_VARIANT`

Since structs are always `VariantIdx(0)`, there's a bunch of files where the only reason they had `VariantIdx` or `vec::Idx` imported at all was to get the first variant.

So this uses a constant for that, and adds some doc-comments to `VariantIdx` while I'm there, since [it doesn't have any today](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/struct.VariantIdx.html).

Author: matthiaskrgr

compiler/rustc_abi/src/layout.rs

@@ -43,7 +43,7 @@ pub trait LayoutCalculator {
             .max_by_key(|niche| niche.available(dl));
 
         LayoutS {
-            variants: Variants::Single { index: VariantIdx::new(0) },
+            variants: Variants::Single { index: FIRST_VARIANT },
             fields: FieldsShape::Arbitrary {
                 offsets: vec![Size::ZERO, b_offset],
                 memory_index: vec![0, 1],
@@ -264,7 +264,7 @@ pub trait LayoutCalculator {
             abi = Abi::Uninhabited;
         }
         Some(LayoutS {
-            variants: Variants::Single { index: VariantIdx::new(0) },
+            variants: Variants::Single { index: FIRST_VARIANT },
             fields: FieldsShape::Arbitrary { offsets, memory_index },
             abi,
             largest_niche,
@@ -277,7 +277,7 @@ pub trait LayoutCalculator {
         let dl = self.current_data_layout();
         let dl = dl.borrow();
         LayoutS {
-            variants: Variants::Single { index: VariantIdx::new(0) },
+            variants: Variants::Single { index: FIRST_VARIANT },
             fields: FieldsShape::Primitive,
             abi: Abi::Uninhabited,
             largest_niche: None,
@@ -331,7 +331,7 @@ pub trait LayoutCalculator {
             }
             // If it's a struct, still compute a layout so that we can still compute the
             // field offsets.
-            None => VariantIdx::new(0),
+            None => FIRST_VARIANT,
         };
 
         let is_struct = !is_enum ||
@@ -467,7 +467,7 @@ pub trait LayoutCalculator {
                 .max_by_key(|(_i, layout)| layout.size.bytes())
                 .map(|(i, _layout)| i)?;
 
-            let all_indices = (0..=variants.len() - 1).map(VariantIdx::new);
+            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().index()
@@ -896,8 +896,8 @@ pub trait LayoutCalculator {
         let optimize = !repr.inhibit_union_abi_opt();
         let mut size = Size::ZERO;
         let mut abi = Abi::Aggregate { sized: true };
-        let index = VariantIdx::new(0);
-        for field in &variants[index] {
+        let only_variant = &variants[FIRST_VARIANT];
+        for field in only_variant {
             assert!(field.0.is_sized());
             align = align.max(field.align());
 
@@ -930,8 +930,8 @@ pub trait LayoutCalculator {
         }
 
         Some(LayoutS {
-            variants: Variants::Single { index },
-            fields: FieldsShape::Union(NonZeroUsize::new(variants[index].len())?),
+            variants: Variants::Single { index: FIRST_VARIANT },
+            fields: FieldsShape::Union(NonZeroUsize::new(only_variant.len())?),
             abi,
             largest_niche: None,
             align,

compiler/rustc_abi/src/lib.rs

@@ -1380,8 +1380,21 @@ impl Niche {
 }
 
 rustc_index::newtype_index! {
+    /// The *source-order* index of a variant in a type.
+    ///
+    /// For enums, these are always `0..variant_count`, regardless of any
+    /// custom discriminants that may have been defined, and including any
+    /// variants that may end up uninhabited due to field types.  (Some of the
+    /// variants may not be present in a monomorphized ABI [`Variants`], but
+    /// those skipped variants are always counted when determining the *index*.)
+    ///
+    /// `struct`s, `tuples`, and `unions`s are considered to have a single variant
+    /// with variant index zero, aka [`FIRST_VARIANT`].
     #[derive(HashStable_Generic)]
-    pub struct VariantIdx {}
+    pub struct VariantIdx {
+        /// Equivalent to `VariantIdx(0)`.
+        const FIRST_VARIANT = 0;
+    }
 }
 
 #[derive(PartialEq, Eq, Hash, Clone)]
@@ -1422,7 +1435,7 @@ impl LayoutS {
         let size = scalar.size(cx);
         let align = scalar.align(cx);
         LayoutS {
-            variants: Variants::Single { index: VariantIdx::new(0) },
+            variants: Variants::Single { index: FIRST_VARIANT },
             fields: FieldsShape::Primitive,
             abi: Abi::Scalar(scalar),
             largest_niche,

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -14,7 +14,7 @@ use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::lang_items::LangItem;
-use rustc_index::vec::{Idx, IndexVec};
+use rustc_index::vec::IndexVec;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::env::RegionBoundPairs;
 use rustc_infer::infer::region_constraints::RegionConstraintData;
@@ -36,7 +36,7 @@ use rustc_middle::ty::{
 };
 use rustc_span::def_id::CRATE_DEF_ID;
 use rustc_span::{Span, DUMMY_SP};
-use rustc_target::abi::VariantIdx;
+use rustc_target::abi::FIRST_VARIANT;
 use rustc_trait_selection::traits::query::type_op::custom::scrape_region_constraints;
 use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;
 use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
@@ -812,7 +812,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             },
             PlaceTy { ty, variant_index: None } => match *ty.kind() {
                 ty::Adt(adt_def, substs) if !adt_def.is_enum() => {
-                    (adt_def.variant(VariantIdx::new(0)), substs)
+                    (adt_def.variant(FIRST_VARIANT), substs)
                 }
                 ty::Closure(_, substs) => {
                     return match substs

compiler/rustc_codegen_cranelift/src/base.rs

@@ -785,7 +785,7 @@ fn codegen_stmt<'tcx>(
                             let variant_dest = lval.downcast_variant(fx, variant_index);
                             (variant_index, variant_dest, active_field_index)
                         }
-                        _ => (VariantIdx::from_u32(0), lval, None),
+                        _ => (FIRST_VARIANT, lval, None),
                     };
                     if active_field_index.is_some() {
                         assert_eq!(operands.len(), 1);

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -86,7 +86,7 @@ mod prelude {
         self, FloatTy, Instance, InstanceDef, IntTy, ParamEnv, Ty, TyCtxt, TypeAndMut,
         TypeFoldable, TypeVisitableExt, UintTy,
     };
-    pub(crate) use rustc_target::abi::{Abi, Scalar, Size, VariantIdx};
+    pub(crate) use rustc_target::abi::{Abi, Scalar, Size, VariantIdx, FIRST_VARIANT};
 
     pub(crate) use rustc_data_structures::fx::FxHashMap;
 

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -146,7 +146,7 @@ pub(crate) fn coerce_unsized_into<'tcx>(
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
             assert_eq!(def_a, def_b);
 
-            for i in 0..def_a.variant(VariantIdx::new(0)).fields.len() {
+            for i in 0..def_a.variant(FIRST_VARIANT).fields.len() {
                 let src_f = src.value_field(fx, mir::Field::new(i));
                 let dst_f = dst.place_field(fx, mir::Field::new(i));
 

compiler/rustc_codegen_ssa/src/base.rs

@@ -24,7 +24,6 @@ use rustc_data_structures::sync::ParallelIterator;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::lang_items::LangItem;
-use rustc_index::vec::Idx;
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::middle::exported_symbols;
@@ -40,7 +39,7 @@ use rustc_session::Session;
 use rustc_span::symbol::sym;
 use rustc_span::Symbol;
 use rustc_span::{DebuggerVisualizerFile, DebuggerVisualizerType};
-use rustc_target::abi::{Align, VariantIdx};
+use rustc_target::abi::{Align, FIRST_VARIANT};
 
 use std::collections::BTreeSet;
 use std::time::{Duration, Instant};
@@ -307,7 +306,7 @@ pub fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
             assert_eq!(def_a, def_b);
 
-            for i in 0..def_a.variant(VariantIdx::new(0)).fields.len() {
+            for i in 0..def_a.variant(FIRST_VARIANT).fields.len() {
                 let src_f = src.project_field(bx, i);
                 let dst_f = dst.project_field(bx, i);
 

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -13,7 +13,7 @@ use rustc_middle::ty::cast::{CastTy, IntTy};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
 use rustc_middle::ty::{self, adjustment::PointerCast, Instance, Ty, TyCtxt};
 use rustc_span::source_map::{Span, DUMMY_SP};
-use rustc_target::abi::{self, VariantIdx};
+use rustc_target::abi::{self, FIRST_VARIANT};
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     #[instrument(level = "trace", skip(self, bx))]
@@ -118,7 +118,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         let variant_dest = dest.project_downcast(bx, variant_index);
                         (variant_index, variant_dest, active_field_index)
                     }
-                    _ => (VariantIdx::from_u32(0), dest, None),
+                    _ => (FIRST_VARIANT, dest, None),
                 };
                 if active_field_index.is_some() {
                     assert_eq!(operands.len(), 1);

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -8,7 +8,7 @@ use crate::interpret::{
 use crate::interpret::{MPlaceTy, Value};
 use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
 use rustc_span::source_map::DUMMY_SP;
-use rustc_target::abi::{Align, VariantIdx};
+use rustc_target::abi::{Align, VariantIdx, FIRST_VARIANT};
 
 #[instrument(skip(ecx), level = "debug")]
 fn branches<'tcx>(
@@ -412,7 +412,7 @@ fn valtree_into_mplace<'tcx>(
 
                         let inner_ty = match ty.kind() {
                             ty::Adt(def, substs) => {
-                                def.variant(VariantIdx::from_u32(0)).fields[i].ty(tcx, substs)
+                                def.variant(FIRST_VARIANT).fields[i].ty(tcx, substs)
                             }
                             ty::Tuple(inner_tys) => inner_tys[i],
                             _ => bug!("unexpected unsized type {:?}", ty),

compiler/rustc_const_eval/src/interpret/place.rs

@@ -8,7 +8,7 @@ use rustc_ast::Mutability;
 use rustc_middle::mir;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
-use rustc_target::abi::{self, Abi, Align, HasDataLayout, Size, VariantIdx};
+use rustc_target::abi::{self, Abi, Align, HasDataLayout, Size, FIRST_VARIANT};
 
 use super::{
     alloc_range, mir_assign_valid_types, AllocId, AllocRef, AllocRefMut, CheckInAllocMsg,
@@ -796,7 +796,7 @@ where
                 let variant_dest = self.place_downcast(&dest, variant_index)?;
                 (variant_index, variant_dest, active_field_index)
             }
-            _ => (VariantIdx::from_u32(0), dest.clone(), None),
+            _ => (FIRST_VARIANT, dest.clone(), None),
         };
         if active_field_index.is_some() {
             assert_eq!(operands.len(), 1);