Don't rely on contiguous VariantIds outside of rustc

Cargo.lock

@@ -4355,6 +4355,7 @@ name = "rustc_pattern_analysis"
 version = "0.0.0"
 dependencies = [
  "derivative",
+ "rustc-hash",
  "rustc_apfloat",
  "rustc_arena",
  "rustc_data_structures",

compiler/rustc_pattern_analysis/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 [dependencies]
 # tidy-alphabetical-start
 derivative = "2.2.0"
+rustc-hash = "1.1.0"
 rustc_apfloat = "0.2.0"
 rustc_arena = { path = "../rustc_arena", optional = true }
 rustc_data_structures = { path = "../rustc_data_structures", optional = true }

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -155,13 +155,13 @@
 use smallvec::SmallVec;
 
 use rustc_apfloat::ieee::{DoubleS, IeeeFloat, SingleS};
-use rustc_index::bit_set::{BitSet, GrowableBitSet};
-use rustc_index::IndexVec;
+use rustc_index::bit_set::GrowableBitSet;
 
 use self::Constructor::*;
 use self::MaybeInfiniteInt::*;
 use self::SliceKind::*;
 
+use crate::index;
 use crate::usefulness::PlaceCtxt;
 use crate::TypeCx;
 
@@ -804,7 +804,10 @@ pub enum ConstructorSet<Cx: TypeCx> {
     Struct { empty: bool },
     /// This type has the following list of constructors. If `variants` is empty and
     /// `non_exhaustive` is false, don't use this; use `NoConstructors` instead.
-    Variants { variants: IndexVec<Cx::VariantIdx, VariantVisibility>, non_exhaustive: bool },
+    Variants {
+        variants: index::IdxContainer<Cx::VariantIdx, VariantVisibility>,
+        non_exhaustive: bool,
+    },
     /// The type is `&T`.
     Ref,
     /// The type is a union.
@@ -904,7 +907,7 @@ pub(crate) fn split<'a>(
                 }
             }
             ConstructorSet::Variants { variants, non_exhaustive } => {
-                let mut seen_set: BitSet<_> = BitSet::new_empty(variants.len());
+                let mut seen_set = index::IdxSet::new_empty(variants.len());
                 for idx in seen.iter().map(|c| c.as_variant().unwrap()) {
                     seen_set.insert(idx);
                 }

compiler/rustc_pattern_analysis/src/lib.rs

@@ -21,7 +21,45 @@
 
 use std::fmt;
 
-use rustc_index::Idx;
+#[cfg(feature = "rustc")]
+pub mod index {
+    // Faster version when the indices of variants are `0..variants.len()`.
+    pub use rustc_index::bit_set::BitSet as IdxSet;
+    pub use rustc_index::Idx;
+    pub use rustc_index::IndexVec as IdxContainer;
+}
+#[cfg(not(feature = "rustc"))]
+pub mod index {
+    // Slower version when the indices of variants are something else.
+    pub trait Idx: Copy + PartialEq + Eq + std::hash::Hash {}
+    impl<T: Copy + PartialEq + Eq + std::hash::Hash> Idx for T {}
+
+    #[derive(Debug)]
+    pub struct IdxContainer<K, V>(pub rustc_hash::FxHashMap<K, V>);
+    impl<K: Idx, V> IdxContainer<K, V> {
+        pub fn len(&self) -> usize {
+            self.0.len()
+        }
+        pub fn iter_enumerated(&self) -> impl Iterator<Item = (K, &V)> {
+            self.0.iter().map(|(k, v)| (*k, v))
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct IdxSet<T>(pub rustc_hash::FxHashSet<T>);
+    impl<T: Idx> IdxSet<T> {
+        pub fn new_empty(_len: usize) -> Self {
+            Self(Default::default())
+        }
+        pub fn contains(&self, elem: T) -> bool {
+            self.0.contains(&elem)
+        }
+        pub fn insert(&mut self, elem: T) {
+            self.0.insert(elem);
+        }
+    }
+}
+
 #[cfg(feature = "rustc")]
 use rustc_middle::ty::Ty;
 #[cfg(feature = "rustc")]
@@ -50,7 +88,7 @@ pub trait TypeCx: Sized + fmt::Debug {
     /// Errors that can abort analysis.
     type Error: fmt::Debug;
     /// The index of an enum variant.
-    type VariantIdx: Clone + Idx;
+    type VariantIdx: Clone + index::Idx + fmt::Debug;
     /// A string literal
     type StrLit: Clone + PartialEq + fmt::Debug;
     /// Extra data to store in a match arm.