Don't rely on contiguous VariantIds outside of rustc

compiler/rustc_pattern_analysis/src/lib.rs

@@ -21,7 +21,45 @@ rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 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.