6 files changed, 412 insertions, 368 deletions

diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 271ab8306..15bc3b4e3 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -1,6 +1,3 @@
-use crate::vec::{Idx, IndexVec};
-use arrayvec::ArrayVec;
-use smallvec::{smallvec, SmallVec};
 use std::fmt;
 use std::iter;
 use std::marker::PhantomData;
@@ -9,8 +6,13 @@ use std::ops::{BitAnd, BitAndAssign, BitOrAssign, Bound, Not, Range, RangeBounds
 use std::rc::Rc;
 use std::slice;
 
+use arrayvec::ArrayVec;
+use smallvec::{smallvec, SmallVec};
+
 use rustc_macros::{Decodable, Encodable};
 
+use crate::{Idx, IndexVec};
+
 use Chunk::*;
 
 #[cfg(test)]
@@ -1542,7 +1544,7 @@ impl<T: Idx> GrowableBitSet<T> {
     #[inline]
     pub fn contains(&self, elem: T) -> bool {
         let (word_index, mask) = word_index_and_mask(elem);
-        self.bit_set.words.get(word_index).map_or(false, |word| (word & mask) != 0)
+        self.bit_set.words.get(word_index).is_some_and(|word| (word & mask) != 0)
     }
 
     #[inline]
@@ -1816,7 +1818,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// if the matrix represents (transitive) reachability, can
     /// `row` reach `column`?
     pub fn contains(&self, row: R, column: C) -> bool {
-        self.row(row).map_or(false, |r| r.contains(column))
+        self.row(row).is_some_and(|r| r.contains(column))
     }
 
     /// Adds the bits from row `read` to the bits from row `write`, and
diff --git a/compiler/rustc_index/src/idx.rs b/compiler/rustc_index/src/idx.rs
new file mode 100644
index 000000000..b85160540
--- /dev/null
+++ b/compiler/rustc_index/src/idx.rs
@@ -0,0 +1,45 @@
+use std::fmt::Debug;
+use std::hash::Hash;
+
+/// Represents some newtyped `usize` wrapper.
+///
+/// Purpose: avoid mixing indexes for different bitvector domains.
+pub trait Idx: Copy + 'static + Eq + PartialEq + Debug + Hash {
+    fn new(idx: usize) -> Self;
+
+    fn index(self) -> usize;
+
+    #[inline]
+    fn increment_by(&mut self, amount: usize) {
+        *self = self.plus(amount);
+    }
+
+    #[inline]
+    #[must_use = "Use `increment_by` if you wanted to update the index in-place"]
+    fn plus(self, amount: usize) -> Self {
+        Self::new(self.index() + amount)
+    }
+}
+
+impl Idx for usize {
+    #[inline]
+    fn new(idx: usize) -> Self {
+        idx
+    }
+    #[inline]
+    fn index(self) -> usize {
+        self
+    }
+}
+
+impl Idx for u32 {
+    #[inline]
+    fn new(idx: usize) -> Self {
+        assert!(idx <= u32::MAX as usize);
+        idx as u32
+    }
+    #[inline]
+    fn index(self) -> usize {
+        self as usize
+    }
+}
diff --git a/compiler/rustc_index/src/interval.rs b/compiler/rustc_index/src/interval.rs
index d809740c6..d3cf267dc 100644
--- a/compiler/rustc_index/src/interval.rs
+++ b/compiler/rustc_index/src/interval.rs
@@ -3,10 +3,11 @@ use std::marker::PhantomData;
 use std::ops::RangeBounds;
 use std::ops::{Bound, Range};
 
-use crate::vec::Idx;
-use crate::vec::IndexVec;
 use smallvec::SmallVec;
 
+use crate::idx::Idx;
+use crate::vec::IndexVec;
+
 #[cfg(test)]
 mod tests;
 
@@ -180,6 +181,30 @@ impl<I: Idx> IntervalSet<I> {
         self.map.is_empty()
     }
 
+    /// Equivalent to `range.iter().find(|i| !self.contains(i))`.
+    pub fn first_unset_in(&self, range: impl RangeBounds<I> + Clone) -> Option<I> {
+        let start = inclusive_start(range.clone());
+        let Some(end) = inclusive_end(self.domain, range) else {
+            // empty range
+            return None;
+        };
+        if start > end {
+            return None;
+        }
+        let Some(last) = self.map.partition_point(|r| r.0 <= start).checked_sub(1) else {
+            // All ranges in the map start after the new range's end
+            return Some(I::new(start as usize));
+        };
+        let (_, prev_end) = self.map[last];
+        if start > prev_end {
+            Some(I::new(start as usize))
+        } else if prev_end < end {
+            Some(I::new(prev_end as usize + 1))
+        } else {
+            None
+        }
+    }
+
     /// Returns the maximum (last) element present in the set from `range`.
     pub fn last_set_in(&self, range: impl RangeBounds<I> + Clone) -> Option<I> {
         let start = inclusive_start(range.clone());
@@ -223,7 +248,7 @@ impl<I: Idx> IntervalSet<I> {
     fn check_invariants(&self) -> bool {
         let mut current: Option<u32> = None;
         for (start, end) in &self.map {
-            if start > end || current.map_or(false, |x| x + 1 >= *start) {
+            if start > end || current.is_some_and(|x| x + 1 >= *start) {
                 return false;
             }
             current = Some(*end);
@@ -261,8 +286,7 @@ impl<R: Idx, C: Step + Idx> SparseIntervalMatrix<R, C> {
     }
 
     fn ensure_row(&mut self, row: R) -> &mut IntervalSet<C> {
-        self.rows.ensure_contains_elem(row, || IntervalSet::new(self.column_size));
-        &mut self.rows[row]
+        self.rows.ensure_contains_elem(row, || IntervalSet::new(self.column_size))
     }
 
     pub fn union_row(&mut self, row: R, from: &IntervalSet<C>) -> bool
@@ -297,6 +321,6 @@ impl<R: Idx, C: Step + Idx> SparseIntervalMatrix<R, C> {
     }
 
     pub fn contains(&self, row: R, point: C) -> bool {
-        self.row(row).map_or(false, |r| r.contains(point))
+        self.row(row).is_some_and(|r| r.contains(point))
     }
 }
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
index db2c79152..6fd9f34b2 100644
--- a/compiler/rustc_index/src/lib.rs
+++ b/compiler/rustc_index/src/lib.rs
@@ -17,7 +17,12 @@
 pub mod bit_set;
 #[cfg(feature = "nightly")]
 pub mod interval;
-pub mod vec;
+
+mod idx;
+mod slice;
+mod vec;
+
+pub use {idx::Idx, slice::IndexSlice, vec::IndexVec};
 
 #[cfg(feature = "rustc_macros")]
 pub use rustc_macros::newtype_index;
diff --git a/compiler/rustc_index/src/slice.rs b/compiler/rustc_index/src/slice.rs
new file mode 100644
index 000000000..0663c7247
--- /dev/null
+++ b/compiler/rustc_index/src/slice.rs
@@ -0,0 +1,256 @@
+use std::{
+    fmt,
+    marker::PhantomData,
+    ops::{Index, IndexMut},
+    slice,
+};
+
+use crate::{Idx, IndexVec};
+
+/// A view into contiguous `T`s, indexed by `I` rather than by `usize`.
+///
+/// One common pattern you'll see is code that uses [`IndexVec::from_elem`]
+/// to create the storage needed for a particular "universe" (aka the set of all
+/// the possible keys that need an associated value) then passes that working
+/// area as `&mut IndexSlice<I, T>` to clarify that nothing will be added nor
+/// removed during processing (and, as a bonus, to chase fewer pointers).
+#[derive(PartialEq, Eq, Hash)]
+#[repr(transparent)]
+pub struct IndexSlice<I: Idx, T> {
+    _marker: PhantomData<fn(&I)>,
+    pub raw: [T],
+}
+
+impl<I: Idx, T> IndexSlice<I, T> {
+    #[inline]
+    pub const fn empty() -> &'static Self {
+        Self::from_raw(&[])
+    }
+
+    #[inline]
+    pub const fn from_raw(raw: &[T]) -> &Self {
+        let ptr: *const [T] = raw;
+        // SAFETY: `IndexSlice` is `repr(transparent)` over a normal slice
+        unsafe { &*(ptr as *const Self) }
+    }
+
+    #[inline]
+    pub fn from_raw_mut(raw: &mut [T]) -> &mut Self {
+        let ptr: *mut [T] = raw;
+        // SAFETY: `IndexSlice` is `repr(transparent)` over a normal slice
+        unsafe { &mut *(ptr as *mut Self) }
+    }
+
+    #[inline]
+    pub const fn len(&self) -> usize {
+        self.raw.len()
+    }
+
+    #[inline]
+    pub const fn is_empty(&self) -> bool {
+        self.raw.is_empty()
+    }
+
+    /// Gives the next index that will be assigned when `push` is called.
+    ///
+    /// Manual bounds checks can be done using `idx < slice.next_index()`
+    /// (as opposed to `idx.index() < slice.len()`).
+    #[inline]
+    pub fn next_index(&self) -> I {
+        I::new(self.len())
+    }
+
+    #[inline]
+    pub fn iter(&self) -> slice::Iter<'_, T> {
+        self.raw.iter()
+    }
+
+    #[inline]
+    pub fn iter_enumerated(
+        &self,
+    ) -> impl DoubleEndedIterator<Item = (I, &T)> + ExactSizeIterator + '_ {
+        self.raw.iter().enumerate().map(|(n, t)| (I::new(n), t))
+    }
+
+    #[inline]
+    pub fn indices(
+        &self,
+    ) -> impl DoubleEndedIterator<Item = I> + ExactSizeIterator + Clone + 'static {
+        (0..self.len()).map(|n| I::new(n))
+    }
+
+    #[inline]
+    pub fn iter_mut(&mut self) -> slice::IterMut<'_, T> {
+        self.raw.iter_mut()
+    }
+
+    #[inline]
+    pub fn iter_enumerated_mut(
+        &mut self,
+    ) -> impl DoubleEndedIterator<Item = (I, &mut T)> + ExactSizeIterator + '_ {
+        self.raw.iter_mut().enumerate().map(|(n, t)| (I::new(n), t))
+    }
+
+    #[inline]
+    pub fn last_index(&self) -> Option<I> {
+        self.len().checked_sub(1).map(I::new)
+    }
+
+    #[inline]
+    pub fn swap(&mut self, a: I, b: I) {
+        self.raw.swap(a.index(), b.index())
+    }
+
+    #[inline]
+    pub fn get(&self, index: I) -> Option<&T> {
+        self.raw.get(index.index())
+    }
+
+    #[inline]
+    pub fn get_mut(&mut self, index: I) -> Option<&mut T> {
+        self.raw.get_mut(index.index())
+    }
+
+    /// Returns mutable references to two distinct elements, `a` and `b`.
+    ///
+    /// Panics if `a == b`.
+    #[inline]
+    pub fn pick2_mut(&mut self, a: I, b: I) -> (&mut T, &mut T) {
+        let (ai, bi) = (a.index(), b.index());
+        assert!(ai != bi);
+
+        if ai < bi {
+            let (c1, c2) = self.raw.split_at_mut(bi);
+            (&mut c1[ai], &mut c2[0])
+        } else {
+            let (c2, c1) = self.pick2_mut(b, a);
+            (c1, c2)
+        }
+    }
+
+    /// Returns mutable references to three distinct elements.
+    ///
+    /// Panics if the elements are not distinct.
+    #[inline]
+    pub fn pick3_mut(&mut self, a: I, b: I, c: I) -> (&mut T, &mut T, &mut T) {
+        let (ai, bi, ci) = (a.index(), b.index(), c.index());
+        assert!(ai != bi && bi != ci && ci != ai);
+        let len = self.raw.len();
+        assert!(ai < len && bi < len && ci < len);
+        let ptr = self.raw.as_mut_ptr();
+        unsafe { (&mut *ptr.add(ai), &mut *ptr.add(bi), &mut *ptr.add(ci)) }
+    }
+
+    #[inline]
+    pub fn binary_search(&self, value: &T) -> Result<I, I>
+    where
+        T: Ord,
+    {
+        match self.raw.binary_search(value) {
+            Ok(i) => Ok(Idx::new(i)),
+            Err(i) => Err(Idx::new(i)),
+        }
+    }
+}
+
+impl<I: Idx, J: Idx> IndexSlice<I, J> {
+    /// Invert a bijective mapping, i.e. `invert(map)[y] = x` if `map[x] = y`,
+    /// assuming the values in `self` are a permutation of `0..self.len()`.
+    ///
+    /// This is used to go between `memory_index` (source field order to memory order)
+    /// and `inverse_memory_index` (memory order to source field order).
+    /// See also `FieldsShape::Arbitrary::memory_index` for more details.
+    // FIXME(eddyb) build a better abstraction for permutations, if possible.
+    pub fn invert_bijective_mapping(&self) -> IndexVec<J, I> {
+        debug_assert_eq!(
+            self.iter().map(|x| x.index() as u128).sum::<u128>(),
+            (0..self.len() as u128).sum::<u128>(),
+            "The values aren't 0..N in input {self:?}",
+        );
+
+        let mut inverse = IndexVec::from_elem_n(Idx::new(0), self.len());
+        for (i1, &i2) in self.iter_enumerated() {
+            inverse[i2] = i1;
+        }
+
+        debug_assert_eq!(
+            inverse.iter().map(|x| x.index() as u128).sum::<u128>(),
+            (0..inverse.len() as u128).sum::<u128>(),
+            "The values aren't 0..N in result {self:?}",
+        );
+
+        inverse
+    }
+}
+
+impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexSlice<I, T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.raw, fmt)
+    }
+}
+
+impl<I: Idx, T> Index<I> for IndexSlice<I, T> {
+    type Output = T;
+
+    #[inline]
+    fn index(&self, index: I) -> &T {
+        &self.raw[index.index()]
+    }
+}
+
+impl<I: Idx, T> IndexMut<I> for IndexSlice<I, T> {
+    #[inline]
+    fn index_mut(&mut self, index: I) -> &mut T {
+        &mut self.raw[index.index()]
+    }
+}
+
+impl<'a, I: Idx, T> IntoIterator for &'a IndexSlice<I, T> {
+    type Item = &'a T;
+    type IntoIter = slice::Iter<'a, T>;
+
+    #[inline]
+    fn into_iter(self) -> slice::Iter<'a, T> {
+        self.raw.iter()
+    }
+}
+
+impl<'a, I: Idx, T> IntoIterator for &'a mut IndexSlice<I, T> {
+    type Item = &'a mut T;
+    type IntoIter = slice::IterMut<'a, T>;
+
+    #[inline]
+    fn into_iter(self) -> slice::IterMut<'a, T> {
+        self.raw.iter_mut()
+    }
+}
+
+impl<I: Idx, T: Clone> ToOwned for IndexSlice<I, T> {
+    type Owned = IndexVec<I, T>;
+
+    fn to_owned(&self) -> IndexVec<I, T> {
+        IndexVec::from_raw(self.raw.to_owned())
+    }
+
+    fn clone_into(&self, target: &mut IndexVec<I, T>) {
+        self.raw.clone_into(&mut target.raw)
+    }
+}
+
+impl<I: Idx, T> Default for &IndexSlice<I, T> {
+    #[inline]
+    fn default() -> Self {
+        IndexSlice::from_raw(Default::default())
+    }
+}
+
+impl<I: Idx, T> Default for &mut IndexSlice<I, T> {
+    #[inline]
+    fn default() -> Self {
+        IndexSlice::from_raw_mut(Default::default())
+    }
+}
+
+// Whether `IndexSlice` is `Send` depends only on the data,
+// not the phantom data.
+unsafe impl<I: Idx, T> Send for IndexSlice<I, T> where T: Send {}
diff --git a/compiler/rustc_index/src/vec.rs b/compiler/rustc_index/src/vec.rs
index ae2f52c51..99e72e49f 100644
--- a/compiler/rustc_index/src/vec.rs
+++ b/compiler/rustc_index/src/vec.rs
@@ -3,55 +3,13 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 
 use std::borrow::{Borrow, BorrowMut};
 use std::fmt;
-use std::fmt::Debug;
 use std::hash::Hash;
 use std::marker::PhantomData;
-use std::ops::{Deref, DerefMut, Index, IndexMut, RangeBounds};
+use std::ops::{Deref, DerefMut, RangeBounds};
 use std::slice;
 use std::vec;
 
-/// Represents some newtyped `usize` wrapper.
-///
-/// Purpose: avoid mixing indexes for different bitvector domains.
-pub trait Idx: Copy + 'static + Eq + PartialEq + Debug + Hash {
-    fn new(idx: usize) -> Self;
-
-    fn index(self) -> usize;
-
-    #[inline]
-    fn increment_by(&mut self, amount: usize) {
-        *self = self.plus(amount);
-    }
-
-    #[inline]
-    #[must_use = "Use `increment_by` if you wanted to update the index in-place"]
-    fn plus(self, amount: usize) -> Self {
-        Self::new(self.index() + amount)
-    }
-}
-
-impl Idx for usize {
-    #[inline]
-    fn new(idx: usize) -> Self {
-        idx
-    }
-    #[inline]
-    fn index(self) -> usize {
-        self
-    }
-}
-
-impl Idx for u32 {
-    #[inline]
-    fn new(idx: usize) -> Self {
-        assert!(idx <= u32::MAX as usize);
-        idx as u32
-    }
-    #[inline]
-    fn index(self) -> usize {
-        self as usize
-    }
-}
+use crate::{Idx, IndexSlice};
 
 /// An owned contiguous collection of `T`s, indexed by `I` rather than by `usize`.
 ///
@@ -66,68 +24,20 @@ pub struct IndexVec<I: Idx, T> {
     _marker: PhantomData<fn(&I)>,
 }
 
-/// A view into contiguous `T`s, indexed by `I` rather than by `usize`.
-///
-/// One common pattern you'll see is code that uses [`IndexVec::from_elem`]
-/// to create the storage needed for a particular "universe" (aka the set of all
-/// the possible keys that need an associated value) then passes that working
-/// area as `&mut IndexSlice<I, T>` to clarify that nothing will be added nor
-/// removed during processing (and, as a bonus, to chase fewer pointers).
-#[derive(PartialEq, Eq, Hash)]
-#[repr(transparent)]
-pub struct IndexSlice<I: Idx, T> {
-    _marker: PhantomData<fn(&I)>,
-    pub raw: [T],
-}
-
-// Whether `IndexVec` is `Send` depends only on the data,
-// not the phantom data.
-unsafe impl<I: Idx, T> Send for IndexVec<I, T> where T: Send {}
-
-// Whether `IndexSlice` is `Send` depends only on the data,
-// not the phantom data.
-unsafe impl<I: Idx, T> Send for IndexSlice<I, T> where T: Send {}
-
-#[cfg(feature = "rustc_serialize")]
-impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for IndexVec<I, T> {
-    fn encode(&self, s: &mut S) {
-        Encodable::encode(&self.raw, s);
-    }
-}
-
-#[cfg(feature = "rustc_serialize")]
-impl<D: Decoder, I: Idx, T: Decodable<D>> Decodable<D> for IndexVec<I, T> {
-    fn decode(d: &mut D) -> Self {
-        IndexVec { raw: Decodable::decode(d), _marker: PhantomData }
-    }
-}
-
-impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexVec<I, T> {
-    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.raw, fmt)
-    }
-}
-
-impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexSlice<I, T> {
-    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.raw, fmt)
-    }
-}
-
 impl<I: Idx, T> IndexVec<I, T> {
     #[inline]
-    pub fn new() -> Self {
-        IndexVec { raw: Vec::new(), _marker: PhantomData }
+    pub const fn new() -> Self {
+        IndexVec::from_raw(Vec::new())
     }
 
     #[inline]
-    pub fn from_raw(raw: Vec<T>) -> Self {
+    pub const fn from_raw(raw: Vec<T>) -> Self {
         IndexVec { raw, _marker: PhantomData }
     }
 
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
-        IndexVec { raw: Vec::with_capacity(capacity), _marker: PhantomData }
+        IndexVec::from_raw(Vec::with_capacity(capacity))
     }
 
     /// Creates a new vector with a copy of `elem` for each index in `universe`.
@@ -146,7 +56,7 @@ impl<I: Idx, T> IndexVec<I, T> {
     where
         T: Clone,
     {
-        IndexVec { raw: vec![elem; universe.len()], _marker: PhantomData }
+        IndexVec::from_raw(vec![elem; universe.len()])
     }
 
     #[inline]
@@ -154,7 +64,7 @@ impl<I: Idx, T> IndexVec<I, T> {
     where
         T: Clone,
     {
-        IndexVec { raw: vec![elem; n], _marker: PhantomData }
+        IndexVec::from_raw(vec![elem; n])
     }
 
     /// Create an `IndexVec` with `n` elements, where the value of each
@@ -162,8 +72,7 @@ impl<I: Idx, T> IndexVec<I, T> {
     /// be allocated only once, with a capacity of at least `n`.)
     #[inline]
     pub fn from_fn_n(func: impl FnMut(I) -> T, n: usize) -> Self {
-        let indices = (0..n).map(I::new);
-        Self::from_raw(indices.map(func).collect())
+        IndexVec::from_raw((0..n).map(I::new).map(func).collect())
     }
 
     #[inline]
@@ -178,7 +87,7 @@ impl<I: Idx, T> IndexVec<I, T> {
 
     #[inline]
     pub fn push(&mut self, d: T) -> I {
-        let idx = I::new(self.len());
+        let idx = self.next_index();
         self.raw.push(d);
         idx
     }
@@ -229,216 +138,37 @@ impl<I: Idx, T> IndexVec<I, T> {
     }
 
     pub fn convert_index_type<Ix: Idx>(self) -> IndexVec<Ix, T> {
-        IndexVec { raw: self.raw, _marker: PhantomData }
+        IndexVec::from_raw(self.raw)
     }
 
     /// Grows the index vector so that it contains an entry for
     /// `elem`; if that is already true, then has no
     /// effect. Otherwise, inserts new values as needed by invoking
     /// `fill_value`.
+    ///
+    /// Returns a reference to the `elem` entry.
     #[inline]
-    pub fn ensure_contains_elem(&mut self, elem: I, fill_value: impl FnMut() -> T) {
+    pub fn ensure_contains_elem(&mut self, elem: I, fill_value: impl FnMut() -> T) -> &mut T {
         let min_new_len = elem.index() + 1;
         if self.len() < min_new_len {
             self.raw.resize_with(min_new_len, fill_value);
         }
-    }
-
-    #[inline]
-    pub fn resize_to_elem(&mut self, elem: I, fill_value: impl FnMut() -> T) {
-        let min_new_len = elem.index() + 1;
-        self.raw.resize_with(min_new_len, fill_value);
-    }
-}
-
-impl<I: Idx, T> Deref for IndexVec<I, T> {
-    type Target = IndexSlice<I, T>;
-
-    #[inline]
-    fn deref(&self) -> &Self::Target {
-        self.as_slice()
-    }
-}
-
-impl<I: Idx, T> DerefMut for IndexVec<I, T> {
-    #[inline]
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        self.as_mut_slice()
-    }
-}
-
-impl<I: Idx, T> Borrow<IndexSlice<I, T>> for IndexVec<I, T> {
-    fn borrow(&self) -> &IndexSlice<I, T> {
-        self
-    }
-}
-
-impl<I: Idx, T> BorrowMut<IndexSlice<I, T>> for IndexVec<I, T> {
-    fn borrow_mut(&mut self) -> &mut IndexSlice<I, T> {
-        self
-    }
-}
-
-impl<I: Idx, T: Clone> ToOwned for IndexSlice<I, T> {
-    type Owned = IndexVec<I, T>;
-
-    fn to_owned(&self) -> IndexVec<I, T> {
-        IndexVec::from_raw(self.raw.to_owned())
-    }
-
-    fn clone_into(&self, target: &mut IndexVec<I, T>) {
-        self.raw.clone_into(&mut target.raw)
-    }
-}
-
-impl<I: Idx, T> IndexSlice<I, T> {
-    #[inline]
-    pub fn empty() -> &'static Self {
-        Default::default()
-    }
-
-    #[inline]
-    pub fn from_raw(raw: &[T]) -> &Self {
-        let ptr: *const [T] = raw;
-        // SAFETY: `IndexSlice` is `repr(transparent)` over a normal slice
-        unsafe { &*(ptr as *const Self) }
-    }
-
-    #[inline]
-    pub fn from_raw_mut(raw: &mut [T]) -> &mut Self {
-        let ptr: *mut [T] = raw;
-        // SAFETY: `IndexSlice` is `repr(transparent)` over a normal slice
-        unsafe { &mut *(ptr as *mut Self) }
-    }
-
-    #[inline]
-    pub fn len(&self) -> usize {
-        self.raw.len()
-    }
-
-    /// Gives the next index that will be assigned when `push` is called.
-    ///
-    /// Manual bounds checks can be done using `idx < slice.next_index()`
-    /// (as opposed to `idx.index() < slice.len()`).
-    #[inline]
-    pub fn next_index(&self) -> I {
-        I::new(self.len())
-    }
-
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        self.raw.is_empty()
-    }
-
-    #[inline]
-    pub fn iter(&self) -> slice::Iter<'_, T> {
-        self.raw.iter()
-    }
 
-    #[inline]
-    pub fn iter_enumerated(
-        &self,
-    ) -> impl DoubleEndedIterator<Item = (I, &T)> + ExactSizeIterator + '_ {
-        self.raw.iter().enumerate().map(|(n, t)| (I::new(n), t))
-    }
-
-    #[inline]
-    pub fn indices(
-        &self,
-    ) -> impl DoubleEndedIterator<Item = I> + ExactSizeIterator + Clone + 'static {
-        (0..self.len()).map(|n| I::new(n))
-    }
-
-    #[inline]
-    pub fn iter_mut(&mut self) -> slice::IterMut<'_, T> {
-        self.raw.iter_mut()
+        &mut self[elem]
     }
 
     #[inline]
-    pub fn iter_enumerated_mut(
-        &mut self,
-    ) -> impl DoubleEndedIterator<Item = (I, &mut T)> + ExactSizeIterator + '_ {
-        self.raw.iter_mut().enumerate().map(|(n, t)| (I::new(n), t))
-    }
-
-    #[inline]
-    pub fn last_index(&self) -> Option<I> {
-        self.len().checked_sub(1).map(I::new)
-    }
-
-    #[inline]
-    pub fn swap(&mut self, a: I, b: I) {
-        self.raw.swap(a.index(), b.index())
-    }
-
-    #[inline]
-    pub fn get(&self, index: I) -> Option<&T> {
-        self.raw.get(index.index())
-    }
-
-    #[inline]
-    pub fn get_mut(&mut self, index: I) -> Option<&mut T> {
-        self.raw.get_mut(index.index())
-    }
-
-    /// Returns mutable references to two distinct elements, `a` and `b`.
-    ///
-    /// Panics if `a == b`.
-    #[inline]
-    pub fn pick2_mut(&mut self, a: I, b: I) -> (&mut T, &mut T) {
-        let (ai, bi) = (a.index(), b.index());
-        assert!(ai != bi);
-
-        if ai < bi {
-            let (c1, c2) = self.raw.split_at_mut(bi);
-            (&mut c1[ai], &mut c2[0])
-        } else {
-            let (c2, c1) = self.pick2_mut(b, a);
-            (c1, c2)
-        }
+    pub fn resize(&mut self, new_len: usize, value: T)
+    where
+        T: Clone,
+    {
+        self.raw.resize(new_len, value)
     }
 
-    /// Returns mutable references to three distinct elements.
-    ///
-    /// Panics if the elements are not distinct.
     #[inline]
-    pub fn pick3_mut(&mut self, a: I, b: I, c: I) -> (&mut T, &mut T, &mut T) {
-        let (ai, bi, ci) = (a.index(), b.index(), c.index());
-        assert!(ai != bi && bi != ci && ci != ai);
-        let len = self.raw.len();
-        assert!(ai < len && bi < len && ci < len);
-        let ptr = self.raw.as_mut_ptr();
-        unsafe { (&mut *ptr.add(ai), &mut *ptr.add(bi), &mut *ptr.add(ci)) }
-    }
-}
-
-impl<I: Idx, J: Idx> IndexSlice<I, J> {
-    /// Invert a bijective mapping, i.e. `invert(map)[y] = x` if `map[x] = y`,
-    /// assuming the values in `self` are a permutation of `0..self.len()`.
-    ///
-    /// This is used to go between `memory_index` (source field order to memory order)
-    /// and `inverse_memory_index` (memory order to source field order).
-    /// See also `FieldsShape::Arbitrary::memory_index` for more details.
-    // FIXME(eddyb) build a better abstraction for permutations, if possible.
-    pub fn invert_bijective_mapping(&self) -> IndexVec<J, I> {
-        debug_assert_eq!(
-            self.iter().map(|x| x.index() as u128).sum::<u128>(),
-            (0..self.len() as u128).sum::<u128>(),
-            "The values aren't 0..N in input {self:?}",
-        );
-
-        let mut inverse = IndexVec::from_elem_n(Idx::new(0), self.len());
-        for (i1, &i2) in self.iter_enumerated() {
-            inverse[i2] = i1;
-        }
-
-        debug_assert_eq!(
-            inverse.iter().map(|x| x.index() as u128).sum::<u128>(),
-            (0..inverse.len() as u128).sum::<u128>(),
-            "The values aren't 0..N in result {self:?}",
-        );
-
-        inverse
+    pub fn resize_to_elem(&mut self, elem: I, fill_value: impl FnMut() -> T) {
+        let min_new_len = elem.index() + 1;
+        self.raw.resize_with(min_new_len, fill_value);
     }
 }
 
@@ -446,74 +176,51 @@ impl<I: Idx, J: Idx> IndexSlice<I, J> {
 impl<I: Idx, T> IndexVec<I, Option<T>> {
     #[inline]
     pub fn insert(&mut self, index: I, value: T) -> Option<T> {
-        self.ensure_contains_elem(index, || None);
-        self[index].replace(value)
+        self.ensure_contains_elem(index, || None).replace(value)
     }
 
     #[inline]
     pub fn get_or_insert_with(&mut self, index: I, value: impl FnOnce() -> T) -> &mut T {
-        self.ensure_contains_elem(index, || None);
-        self[index].get_or_insert_with(value)
+        self.ensure_contains_elem(index, || None).get_or_insert_with(value)
     }
 
     #[inline]
     pub fn remove(&mut self, index: I) -> Option<T> {
-        self.ensure_contains_elem(index, || None);
-        self[index].take()
-    }
-}
-
-impl<I: Idx, T: Clone> IndexVec<I, T> {
-    #[inline]
-    pub fn resize(&mut self, new_len: usize, value: T) {
-        self.raw.resize(new_len, value)
+        self.get_mut(index)?.take()
     }
 }
 
-impl<I: Idx, T: Ord> IndexSlice<I, T> {
-    #[inline]
-    pub fn binary_search(&self, value: &T) -> Result<I, I> {
-        match self.raw.binary_search(value) {
-            Ok(i) => Ok(Idx::new(i)),
-            Err(i) => Err(Idx::new(i)),
-        }
+impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexVec<I, T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.raw, fmt)
     }
 }
 
-impl<I: Idx, T> Index<I> for IndexSlice<I, T> {
-    type Output = T;
-
-    #[inline]
-    fn index(&self, index: I) -> &T {
-        &self.raw[index.index()]
-    }
-}
+impl<I: Idx, T> Deref for IndexVec<I, T> {
+    type Target = IndexSlice<I, T>;
 
-impl<I: Idx, T> IndexMut<I> for IndexSlice<I, T> {
     #[inline]
-    fn index_mut(&mut self, index: I) -> &mut T {
-        &mut self.raw[index.index()]
+    fn deref(&self) -> &Self::Target {
+        self.as_slice()
     }
 }
 
-impl<I: Idx, T> Default for IndexVec<I, T> {
+impl<I: Idx, T> DerefMut for IndexVec<I, T> {
     #[inline]
-    fn default() -> Self {
-        Self::new()
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.as_mut_slice()
     }
 }
 
-impl<I: Idx, T> Default for &IndexSlice<I, T> {
-    #[inline]
-    fn default() -> Self {
-        IndexSlice::from_raw(Default::default())
+impl<I: Idx, T> Borrow<IndexSlice<I, T>> for IndexVec<I, T> {
+    fn borrow(&self) -> &IndexSlice<I, T> {
+        self
     }
 }
 
-impl<I: Idx, T> Default for &mut IndexSlice<I, T> {
-    #[inline]
-    fn default() -> Self {
-        IndexSlice::from_raw_mut(Default::default())
+impl<I: Idx, T> BorrowMut<IndexSlice<I, T>> for IndexVec<I, T> {
+    fn borrow_mut(&mut self) -> &mut IndexSlice<I, T> {
+        self
     }
 }
 
@@ -542,14 +249,7 @@ impl<I: Idx, T> FromIterator<T> for IndexVec<I, T> {
     where
         J: IntoIterator<Item = T>,
     {
-        IndexVec { raw: FromIterator::from_iter(iter), _marker: PhantomData }
-    }
-}
-
-impl<I: Idx, T, const N: usize> From<[T; N]> for IndexVec<I, T> {
-    #[inline]
-    fn from(array: [T; N]) -> Self {
-        IndexVec::from_raw(array.into())
+        IndexVec::from_raw(Vec::from_iter(iter))
     }
 }
 
@@ -569,7 +269,7 @@ impl<'a, I: Idx, T> IntoIterator for &'a IndexVec<I, T> {
 
     #[inline]
     fn into_iter(self) -> slice::Iter<'a, T> {
-        self.raw.iter()
+        self.iter()
     }
 }
 
@@ -579,29 +279,41 @@ impl<'a, I: Idx, T> IntoIterator for &'a mut IndexVec<I, T> {
 
     #[inline]
     fn into_iter(self) -> slice::IterMut<'a, T> {
-        self.raw.iter_mut()
+        self.iter_mut()
     }
 }
 
-impl<'a, I: Idx, T> IntoIterator for &'a IndexSlice<I, T> {
-    type Item = &'a T;
-    type IntoIter = slice::Iter<'a, T>;
+impl<I: Idx, T> Default for IndexVec<I, T> {
+    #[inline]
+    fn default() -> Self {
+        IndexVec::new()
+    }
+}
 
+impl<I: Idx, T, const N: usize> From<[T; N]> for IndexVec<I, T> {
     #[inline]
-    fn into_iter(self) -> slice::Iter<'a, T> {
-        self.raw.iter()
+    fn from(array: [T; N]) -> Self {
+        IndexVec::from_raw(array.into())
     }
 }
 
-impl<'a, I: Idx, T> IntoIterator for &'a mut IndexSlice<I, T> {
-    type Item = &'a mut T;
-    type IntoIter = slice::IterMut<'a, T>;
+#[cfg(feature = "rustc_serialize")]
+impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for IndexVec<I, T> {
+    fn encode(&self, s: &mut S) {
+        Encodable::encode(&self.raw, s);
+    }
+}
 
-    #[inline]
-    fn into_iter(self) -> slice::IterMut<'a, T> {
-        self.raw.iter_mut()
+#[cfg(feature = "rustc_serialize")]
+impl<D: Decoder, I: Idx, T: Decodable<D>> Decodable<D> for IndexVec<I, T> {
+    fn decode(d: &mut D) -> Self {
+        IndexVec::from_raw(Vec::<T>::decode(d))
     }
 }
 
+// Whether `IndexVec` is `Send` depends only on the data,
+// not the phantom data.
+unsafe impl<I: Idx, T> Send for IndexVec<I, T> where T: Send {}
+
 #[cfg(test)]
 mod tests;