Use &IndexSlice instead of &IndexVec where possible

All the same reasons as for `[T]`: more general, less pointer chasing, and `&mut IndexSlice` emphasizes that it doesn't change *length*.

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -21,7 +21,7 @@ use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::{self, List, TyCtxt, TypeVisitableExt};
 use rustc_span::Span;
 
-use rustc_index::vec::{Idx, IndexVec};
+use rustc_index::vec::{Idx, IndexSlice, IndexVec};
 
 use std::cell::Cell;
 use std::{cmp, iter, mem};
@@ -184,7 +184,7 @@ pub fn collect_temps_and_candidates<'tcx>(
 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 struct Validator<'a, 'tcx> {
     ccx: &'a ConstCx<'a, 'tcx>,
-    temps: &'a mut IndexVec<Local, TempState>,
+    temps: &'a mut IndexSlice<Local, TempState>,
 }
 
 impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> {
@@ -669,7 +669,7 @@ impl<'tcx> Validator<'_, 'tcx> {
 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 pub fn validate_candidates(
     ccx: &ConstCx<'_, '_>,
-    temps: &mut IndexVec<Local, TempState>,
+    temps: &mut IndexSlice<Local, TempState>,
     candidates: &[Candidate],
 ) -> Vec<Candidate> {
     let mut validator = Validator { ccx, temps };