Auto merge of #109915 - scottmcm:layout-indexvec, r=oli-obk

Use `FieldIdx` in `FieldsShape`

Finally got to the main motivating example from https://github.com/rust-lang/compiler-team/issues/606 :)

compiler/rustc_middle/src/ty/layout.rs

@@ -5,6 +5,7 @@ use crate::ty::{self, ReprOptions, Ty, TyCtxt, TypeVisitableExt};
 use rustc_errors::{DiagnosticBuilder, Handler, IntoDiagnostic};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_index::vec::IndexVec;
 use rustc_session::config::OptLevel;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
@@ -635,7 +636,7 @@ where
                     variants: Variants::Single { index: variant_index },
                     fields: match NonZeroUsize::new(fields) {
                         Some(fields) => FieldsShape::Union(fields),
-                        None => FieldsShape::Arbitrary { offsets: vec![], memory_index: vec![] },
+                        None => FieldsShape::Arbitrary { offsets: IndexVec::new(), memory_index: IndexVec::new() },
                     },
                     abi: Abi::Uninhabited,
                     largest_niche: None,