Refactor and document the repeat length check

compiler/rustc_typeck/src/check/expr.rs

@@ -1304,31 +1304,34 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  element_ty: Ty<'tcx>,
  ) {
  let tcx = self.tcx;
- let is_const = match &element.kind {
- hir::ExprKind::ConstBlock(..) => true,
+ // Actual constants as the repeat element get inserted repeatedly instead of getting copied via Copy.
+ match &element.kind {
+ hir::ExprKind::ConstBlock(..) => return,
  hir::ExprKind::Path(qpath) => {
  let res = self.typeck_results.borrow().qpath_res(qpath, element.hir_id);
- matches!(
-  res,
- Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::AnonConst, _)
- )
+ if let Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::AnonConst, _) = res
+ {
+ return;
+ }
  }
+ _ => {}
+ }
+ // If someone calls a const fn, they can extract that call out into a separate constant (or a const
+ // block in the future), so we check that to tell them that in the diagnostic. Does not affect typeck.
+ let is_const_fn = match element.kind {
+ hir::ExprKind::Call(func, _args) => match *self.node_ty(func.hir_id).kind() {
+ ty::FnDef(def_id, _) => tcx.is_const_fn(def_id),
+ _ => false,
+ },
  _ => false,
  };
- if !is_const {
- let is_const_fn = match element.kind {
- hir::ExprKind::Call(func, _args) => match *self.node_ty(func.hir_id).kind() {
- ty::FnDef(def_id, _) => tcx.is_const_fn(def_id),
- _ => false,
- },
- _ => false,
- };
 
- if count.try_eval_usize(tcx, self.param_env).map_or(true, |len| len > 1) {
- let lang_item = self.tcx.require_lang_item(LangItem::Copy, None);
- let code = traits::ObligationCauseCode::RepeatElementCopy { is_const_fn };
- self.require_type_meets(element_ty, element.span, code, lang_item);
- }
+ // If the length is 0, we don't create any elements, so we don't copy any. If the length is 1, we
+ // don't copy that one element, we move it. Only check for Copy if the length is larger.
+ if count.try_eval_usize(tcx, self.param_env).map_or(true, |len| len > 1) {
+ let lang_item = self.tcx.require_lang_item(LangItem::Copy, None);
+ let code = traits::ObligationCauseCode::RepeatElementCopy { is_const_fn };
+ self.require_type_meets(element_ty, element.span, code, lang_item);
  }
  }