compiler/rustc_const_eval/src/const_eval/fn_queries.rs

   1 use rustc_hir as hir;
   2 use rustc_hir::def::DefKind;
   3 use rustc_hir::def_id::{DefId, LocalDefId};
   4 use rustc_middle::ty::query::Providers;
   5 use rustc_middle::ty::{DefIdTree, TyCtxt};
   6 use rustc_span::symbol::Symbol;
   7
   8 /// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it
   9 pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
  10     if tcx.is_const_fn_raw(def_id) {
  11         let const_stab = tcx.lookup_const_stability(def_id)?;
  12         if const_stab.is_const_unstable() { Some(const_stab.feature) } else { None }
  13     } else {
  14         None
  15     }
  16 }
  17
  18 pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
  19     let parent_id = tcx.local_parent(def_id);
  20     matches!(tcx.def_kind(parent_id), DefKind::Impl { .. })
  21         && tcx.constness(parent_id) == hir::Constness::Const
  22 }
  23
  24 /// Checks whether an item is considered to be `const`. If it is a constructor, it is const. If
  25 /// it is a trait impl/function, return if it has a `const` modifier. If it is an intrinsic,
  26 /// report whether said intrinsic has a `rustc_const_{un,}stable` attribute. Otherwise, return
  27 /// `Constness::NotConst`.
  28 fn constness(tcx: TyCtxt<'_>, def_id: DefId) -> hir::Constness {
  29     let def_id = def_id.expect_local();
  30     let node = tcx.hir().get_by_def_id(def_id);
  31
  32     match node {
  33         hir::Node::Ctor(_) => hir::Constness::Const,
  34         hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.constness,
  35         hir::Node::ForeignItem(hir::ForeignItem { kind: hir::ForeignItemKind::Fn(..), .. }) => {
  36             // Intrinsics use `rustc_const_{un,}stable` attributes to indicate constness. All other
  37             // foreign items cannot be evaluated at compile-time.
  38             let is_const = if tcx.is_intrinsic(def_id) {
  39                 tcx.lookup_const_stability(def_id).is_some()
  40             } else {
  41                 false
  42             };
  43             if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
  44         }
  45         hir::Node::Expr(e) if let hir::ExprKind::Closure(c) = e.kind => c.constness,
  46         _ => {
  47             if let Some(fn_kind) = node.fn_kind() {
  48                 if fn_kind.constness() == hir::Constness::Const {
  49                     return hir::Constness::Const;
  50                 }
  51
  52                 // If the function itself is not annotated with `const`, it may still be a `const fn`
  53                 // if it resides in a const trait impl.
  54                 let is_const = is_parent_const_impl_raw(tcx, def_id);
  55                 if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
  56             } else {
  57                 hir::Constness::NotConst
  58             }
  59         }
  60     }
  61 }
  62
  63 fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
  64     tcx.is_const_fn(def_id)
  65         && match tcx.lookup_const_stability(def_id) {
  66             Some(stab) => {
  67                 if cfg!(debug_assertions) && stab.promotable {
  68                     let sig = tcx.fn_sig(def_id);
  69                     assert_eq!(
  70                         sig.skip_binder().unsafety(),
  71                         hir::Unsafety::Normal,
  72                         "don't mark const unsafe fns as promotable",
  73                         // https://github.com/rust-lang/rust/pull/53851#issuecomment-418760682
  74                     );
  75                 }
  76                 stab.promotable
  77             }
  78             None => false,
  79         }
  80 }
  81
  82 pub fn provide(providers: &mut Providers) {
  83     *providers = Providers { constness, is_promotable_const_fn, ..*providers };
  84 }