src/librustc_mir/const_eval/fn_queries.rs

   1 use rustc_attr as attr;
   2 use rustc_hir as hir;
   3 use rustc_hir::def_id::{DefId, LocalDefId};
   4 use rustc_middle::hir::map::blocks::FnLikeNode;
   5 use rustc_middle::ty::query::Providers;
   6 use rustc_middle::ty::TyCtxt;
   7 use rustc_span::symbol::Symbol;
   8 use rustc_target::spec::abi::Abi;
   9
  10 /// Whether the `def_id` counts as const fn in your current crate, considering all active
  11 /// feature gates
  12 pub fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
  13     tcx.is_const_fn_raw(def_id)
  14         && match is_unstable_const_fn(tcx, def_id) {
  15             Some(feature_name) => {
  16                 // has a `rustc_const_unstable` attribute, check whether the user enabled the
  17                 // corresponding feature gate.
  18                 tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == feature_name)
  19             }
  20             // functions without const stability are either stable user written
  21             // const fn or the user is using feature gates and we thus don't
  22             // care what they do
  23             None => true,
  24         }
  25 }
  26
  27 /// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it
  28 pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
  29     if tcx.is_const_fn_raw(def_id) {
  30         let const_stab = tcx.lookup_const_stability(def_id)?;
  31         if const_stab.level.is_unstable() { Some(const_stab.feature) } else { None }
  32     } else {
  33         None
  34     }
  35 }
  36
  37 /// Returns `true` if this function must conform to `min_const_fn`
  38 pub fn is_min_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
  39     // Bail out if the signature doesn't contain `const`
  40     if !tcx.is_const_fn_raw(def_id) {
  41         return false;
  42     }
  43
  44     if tcx.features().staged_api {
  45         // In order for a libstd function to be considered min_const_fn
  46         // it needs to be stable and have no `rustc_const_unstable` attribute.
  47         match tcx.lookup_const_stability(def_id) {
  48             // `rustc_const_unstable` functions don't need to conform.
  49             Some(&attr::ConstStability { ref level, .. }) if level.is_unstable() => false,
  50             None => {
  51                 if let Some(stab) = tcx.lookup_stability(def_id) {
  52                     if stab.level.is_stable() {
  53                         tcx.sess.span_err(
  54                             tcx.def_span(def_id),
  55                             "stable const functions must have either `rustc_const_stable` or \
  56                              `rustc_const_unstable` attribute",
  57                         );
  58                         // While we errored above, because we don't know if we need to conform, we
  59                         // err on the "safe" side and require min_const_fn.
  60                         true
  61                     } else {
  62                         // Unstable functions need not conform to min_const_fn.
  63                         false
  64                     }
  65                 } else {
  66                     // Internal functions are forced to conform to min_const_fn.
  67                     // Annotate the internal function with a const stability attribute if
  68                     // you need to use unstable features.
  69                     // Note: this is an arbitrary choice that does not affect stability or const
  70                     // safety or anything, it just changes whether we need to annotate some
  71                     // internal functions with `rustc_const_stable` or with `rustc_const_unstable`
  72                     true
  73                 }
  74             }
  75             // Everything else needs to conform, because it would be callable from
  76             // other `min_const_fn` functions.
  77             _ => true,
  78         }
  79     } else {
  80         // users enabling the `const_fn` feature gate can do what they want
  81         !tcx.features().const_fn
  82     }
  83 }
  84
  85 pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
  86     let parent_id = tcx.hir().get_parent_did(hir_id);
  87     if !parent_id.is_top_level_module() { is_const_impl_raw(tcx, parent_id) } else { false }
  88 }
  89
  90 /// Checks whether the function has a `const` modifier or, in case it is an intrinsic, whether
  91 /// said intrinsic has a `rustc_const_{un,}stable` attribute.
  92 fn is_const_fn_raw(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
  93     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
  94
  95     let node = tcx.hir().get(hir_id);
  96
  97     if let hir::Node::ForeignItem(hir::ForeignItem { kind: hir::ForeignItemKind::Fn(..), .. }) =
  98         node
  99     {
 100         // Intrinsics use `rustc_const_{un,}stable` attributes to indicate constness. All other
 101         // foreign items cannot be evaluated at compile-time.
 102         if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = tcx.hir().get_foreign_abi(hir_id) {
 103             tcx.lookup_const_stability(def_id).is_some()
 104         } else {
 105             false
 106         }
 107     } else if let Some(fn_like) = FnLikeNode::from_node(node) {
 108         if fn_like.constness() == hir::Constness::Const {
 109             return true;
 110         }
 111
 112         // If the function itself is not annotated with `const`, it may still be a `const fn`
 113         // if it resides in a const trait impl.
 114         is_parent_const_impl_raw(tcx, hir_id)
 115     } else if let hir::Node::Ctor(_) = node {
 116         true
 117     } else {
 118         false
 119     }
 120 }
 121
 122 /// Checks whether the given item is an `impl` that has a `const` modifier.
 123 fn is_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
 124     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 125     let node = tcx.hir().get(hir_id);
 126     matches!(
 127         node,
 128         hir::Node::Item(hir::Item {
 129             kind: hir::ItemKind::Impl { constness: hir::Constness::Const, .. },
 130             ..
 131         })
 132     )
 133 }
 134
 135 fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 136     is_const_fn(tcx, def_id)
 137         && match tcx.lookup_const_stability(def_id) {
 138             Some(stab) => {
 139                 if cfg!(debug_assertions) && stab.promotable {
 140                     let sig = tcx.fn_sig(def_id);
 141                     assert_eq!(
 142                         sig.unsafety(),
 143                         hir::Unsafety::Normal,
 144                         "don't mark const unsafe fns as promotable",
 145                         // https://github.com/rust-lang/rust/pull/53851#issuecomment-418760682
 146                     );
 147                 }
 148                 stab.promotable
 149             }
 150             None => false,
 151         }
 152 }
 153
 154 fn const_fn_is_allowed_fn_ptr(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 155     is_const_fn(tcx, def_id)
 156         && tcx.lookup_const_stability(def_id).map(|stab| stab.allow_const_fn_ptr).unwrap_or(false)
 157 }
 158
 159 pub fn provide(providers: &mut Providers) {
 160     *providers = Providers {
 161         is_const_fn_raw,
 162         is_const_impl_raw: |tcx, def_id| is_const_impl_raw(tcx, def_id.expect_local()),
 163         is_promotable_const_fn,
 164         const_fn_is_allowed_fn_ptr,
 165         ..*providers
 166     };
 167 }