1 files changed, 82 insertions, 0 deletions
diff --git a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
new file mode 100644
index 000000000..f1674d04f
--- /dev/null
+++ b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
@@ -0,0 +1,82 @@
+use rustc_hir as hir;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_middle::ty::query::Providers;
+use rustc_middle::ty::{DefIdTree, TyCtxt};
+use rustc_span::symbol::Symbol;
+
+/// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it
+pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
+    if tcx.is_const_fn_raw(def_id) {
+        let const_stab = tcx.lookup_const_stability(def_id)?;
+        if const_stab.is_const_unstable() { Some(const_stab.feature) } else { None }
+    } else {
+        None
+    }
+}
+
+pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
+    let parent_id = tcx.local_parent(def_id);
+    tcx.def_kind(parent_id) == DefKind::Impl && tcx.constness(parent_id) == hir::Constness::Const
+}
+
+/// Checks whether an item is considered to be `const`. If it is a constructor, it is const. If
+/// it is a trait impl/function, return if it has a `const` modifier. If it is an intrinsic,
+/// report whether said intrinsic has a `rustc_const_{un,}stable` attribute. Otherwise, return
+/// `Constness::NotConst`.
+fn constness(tcx: TyCtxt<'_>, def_id: DefId) -> hir::Constness {
+    let def_id = def_id.expect_local();
+    let node = tcx.hir().get_by_def_id(def_id);
+
+    match node {
+        hir::Node::Ctor(_) => hir::Constness::Const,
+        hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.constness,
+        hir::Node::ForeignItem(hir::ForeignItem { kind: hir::ForeignItemKind::Fn(..), .. }) => {
+            // Intrinsics use `rustc_const_{un,}stable` attributes to indicate constness. All other
+            // foreign items cannot be evaluated at compile-time.
+            let is_const = if tcx.is_intrinsic(def_id) {
+                tcx.lookup_const_stability(def_id).is_some()
+            } else {
+                false
+            };
+            if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
+        }
+        _ => {
+            if let Some(fn_kind) = node.fn_kind() {
+                if fn_kind.constness() == hir::Constness::Const {
+                    return hir::Constness::Const;
+                }
+
+                // If the function itself is not annotated with `const`, it may still be a `const fn`
+                // if it resides in a const trait impl.
+                let is_const = is_parent_const_impl_raw(tcx, def_id);
+                if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
+            } else {
+                hir::Constness::NotConst
+            }
+        }
+    }
+}
+
+fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    tcx.is_const_fn(def_id)
+        && match tcx.lookup_const_stability(def_id) {
+            Some(stab) => {
+                if cfg!(debug_assertions) && stab.promotable {
+                    let sig = tcx.fn_sig(def_id);
+                    assert_eq!(
+                        sig.unsafety(),
+                        hir::Unsafety::Normal,
+                        "don't mark const unsafe fns as promotable",
+                        // https://github.com/rust-lang/rust/pull/53851#issuecomment-418760682
+                    );
+                }
+                stab.promotable
+            }
+            None => false,
+        }
+}
+
+pub fn provide(providers: &mut Providers) {
+    *providers = Providers { constness, is_promotable_const_fn, ..*providers };
+}