Limit the promotion of const fns to the libstd and the rustc_promotable attribute

src/libcore/mem.rs

@@ -285,6 +285,7 @@ pub fn forget<T>(t: T) {
 /// [alignment]: ./fn.align_of.html
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_promotable)]
 pub const fn size_of<T>() -> usize {
     intrinsics::size_of::<T>()
 }
@@ -376,6 +377,7 @@ pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_promotable)]
 pub const fn align_of<T>() -> usize {
     intrinsics::min_align_of::<T>()
 }

src/libcore/num/mod.rs

@@ -216,6 +216,7 @@ $EndFeature, "
 ```"),
             #[stable(feature = "rust1", since = "1.0.0")]
             #[inline]
+            #[cfg_attr(not(stage0), rustc_promotable)]
             pub const fn min_value() -> Self {
                 !0 ^ ((!0 as $UnsignedT) >> 1) as Self
             }
@@ -234,6 +235,7 @@ $EndFeature, "
 ```"),
             #[stable(feature = "rust1", since = "1.0.0")]
             #[inline]
+            #[cfg_attr(not(stage0), rustc_promotable)]
             pub const fn max_value() -> Self {
                 !Self::min_value()
             }

src/libcore/ops/range.rs

@@ -391,6 +391,7 @@ impl<Idx> RangeInclusive<Idx> {
     /// ```
     #[stable(feature = "inclusive_range_methods", since = "1.27.0")]
     #[inline]
+    #[cfg_attr(not(stage0), rustc_promotable)]
     pub const fn new(start: Idx, end: Idx) -> Self {
         Self { start, end, is_empty: None }
     }

src/libcore/ptr.rs

@@ -209,6 +209,7 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_promotable)]
 pub const fn null<T>() -> *const T { 0 as *const T }
 
 /// Creates a null mutable raw pointer.
@@ -223,6 +224,7 @@ pub const fn null<T>() -> *const T { 0 as *const T }
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_promotable)]
 pub const fn null_mut<T>() -> *mut T { 0 as *mut T }
 
 /// Swaps the values at two mutable locations of the same type, without

src/libcore/time.rs

@@ -109,6 +109,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
+    #[cfg_attr(not(stage0), rustc_promotable)]
     pub const fn from_secs(secs: u64) -> Duration {
         Duration { secs, nanos: 0 }
     }
@@ -127,6 +128,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
+    #[cfg_attr(not(stage0), rustc_promotable)]
     pub const fn from_millis(millis: u64) -> Duration {
         Duration {
             secs: millis / MILLIS_PER_SEC,
@@ -148,6 +150,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration_from_micros", since = "1.27.0")]
     #[inline]
+    #[cfg_attr(not(stage0), rustc_promotable)]
     pub const fn from_micros(micros: u64) -> Duration {
         Duration {
             secs: micros / MICROS_PER_SEC,
@@ -169,6 +172,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration_extras", since = "1.27.0")]
     #[inline]
+    #[cfg_attr(not(stage0), rustc_promotable)]
     pub const fn from_nanos(nanos: u64) -> Duration {
         Duration {
             secs: nanos / (NANOS_PER_SEC as u64),

src/librustc/dep_graph/dep_node.rs

@@ -515,6 +515,7 @@ define_dep_nodes!( <'tcx>
     [] ItemVarianceConstraints(DefId),
     [] ItemVariances(DefId),
     [] IsConstFn(DefId),
+    [] IsPromotableConstFn(DefId),
     [] IsForeignItem(DefId),
     [] TypeParamPredicates { item_id: DefId, param_id: DefId },
     [] SizedConstraint(DefId),

src/librustc/ich/impls_syntax.rs

@@ -130,6 +130,7 @@ impl_stable_hash_for!(struct ::syntax::attr::Stability {
     level,
     feature,
     rustc_depr,
+    promotable,
     const_stability
 });
 

src/librustc/middle/stability.rs

@@ -441,6 +441,7 @@ impl<'a, 'tcx> Index<'tcx> {
                     feature: Symbol::intern("rustc_private"),
                     rustc_depr: None,
                     const_stability: None,
+                    promotable: false,
                 });
                 annotator.parent_stab = Some(stability);
             }

src/librustc/ty/constness.rs

@@ -0,0 +1,111 @@
+use ty::query::Providers;
+use hir::def_id::DefId;
+use hir;
+use ty::TyCtxt;
+use syntax_pos::symbol::Symbol;
+use hir::map::blocks::FnLikeNode;
+use syntax::attr;
+use rustc_target::spec::abi;
+
+impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
+    /// Whether the `def_id` counts as const fn in your current crate, considering all active
+    /// feature gates
+    pub fn is_const_fn(self, def_id: DefId) -> bool {
+        self.is_const_fn_raw(def_id) && match self.lookup_stability(def_id) {
+            Some(stab) => match stab.const_stability {
+                // has a `rustc_const_unstable` attribute, check whether the user enabled the
+                // corresponding feature gate
+                Some(feature_name) => self.features()
+                    .declared_lib_features
+                    .iter()
+                    .any(|&(sym, _)| sym == feature_name),
+                // the function has no stability attribute, it is stable as const fn or the user
+                // needs to use feature gates to use the function at all
+                None => true,
+            },
+            // functions without stability are either stable user written const fn or the user is
+            // using feature gates and we thus don't care what they do
+            None => true,
+        }
+    }
+
+    /// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it
+    pub fn is_unstable_const_fn(self, def_id: DefId) -> Option<Symbol> {
+        if self.is_const_fn_raw(def_id) {
+            self.lookup_stability(def_id)?.const_stability
+        } else {
+            None
+        }
+    }
+
+    /// Returns true if this function must conform to `min_const_fn`
+    pub fn is_min_const_fn(self, def_id: DefId) -> bool {
+        if self.features().staged_api {
+            // some intrinsics are waved through if called inside the
+            // standard library. Users never need to call them directly
+            if let abi::Abi::RustIntrinsic = self.fn_sig(def_id).abi() {
+                assert!(!self.is_const_fn(def_id));
+                match &self.item_name(def_id).as_str()[..] {
+                    | "size_of"
+                    | "min_align_of"
+                    | "needs_drop"
+                    => return true,
+                    _ => {},
+                }
+            }
+            // in order for a libstd function to be considered min_const_fn
+            // it needs to be stable and have no `rustc_const_unstable` attribute
+            match self.lookup_stability(def_id) {
+                // stable functions with unstable const fn aren't `min_const_fn`
+                Some(&attr::Stability { const_stability: Some(_), .. }) => false,
+                // unstable functions don't need to conform
+                Some(&attr::Stability { ref level, .. }) if level.is_unstable() => false,
+                // everything else needs to conform, because it would be callable from
+                // other `min_const_fn` functions
+                _ => true,
+            }
+        } else {
+            // users enabling the `const_fn` can do what they want
+            !self.sess.features_untracked().const_fn
+        }
+    }
+}
+
+
+pub fn provide<'tcx>(providers: &mut Providers<'tcx>) {
+    /// only checks whether the function has a `const` modifier
+    fn is_const_fn_raw<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> bool {
+        let node_id = tcx.hir.as_local_node_id(def_id)
+                             .expect("Non-local call to local provider is_const_fn");
+
+        if let Some(fn_like) = FnLikeNode::from_node(tcx.hir.get(node_id)) {
+            fn_like.constness() == hir::Constness::Const
+        } else {
+            false
+        }
+    }
+
+    fn is_promotable_const_fn<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> bool {
+        tcx.is_const_fn(def_id) && match tcx.lookup_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,
+        }
+    }
+
+    *providers = Providers {
+        is_const_fn_raw,
+        is_promotable_const_fn,
+        ..*providers
+    };
+}

src/librustc/ty/context.rs

@@ -1134,38 +1134,6 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         local as usize == global as usize
     }
 
-    /// Returns true if this function must conform to `min_const_fn`
-    pub fn is_min_const_fn(self, def_id: DefId) -> bool {
-        if self.features().staged_api {
-            // some intrinsics are waved through if called inside the
-            // standard library. Users never need to call them directly
-            if let abi::Abi::RustIntrinsic = self.fn_sig(def_id).abi() {
-                assert!(!self.is_const_fn(def_id));
-                match &self.item_name(def_id).as_str()[..] {
-                    | "size_of"
-                    | "min_align_of"
-                    | "needs_drop"
-                    => return true,
-                    _ => {},
-                }
-            }
-            // in order for a libstd function to be considered min_const_fn
-            // it needs to be stable and have no `rustc_const_unstable` attribute
-            match self.lookup_stability(def_id) {
-                // stable functions with unstable const fn aren't `min_const_fn`
-                Some(&attr::Stability { const_stability: Some(_), .. }) => false,
-                // unstable functions don't need to conform
-                Some(&attr::Stability { ref level, .. }) if level.is_unstable() => false,
-                // everything else needs to conform, because it would be callable from
-                // other `min_const_fn` functions
-                _ => true,
-            }
-        } else {
-            // users enabling the `const_fn` can do what they want
-            !self.sess.features_untracked().const_fn
-        }
-    }
-
     /// Create a type context and call the closure with a `TyCtxt` reference
     /// to the context. The closure enforces that the type context and any interned
     /// value (types, substs, etc.) can only be used while `ty::tls` has a valid

src/librustc/ty/instance.rs

@@ -109,7 +109,9 @@ impl<'tcx> InstanceDef<'tcx> {
             return true
         }
         let codegen_fn_attrs = tcx.codegen_fn_attrs(self.def_id());
-        codegen_fn_attrs.requests_inline() || tcx.is_const_fn(self.def_id())
+        // need to use `is_const_fn_raw` since we don't really care if the user can use it as a
+        // const fn, just whether the function should be inlined
+        codegen_fn_attrs.requests_inline() || tcx.is_const_fn_raw(self.def_id())
     }
 }
 

src/librustc/ty/mod.rs

@@ -94,6 +94,7 @@ pub mod binding;
 pub mod cast;
 #[macro_use]
 pub mod codec;
+mod constness;
 pub mod error;
 mod erase_regions;
 pub mod fast_reject;
@@ -3056,6 +3057,7 @@ pub fn provide(providers: &mut ty::query::Providers<'_>) {
     erase_regions::provide(providers);
     layout::provide(providers);
     util::provide(providers);
+    constness::provide(providers);
     *providers = ty::query::Providers {
         associated_item,
         associated_item_def_ids,

src/librustc/ty/query/config.rs

@@ -437,7 +437,7 @@ impl<'tcx> QueryDescription<'tcx> for queries::is_object_safe<'tcx> {
     }
 }
 
-impl<'tcx> QueryDescription<'tcx> for queries::is_const_fn<'tcx> {
+impl<'tcx> QueryDescription<'tcx> for queries::is_const_fn_raw<'tcx> {
     fn describe(tcx: TyCtxt<'_, '_, '_>, def_id: DefId) -> String {
         format!("checking if item is const fn: `{}`", tcx.item_path_str(def_id))
     }

src/librustc/ty/query/mod.rs

@@ -160,8 +160,23 @@ define_queries! { <'tcx>
             DefId
         ) -> Result<DtorckConstraint<'tcx>, NoSolution>,
 
-        /// True if this is a const fn
-        [] fn is_const_fn: IsConstFn(DefId) -> bool,
+        /// True if this is a const fn, use the `is_const_fn` to know whether your crate actually
+        /// sees it as const fn (e.g. the const-fn-ness might be unstable and you might not have
+        /// the feature gate active)
+        ///
+        /// DO NOT CALL MANUALLY, it is only meant to cache the base data for the `is_const_fn`
+        /// function
+        [] fn is_const_fn_raw: IsConstFn(DefId) -> bool,
+
+
+        /// Returns true if calls to the function may be promoted
+        ///
+        /// This is either because the function is e.g. a tuple-struct or tuple-variant constructor,
+        /// or because it has the `#[rustc_promotable]` attribute. The attribute should be removed
+        /// in the future in favour of some form of check which figures out whether the function
+        /// does not inspect the bits of any of its arguments (so is essentially just a constructor
+        /// function)
+        [] fn is_promotable_const_fn: IsPromotableConstFn(DefId) -> bool,
 
         /// True if this is a foreign item (i.e., linked via `extern { ... }`).
         [] fn is_foreign_item: IsForeignItem(DefId) -> bool,

src/librustc/ty/query/plumbing.rs

@@ -1136,7 +1136,8 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::FnSignature => { force!(fn_sig, def_id!()); }
         DepKind::CoerceUnsizedInfo => { force!(coerce_unsized_info, def_id!()); }
         DepKind::ItemVariances => { force!(variances_of, def_id!()); }
-        DepKind::IsConstFn => { force!(is_const_fn, def_id!()); }
+        DepKind::IsConstFn => { force!(is_const_fn_raw, def_id!()); }
+        DepKind::IsPromotableConstFn => { force!(is_promotable_const_fn, def_id!()); }
         DepKind::IsForeignItem => { force!(is_foreign_item, def_id!()); }
         DepKind::SizedConstraint => { force!(adt_sized_constraint, def_id!()); }
         DepKind::DtorckConstraint => { force!(adt_dtorck_constraint, def_id!()); }

src/librustc_metadata/cstore_impl.rs

@@ -26,7 +26,6 @@ use rustc::ty::{self, TyCtxt};
 use rustc::ty::query::Providers;
 use rustc::hir::def_id::{CrateNum, DefId, LOCAL_CRATE, CRATE_DEF_INDEX};
 use rustc::hir::map::{DefKey, DefPath, DefPathHash};
-use rustc::hir::map::blocks::FnLikeNode;
 use rustc::hir::map::definitions::DefPathTable;
 use rustc::util::nodemap::DefIdMap;
 use rustc_data_structures::svh::Svh;
@@ -43,7 +42,6 @@ use syntax::parse::source_file_to_stream;
 use syntax::symbol::Symbol;
 use syntax_pos::{Span, NO_EXPANSION, FileName};
 use rustc_data_structures::bit_set::BitSet;
-use rustc::hir;
 
 macro_rules! provide {
     (<$lt:tt> $tcx:ident, $def_id:ident, $other:ident, $cdata:ident,
@@ -145,7 +143,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     }
     fn_sig => { cdata.fn_sig(def_id.index, tcx) }
     inherent_impls => { Lrc::new(cdata.get_inherent_implementations_for_type(def_id.index)) }
-    is_const_fn => { cdata.is_const_fn(def_id.index) }
+    is_const_fn_raw => { cdata.is_const_fn_raw(def_id.index) }
     is_foreign_item => { cdata.is_foreign_item(def_id.index) }
     describe_def => { cdata.get_def(def_id.index) }
     def_span => { cdata.get_span(def_id.index, &tcx.sess) }
@@ -264,22 +262,10 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 }
 
 pub fn provide<'tcx>(providers: &mut Providers<'tcx>) {
-    fn is_const_fn<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> bool {
-        let node_id = tcx.hir.as_local_node_id(def_id)
-                             .expect("Non-local call to local provider is_const_fn");
-
-        if let Some(fn_like) = FnLikeNode::from_node(tcx.hir.get(node_id)) {
-            fn_like.constness() == hir::Constness::Const
-        } else {
-            false
-        }
-    }
-
     // FIXME(#44234) - almost all of these queries have no sub-queries and
     // therefore no actual inputs, they're just reading tables calculated in
     // resolve! Does this work? Unsure! That's what the issue is about
     *providers = Providers {
-        is_const_fn,
         is_dllimport_foreign_item: |tcx, id| {
             tcx.native_library_kind(id) == Some(NativeLibraryKind::NativeUnknown)
         },

src/librustc_metadata/decoder.rs

@@ -1114,7 +1114,7 @@ impl<'a, 'tcx> CrateMetadata {
         }
     }
 
-    pub fn is_const_fn(&self, id: DefIndex) -> bool {
+    crate fn is_const_fn_raw(&self, id: DefIndex) -> bool {
         let constness = match self.entry(id).kind {
             EntryKind::Method(data) => data.decode(self).fn_data.constness,
             EntryKind::Fn(data) => data.decode(self).constness,