Auto merge of #53851 - oli-obk:local_promotion, r=eddyb

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

There are so many questions around promoting const fn calls... it seems saner to try to limit automatic promotion to const fns which were explicitly opted in for promotion.

I added the attribute to all public stable const fns that were already promotable (e.g. not Cell::new) in order to not cause any breakage

r? @eddyb

cc @nikomatsakis

Author: bors

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