reserve impl<T> From<!> for T

src/libcore/convert.rs

@@ -554,6 +554,18 @@ impl<T> From<T> for T {
     fn from(t: T) -> T { t }
 }
 
+/// **Stability note:** This impl does not yet exist, but we are
+/// "reserving space" to add it in the future. See
+/// [rust-lang/rust#64715][#64715] for details.
+///
+/// [#64715]: https://github.com/rust-lang/rust/issues/64715
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+#[cfg(not(bootstrap))]
+#[rustc_reservation_impl="permitting this impl would forbid us from adding \
+`impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
+impl<T> From<!> for T {
+    fn from(t: !) -> T { t }
+}
 
 // TryFrom implies TryInto
 #[stable(feature = "try_from", since = "1.34.0")]

src/librustc/query/mod.rs

@@ -286,7 +286,7 @@ rustc_queries! {
         query associated_item(_: DefId) -> ty::AssocItem {}
 
         query impl_trait_ref(_: DefId) -> Option<ty::TraitRef<'tcx>> {}
-        query impl_polarity(_: DefId) -> hir::ImplPolarity {}
+        query impl_polarity(_: DefId) -> ty::ImplPolarity {}
 
         query issue33140_self_ty(_: DefId) -> Option<ty::Ty<'tcx>> {}
     }

src/librustc/traits/auto_trait.rs

@@ -321,7 +321,7 @@ impl AutoTraitFinder<'tcx> {
                     match vtable {
                         Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => {
                             // Blame tidy for the weird bracket placement
-                            if infcx.tcx.impl_polarity(*impl_def_id) == hir::ImplPolarity::Negative
+                            if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative
                             {
                                 debug!("evaluate_nested_obligations: Found explicit negative impl\
                                         {:?}, bailing out", impl_def_id);

src/librustc/traits/select.rs

@@ -43,6 +43,8 @@ use crate::hir;
 use rustc_data_structures::bit_set::GrowableBitSet;
 use rustc_data_structures::sync::Lock;
 use rustc_target::spec::abi::Abi;
+use syntax::attr;
+use syntax::symbol::sym;
 use std::cell::{Cell, RefCell};
 use std::cmp;
 use std::fmt::{self, Display};
@@ -99,6 +101,9 @@ pub enum IntercrateAmbiguityCause {
         trait_desc: String,
         self_desc: Option<String>,
     },
+    ReservationImpl {
+        message: String
+    },
 }
 
 impl IntercrateAmbiguityCause {
@@ -139,6 +144,11 @@ impl IntercrateAmbiguityCause {
                     trait_desc, self_desc
                 )
             }
+            &IntercrateAmbiguityCause::ReservationImpl {
+                ref message
+            } => {
+                message.clone()
+            }
         }
     }
 }
@@ -1326,17 +1336,38 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         (result, dep_node)
     }
 
-    // Treat negative impls as unimplemented
-    fn filter_negative_impls(
-        &self,
+    // Treat negative impls as unimplemented, and reservation impls as ambiguity.
+    fn filter_negative_and_reservation_impls(
+        &mut self,
         candidate: SelectionCandidate<'tcx>,
     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
         if let ImplCandidate(def_id) = candidate {
-            if !self.allow_negative_impls
-                && self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative
-            {
-                return Err(Unimplemented);
-            }
+            let tcx = self.tcx();
+            match tcx.impl_polarity(def_id) {
+                ty::ImplPolarity::Negative if !self.allow_negative_impls => {
+                    return Err(Unimplemented);
+                }
+                ty::ImplPolarity::Reservation => {
+                    if let Some(intercrate_ambiguity_clauses)
+                        = &mut self.intercrate_ambiguity_causes
+                    {
+                        let attrs = tcx.get_attrs(def_id);
+                        let attr = attr::find_by_name(&attrs, sym::rustc_reservation_impl);
+                        let value = attr.and_then(|a| a.value_str());
+                        if let Some(value) = value {
+                            debug!("filter_negative_and_reservation_impls: \
+                                    reservation impl ambiguity on {:?}", def_id);
+                            intercrate_ambiguity_clauses.push(
+                                IntercrateAmbiguityCause::ReservationImpl {
+                                    message: value.to_string()
+                                }
+                            );
+                        }
+                    }
+                    return Ok(None);
+                }
+                _ => {}
+            };
         }
         Ok(Some(candidate))
     }
@@ -1453,7 +1484,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // Instead, we select the right impl now but report `Bar does
         // not implement Clone`.
         if candidates.len() == 1 {
-            return self.filter_negative_impls(candidates.pop().unwrap());
+            return self.filter_negative_and_reservation_impls(candidates.pop().unwrap());
         }
 
         // Winnow, but record the exact outcome of evaluation, which
@@ -1528,7 +1559,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         // Just one candidate left.
-        self.filter_negative_impls(candidates.pop().unwrap().candidate)
+        self.filter_negative_and_reservation_impls(candidates.pop().unwrap().candidate)
     }
 
     fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
@@ -3728,6 +3759,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return Err(());
         }
 
+        if self.intercrate.is_none()
+            && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
+        {
+            debug!("match_impl: reservation impls only apply in intercrate mode");
+            return Err(());
+        }
+
         debug!("match_impl: success impl_substs={:?}", impl_substs);
         Ok(Normalized {
             value: impl_substs,

src/librustc/ty/mod.rs

@@ -167,6 +167,19 @@ pub struct ImplHeader<'tcx> {
     pub predicates: Vec<Predicate<'tcx>>,
 }
 
+#[derive(Copy, Clone, PartialEq, RustcEncodable, RustcDecodable, HashStable)]
+pub enum ImplPolarity {
+    /// `impl Trait for Type`
+    Positive,
+    /// `impl !Trait for Type`
+    Negative,
+    /// `#[rustc_reservation_impl] impl Trait for Type`
+    ///
+    /// This is a "stability hack", not a real Rust feature.
+    /// See #64631 for details.
+    Reservation,
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, HashStable)]
 pub struct AssocItem {
     pub def_id: DefId,
@@ -2911,7 +2924,26 @@ impl<'tcx> TyCtxt<'tcx> {
             return Some(ImplOverlapKind::Permitted);
         }
 
-        let is_legit = if self.features().overlapping_marker_traits {
+        match (self.impl_polarity(def_id1), self.impl_polarity(def_id2)) {
+            (ImplPolarity::Reservation, _) |
+            (_, ImplPolarity::Reservation) => {
+                // `#[rustc_reservation_impl]` impls don't overlap with anything
+                debug!("impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted) (reservations)",
+                       def_id1, def_id2);
+                return Some(ImplOverlapKind::Permitted);
+            }
+            (ImplPolarity::Positive, ImplPolarity::Negative) |
+            (ImplPolarity::Negative, ImplPolarity::Positive) => {
+                // `impl AutoTrait for Type` + `impl !AutoTrait for Type`
+                debug!("impls_are_allowed_to_overlap({:?}, {:?}) - None (differing polarities)",
+                       def_id1, def_id2);
+                return None;
+            }
+            (ImplPolarity::Positive, ImplPolarity::Positive) |
+            (ImplPolarity::Negative, ImplPolarity::Negative) => {}
+        };
+
+        let is_marker_overlap = if self.features().overlapping_marker_traits {
             let trait1_is_empty = self.impl_trait_ref(def_id1)
                 .map_or(false, |trait_ref| {
                     self.associated_item_def_ids(trait_ref.def_id).is_empty()
@@ -2920,22 +2952,19 @@ impl<'tcx> TyCtxt<'tcx> {
                 .map_or(false, |trait_ref| {
                     self.associated_item_def_ids(trait_ref.def_id).is_empty()
                 });
-            self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
-                && trait1_is_empty
-                && trait2_is_empty
+            trait1_is_empty && trait2_is_empty
         } else {
             let is_marker_impl = |def_id: DefId| -> bool {
                 let trait_ref = self.impl_trait_ref(def_id);
                 trait_ref.map_or(false, |tr| self.trait_def(tr.def_id).is_marker)
             };
-            self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
-                && is_marker_impl(def_id1)
-                && is_marker_impl(def_id2)
+            is_marker_impl(def_id1) && is_marker_impl(def_id2)
         };
 
-        if is_legit {
-            debug!("impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted)",
-                  def_id1, def_id2);
+
+        if is_marker_overlap {
+            debug!("impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted) (marker overlap)",
+                   def_id1, def_id2);
             Some(ImplOverlapKind::Permitted)
         } else {
             if let Some(self_ty1) = self.issue33140_self_ty(def_id1) {
@@ -3317,7 +3346,7 @@ fn issue33140_self_ty(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Ty<'_>> {
     debug!("issue33140_self_ty({:?}), trait-ref={:?}", def_id, trait_ref);
 
     let is_marker_like =
-        tcx.impl_polarity(def_id) == hir::ImplPolarity::Positive &&
+        tcx.impl_polarity(def_id) == ty::ImplPolarity::Positive &&
         tcx.associated_item_def_ids(trait_ref.def_id).is_empty();
 
     // Check whether these impls would be ok for a marker trait.

src/librustc_metadata/decoder.rs

@@ -722,7 +722,7 @@ impl<'a, 'tcx> CrateMetadata {
         self.get_impl_data(id).parent_impl
     }
 
-    pub fn get_impl_polarity(&self, id: DefIndex) -> hir::ImplPolarity {
+    pub fn get_impl_polarity(&self, id: DefIndex) -> ty::ImplPolarity {
         self.get_impl_data(id).polarity
     }
 

src/librustc_metadata/encoder.rs

@@ -1172,8 +1172,9 @@ impl EncodeContext<'tcx> {
                     ctor_sig: None,
                 }), repr_options)
             }
-            hir::ItemKind::Impl(_, polarity, defaultness, ..) => {
+            hir::ItemKind::Impl(_, _, defaultness, ..) => {
                 let trait_ref = tcx.impl_trait_ref(def_id);
+                let polarity = tcx.impl_polarity(def_id);
                 let parent = if let Some(trait_ref) = trait_ref {
                     let trait_def = tcx.trait_def(trait_ref.def_id);
                     trait_def.ancestors(tcx, def_id).nth(1).and_then(|node| {

src/librustc_metadata/schema.rs

@@ -327,7 +327,7 @@ pub struct TraitAliasData<'tcx> {
 
 #[derive(RustcEncodable, RustcDecodable)]
 pub struct ImplData<'tcx> {
-    pub polarity: hir::ImplPolarity,
+    pub polarity: ty::ImplPolarity,
     pub defaultness: hir::Defaultness,
     pub parent_impl: Option<DefId>,
 

src/librustc_traits/lowering/mod.rs

@@ -4,7 +4,7 @@ use rustc::hir::def::DefKind;
 use rustc::hir::def_id::DefId;
 use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc::hir::map::definitions::DefPathData;
-use rustc::hir::{self, ImplPolarity};
+use rustc::hir;
 use rustc::traits::{
     Clause,
     Clauses,
@@ -295,7 +295,7 @@ fn program_clauses_for_trait(tcx: TyCtxt<'_>, def_id: DefId) -> Clauses<'_> {
 }
 
 fn program_clauses_for_impl(tcx: TyCtxt<'tcx>, def_id: DefId) -> Clauses<'tcx> {
-    if let ImplPolarity::Negative = tcx.impl_polarity(def_id) {
+    if let ty::ImplPolarity::Negative = tcx.impl_polarity(def_id) {
         return List::empty();
     }
 

src/librustc_typeck/check/wfcheck.rs

@@ -94,20 +94,27 @@ pub fn check_item_well_formed(tcx: TyCtxt<'_>, def_id: DefId) {
         //
         // won't be allowed unless there's an *explicit* implementation of `Send`
         // for `T`
-        hir::ItemKind::Impl(_, polarity, defaultness, _, ref trait_ref, ref self_ty, _) => {
+        hir::ItemKind::Impl(_, _, defaultness, _, ref trait_ref, ref self_ty, _) => {
             let is_auto = tcx.impl_trait_ref(tcx.hir().local_def_id(item.hir_id))
-                                .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
+                .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
+            let polarity = tcx.impl_polarity(def_id);
             if let (hir::Defaultness::Default { .. }, true) = (defaultness, is_auto) {
                 tcx.sess.span_err(item.span, "impls of auto traits cannot be default");
             }
-            if polarity == hir::ImplPolarity::Positive {
-                check_impl(tcx, item, self_ty, trait_ref);
-            } else {
-                // FIXME(#27579): what amount of WF checking do we need for neg impls?
-                if trait_ref.is_some() && !is_auto {
-                    span_err!(tcx.sess, item.span, E0192,
-                              "negative impls are only allowed for \
-                               auto traits (e.g., `Send` and `Sync`)")
+            match polarity {
+                ty::ImplPolarity::Positive => {
+                    check_impl(tcx, item, self_ty, trait_ref);
+                }
+                ty::ImplPolarity::Negative => {
+                    // FIXME(#27579): what amount of WF checking do we need for neg impls?
+                    if trait_ref.is_some() && !is_auto {
+                        span_err!(tcx.sess, item.span, E0192,
+                                  "negative impls are only allowed for \
+                                   auto traits (e.g., `Send` and `Sync`)")
+                    }
+                }
+                ty::ImplPolarity::Reservation => {
+                    // FIXME: what amount of WF checking do we need for reservation impls?
                 }
             }
         }
@@ -398,16 +405,19 @@ fn check_impl<'tcx>(
 
         match *ast_trait_ref {
             Some(ref ast_trait_ref) => {
+                // `#[rustc_reservation_impl]` impls are not real impls and
+                // therefore don't need to be WF (the trait's `Self: Trait` predicate
+                // won't hold).
                 let trait_ref = fcx.tcx.impl_trait_ref(item_def_id).unwrap();
                 let trait_ref =
                     fcx.normalize_associated_types_in(
                         ast_trait_ref.path.span, &trait_ref);
                 let obligations =
                     ty::wf::trait_obligations(fcx,
-                                                fcx.param_env,
-                                                fcx.body_id,
-                                                &trait_ref,
-                                                ast_trait_ref.path.span);
+                                              fcx.param_env,
+                                              fcx.body_id,
+                                              &trait_ref,
+                                              ast_trait_ref.path.span);
                 for obligation in obligations {
                     fcx.register_predicate(obligation);
                 }

src/librustc_typeck/collect.rs

@@ -1866,10 +1866,30 @@ fn impl_trait_ref(tcx: TyCtxt<'_>, def_id: DefId) -> Option<ty::TraitRef<'_>> {
     }
 }
 
-fn impl_polarity(tcx: TyCtxt<'_>, def_id: DefId) -> hir::ImplPolarity {
+fn impl_polarity(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ImplPolarity {
     let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
-    match tcx.hir().expect_item(hir_id).node {
-        hir::ItemKind::Impl(_, polarity, ..) => polarity,
+    let is_rustc_reservation = tcx.has_attr(def_id, sym::rustc_reservation_impl);
+    let item = tcx.hir().expect_item(hir_id);
+    match &item.node {
+        hir::ItemKind::Impl(_, hir::ImplPolarity::Negative, ..) => {
+            if is_rustc_reservation {
+                tcx.sess.span_err(item.span, "reservation impls can't be negative");
+            }
+            ty::ImplPolarity::Negative
+        }
+        hir::ItemKind::Impl(_, hir::ImplPolarity::Positive, _, _, None, _, _) => {
+            if is_rustc_reservation {
+                tcx.sess.span_err(item.span, "reservation impls can't be inherent");
+            }
+            ty::ImplPolarity::Positive
+        }
+        hir::ItemKind::Impl(_, hir::ImplPolarity::Positive, _, _, Some(_tr), _, _) => {
+            if is_rustc_reservation {
+                ty::ImplPolarity::Reservation
+            } else {
+                ty::ImplPolarity::Positive
+            }
+        }
         ref item => bug!("impl_polarity: {:?} not an impl", item),
     }
 }