reserve impl<T> From<!> for T

this is necessary for never-type stabilization

src/libcore/convert.rs

@@ -554,6 +554,12 @@ impl<T> From<T> for T {
     fn from(t: T) -> T { t }
 }
 
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+#[cfg(not(boostrap_stdarch_ignore_this))]
+#[rustc_reservation_impl]
+impl<T> From<!> for T {
+    fn from(t: !) -> T { t }
+}
 
 // TryFrom implies TryInto
 #[stable(feature = "try_from", since = "1.34.0")]

src/librustc/traits/select.rs

@@ -50,6 +50,8 @@ use std::iter;
 use std::rc::Rc;
 use crate::util::nodemap::{FxHashMap, FxHashSet};
 
+use syntax::symbol::sym;
+
 pub struct SelectionContext<'cx, 'tcx> {
     infcx: &'cx InferCtxt<'cx, 'tcx>,
 
@@ -1326,8 +1328,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         (result, dep_node)
     }
 
-    // Treat negative impls as unimplemented
-    fn filter_negative_impls(
+    // Treat negative impls as unimplemented, and reservation impls as Ok(None)
+    fn filter_negative_and_reservation_impls(
         &self,
         candidate: SelectionCandidate<'tcx>,
     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
@@ -1337,6 +1339,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             {
                 return Err(Unimplemented);
             }
+
+            if self.tcx().has_attr(def_id, sym::rustc_reservation_impl) {
+                return Ok(None);
+            }
         }
         Ok(Some(candidate))
     }
@@ -1453,7 +1459,7 @@ 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 +1534,7 @@ 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 +3734,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return Err(());
         }
 
+        if self.intercrate.is_none() &&
+            self.tcx().has_attr(impl_def_id, sym::rustc_reservation_impl)
+        {
+            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

@@ -2911,7 +2911,13 @@ impl<'tcx> TyCtxt<'tcx> {
             return Some(ImplOverlapKind::Permitted);
         }
 
-        let is_legit = if self.features().overlapping_marker_traits {
+        if self.impl_polarity(def_id1) != self.impl_polarity(def_id2) {
+            debug!("impls_are_allowed_to_overlap({:?}, {:?}) - different polarities, None",
+                   def_id1, def_id2);
+            return None;
+        }
+
+        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 +2926,24 @@ 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);
+        // `#[rustc_reservation_impl]` impls don't overlap with anything
+        let is_reserve_overlap = {
+            self.has_attr(def_id1, sym::rustc_reservation_impl) ||
+            self.has_attr(def_id2, sym::rustc_reservation_impl)
+        };
+
+        if is_marker_overlap || is_reserve_overlap {
+            debug!("impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted) ({:?}/{:?})",
+                  def_id1, def_id2, is_marker_overlap, is_reserve_overlap);
             Some(ImplOverlapKind::Permitted)
         } else {
             if let Some(self_ty1) = self.issue33140_self_ty(def_id1) {

src/librustc_typeck/check/wfcheck.rs

@@ -398,18 +398,23 @@ fn check_impl<'tcx>(
 
         match *ast_trait_ref {
             Some(ref ast_trait_ref) => {
-                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);
-                for obligation in obligations {
-                    fcx.register_predicate(obligation);
+                // `#[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).
+                if !fcx.tcx.has_attr(item_def_id, sym::rustc_reservation_impl) {
+                    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);
+                    for obligation in obligations {
+                        fcx.register_predicate(obligation);
+                    }
                 }
             }
             None => {

src/libsyntax/feature_gate/builtin_attrs.rs

@@ -498,6 +498,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         overflow checking behavior of several libcore functions that are inlined \
         across crates and will never be stable",
     ),
+    rustc_attr!(rustc_reservation_impl, Normal, template!(Word),
+                "the `#[rustc_reservation_impl]` attribute is internally used \
+                 for reserving for `for<T> From<!> for T` impl"
+    ),
     rustc_attr!(
         rustc_test_marker, Normal, template!(Word),
         "the `#[rustc_test_marker]` attribute is used internally to track tests",

src/libsyntax_pos/symbol.rs

@@ -606,6 +606,7 @@ symbols! {
         rustc_std_internal_symbol,
         rustc_symbol_name,
         rustc_synthetic,
+        rustc_reservation_impl,
         rustc_test_marker,
         rustc_then_this_would_need,
         rustc_variance,

src/test/ui/never-impl-is-reserved.rs

@@ -0,0 +1,12 @@
+// check that the `for<T> T: From<!>` impl is reserved
+
+#![feature(never_type)]
+
+pub struct MyFoo;
+pub trait MyTrait {}
+
+impl MyTrait for MyFoo {}
+// This will conflict with the first impl if we impl `for<T> T: From<!>`.
+impl<T> MyTrait for T where T: From<!> {} //~ ERROR conflicting implementation
+
+fn main() {}

src/test/ui/never-impl-is-reserved.stderr

@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `MyTrait` for type `MyFoo`:
+  --> $DIR/never-impl-is-reserved.rs:10:1
+   |
+LL | impl MyTrait for MyFoo {}
+   | ---------------------- first implementation here
+LL | // This will conflict with the first impl if we impl `for<T> T: From<!>`.
+LL | impl<T> MyTrait for T where T: From<!> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `MyFoo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.