Generate documentation for auto-trait impls

A new section is added to both both struct and trait doc pages.

On struct/enum pages, a new 'Auto Trait Implementations' section displays any
synthetic implementations for auto traits. Currently, this is only done
for Send and Sync.

On trait pages, a new 'Auto Implementors' section displays all types
which automatically implement the trait. Effectively, this is a list of
all public types in the standard library.

Synthesized impls for a particular auto trait ('synthetic impls') take
into account generic bounds. For example, a type 'struct Foo<T>(T)' will
have 'impl<T> Send for Foo<T> where T: Send' generated for it.

Manual implementations of auto traits are also taken into account. If we have
the following types:

'struct Foo<T>(T)'
'struct Wrapper<T>(Foo<T>)'
'unsafe impl<T> Send for Wrapper<T>' // pretend that Wrapper<T> makes
this sound somehow

Then Wrapper will have the following impl generated:
'impl<T> Send for Wrapper<T>'
reflecting the fact that 'T: Send' need not hold for 'Wrapper<T>: Send'
to hold

Lifetimes, HRTBS, and projections (e.g. '<T as Iterator>::Item') are
taken into account by synthetic impls

However, if a type can *never* implement a particular auto trait
(e.g. 'struct MyStruct<T>(*const T)'), then a negative impl will be
generated (in this case, 'impl<T> !Send for MyStruct<T>')

All of this means that a user should be able to copy-paste a synthetic
impl into their code, without any observable changes in behavior
(assuming the rest of the program remains unchanged).

src/librustc/hir/map/definitions.rs

@@ -72,6 +72,10 @@ impl DefPathTable {
         index
     }
 
+    pub fn next_id(&self, address_space: DefIndexAddressSpace) -> DefIndex {
+        DefIndex::from_array_index(self.index_to_key[address_space.index()].len(), address_space)
+    }
+
     #[inline(always)]
     pub fn def_key(&self, index: DefIndex) -> DefKey {
         self.index_to_key[index.address_space().index()]

src/librustc/infer/mod.rs

@@ -180,7 +180,7 @@ pub struct InferCtxt<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     // for each body-id in this map, which will process the
     // obligations within. This is expected to be done 'late enough'
     // that all type inference variables have been bound and so forth.
-    region_obligations: RefCell<Vec<(ast::NodeId, RegionObligation<'tcx>)>>,
+    pub region_obligations: RefCell<Vec<(ast::NodeId, RegionObligation<'tcx>)>>,
 }
 
 /// A map returned by `skolemize_late_bound_regions()` indicating the skolemized
@@ -1555,11 +1555,20 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         InferOk { value, obligations }
     }
 
-    fn borrow_region_constraints(&self) -> RefMut<'_, RegionConstraintCollector<'tcx>> {
+    pub fn borrow_region_constraints(&self) -> RefMut<'_, RegionConstraintCollector<'tcx>> {
         RefMut::map(
             self.region_constraints.borrow_mut(),
             |c| c.as_mut().expect("region constraints already solved"))
     }
+
+    /// Clears the selection, evaluation, and projection cachesThis is useful when
+    /// repeatedly attemping to select an Obligation while chagning only
+    /// its ParamEnv, since FulfillmentContext doesn't use 'probe'
+    pub fn clear_caches(&self) {
+        self.selection_cache.clear();
+        self.evaluation_cache.clear();
+        self.projection_cache.borrow_mut().clear();
+    }
 }
 
 impl<'a, 'gcx, 'tcx> TypeTrace<'tcx> {

src/librustc/infer/region_constraints/mod.rs

@@ -82,7 +82,7 @@ pub type VarOrigins = IndexVec<RegionVid, RegionVariableOrigin>;
 /// Describes constraints between the region variables and other
 /// regions, as well as other conditions that must be verified, or
 /// assumptions that can be made.
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone)]
 pub struct RegionConstraintData<'tcx> {
     /// Constraints of the form `A <= B`, where either `A` or `B` can
     /// be a region variable (or neither, as it happens).
@@ -142,7 +142,7 @@ pub enum Constraint<'tcx> {
 /// outlive `RS`. Therefore verify that `R <= RS[i]` for some
 /// `i`. Inference variables may be involved (but this verification
 /// step doesn't influence inference).
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct Verify<'tcx> {
     pub kind: GenericKind<'tcx>,
     pub origin: SubregionOrigin<'tcx>,
@@ -159,7 +159,7 @@ pub enum GenericKind<'tcx> {
 /// When we introduce a verification step, we wish to test that a
 /// particular region (let's call it `'min`) meets some bound.
 /// The bound is described the by the following grammar:
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum VerifyBound<'tcx> {
     /// B = exists {R} --> some 'r in {R} must outlive 'min
     ///
@@ -288,6 +288,10 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         &self.var_origins
     }
 
+    pub fn region_constraint_data(&self) -> &RegionConstraintData<'tcx> {
+        &self.data
+    }
+
     /// Once all the constraints have been gathered, extract out the final data.
     ///
     /// Not legal during a snapshot.

src/librustc/traits/mod.rs

@@ -32,8 +32,8 @@ use syntax_pos::{Span, DUMMY_SP};
 pub use self::coherence::{orphan_check, overlapping_impls, OrphanCheckErr, OverlapResult};
 pub use self::fulfill::FulfillmentContext;
 pub use self::project::MismatchedProjectionTypes;
-pub use self::project::{normalize, normalize_projection_type, Normalized};
-pub use self::project::{ProjectionCache, ProjectionCacheSnapshot, Reveal};
+pub use self::project::{normalize, normalize_projection_type, poly_project_and_unify_type};
+pub use self::project::{ProjectionCache, ProjectionCacheSnapshot, Reveal, Normalized};
 pub use self::object_safety::ObjectSafetyViolation;
 pub use self::object_safety::MethodViolationCode;
 pub use self::on_unimplemented::{OnUnimplementedDirective, OnUnimplementedNote};

src/librustc/traits/project.rs

@@ -1596,6 +1596,10 @@ impl<'tcx> ProjectionCache<'tcx> {
         }
     }
 
+    pub fn clear(&mut self) {
+        self.map.clear();
+    }
+
     pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
         ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
     }

src/librustc/traits/select.rs

@@ -93,6 +93,11 @@ pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
     inferred_obligations: SnapshotVec<InferredObligationsSnapshotVecDelegate<'tcx>>,
 
     intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
+
+    /// Controls whether or not to filter out negative impls when selecting.
+    /// This is used in librustdoc to distinguish between the lack of an impl
+    /// and a negative impl
+    allow_negative_impls: bool
 }
 
 #[derive(Clone, Debug)]
@@ -424,6 +429,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             intercrate: None,
             inferred_obligations: SnapshotVec::new(),
             intercrate_ambiguity_causes: None,
+            allow_negative_impls: false
         }
     }
 
@@ -436,6 +442,20 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             intercrate: Some(mode),
             inferred_obligations: SnapshotVec::new(),
             intercrate_ambiguity_causes: None,
+            allow_negative_impls: false
+        }
+    }
+
+    pub fn with_negative(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
+                         allow_negative_impls: bool) -> SelectionContext<'cx, 'gcx, 'tcx> {
+        debug!("with_negative({:?})", allow_negative_impls);
+        SelectionContext {
+            infcx,
+            freshener: infcx.freshener(),
+            intercrate: None,
+            inferred_obligations: SnapshotVec::new(),
+            intercrate_ambiguity_causes: Vec::new(),
+            allow_negative_impls
         }
     }
 
@@ -1086,7 +1106,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     fn filter_negative_impls(&self, candidate: SelectionCandidate<'tcx>)
                              -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
         if let ImplCandidate(def_id) = candidate {
-            if self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative {
+            if !self.allow_negative_impls &&
+                self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative {
                 return Err(Unimplemented)
             }
         }
@@ -3337,6 +3358,10 @@ impl<'tcx> SelectionCache<'tcx> {
             hashmap: RefCell::new(FxHashMap())
         }
     }
+
+    pub fn clear(&self) {
+        *self.hashmap.borrow_mut() = FxHashMap()
+    }
 }
 
 impl<'tcx> EvaluationCache<'tcx> {
@@ -3345,6 +3370,10 @@ impl<'tcx> EvaluationCache<'tcx> {
             hashmap: RefCell::new(FxHashMap())
         }
     }
+
+    pub fn clear(&self) {
+        *self.hashmap.borrow_mut() = FxHashMap()
+    }
 }
 
 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {

src/librustc_data_structures/snapshot_map/mod.rs

@@ -45,6 +45,11 @@ impl<K, V> SnapshotMap<K, V>
         }
     }
 
+    pub fn clear(&mut self) {
+        self.map.clear();
+        self.undo_log.clear();
+    }
+
     pub fn insert(&mut self, key: K, value: V) -> bool {
         match self.map.insert(key.clone(), value) {
             None => {