Auto merge of #46733 - nikomatsakis:nll-master-to-rust-master-5, r=arielb1

nll part 5

Next round of changes from the nll-master branch.

Extensions:

- we now propagate ty-region-outlives constraints out of closures and into their creator when necessary
- we fix a few ICEs that can occur by doing liveness analysis (and the resulting normalization) during type-checking
- we handle the implicit region bound that assumes that each type `T` outlives the fn body
- we handle normalization of inputs/outputs in fn signatures

Not included in this PR (will come next):

- handling `impl Trait`
- tracking causal information
- extended errors

r? @arielb1

Author: bors

src/librustc/ich/impls_mir.rs

@@ -536,14 +536,29 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for mir::Literal<'gcx> {
 
 impl_stable_hash_for!(struct mir::Location { block, statement_index });
 
-impl_stable_hash_for!(struct mir::ClosureRegionRequirements {
+impl_stable_hash_for!(struct mir::ClosureRegionRequirements<'tcx> {
     num_external_vids,
     outlives_requirements
 });
 
-impl_stable_hash_for!(struct mir::ClosureOutlivesRequirement {
-    free_region,
+impl_stable_hash_for!(struct mir::ClosureOutlivesRequirement<'tcx> {
+    subject,
     outlived_free_region,
     blame_span
 });
 
+impl<'gcx> HashStable<StableHashingContext<'gcx>> for mir::ClosureOutlivesSubject<'gcx> {
+    fn hash_stable<W: StableHasherResult>(&self,
+                                          hcx: &mut StableHashingContext<'gcx>,
+                                          hasher: &mut StableHasher<W>) {
+        mem::discriminant(self).hash_stable(hcx, hasher);
+        match *self {
+            mir::ClosureOutlivesSubject::Ty(ref ty) => {
+                ty.hash_stable(hcx, hasher);
+            }
+            mir::ClosureOutlivesSubject::Region(ref region) => {
+                region.hash_stable(hcx, hasher);
+            }
+        }
+    }
+}

src/librustc/ich/impls_ty.rs

@@ -75,6 +75,9 @@ for ty::RegionKind {
             ty::ReFree(ref free_region) => {
                 free_region.hash_stable(hcx, hasher);
             }
+            ty::ReClosureBound(vid) => {
+                vid.hash_stable(hcx, hasher);
+            }
             ty::ReLateBound(..) |
             ty::ReVar(..) |
             ty::ReSkolemized(..) => {

src/librustc/infer/combine.rs

@@ -475,6 +475,14 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                     ty::Bivariant | ty::Covariant | ty::Contravariant => (),
                 }
             }
+
+            ty::ReClosureBound(..) => {
+                span_bug!(
+                    self.span,
+                    "encountered unexpected ReClosureBound: {:?}",
+                    r,
+                );
+            }
         }
 
         // FIXME: This is non-ideal because we don't give a

src/librustc/infer/error_reporting/mod.rs

@@ -240,6 +240,14 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             ty::ReErased => {
                 (format!("lifetime {:?}", region), None)
             }
+
+            // We shouldn't encounter an error message with ReClosureBound.
+            ty::ReClosureBound(..) => {
+                bug!(
+                    "encountered unexpected ReClosureBound: {:?}",
+                    region,
+                );
+            }
         };
         let message = format!("{}{}{}", prefix, description, suffix);
         if let Some(span) = span {

src/librustc/infer/freshen.rs

@@ -113,6 +113,13 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
                 // replace all free regions with 'erased
                 self.tcx().types.re_erased
             }
+
+            ty::ReClosureBound(..) => {
+                bug!(
+                    "encountered unexpected ReClosureBound: {:?}",
+                    r,
+                );
+            }
         }
     }
 

src/librustc/infer/lexical_region_resolve/mod.rs

@@ -258,7 +258,12 @@ impl<'cx, 'gcx, 'tcx> LexicalResolver<'cx, 'gcx, 'tcx> {
     fn lub_concrete_regions(&self, a: Region<'tcx>, b: Region<'tcx>) -> Region<'tcx> {
         let tcx = self.region_rels.tcx;
         match (a, b) {
-            (&ReLateBound(..), _) | (_, &ReLateBound(..)) | (&ReErased, _) | (_, &ReErased) => {
+            (&ty::ReClosureBound(..), _) |
+            (_, &ty::ReClosureBound(..)) |
+            (&ReLateBound(..), _) |
+            (_, &ReLateBound(..)) |
+            (&ReErased, _) |
+            (_, &ReErased) => {
                 bug!("cannot relate region: LUB({:?}, {:?})", a, b);
             }
 

src/librustc/mir/mod.rs

@@ -1832,8 +1832,17 @@ pub struct GeneratorLayout<'tcx> {
 /// instance of the closure is created, the corresponding free regions
 /// can be extracted from its type and constrained to have the given
 /// outlives relationship.
+///
+/// In some cases, we have to record outlives requirements between
+/// types and regions as well. In that case, if those types include
+/// any regions, those regions are recorded as `ReClosureBound`
+/// instances assigned one of these same indices. Those regions will
+/// be substituted away by the creator. We use `ReClosureBound` in
+/// that case because the regions must be allocated in the global
+/// TyCtxt, and hence we cannot use `ReVar` (which is what we use
+/// internally within the rest of the NLL code).
 #[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
-pub struct ClosureRegionRequirements {
+pub struct ClosureRegionRequirements<'gcx> {
     /// The number of external regions defined on the closure.  In our
     /// example above, it would be 3 -- one for `'static`, then `'1`
     /// and `'2`. This is just used for a sanity check later on, to
@@ -1843,15 +1852,15 @@ pub struct ClosureRegionRequirements {
 
     /// Requirements between the various free regions defined in
     /// indices.
-    pub outlives_requirements: Vec<ClosureOutlivesRequirement>,
+    pub outlives_requirements: Vec<ClosureOutlivesRequirement<'gcx>>,
 }
 
-/// Indicates an outlives constraint between two free-regions declared
-/// on the closure.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
-pub struct ClosureOutlivesRequirement {
-    // This region ...
-    pub free_region: ty::RegionVid,
+/// Indicates an outlives constraint between a type or between two
+/// free-regions declared on the closure.
+#[derive(Copy, Clone, Debug, RustcEncodable, RustcDecodable)]
+pub struct ClosureOutlivesRequirement<'tcx> {
+    // This region or type ...
+    pub subject: ClosureOutlivesSubject<'tcx>,
 
     // .. must outlive this one.
     pub outlived_free_region: ty::RegionVid,
@@ -1860,6 +1869,23 @@ pub struct ClosureOutlivesRequirement {
     pub blame_span: Span,
 }
 
+/// The subject of a ClosureOutlivesRequirement -- that is, the thing
+/// that must outlive some region.
+#[derive(Copy, Clone, Debug, RustcEncodable, RustcDecodable)]
+pub enum ClosureOutlivesSubject<'tcx> {
+    /// Subject is a type, typically a type parameter, but could also
+    /// be a projection. Indicates a requirement like `T: 'a` being
+    /// passed to the caller, where the type here is `T`.
+    ///
+    /// The type here is guaranteed not to contain any free regions at
+    /// present.
+    Ty(Ty<'tcx>),
+
+    /// Subject is a free region from the closure. Indicates a requirement
+    /// like `'a: 'b` being passed to the caller; the region here is `'a`.
+    Region(ty::RegionVid),
+}
+
 /*
  * TypeFoldable implementations for MIR types
  */

src/librustc/ty/fold.rs

@@ -97,14 +97,19 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     fn has_closure_types(&self) -> bool {
         self.has_type_flags(TypeFlags::HAS_TY_CLOSURE)
     }
+    /// "Free" regions in this context means that it has any region
+    /// that is not (a) erased or (b) late-bound.
+    fn has_free_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
+    }
+
+    /// True if there any any un-erased free regions.
     fn has_erasable_regions(&self) -> bool {
-        self.has_type_flags(TypeFlags::HAS_RE_EARLY_BOUND |
-                            TypeFlags::HAS_RE_INFER |
-                            TypeFlags::HAS_FREE_REGIONS)
+        self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
     }
+
     fn is_normalized_for_trans(&self) -> bool {
-        !self.has_type_flags(TypeFlags::HAS_RE_EARLY_BOUND |
-                             TypeFlags::HAS_RE_INFER |
+        !self.has_type_flags(TypeFlags::HAS_RE_INFER |
                              TypeFlags::HAS_FREE_REGIONS |
                              TypeFlags::HAS_TY_INFER |
                              TypeFlags::HAS_PARAMS |

src/librustc/ty/maps/mod.rs

@@ -193,7 +193,7 @@ define_maps! { <'tcx>
 
     /// Borrow checks the function body. If this is a closure, returns
     /// additional requirements that the closure's creator must verify.
-    [] fn mir_borrowck: MirBorrowCheck(DefId) -> Option<mir::ClosureRegionRequirements>,
+    [] fn mir_borrowck: MirBorrowCheck(DefId) -> Option<mir::ClosureRegionRequirements<'tcx>>,
 
     /// Gets a complete map from all types to their inherent impls.
     /// Not meant to be used directly outside of coherence.

src/librustc/ty/mod.rs

@@ -421,8 +421,18 @@ bitflags! {
         const HAS_TY_INFER       = 1 << 2;
         const HAS_RE_INFER       = 1 << 3;
         const HAS_RE_SKOL        = 1 << 4;
+
+        /// Does this have any `ReEarlyBound` regions? Used to
+        /// determine whether substitition is required, since those
+        /// represent regions that are bound in a `ty::Generics` and
+        /// hence may be substituted.
         const HAS_RE_EARLY_BOUND = 1 << 5;
+
+        /// Does this have any region that "appears free" in the type?
+        /// Basically anything but `ReLateBound` and `ReErased`.
         const HAS_FREE_REGIONS   = 1 << 6;
+
+        /// Is an error type reachable?
         const HAS_TY_ERR         = 1 << 7;
         const HAS_PROJECTION     = 1 << 8;