Handle bindings in substructure of patterns with type ascriptions

src/librustc/ich/impls_mir.rs

@@ -606,3 +606,6 @@ impl<'a, 'gcx> HashStable<StableHashingContext<'a>> for mir::UserTypeAnnotation<
         }
     }
 }
+
+impl_stable_hash_for!(struct mir::UserTypeProjection<'tcx> { base, projs });
+impl_stable_hash_for!(struct mir::UserTypeProjections<'tcx> { contents });

src/librustc/mir/mod.rs

@@ -710,7 +710,7 @@ pub struct LocalDecl<'tcx> {
     /// e.g. via `let x: T`, then we carry that type here. The MIR
     /// borrow checker needs this information since it can affect
     /// region inference.
-    pub user_ty: Option<(UserTypeAnnotation<'tcx>, Span)>,
+    pub user_ty: UserTypeProjections<'tcx>,
 
     /// Name of the local, used in debuginfo and pretty-printing.
     ///
@@ -882,7 +882,7 @@ impl<'tcx> LocalDecl<'tcx> {
         LocalDecl {
             mutability,
             ty,
-            user_ty: None,
+            user_ty: UserTypeProjections::none(),
             name: None,
             source_info: SourceInfo {
                 span,
@@ -903,7 +903,7 @@ impl<'tcx> LocalDecl<'tcx> {
         LocalDecl {
             mutability: Mutability::Mut,
             ty: return_ty,
-            user_ty: None,
+            user_ty: UserTypeProjections::none(),
             source_info: SourceInfo {
                 span,
                 scope: OUTERMOST_SOURCE_SCOPE,
@@ -1741,7 +1741,7 @@ pub enum StatementKind<'tcx> {
     /// - `Contravariant` -- requires that `T_y :> T`
     /// - `Invariant` -- requires that `T_y == T`
     /// - `Bivariant` -- no effect
-    AscribeUserType(Place<'tcx>, ty::Variance, Box<UserTypeAnnotation<'tcx>>),
+    AscribeUserType(Place<'tcx>, ty::Variance, Box<UserTypeProjection<'tcx>>),
 
     /// No-op. Useful for deleting instructions without affecting statement indices.
     Nop,
@@ -1944,6 +1944,10 @@ pub type PlaceProjection<'tcx> = Projection<'tcx, Place<'tcx>, Local, Ty<'tcx>>;
 /// and the index is a local.
 pub type PlaceElem<'tcx> = ProjectionElem<'tcx, Local, Ty<'tcx>>;
 
+/// Alias for projections as they appear in `UserTypeProjection`, where we
+/// need neither the `V` parameter for `Index` nor the `T` for `Field`.
+pub type ProjectionKind<'tcx> = ProjectionElem<'tcx, (), ()>;
+
 newtype_index! {
     pub struct Field {
         DEBUG_FORMAT = "field[{}]"
@@ -2449,6 +2453,117 @@ EnumLiftImpl! {
     }
 }
 
+/// A collection of projections into user types.
+///
+/// They are projections because a binding can occur a part of a
+/// parent pattern that has been ascribed a type.
+///
+/// Its a collection because there can be multiple type ascriptions on
+/// the path from the root of the pattern down to the binding itself.
+///
+/// An example:
+///
+/// ```rust
+/// struct S<'a>((i32, &'a str), String);
+/// let S((_, w): (i32, &'static str), _): S = ...;
+/// //    ------  ^^^^^^^^^^^^^^^^^^^ (1)
+/// //  ---------------------------------  ^ (2)
+/// ```
+///
+/// The highlights labelled `(1)` show the subpattern `(_, w)` being
+/// ascribed the type `(i32, &'static str)`.
+///
+/// The highlights labelled `(2)` show the whole pattern being
+/// ascribed the type `S`.
+///
+/// In this example, when we descend to `w`, we will have built up the
+/// following two projected types:
+///
+///   * base: `S`,                   projection: `(base.0).1`
+///   * base: `(i32, &'static str)`, projection: `base.1`
+///
+/// The first will lead to the constraint `w: &'1 str` (for some
+/// inferred region `'1`). The second will lead to the constraint `w:
+/// &'static str`.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
+pub struct UserTypeProjections<'tcx> {
+    pub(crate) contents: Vec<(UserTypeProjection<'tcx>, Span)>,
+}
+
+BraceStructTypeFoldableImpl! {
+    impl<'tcx> TypeFoldable<'tcx> for UserTypeProjections<'tcx> {
+        contents
+    }
+}
+
+impl<'tcx> UserTypeProjections<'tcx> {
+    pub fn none() -> Self {
+        UserTypeProjections { contents: vec![] }
+    }
+
+    pub fn from_projections(projs: impl Iterator<Item=(UserTypeProjection<'tcx>, Span)>) -> Self {
+        UserTypeProjections { contents: projs.collect() }
+    }
+
+    pub fn projections_and_spans(&self) -> impl Iterator<Item=&(UserTypeProjection<'tcx>, Span)> {
+        self.contents.iter()
+    }
+
+    pub fn projections(&self) -> impl Iterator<Item=&UserTypeProjection<'tcx>> {
+        self.contents.iter().map(|&(ref user_type, _span)| user_type)
+    }
+}
+
+/// Encodes the effect of a user-supplied type annotation on the
+/// subcomponents of a pattern. The effect is determined by applying the
+/// given list of proejctions to some underlying base type. Often,
+/// the projection element list `projs` is empty, in which case this
+/// directly encodes a type in `base`. But in the case of complex patterns with
+/// subpatterns and bindings, we want to apply only a *part* of the type to a variable,
+/// in which case the `projs` vector is used.
+///
+/// Examples:
+///
+/// * `let x: T = ...` -- here, the `projs` vector is empty.
+///
+/// * `let (x, _): T = ...` -- here, the `projs` vector would contain
+///   `field[0]` (aka `.0`), indicating that the type of `s` is
+///   determined by finding the type of the `.0` field from `T`.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
+pub struct UserTypeProjection<'tcx> {
+    pub base: UserTypeAnnotation<'tcx>,
+    pub projs: Vec<ProjectionElem<'tcx, (), ()>>,
+}
+
+impl<'tcx> Copy for ProjectionKind<'tcx> { }
+
+CloneTypeFoldableAndLiftImpls! { ProjectionKind<'tcx>, }
+
+impl<'tcx> TypeFoldable<'tcx> for UserTypeProjection<'tcx> {
+    fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
+        use mir::ProjectionElem::*;
+
+        let base = self.base.fold_with(folder);
+        let projs: Vec<_> = self.projs
+            .iter()
+            .map(|elem| {
+                match elem {
+                    Deref => Deref,
+                    Field(f, ()) => Field(f.clone(), ()),
+                    Index(()) => Index(()),
+                    elem => elem.clone(),
+                }})
+            .collect();
+
+        UserTypeProjection { base, projs }
+    }
+
+    fn super_visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> bool {
+        self.base.visit_with(visitor)
+        // Note: there's nothing in `self.proj` to visit.
+    }
+}
+
 newtype_index! {
     pub struct Promoted {
         DEBUG_FORMAT = "promoted[{}]"

src/librustc/mir/tcx.rs

@@ -44,11 +44,59 @@ impl<'a, 'gcx, 'tcx> PlaceTy<'tcx> {
         }
     }
 
+    /// `place_ty.field_ty(tcx, f)` computes the type at a given field
+    /// of a record or enum-variant. (Most clients of `PlaceTy` can
+    /// instead just extract the relevant type directly from their
+    /// `PlaceElem`, but some instances of `ProjectionElem<V, T>` do
+    /// not carry a `Ty` for `T`.)
+    ///
+    /// Note that the resulting type has not been normalized.
+    pub fn field_ty(self, tcx: TyCtxt<'a, 'gcx, 'tcx>, f: &Field) -> Ty<'tcx>
+    {
+        // Pass `0` here so it can be used as a "default" variant_index in first arm below
+        let answer = match (self, 0) {
+            (PlaceTy::Ty {
+                ty: &ty::TyS { sty: ty::TyKind::Adt(adt_def, substs), .. } }, variant_index) |
+            (PlaceTy::Downcast { adt_def, substs, variant_index }, _) => {
+                let variant_def = &adt_def.variants[variant_index];
+                let field_def = &variant_def.fields[f.index()];
+                field_def.ty(tcx, substs)
+            }
+            (PlaceTy::Ty { ty }, _) => {
+                match ty.sty {
+                    ty::Tuple(ref tys) => tys[f.index()],
+                    _ => bug!("extracting field of non-tuple non-adt: {:?}", self),
+                }
+            }
+        };
+        debug!("field_ty self: {:?} f: {:?} yields: {:?}", self, f, answer);
+        answer
+    }
+
+    /// Convenience wrapper around `projection_ty_core` for
+    /// `PlaceElem`, where we can just use the `Ty` that is already
+    /// stored inline on field projection elems.
     pub fn projection_ty(self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
                          elem: &PlaceElem<'tcx>)
                          -> PlaceTy<'tcx>
     {
-        match *elem {
+        self.projection_ty_core(tcx, elem, |_, _, ty| ty)
+    }
+
+    /// `place_ty.projection_ty_core(tcx, elem, |...| { ... })`
+    /// projects `place_ty` onto `elem`, returning the appropriate
+    /// `Ty` or downcast variant corresponding to that projection.
+    /// The `handle_field` callback must map a `Field` to its `Ty`,
+    /// (which should be trivial when `T` = `Ty`).
+    pub fn projection_ty_core<V, T>(self,
+                                    tcx: TyCtxt<'a, 'gcx, 'tcx>,
+                                    elem: &ProjectionElem<'tcx, V, T>,
+                                    mut handle_field: impl FnMut(&Self, &Field, &T) -> Ty<'tcx>)
+                                    -> PlaceTy<'tcx>
+    where
+        V: ::std::fmt::Debug, T: ::std::fmt::Debug
+    {
+        let answer = match *elem {
             ProjectionElem::Deref => {
                 let ty = self.to_ty(tcx)
                              .builtin_deref(true)
@@ -94,8 +142,10 @@ impl<'a, 'gcx, 'tcx> PlaceTy<'tcx> {
                         bug!("cannot downcast non-ADT type: `{:?}`", self)
                     }
                 },
-            ProjectionElem::Field(_, fty) => PlaceTy::Ty { ty: fty }
-        }
+            ProjectionElem::Field(ref f, ref fty) => PlaceTy::Ty { ty: handle_field(&self, f, fty) }
+        };
+        debug!("projection_ty self: {:?} elem: {:?} yields: {:?}", self, elem, answer);
+        answer
     }
 }
 

src/librustc/mir/visit.rs

@@ -147,7 +147,7 @@ macro_rules! make_mir_visitor {
             fn visit_ascribe_user_ty(&mut self,
                                      place: & $($mutability)* Place<'tcx>,
                                      variance: & $($mutability)* ty::Variance,
-                                     user_ty: & $($mutability)* UserTypeAnnotation<'tcx>,
+                                     user_ty: & $($mutability)* UserTypeProjection<'tcx>,
                                      location: Location) {
                 self.super_ascribe_user_ty(place, variance, user_ty, location);
             }
@@ -175,9 +175,8 @@ macro_rules! make_mir_visitor {
 
             fn visit_projection_elem(&mut self,
                                      place: & $($mutability)* PlaceElem<'tcx>,
-                                     context: PlaceContext<'tcx>,
                                      location: Location) {
-                self.super_projection_elem(place, context, location);
+                self.super_projection_elem(place, location);
             }
 
             fn visit_branch(&mut self,
@@ -214,6 +213,13 @@ macro_rules! make_mir_visitor {
                 self.super_ty(ty);
             }
 
+            fn visit_user_type_projection(
+                &mut self,
+                ty: & $($mutability)* UserTypeProjection<'tcx>,
+            ) {
+                self.super_user_type_projection(ty);
+            }
+
             fn visit_user_type_annotation(
                 &mut self,
                 ty: & $($mutability)* UserTypeAnnotation<'tcx>,
@@ -640,10 +646,10 @@ macro_rules! make_mir_visitor {
             fn super_ascribe_user_ty(&mut self,
                                      place: & $($mutability)* Place<'tcx>,
                                      _variance: & $($mutability)* ty::Variance,
-                                     user_ty: & $($mutability)* UserTypeAnnotation<'tcx>,
+                                     user_ty: & $($mutability)* UserTypeProjection<'tcx>,
                                      location: Location) {
                 self.visit_place(place, PlaceContext::Validate, location);
-                self.visit_user_type_annotation(user_ty);
+                self.visit_user_type_projection(user_ty);
             }
 
             fn super_place(&mut self,
@@ -692,12 +698,11 @@ macro_rules! make_mir_visitor {
                     PlaceContext::Projection(Mutability::Not)
                 };
                 self.visit_place(base, context, location);
-                self.visit_projection_elem(elem, context, location);
+                self.visit_projection_elem(elem, location);
             }
 
             fn super_projection_elem(&mut self,
                                      proj: & $($mutability)* PlaceElem<'tcx>,
-                                     _context: PlaceContext<'tcx>,
                                      location: Location) {
                 match *proj {
                     ProjectionElem::Deref => {
@@ -738,8 +743,8 @@ macro_rules! make_mir_visitor {
                     local,
                     source_info: *source_info,
                 });
-                if let Some((user_ty, _)) = user_ty {
-                    self.visit_user_type_annotation(user_ty);
+                for (user_ty, _) in & $($mutability)* user_ty.contents {
+                    self.visit_user_type_projection(user_ty);
                 }
                 self.visit_source_info(source_info);
                 self.visit_source_scope(visibility_scope);
@@ -786,6 +791,17 @@ macro_rules! make_mir_visitor {
                 self.visit_source_scope(scope);
             }
 
+            fn super_user_type_projection(
+                &mut self,
+                ty: & $($mutability)* UserTypeProjection<'tcx>,
+            ) {
+                let UserTypeProjection {
+                    ref $($mutability)* base,
+                    projs: _, // Note: Does not visit projection elems!
+                } = *ty;
+                self.visit_user_type_annotation(base);
+            }
+
             fn super_user_type_annotation(
                 &mut self,
                 _ty: & $($mutability)* UserTypeAnnotation<'tcx>,

src/librustc/traits/query/type_op/ascribe_user_type.rs

@@ -11,6 +11,7 @@
 use infer::canonical::{Canonical, Canonicalized, CanonicalizedQueryResponse, QueryResponse};
 use traits::query::Fallible;
 use hir::def_id::DefId;
+use mir::ProjectionKind;
 use ty::{self, ParamEnvAnd, Ty, TyCtxt};
 use ty::subst::UserSubsts;
 
@@ -20,6 +21,7 @@ pub struct AscribeUserType<'tcx> {
     pub variance: ty::Variance,
     pub def_id: DefId,
     pub user_substs: UserSubsts<'tcx>,
+    pub projs: &'tcx ty::List<ProjectionKind<'tcx>>,
 }
 
 impl<'tcx> AscribeUserType<'tcx> {
@@ -28,8 +30,9 @@ impl<'tcx> AscribeUserType<'tcx> {
         variance: ty::Variance,
         def_id: DefId,
         user_substs: UserSubsts<'tcx>,
+        projs: &'tcx ty::List<ProjectionKind<'tcx>>,
     ) -> Self {
-        AscribeUserType { mir_ty, variance, def_id, user_substs }
+        AscribeUserType { mir_ty, variance, def_id, user_substs, projs }
     }
 }
 
@@ -59,19 +62,19 @@ impl<'gcx: 'tcx, 'tcx> super::QueryTypeOp<'gcx, 'tcx> for AscribeUserType<'tcx>
 
 BraceStructTypeFoldableImpl! {
     impl<'tcx> TypeFoldable<'tcx> for AscribeUserType<'tcx> {
-        mir_ty, variance, def_id, user_substs
+        mir_ty, variance, def_id, user_substs, projs
     }
 }
 
 BraceStructLiftImpl! {
     impl<'a, 'tcx> Lift<'tcx> for AscribeUserType<'a> {
         type Lifted = AscribeUserType<'tcx>;
-        mir_ty, variance, def_id, user_substs
+        mir_ty, variance, def_id, user_substs, projs
     }
 }
 
 impl_stable_hash_for! {
     struct AscribeUserType<'tcx> {
-        mir_ty, variance, def_id, user_substs
+        mir_ty, variance, def_id, user_substs, projs
     }
 }