rustc_typeck: don't use Result for get_type_parameter_bounds and ensure_super_predicates.

Author: eddyb

src/librustc/dep_graph/dep_node.rs

@@ -109,6 +109,7 @@ pub enum DepNode<D: Clone + Debug> {
     // predicates for an item wind up in `ItemSignature`).
     AssociatedItems(D),
     ItemSignature(D),
+    TypeParamPredicates((D, D)),
     SizedConstraint(D),
     AssociatedItemDefIds(D),
     InherentImpls(D),
@@ -259,6 +260,9 @@ impl<D: Clone + Debug> DepNode<D> {
             TransInlinedItem(ref d) => op(d).map(TransInlinedItem),
             AssociatedItems(ref d) => op(d).map(AssociatedItems),
             ItemSignature(ref d) => op(d).map(ItemSignature),
+            TypeParamPredicates((ref item, ref param)) => {
+                Some(TypeParamPredicates((try_opt!(op(item)), try_opt!(op(param)))))
+            }
             SizedConstraint(ref d) => op(d).map(SizedConstraint),
             AssociatedItemDefIds(ref d) => op(d).map(AssociatedItemDefIds),
             InherentImpls(ref d) => op(d).map(InherentImpls),

src/librustc/diagnostics.rs

@@ -1390,6 +1390,23 @@ error. To resolve it, add an `else` block having the same type as the `if`
 block.
 "##,
 
+E0391: r##"
+This error indicates that some types or traits depend on each other
+and therefore cannot be constructed.
+
+The following example contains a circular dependency between two traits:
+
+```compile_fail,E0391
+trait FirstTrait : SecondTrait {
+
+}
+
+trait SecondTrait : FirstTrait {
+
+}
+```
+"##,
+
 E0398: r##"
 In Rust 1.3, the default object lifetime bounds are expected to change, as
 described in RFC #1156 [1]. You are getting a warning because the compiler

src/librustc/hir/map/mod.rs

@@ -437,6 +437,30 @@ impl<'hir> Map<'hir> {
         self.local_def_id(self.body_owner(id))
     }
 
+    pub fn ty_param_owner(&self, id: NodeId) -> NodeId {
+        match self.get(id) {
+            NodeItem(&Item { node: ItemTrait(..), .. }) => id,
+            NodeTyParam(_) => self.get_parent_node(id),
+            _ => {
+                bug!("ty_param_owner: {} not a type parameter",
+                    self.node_to_string(id))
+            }
+        }
+    }
+
+    pub fn ty_param_name(&self, id: NodeId) -> Name {
+        match self.get(id) {
+            NodeItem(&Item { node: ItemTrait(..), .. }) => {
+                keywords::SelfType.name()
+            }
+            NodeTyParam(tp) => tp.name,
+            _ => {
+                bug!("ty_param_name: {} not a type parameter",
+                    self.node_to_string(id))
+            }
+        }
+    }
+
     /// Get the attributes on the krate. This is preferable to
     /// invoking `krate.attrs` because it registers a tighter
     /// dep-graph access.

src/librustc/lib.rs

@@ -39,6 +39,7 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(rustc_private)]
 #![feature(slice_patterns)]
+#![feature(specialization)]
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
 

src/librustc/ty/maps.rs

@@ -20,6 +20,7 @@ use rustc_data_structures::indexed_vec::IndexVec;
 use std::cell::RefCell;
 use std::rc::Rc;
 use syntax::attr;
+use syntax_pos::Span;
 
 trait Key {
     fn map_crate(&self) -> CrateNum;
@@ -31,13 +32,105 @@ impl Key for DefId {
     }
 }
 
+impl Key for (DefId, DefId) {
+    fn map_crate(&self) -> CrateNum {
+        self.0.krate
+    }
+}
+
+trait Value<'tcx>: Sized {
+    fn from_cycle_error<'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> Self;
+}
+
+impl<'tcx, T> Value<'tcx> for T {
+    default fn from_cycle_error<'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> T {
+        tcx.sess.abort_if_errors();
+        bug!("Value::from_cycle_error called without errors");
+    }
+}
+
+impl<'tcx, T: Default> Value<'tcx> for T {
+    default fn from_cycle_error<'a>(_: TyCtxt<'a, 'tcx, 'tcx>) -> T {
+        T::default()
+    }
+}
+
+impl<'tcx> Value<'tcx> for Ty<'tcx> {
+    fn from_cycle_error<'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> Ty<'tcx> {
+        tcx.types.err
+    }
+}
+
+impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
+    fn report_cycle(self, span: Span, cycle: &[(Span, Query)]) {
+        assert!(!cycle.is_empty());
+
+        let mut err = struct_span_err!(self.sess, span, E0391,
+            "unsupported cyclic reference between types/traits detected");
+        err.span_label(span, &format!("cyclic reference"));
+
+        err.span_note(cycle[0].0, &format!("the cycle begins when {}...",
+                                           cycle[0].1.describe(self)));
+
+        for &(span, ref query) in &cycle[1..] {
+            err.span_note(span, &format!("...which then requires {}...",
+                                         query.describe(self)));
+        }
+
+        err.note(&format!("...which then again requires {}, completing the cycle.",
+                          cycle[0].1.describe(self)));
+
+        err.emit();
+    }
+
+    pub fn cycle_check<F, R>(self, span: Span, query: Query, compute: F) -> R
+        where F: FnOnce() -> R
+    {
+        {
+            let mut stack = self.maps.query_stack.borrow_mut();
+            if let Some((i, _)) = stack.iter().enumerate().rev()
+                                       .find(|&(_, &(_, ref q))| *q == query) {
+                let cycle = &stack[i..];
+                self.report_cycle(span, cycle);
+                return R::from_cycle_error(self.global_tcx());
+            }
+            stack.push((span, query));
+        }
+
+        let result = compute();
+
+        self.maps.query_stack.borrow_mut().pop();
+        result
+    }
+}
+
+impl Query {
+    fn describe(&self, tcx: TyCtxt) -> String {
+        match *self {
+            Query::ty(def_id) => {
+                format!("processing `{}`", tcx.item_path_str(def_id))
+            }
+            Query::super_predicates(def_id) => {
+                format!("computing the supertraits of `{}`",
+                        tcx.item_path_str(def_id))
+            }
+            Query::type_param_predicates((_, def_id)) => {
+                let id = tcx.hir.as_local_node_id(def_id).unwrap();
+                format!("computing the bounds for type parameter `{}`",
+                        tcx.hir.ty_param_name(id))
+            }
+            _ => bug!("unexpected `{:?}`", self)
+        }
+    }
+}
+
 macro_rules! define_maps {
     (<$tcx:tt>
      $($(#[$attr:meta])*
        pub $name:ident: $node:ident($K:ty) -> $V:ty),*) => {
         pub struct Maps<$tcx> {
             providers: IndexVec<CrateNum, Providers<$tcx>>,
-            pub query_stack: RefCell<Vec<Query>>,
+            pub query_stack: RefCell<Vec<(Span, Query)>>,
             $($(#[$attr])* pub $name: RefCell<DepTrackingMap<queries::$name<$tcx>>>),*
         }
 
@@ -129,7 +222,7 @@ define_maps! { <'tcx>
 
     /// To avoid cycles within the predicates of a single item we compute
     /// per-type-parameter predicates for resolving `T::AssocTy`.
-    pub type_param_predicates: ItemSignature(DefId)
+    pub type_param_predicates: TypeParamPredicates((DefId, DefId))
         -> ty::GenericPredicates<'tcx>,
 
     pub trait_def: ItemSignature(DefId) -> &'tcx ty::TraitDef,

src/librustc/ty/mod.rs

@@ -669,7 +669,7 @@ impl Generics {
 }
 
 /// Bounds on generics.
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub struct GenericPredicates<'tcx> {
     pub parent: Option<DefId>,
     pub predicates: Vec<Predicate<'tcx>>,

src/librustc_typeck/astconv.rs

@@ -64,13 +64,12 @@ pub trait AstConv<'gcx, 'tcx> {
     /// Ensure that the super-predicates for the trait with the given
     /// id are available and also for the transitive set of
     /// super-predicates.
-    fn ensure_super_predicates(&self, span: Span, id: DefId)
-                               -> Result<(), ErrorReported>;
+    fn ensure_super_predicates(&self, span: Span, id: DefId);
 
     /// Returns the set of bounds in scope for the type parameter with
     /// the given id.
-    fn get_type_parameter_bounds(&self, span: Span, def_id: ast::NodeId)
-                                 -> Result<Vec<ty::PolyTraitRef<'tcx>>, ErrorReported>;
+    fn get_type_parameter_bounds(&self, span: Span, def_id: DefId)
+                                 -> Vec<ty::Predicate<'tcx>>;
 
     /// Return an (optional) substitution to convert bound type parameters that
     /// are in scope into free ones. This function should only return Some
@@ -599,7 +598,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
 
         // Otherwise, we have to walk through the supertraits to find
         // those that do.
-        self.ensure_super_predicates(binding.span, trait_ref.def_id())?;
+        self.ensure_super_predicates(binding.span, trait_ref.def_id());
 
         let candidates =
             traits::supertraits(tcx, trait_ref.clone())
@@ -685,10 +684,8 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
             })
         });
 
-        // ensure the super predicates and stop if we encountered an error
-        if self.ensure_super_predicates(span, principal.def_id()).is_err() {
-            return tcx.types.err;
-        }
+        // ensure the super predicates
+        self.ensure_super_predicates(span, principal.def_id());
 
         // check that there are no gross object safety violations,
         // most importantly, that the supertraits don't contain Self,
@@ -774,29 +771,23 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
     }
 
     // Search for a bound on a type parameter which includes the associated item
-    // given by assoc_name. ty_param_node_id is the node id for the type parameter
-    // (which might be `Self`, but only if it is the `Self` of a trait, not an
-    // impl). This function will fail if there are no suitable bounds or there is
+    // given by `assoc_name`. `ty_param_def_id` is the `DefId` for the type parameter
+    // This function will fail if there are no suitable bounds or there is
     // any ambiguity.
     fn find_bound_for_assoc_item(&self,
-                                 ty_param_node_id: ast::NodeId,
-                                 ty_param_name: ast::Name,
+                                 ty_param_def_id: DefId,
                                  assoc_name: ast::Name,
                                  span: Span)
                                  -> Result<ty::PolyTraitRef<'tcx>, ErrorReported>
     {
         let tcx = self.tcx();
 
-        let bounds = match self.get_type_parameter_bounds(span, ty_param_node_id) {
-            Ok(v) => v,
-            Err(ErrorReported) => {
-                return Err(ErrorReported);
-            }
-        };
+        let bounds: Vec<_> = self.get_type_parameter_bounds(span, ty_param_def_id)
+            .into_iter().filter_map(|p| p.to_opt_poly_trait_ref()).collect();
 
-        // Ensure the super predicates and stop if we encountered an error.
-        if bounds.iter().any(|b| self.ensure_super_predicates(span, b.def_id()).is_err()) {
-            return Err(ErrorReported);
+        // Ensure the super predicates.
+        for b in &bounds {
+            self.ensure_super_predicates(span, b.def_id());
         }
 
         // Check that there is exactly one way to find an associated type with the
@@ -805,8 +796,10 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
             traits::transitive_bounds(tcx, &bounds)
             .filter(|b| self.trait_defines_associated_type_named(b.def_id(), assoc_name));
 
+        let param_node_id = tcx.hir.as_local_node_id(ty_param_def_id).unwrap();
+        let param_name = tcx.hir.ty_param_name(param_node_id);
         self.one_bound_for_assoc_type(suitable_bounds,
-                                      &ty_param_name.as_str(),
+                                      &param_name.as_str(),
                                       &assoc_name.as_str(),
                                       span)
     }
@@ -914,9 +907,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                     trait_ref
                 };
 
-                if self.ensure_super_predicates(span, trait_ref.def_id).is_err() {
-                    return (tcx.types.err, Def::Err);
-                }
+                self.ensure_super_predicates(span, trait_ref.def_id);
 
                 let candidates =
                     traits::supertraits(tcx, ty::Binder(trait_ref))
@@ -933,12 +924,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
             }
             (&ty::TyParam(_), Def::SelfTy(Some(param_did), None)) |
             (&ty::TyParam(_), Def::TyParam(param_did)) => {
-                let param_node_id = tcx.hir.as_local_node_id(param_did).unwrap();
-                let param_name = ::ty_param_name(tcx, param_node_id);
-                match self.find_bound_for_assoc_item(param_node_id,
-                                                     param_name,
-                                                     assoc_name,
-                                                     span) {
+                match self.find_bound_for_assoc_item(param_did, assoc_name, span) {
                     Ok(bound) => bound,
                     Err(ErrorReported) => return (tcx.types.err, Def::Err),
                 }
@@ -1375,9 +1361,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                existential_predicates);
 
         if let Some(principal) = existential_predicates.principal() {
-            if let Err(ErrorReported) = self.ensure_super_predicates(span, principal.def_id()) {
-                return Some(tcx.mk_region(ty::ReStatic));
-            }
+            self.ensure_super_predicates(span, principal.def_id());
         }
 
         // No explicit region bound specified. Therefore, examine trait