Auto merge of #30652 - aturon:specialization, r=nikomatsakis

Implement RFC 1210: impl specialization

This PR implements [impl specialization](rust-lang/rfcs#1210),
carefully following the proposal laid out in the RFC.

The implementation covers the bulk of the RFC. The remaining gaps I know of are:

- no checking for lifetime-dependent specialization (a soundness hole);
- no `default impl` yet;
- no support for `default` with associated consts;

I plan to cover these gaps in follow-up PRs, as per @nikomatsakis's preference.

The basic strategy is to build up a *specialization graph* during
coherence checking. Insertion into the graph locates the right place
to put an impl in the specialization hierarchy; if there is no right
place (due to partial overlap but no containment), you get an overlap
error. Specialization is consulted when selecting an impl (of course),
and the graph is consulted when propagating defaults down the
specialization hierarchy.

You might expect that the specialization graph would be used during
selection -- i.e., when actually performing specialization. This is
not done for two reasons:

- It's merely an optimization: given a set of candidates that apply,
  we can determine the most specialized one by comparing them directly
  for specialization, rather than consulting the graph. Given that we
  also cache the results of selection, the benefit of this
  optimization is questionable.

- To build the specialization graph in the first place, we need to use
  selection (because we need to determine whether one impl specializes
  another). Dealing with this reentrancy would require some additional
  mode switch for selection. Given that there seems to be no strong
  reason to use the graph anyway, we stick with a simpler approach in
  selection, and use the graph only for propagating default
  implementations.

Trait impl selection can succeed even when multiple impls can apply,
as long as they are part of the same specialization family. In that
case, it returns a *single* impl on success -- this is the most
specialized impl *known* to apply. However, if there are any inference
variables in play, the returned impl may not be the actual impl we
will use at trans time. Thus, we take special care to avoid projecting
associated types unless either (1) the associated type does not use
`default` and thus cannot be overridden or (2) all input types are
known concretely.

r? @nikomatsakis

Author: bors

Diff for: src/librustc/dep_graph/README.md

@@ -51,7 +51,7 @@ could invalidate work done for other items. So, for example:
    not shared state, because if it changes it does not itself
    invalidate other functions (though it may be that it causes new
    monomorphizations to occur, but that's handled independently).
-   
+
 Put another way: if the HIR for an item changes, we are going to
 recompile that item for sure. But we need the dep tracking map to tell
 us what *else* we have to recompile. Shared state is anything that is
@@ -177,7 +177,7 @@ reads from `item`, there would be missing edges in the graph:
       |                                 ^
       |                                 |
       +---------------------------------+ // added by `visit_all_items_in_krate`
-    
+
 In particular, the edge from `Hir(X)` to `ItemSignature(X)` is only
 present because we called `read` ourselves when entering the `ItemSignature(X)`
 task.
@@ -273,8 +273,8 @@ should not exist.  In contrast, using the memoized helper, you get:
     ... -> MapVariant(key) -> A
                  |
                  +----------> B
-                 
-which is much cleaner.                 
+
+which is much cleaner.
 
 **Be aware though that the closure is executed with `MapVariant(key)`
 pushed onto the stack as the current task!** That means that you must
@@ -387,4 +387,3 @@ RUST_DEP_GRAPH_FILTER='Hir&foo -> TypeckItemBody & bar'
 This will dump out all the nodes that lead from `Hir(foo)` to
 `TypeckItemBody(bar)`, from which you can (hopefully) see the source
 of the erroneous edge.
-

Diff for: src/librustc/middle/check_match.rs

@@ -25,6 +25,7 @@ use middle::expr_use_visitor as euv;
 use middle::infer;
 use middle::mem_categorization::{cmt};
 use middle::pat_util::*;
+use middle::traits::ProjectionMode;
 use middle::ty::*;
 use middle::ty;
 use std::cmp::Ordering;
@@ -1101,7 +1102,8 @@ fn check_legality_of_move_bindings(cx: &MatchCheckCtxt,
                         //FIXME: (@jroesch) this code should be floated up as well
                         let infcx = infer::new_infer_ctxt(cx.tcx,
                                                           &cx.tcx.tables,
-                                                          Some(cx.param_env.clone()));
+                                                          Some(cx.param_env.clone()),
+                                                          ProjectionMode::AnyFinal);
                         if infcx.type_moves_by_default(pat_ty, pat.span) {
                             check_move(p, sub.as_ref().map(|p| &**p));
                         }
@@ -1133,7 +1135,8 @@ fn check_for_mutation_in_guard<'a, 'tcx>(cx: &'a MatchCheckCtxt<'a, 'tcx>,
 
     let infcx = infer::new_infer_ctxt(cx.tcx,
                                       &cx.tcx.tables,
-                                      Some(checker.cx.param_env.clone()));
+                                      Some(checker.cx.param_env.clone()),
+                                      ProjectionMode::AnyFinal);
 
     let mut visitor = ExprUseVisitor::new(&mut checker, &infcx);
     visitor.walk_expr(guard);

Diff for: src/librustc/middle/const_eval.rs

@@ -24,6 +24,7 @@ use middle::def_id::DefId;
 use middle::pat_util::def_to_path;
 use middle::ty::{self, Ty, TyCtxt};
 use middle::ty::util::IntTypeExt;
+use middle::traits::ProjectionMode;
 use middle::astconv_util::ast_ty_to_prim_ty;
 use util::nodemap::NodeMap;
 
@@ -1049,7 +1050,7 @@ fn resolve_trait_associated_const<'a, 'tcx: 'a>(tcx: &'a TyCtxt<'tcx>,
            trait_ref);
 
     tcx.populate_implementations_for_trait_if_necessary(trait_ref.def_id());
-    let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, None);
+    let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, None, ProjectionMode::AnyFinal);
 
     let mut selcx = traits::SelectionContext::new(&infcx);
     let obligation = traits::Obligation::new(traits::ObligationCause::dummy(),
@@ -1067,6 +1068,11 @@ fn resolve_trait_associated_const<'a, 'tcx: 'a>(tcx: &'a TyCtxt<'tcx>,
         }
     };
 
+    // NOTE: this code does not currently account for specialization, but when
+    // it does so, it should hook into the ProjectionMode to determine when the
+    // constant should resolve; this will also require plumbing through to this
+    // function whether we are in "trans mode" to pick the right ProjectionMode
+    // when constructing the inference context above.
     match selection {
         traits::VtableImpl(ref impl_data) => {
             match tcx.associated_consts(impl_data.impl_def_id)

Diff for: src/librustc/middle/cstore.rs

@@ -176,6 +176,7 @@ pub trait CrateStore<'tcx> : Any {
                                   -> Option<ty::adjustment::CustomCoerceUnsized>;
     fn associated_consts(&self, tcx: &TyCtxt<'tcx>, def: DefId)
                          -> Vec<Rc<ty::AssociatedConst<'tcx>>>;
+    fn impl_parent(&self, impl_def_id: DefId) -> Option<DefId>;
 
     // trait/impl-item info
     fn trait_of_item(&self, tcx: &TyCtxt<'tcx>, def_id: DefId)
@@ -346,6 +347,7 @@ impl<'tcx> CrateStore<'tcx> for DummyCrateStore {
         { unimplemented!() }
     fn associated_consts(&self, tcx: &TyCtxt<'tcx>, def: DefId)
                          -> Vec<Rc<ty::AssociatedConst<'tcx>>> { unimplemented!() }
+    fn impl_parent(&self, def: DefId) -> Option<DefId> { unimplemented!() }
 
     // trait/impl-item info
     fn trait_of_item(&self, tcx: &TyCtxt<'tcx>, def_id: DefId)

Diff for: src/librustc/middle/infer/mod.rs

@@ -27,7 +27,7 @@ use middle::region::CodeExtent;
 use middle::subst;
 use middle::subst::Substs;
 use middle::subst::Subst;
-use middle::traits;
+use middle::traits::{self, ProjectionMode};
 use middle::ty::adjustment;
 use middle::ty::{TyVid, IntVid, FloatVid};
 use middle::ty::{self, Ty, TyCtxt};
@@ -99,6 +99,11 @@ pub struct InferCtxt<'a, 'tcx: 'a> {
     // directly.
     normalize: bool,
 
+    // Sadly, the behavior of projection varies a bit depending on the
+    // stage of compilation. The specifics are given in the
+    // documentation for `ProjectionMode`.
+    projection_mode: ProjectionMode,
+
     err_count_on_creation: usize,
 }
 
@@ -354,7 +359,8 @@ pub fn fixup_err_to_string(f: FixupError) -> String {
 
 pub fn new_infer_ctxt<'a, 'tcx>(tcx: &'a TyCtxt<'tcx>,
                                 tables: &'a RefCell<ty::Tables<'tcx>>,
-                                param_env: Option<ty::ParameterEnvironment<'a, 'tcx>>)
+                                param_env: Option<ty::ParameterEnvironment<'a, 'tcx>>,
+                                projection_mode: ProjectionMode)
                                 -> InferCtxt<'a, 'tcx> {
     InferCtxt {
         tcx: tcx,
@@ -366,14 +372,16 @@ pub fn new_infer_ctxt<'a, 'tcx>(tcx: &'a TyCtxt<'tcx>,
         parameter_environment: param_env.unwrap_or(tcx.empty_parameter_environment()),
         reported_trait_errors: RefCell::new(FnvHashSet()),
         normalize: false,
+        projection_mode: projection_mode,
         err_count_on_creation: tcx.sess.err_count()
     }
 }
 
 pub fn normalizing_infer_ctxt<'a, 'tcx>(tcx: &'a TyCtxt<'tcx>,
-                                        tables: &'a RefCell<ty::Tables<'tcx>>)
+                                        tables: &'a RefCell<ty::Tables<'tcx>>,
+                                        projection_mode: ProjectionMode)
                                         -> InferCtxt<'a, 'tcx> {
-    let mut infcx = new_infer_ctxt(tcx, tables, None);
+    let mut infcx = new_infer_ctxt(tcx, tables, None, projection_mode);
     infcx.normalize = true;
     infcx
 }
@@ -514,6 +522,7 @@ pub struct CombinedSnapshot {
     region_vars_snapshot: RegionSnapshot,
 }
 
+// NOTE: Callable from trans only!
 pub fn normalize_associated_type<'tcx,T>(tcx: &TyCtxt<'tcx>, value: &T) -> T
     where T : TypeFoldable<'tcx>
 {
@@ -525,7 +534,7 @@ pub fn normalize_associated_type<'tcx,T>(tcx: &TyCtxt<'tcx>, value: &T) -> T
         return value;
     }
 
-    let infcx = new_infer_ctxt(tcx, &tcx.tables, None);
+    let infcx = new_infer_ctxt(tcx, &tcx.tables, None, ProjectionMode::Any);
     let mut selcx = traits::SelectionContext::new(&infcx);
     let cause = traits::ObligationCause::dummy();
     let traits::Normalized { value: result, obligations } =
@@ -593,6 +602,10 @@ pub fn drain_fulfillment_cx<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
 }
 
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    pub fn projection_mode(&self) -> ProjectionMode {
+        self.projection_mode
+    }
+
     pub fn freshen<T:TypeFoldable<'tcx>>(&self, t: T) -> T {
         t.fold_with(&mut self.freshener())
     }
@@ -1025,8 +1038,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                               substs: &mut Substs<'tcx>,
                               defs: &[ty::TypeParameterDef<'tcx>]) {
 
-        let mut vars = Vec::with_capacity(defs.len());
-
         for def in defs.iter() {
             let default = def.default.map(|default| {
                 type_variable::Default {
@@ -1038,7 +1049,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
             let ty_var = self.next_ty_var_with_default(default);
             substs.types.push(space, ty_var);
-            vars.push(ty_var)
         }
     }
 

Diff for: src/librustc/middle/subst.rs

@@ -160,6 +160,15 @@ impl<'tcx> Substs<'tcx> {
         Substs { types: types, regions: regions }
     }
 
+    pub fn with_method_from_subst(self, other: &Substs<'tcx>) -> Substs<'tcx> {
+        let Substs { types, regions } = self;
+        let types = types.with_slice(FnSpace, other.types.get_slice(FnSpace));
+        let regions = regions.map(|r| {
+            r.with_slice(FnSpace, other.regions().get_slice(FnSpace))
+        });
+        Substs { types: types, regions: regions }
+    }
+
     /// Creates a trait-ref out of this substs, ignoring the FnSpace substs
     pub fn to_trait_ref(&self, tcx: &TyCtxt<'tcx>, trait_id: DefId)
                         -> ty::TraitRef<'tcx> {

Diff for: src/librustc/middle/traits/README.md

@@ -428,3 +428,43 @@ We used to try and draw finer-grained distinctions, but that led to a
 serious of annoying and weird bugs like #22019 and #18290. This simple
 rule seems to be pretty clearly safe and also still retains a very
 high hit rate (~95% when compiling rustc).
+
+# Specialization
+
+Defined in the `specialize` module.
+
+The basic strategy is to build up a *specialization graph* during
+coherence checking. Insertion into the graph locates the right place
+to put an impl in the specialization hierarchy; if there is no right
+place (due to partial overlap but no containment), you get an overlap
+error. Specialization is consulted when selecting an impl (of course),
+and the graph is consulted when propagating defaults down the
+specialization hierarchy.
+
+You might expect that the specialization graph would be used during
+selection -- i.e., when actually performing specialization. This is
+not done for two reasons:
+
+- It's merely an optimization: given a set of candidates that apply,
+  we can determine the most specialized one by comparing them directly
+  for specialization, rather than consulting the graph. Given that we
+  also cache the results of selection, the benefit of this
+  optimization is questionable.
+
+- To build the specialization graph in the first place, we need to use
+  selection (because we need to determine whether one impl specializes
+  another). Dealing with this reentrancy would require some additional
+  mode switch for selection. Given that there seems to be no strong
+  reason to use the graph anyway, we stick with a simpler approach in
+  selection, and use the graph only for propagating default
+  implementations.
+
+Trait impl selection can succeed even when multiple impls can apply,
+as long as they are part of the same specialization family. In that
+case, it returns a *single* impl on success -- this is the most
+specialized impl *known* to apply. However, if there are any inference
+variables in play, the returned impl may not be the actual impl we
+will use at trans time. Thus, we take special care to avoid projecting
+associated types unless either (1) the associated type does not use
+`default` and thus cannot be overridden or (2) all input types are
+known concretely.

Diff for: src/librustc/middle/traits/coherence.rs

@@ -10,8 +10,7 @@
 
 //! See `README.md` for high-level documentation
 
-use super::{SelectionContext};
-use super::{Obligation, ObligationCause};
+use super::{SelectionContext, Obligation, ObligationCause};
 
 use middle::cstore::LOCAL_CRATE;
 use middle::def_id::DefId;
@@ -23,8 +22,8 @@ use syntax::codemap::DUMMY_SP;
 #[derive(Copy, Clone)]
 struct InferIsLocal(bool);
 
-/// If there are types that satisfy both impls, returns an `ImplTy`
-/// with those types substituted (by updating the given `infcx`)
+/// If there are types that satisfy both impls, returns a suitably-freshened
+/// `ImplHeader` with those types substituted
 pub fn overlapping_impls<'cx, 'tcx>(infcx: &InferCtxt<'cx, 'tcx>,
                                     impl1_def_id: DefId,
                                     impl2_def_id: DefId)

Diff for: src/librustc/middle/traits/mod.rs

@@ -36,20 +36,18 @@ pub use self::coherence::orphan_check;
 pub use self::coherence::overlapping_impls;
 pub use self::coherence::OrphanCheckErr;
 pub use self::fulfill::{FulfillmentContext, GlobalFulfilledPredicates, RegionObligation};
-pub use self::project::MismatchedProjectionTypes;
-pub use self::project::normalize;
-pub use self::project::Normalized;
+pub use self::project::{MismatchedProjectionTypes, ProjectionMode};
+pub use self::project::{normalize, Normalized};
 pub use self::object_safety::is_object_safe;
 pub use self::object_safety::astconv_object_safety_violations;
 pub use self::object_safety::object_safety_violations;
 pub use self::object_safety::ObjectSafetyViolation;
 pub use self::object_safety::MethodViolationCode;
 pub use self::object_safety::is_vtable_safe_method;
-pub use self::select::EvaluationCache;
-pub use self::select::SelectionContext;
-pub use self::select::SelectionCache;
+pub use self::select::{EvaluationCache, SelectionContext, SelectionCache};
 pub use self::select::{MethodMatchResult, MethodMatched, MethodAmbiguous, MethodDidNotMatch};
 pub use self::select::{MethodMatchedData}; // intentionally don't export variants
+pub use self::specialize::{Overlap, specialization_graph, specializes, translate_substs};
 pub use self::util::elaborate_predicates;
 pub use self::util::get_vtable_index_of_object_method;
 pub use self::util::trait_ref_for_builtin_bound;
@@ -67,6 +65,7 @@ mod fulfill;
 mod project;
 mod object_safety;
 mod select;
+mod specialize;
 mod structural_impls;
 mod util;
 
@@ -434,7 +433,10 @@ pub fn normalize_param_env_or_error<'a,'tcx>(unnormalized_env: ty::ParameterEnvi
 
     let elaborated_env = unnormalized_env.with_caller_bounds(predicates);
 
-    let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, Some(elaborated_env));
+    let infcx = infer::new_infer_ctxt(tcx,
+                                      &tcx.tables,
+                                      Some(elaborated_env),
+                                      ProjectionMode::AnyFinal);
     let predicates = match fully_normalize(&infcx,
                                            cause,
                                            &infcx.parameter_environment.caller_bounds) {