move projection mode into parameter environment

[rust.git] / src / librustc_typeck / coherence / builtin.rs

diff --git a/src/librustc_typeck/coherence/builtin.rs b/src/librustc_typeck/coherence/builtin.rs
index 743bfbb44ab7504ef2af4017a9f35fea53b22ae8..89f2595d1a8fb6a95338c66510ff40165baa192e 100644 (file)
--- a/src/librustc_typeck/coherence/builtin.rs
+++ b/src/librustc_typeck/coherence/builtin.rs
@@ -15,12 +15,10 @@
 use rustc::middle::region::RegionMaps;
 use rustc::middle::lang_items::UnsizeTraitLangItem;
 
-use rustc::traits::{self, ObligationCause, Reveal};
+use rustc::traits::{self, ObligationCause};
 use rustc::ty::{self, Ty, TyCtxt};
-use rustc::ty::ParameterEnvironment;
 use rustc::ty::TypeFoldable;
 use rustc::ty::adjustment::CoerceUnsizedInfo;
-use rustc::ty::subst::Subst;
 use rustc::ty::util::CopyImplementationError;
 use rustc::infer;
 
@@ -107,8 +105,7 @@ fn visit_implementation_of_copy<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
            self_type);
 
     let span = tcx.hir.span(impl_node_id);
-    let param_env = ParameterEnvironment::for_item(tcx, impl_node_id);
-    let self_type = self_type.subst(tcx, &param_env.free_substs);
+    let param_env = tcx.param_env(impl_did);
     assert!(!self_type.has_escaping_regions());
 
     debug!("visit_implementation_of_copy: self_type={:?} (free)",
@@ -202,9 +199,7 @@ pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
            target);
 
     let span = tcx.hir.span(impl_node_id);
-    let param_env = ParameterEnvironment::for_item(tcx, impl_node_id);
-    let source = source.subst(tcx, &param_env.free_substs);
-    let target = target.subst(tcx, &param_env.free_substs);
+    let param_env = tcx.param_env(impl_did);
     assert!(!source.has_escaping_regions());
 
     let err_info = CoerceUnsizedInfo { custom_kind: None };
@@ -213,7 +208,7 @@ pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
            source,
            target);
 
-    tcx.infer_ctxt(param_env, Reveal::UserFacing).enter(|infcx| {
+    tcx.infer_ctxt(param_env).enter(|infcx| {
         let cause = ObligationCause::misc(span, impl_node_id);
         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
                            mt_b: ty::TypeAndMut<'tcx>,
@@ -254,6 +249,45 @@ pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                     return err_info;
                 }
 
+                // Here we are considering a case of converting
+                // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
+                // which acts like a pointer to `U`, but carries along some extra data of type `T`:
+                //
+                //     struct Foo<T, U> {
+                //         extra: T,
+                //         ptr: *mut U,
+                //     }
+                //
+                // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
+                // to `Foo<T, [i32]>`. That impl would look like:
+                //
+                //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
+                //
+                // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
+                // when this coercion occurs, we would be changing the
+                // field `ptr` from a thin pointer of type `*mut [i32;
+                // 3]` to a fat pointer of type `*mut [i32]` (with
+                // extra data `3`).  **The purpose of this check is to
+                // make sure that we know how to do this conversion.**
+                //
+                // To check if this impl is legal, we would walk down
+                // the fields of `Foo` and consider their types with
+                // both substitutes. We are looking to find that
+                // exactly one (non-phantom) field has changed its
+                // type, which we will expect to be the pointer that
+                // is becoming fat (we could probably generalize this
+                // to mutiple thin pointers of the same type becoming
+                // fat, but we don't). In this case:
+                //
+                // - `extra` has type `T` before and type `T` after
+                // - `ptr` has type `*mut U` before and type `*mut V` after
+                //
+                // Since just one field changed, we would then check
+                // that `*mut U: CoerceUnsized<*mut V>` is implemented
+                // (in other words, that we know how to do this
+                // conversion). This will work out because `U:
+                // Unsize<V>`, and we have a builtin rule that `*mut
+                // U` can be coerced to `*mut V` if `U: Unsize<V>`.
                 let fields = &def_a.struct_variant().fields;
                 let diff_fields = fields.iter()
                     .enumerate()
@@ -265,8 +299,16 @@ pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                             return None;
                         }
 
-                        // Ignore fields that aren't significantly changed
-                        if let Ok(ok) = infcx.sub_types(false, &cause, b, a) {
+                        // Ignore fields that aren't changed; it may
+                        // be that we could get away with subtyping or
+                        // something more accepting, but we use
+                        // equality because we want to be able to
+                        // perform this check without computing
+                        // variance where possible. (This is because
+                        // we may have to evaluate constraint
+                        // expressions in the course of execution.)
+                        // See e.g. #41936.
+                        if let Ok(ok) = infcx.eq_types(false, &cause, b, a) {
                             if ok.obligations.is_empty() {
                                 return None;
                             }
@@ -345,7 +387,7 @@ pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         // Finally, resolve all regions.
         let region_maps = RegionMaps::new();
         let mut free_regions = FreeRegionMap::new();
-        free_regions.relate_free_regions_from_predicates(&infcx.parameter_environment
+        free_regions.relate_free_regions_from_predicates(&infcx.param_env
             .caller_bounds);
         infcx.resolve_regions_and_report_errors(impl_did, &region_maps, &free_regions);