Rollup merge of #22785 - nikomatsakis:issue-21750-normalization-with-regions, r=pnkfelix

 Two changes:

1. Make traits with assoc types invariant w/r/t their inputs.
2. Fully normalize parameter environments, including any region variables (which were being overlooked).

The former supports the latter, but also just seems like a reasonably good idea.

Fixes #21750

cc @edwardw
r? @pnkfelix

4 files changed, 134 insertions, 53 deletions

diff --git a/src/librustc/middle/infer/combine.rs b/src/librustc/middle/infer/combine.rs
index 38dd9a4d6ec..b782a655d89 100644
--- a/src/librustc/middle/infer/combine.rs
+++ b/src/librustc/middle/infer/combine.rs
@@ -265,13 +265,7 @@ pub trait Combine<'tcx> : Sized {
             Err(ty::terr_projection_name_mismatched(
                 expected_found(self, a.item_name, b.item_name)))
         } else {
-            // Note that the trait refs for the projection must be
-            // *equal*. This is because there is no inherent
-            // relationship between `<T as Foo>::Bar` and `<U as
-            // Foo>::Bar` that we can derive based on how `T` relates
-            // to `U`. Issue #21726 contains further discussion and
-            // in-depth examples.
-            let trait_ref = try!(self.equate().trait_refs(&*a.trait_ref, &*b.trait_ref));
+            let trait_ref = try!(self.trait_refs(&*a.trait_ref, &*b.trait_ref));
             Ok(ty::ProjectionTy { trait_ref: Rc::new(trait_ref), item_name: a.item_name })
         }
     }
diff --git a/src/librustc/middle/traits/coherence.rs b/src/librustc/middle/traits/coherence.rs
index 4d45bb841f4..62b81f0ebe7 100644
--- a/src/librustc/middle/traits/coherence.rs
+++ b/src/librustc/middle/traits/coherence.rs
@@ -52,9 +52,16 @@ fn overlap(selcx: &mut SelectionContext,
            b_def_id: ast::DefId)
            -> bool
 {
+    debug!("overlap(a_def_id={}, b_def_id={})",
+           a_def_id.repr(selcx.tcx()),
+           b_def_id.repr(selcx.tcx()));
+
     let (a_trait_ref, a_obligations) = impl_trait_ref_and_oblig(selcx, a_def_id);
     let (b_trait_ref, b_obligations) = impl_trait_ref_and_oblig(selcx, b_def_id);
 
+    debug!("overlap: a_trait_ref={}", a_trait_ref.repr(selcx.tcx()));
+    debug!("overlap: b_trait_ref={}", b_trait_ref.repr(selcx.tcx()));
+
     // Does `a <: b` hold? If not, no overlap.
     if let Err(_) = infer::mk_sub_poly_trait_refs(selcx.infcx(),
                                                   true,
@@ -64,10 +71,20 @@ fn overlap(selcx: &mut SelectionContext,
         return false;
     }
 
+    debug!("overlap: subtraitref check succeeded");
+
     // Are any of the obligations unsatisfiable? If so, no overlap.
-    a_obligations.iter()
-                 .chain(b_obligations.iter())
-                 .all(|o| selcx.evaluate_obligation(o))
+    let opt_failing_obligation =
+        a_obligations.iter()
+                     .chain(b_obligations.iter())
+                     .find(|o| !selcx.evaluate_obligation(o));
+
+    if let Some(failing_obligation) = opt_failing_obligation {
+        debug!("overlap: obligation unsatisfiable {}", failing_obligation.repr(selcx.tcx()));
+        return false;
+    }
+
+    true
 }
 
 /// Instantiate fresh variables for all bound parameters of the impl
diff --git a/src/librustc/middle/traits/mod.rs b/src/librustc/middle/traits/mod.rs
index 8b836fd322e..5a5639c7012 100644
--- a/src/librustc/middle/traits/mod.rs
+++ b/src/librustc/middle/traits/mod.rs
@@ -18,7 +18,7 @@ pub use self::ObligationCauseCode::*;
 use middle::subst;
 use middle::ty::{self, HasProjectionTypes, Ty};
 use middle::ty_fold::TypeFoldable;
-use middle::infer::{self, InferCtxt};
+use middle::infer::{self, fixup_err_to_string, InferCtxt};
 use std::slice::Iter;
 use std::rc::Rc;
 use syntax::ast;
@@ -395,53 +395,64 @@ pub fn type_known_to_meet_builtin_bound<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
     }
 }
 
+/// Normalizes the parameter environment, reporting errors if they occur.
 pub fn normalize_param_env_or_error<'a,'tcx>(unnormalized_env: ty::ParameterEnvironment<'a,'tcx>,
                                              cause: ObligationCause<'tcx>)
                                              -> ty::ParameterEnvironment<'a,'tcx>
 {
-    match normalize_param_env(&unnormalized_env, cause) {
-        Ok(p) => p,
+    // I'm not wild about reporting errors here; I'd prefer to
+    // have the errors get reported at a defined place (e.g.,
+    // during typeck). Instead I have all parameter
+    // environments, in effect, going through this function
+    // and hence potentially reporting errors. This ensurse of
+    // course that we never forget to normalize (the
+    // alternative seemed like it would involve a lot of
+    // manual invocations of this fn -- and then we'd have to
+    // deal with the errors at each of those sites).
+    //
+    // In any case, in practice, typeck constructs all the
+    // parameter environments once for every fn as it goes,
+    // and errors will get reported then; so after typeck we
+    // can be sure that no errors should occur.
+
+    let tcx = unnormalized_env.tcx;
+    let span = cause.span;
+    let body_id = cause.body_id;
+
+    debug!("normalize_param_env_or_error(unnormalized_env={})",
+           unnormalized_env.repr(tcx));
+
+    let infcx = infer::new_infer_ctxt(tcx);
+    let predicates = match fully_normalize(&infcx, &unnormalized_env, cause,
+                                           &unnormalized_env.caller_bounds) {
+        Ok(predicates) => predicates,
         Err(errors) => {
-            // I'm not wild about reporting errors here; I'd prefer to
-            // have the errors get reported at a defined place (e.g.,
-            // during typeck). Instead I have all parameter
-            // environments, in effect, going through this function
-            // and hence potentially reporting errors. This ensurse of
-            // course that we never forget to normalize (the
-            // alternative seemed like it would involve a lot of
-            // manual invocations of this fn -- and then we'd have to
-            // deal with the errors at each of those sites).
-            //
-            // In any case, in practice, typeck constructs all the
-            // parameter environments once for every fn as it goes,
-            // and errors will get reported then; so after typeck we
-            // can be sure that no errors should occur.
-            let infcx = infer::new_infer_ctxt(unnormalized_env.tcx);
             report_fulfillment_errors(&infcx, &errors);
-
-            // Normalized failed? use what they gave us, it's better than nothing.
-            unnormalized_env
+            return unnormalized_env; // an unnormalized env is better than nothing
         }
-    }
-}
-
-pub fn normalize_param_env<'a,'tcx>(param_env: &ty::ParameterEnvironment<'a,'tcx>,
-                                    cause: ObligationCause<'tcx>)
-                                    -> Result<ty::ParameterEnvironment<'a,'tcx>,
-                                              Vec<FulfillmentError<'tcx>>>
-{
-    let tcx = param_env.tcx;
-
-    debug!("normalize_param_env(param_env={})",
-           param_env.repr(tcx));
+    };
 
-    let infcx = infer::new_infer_ctxt(tcx);
-    let predicates = try!(fully_normalize(&infcx, param_env, cause, &param_env.caller_bounds));
+    infcx.resolve_regions_and_report_errors(body_id);
+    let predicates = match infcx.fully_resolve(&predicates) {
+        Ok(predicates) => predicates,
+        Err(fixup_err) => {
+            // If we encounter a fixup error, it means that some type
+            // variable wound up unconstrained. I actually don't know
+            // if this can happen, and I certainly don't expect it to
+            // happen often, but if it did happen it probably
+            // represents a legitimate failure due to some kind of
+            // unconstrained variable, and it seems better not to ICE,
+            // all things considered.
+            let err_msg = fixup_err_to_string(fixup_err);
+            tcx.sess.span_err(span, &err_msg);
+            return unnormalized_env; // an unnormalized env is better than nothing
+        }
+    };
 
-    debug!("normalize_param_env: predicates={}",
+    debug!("normalize_param_env_or_error: predicates={}",
            predicates.repr(tcx));
 
-    Ok(param_env.with_caller_bounds(predicates))
+    unnormalized_env.with_caller_bounds(predicates)
 }
 
 pub fn fully_normalize<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
@@ -453,8 +464,7 @@ pub fn fully_normalize<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
 {
     let tcx = closure_typer.tcx();
 
-    debug!("normalize_param_env(value={})",
-           value.repr(tcx));
+    debug!("normalize_param_env(value={})", value.repr(tcx));
 
     let mut selcx = &mut SelectionContext::new(infcx, closure_typer);
     let mut fulfill_cx = FulfillmentContext::new();
@@ -468,8 +478,7 @@ pub fn fully_normalize<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
     }
     try!(fulfill_cx.select_all_or_error(infcx, closure_typer));
     let resolved_value = infcx.resolve_type_vars_if_possible(&normalized_value);
-    debug!("normalize_param_env: resolved_value={}",
-           resolved_value.repr(tcx));
+    debug!("normalize_param_env: resolved_value={}", resolved_value.repr(tcx));
     Ok(resolved_value)
 }
 
diff --git a/src/librustc_typeck/variance.rs b/src/librustc_typeck/variance.rs
index 90ca6a79805..1fba4a21ccd 100644
--- a/src/librustc_typeck/variance.rs
+++ b/src/librustc_typeck/variance.rs
@@ -203,6 +203,56 @@
 //! failure, but rather because the target type `Foo<Y>` is itself just
 //! not well-formed. Basically we get to assume well-formedness of all
 //! types involved before considering variance.
+//!
+//! ### Associated types
+//!
+//! Any trait with an associated type is invariant with respect to all
+//! of its inputs. To see why this makes sense, consider what
+//! subtyping for a trait reference means:
+//!
+//!    <T as Trait> <: <U as Trait>
+//!
+//! means that if I know that `T as Trait`,
+//! I also know that `U as
+//! Trait`. Moreover, if you think of it as
+//! dictionary passing style, it means that
+//! a dictionary for `<T as Trait>` is safe
+//! to use where a dictionary for `<U as
+//! Trait>` is expected.
+//!
+//! The problem is that when you can
+//! project types out from `<T as Trait>`,
+//! the relationship to types projected out
+//! of `<U as Trait>` is completely unknown
+//! unless `T==U` (see #21726 for more
+//! details). Making `Trait` invariant
+//! ensures that this is true.
+//!
+//! *Historical note: we used to preserve this invariant another way,
+//! by tweaking the subtyping rules and requiring that when a type `T`
+//! appeared as part of a projection, that was considered an invariant
+//! location, but this version does away with the need for those
+//! somewhat "special-case-feeling" rules.*
+//!
+//! Another related reason is that if we didn't make traits with
+//! associated types invariant, then projection is no longer a
+//! function with a single result. Consider:
+//!
+//! ```
+//! trait Identity { type Out; fn foo(&self); }
+//! impl<T> Identity for T { type Out = T; ... }
+//! ```
+//!
+//! Now if I have `<&'static () as Identity>::Out`, this can be
+//! validly derived as `&'a ()` for any `'a`:
+//!
+//!    <&'a () as Identity> <: <&'static () as Identity>
+//!    if &'static () < : &'a ()   -- Identity is contravariant in Self
+//!    if 'static : 'a             -- Subtyping rules for relations
+//!
+//! This change otoh means that `<'static () as Identity>::Out` is
+//! always `&'static ()` (which might then be upcast to `'a ()`,
+//! separately). This was helpful in solving #21750.
 
 use self::VarianceTerm::*;
 use self::ParamKind::*;
@@ -613,7 +663,18 @@ impl<'a, 'tcx, 'v> Visitor<'v> for ConstraintContext<'a, 'tcx> {
                                 &method.fty.sig,
                                 self.covariant);
                         }
-                        ty::TypeTraitItem(_) => {}
+                        ty::TypeTraitItem(ref data) => {
+                            // Any trait with an associated type is
+                            // invariant with respect to all of its
+                            // inputs. See length discussion in the comment
+                            // on this module.
+                            let projection_ty = ty::mk_projection(tcx,
+                                                                  trait_def.trait_ref.clone(),
+                                                                  data.name);
+                            self.add_constraints_from_ty(&trait_def.generics,
+                                                         projection_ty,
+                                                         self.invariant);
+                        }
                     }
                 }
             }
@@ -893,7 +954,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
                     trait_def.generics.types.as_slice(),
                     trait_def.generics.regions.as_slice(),
                     trait_ref.substs,
-                    self.invariant);
+                    variance);
             }
 
             ty::ty_trait(ref data) => {