Treat associated types the same as type parameters when it comes to region bounding. Fixes rust-lang#20303.

Strictly speaking, this is a [breaking-change] (if you are using
associated types). You are no longer free to wantonly violate the type
system rules by closing associated types into objects without any form
of region bound. Instead you should add region bounds like `T::X :
'a`, just as you would with a normal type parameter.

Author: nikomatsakis

src/librustc/middle/infer/error_reporting.rs

@@ -66,7 +66,8 @@ use super::region_inference::RegionResolutionError;
 use super::region_inference::ConcreteFailure;
 use super::region_inference::SubSupConflict;
 use super::region_inference::SupSupConflict;
-use super::region_inference::ParamBoundFailure;
+use super::region_inference::GenericBoundFailure;
+use super::region_inference::GenericKind;
 use super::region_inference::ProcessedErrors;
 use super::region_inference::SameRegions;
 
@@ -120,11 +121,11 @@ pub trait ErrorReporting<'tcx> {
                                sub: Region,
                                sup: Region);
 
-    fn report_param_bound_failure(&self,
-                                  origin: SubregionOrigin<'tcx>,
-                                  param_ty: ty::ParamTy,
-                                  sub: Region,
-                                  sups: Vec<Region>);
+    fn report_generic_bound_failure(&self,
+                                    origin: SubregionOrigin<'tcx>,
+                                    kind: GenericKind<'tcx>,
+                                    sub: Region,
+                                    sups: Vec<Region>);
 
     fn report_sub_sup_conflict(&self,
                                var_origin: RegionVariableOrigin,
@@ -175,8 +176,8 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
                     self.report_concrete_failure(origin, sub, sup);
                 }
 
-                ParamBoundFailure(origin, param_ty, sub, sups) => {
-                    self.report_param_bound_failure(origin, param_ty, sub, sups);
+                GenericBoundFailure(kind, param_ty, sub, sups) => {
+                    self.report_generic_bound_failure(kind, param_ty, sub, sups);
                 }
 
                 SubSupConflict(var_origin,
@@ -421,63 +422,65 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
                      found.user_string(self.tcx)))
     }
 
-    fn report_param_bound_failure(&self,
-                                  origin: SubregionOrigin<'tcx>,
-                                  param_ty: ty::ParamTy,
-                                  sub: Region,
-                                  _sups: Vec<Region>) {
-
+    fn report_generic_bound_failure(&self,
+                                    origin: SubregionOrigin<'tcx>,
+                                    bound_kind: GenericKind<'tcx>,
+                                    sub: Region,
+                                    _sups: Vec<Region>)
+    {
         // FIXME: it would be better to report the first error message
         // with the span of the parameter itself, rather than the span
         // where the error was detected. But that span is not readily
         // accessible.
 
+        let labeled_user_string = match bound_kind {
+            GenericKind::Param(ref p) =>
+                format!("the parameter type `{}`", p.user_string(self.tcx)),
+            GenericKind::Projection(ref p) =>
+                format!("the associated type `{}`", p.user_string(self.tcx)),
+        };
+
         match sub {
             ty::ReFree(ty::FreeRegion {bound_region: ty::BrNamed(..), ..}) => {
                 // Does the required lifetime have a nice name we can print?
                 self.tcx.sess.span_err(
                     origin.span(),
-                    format!(
-                        "the parameter type `{}` may not live long enough",
-                        param_ty.user_string(self.tcx))[]);
+                    format!("{} may not live long enough", labeled_user_string)[]);
                 self.tcx.sess.span_help(
                     origin.span(),
                     format!(
                         "consider adding an explicit lifetime bound `{}: {}`...",
-                        param_ty.user_string(self.tcx),
+                        bound_kind.user_string(self.tcx),
                         sub.user_string(self.tcx))[]);
             }
 
             ty::ReStatic => {
                 // Does the required lifetime have a nice name we can print?
                 self.tcx.sess.span_err(
                     origin.span(),
-                    format!(
-                        "the parameter type `{}` may not live long enough",
-                        param_ty.user_string(self.tcx))[]);
+                    format!("{} may not live long enough", labeled_user_string)[]);
                 self.tcx.sess.span_help(
                     origin.span(),
                     format!(
                         "consider adding an explicit lifetime bound `{}: 'static`...",
-                        param_ty.user_string(self.tcx))[]);
+                        bound_kind.user_string(self.tcx))[]);
             }
 
             _ => {
                 // If not, be less specific.
                 self.tcx.sess.span_err(
                     origin.span(),
                     format!(
-                        "the parameter type `{}` may not live long enough",
-                        param_ty.user_string(self.tcx))[]);
+                        "{} may not live long enough",
+                        labeled_user_string)[]);
                 self.tcx.sess.span_help(
                     origin.span(),
                     format!(
-                        "consider adding an explicit lifetime bound to `{}`",
-                        param_ty.user_string(self.tcx))[]);
+                        "consider adding an explicit lifetime bound for `{}`",
+                        bound_kind.user_string(self.tcx))[]);
                 note_and_explain_region(
                     self.tcx,
-                    format!("the parameter type `{}` must be valid for ",
-                            param_ty.user_string(self.tcx))[],
+                    format!("{} must be valid for ", labeled_user_string)[],
                     sub,
                     "...");
             }

src/librustc/middle/infer/mod.rs

@@ -20,6 +20,7 @@ pub use self::ValuePairs::*;
 pub use self::fixup_err::*;
 pub use middle::ty::IntVarValue;
 pub use self::freshen::TypeFreshener;
+pub use self::region_inference::GenericKind;
 
 use middle::subst;
 use middle::subst::Substs;
@@ -382,19 +383,6 @@ pub fn mk_subr<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
     cx.region_vars.commit(snapshot);
 }
 
-pub fn verify_param_bound<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
-                                    origin: SubregionOrigin<'tcx>,
-                                    param_ty: ty::ParamTy,
-                                    a: ty::Region,
-                                    bs: Vec<ty::Region>) {
-    debug!("verify_param_bound({}, {} <: {})",
-           param_ty.repr(cx.tcx),
-           a.repr(cx.tcx),
-           bs.repr(cx.tcx));
-
-    cx.region_vars.verify_param_bound(origin, param_ty, a, bs);
-}
-
 pub fn mk_eqty<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
                          a_is_expected: bool,
                          origin: TypeOrigin,
@@ -1070,6 +1058,20 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             value,
             |br, _| self.next_region_var(LateBoundRegion(span, br, lbrct)))
     }
+
+    /// See `verify_generic_bound` method in `region_inference`
+    pub fn verify_generic_bound(&self,
+                                origin: SubregionOrigin<'tcx>,
+                                kind: GenericKind<'tcx>,
+                                a: ty::Region,
+                                bs: Vec<ty::Region>) {
+        debug!("verify_generic_bound({}, {} <: {})",
+               kind.repr(self.tcx),
+               a.repr(self.tcx),
+               bs.repr(self.tcx));
+
+        self.region_vars.verify_generic_bound(origin, kind, a, bs);
+    }
 }
 
 impl<'tcx> TypeTrace<'tcx> {

src/librustc/middle/infer/region_inference/mod.rs

@@ -22,15 +22,15 @@ use super::cres;
 use super::{RegionVariableOrigin, SubregionOrigin, TypeTrace, MiscVariable};
 
 use middle::region;
-use middle::ty;
+use middle::ty::{mod, Ty};
 use middle::ty::{BoundRegion, FreeRegion, Region, RegionVid};
 use middle::ty::{ReEmpty, ReStatic, ReInfer, ReFree, ReEarlyBound};
 use middle::ty::{ReLateBound, ReScope, ReVar, ReSkolemized, BrFresh};
 use middle::graph;
 use middle::graph::{Direction, NodeIndex};
 use util::common::indenter;
 use util::nodemap::{FnvHashMap, FnvHashSet};
-use util::ppaux::Repr;
+use util::ppaux::{Repr, UserString};
 
 use std::cell::{Cell, RefCell};
 use std::cmp::Ordering::{self, Less, Greater, Equal};
@@ -61,12 +61,18 @@ pub enum Verify<'tcx> {
     // `b` are inference variables.
     VerifyRegSubReg(SubregionOrigin<'tcx>, Region, Region),
 
-    // VerifyParamBound(T, _, R, RS): The parameter type `T` must
-    // outlive the region `R`. `T` is known to outlive `RS`. Therefore
-    // verify that `R <= RS[i]` for some `i`. Inference variables may
-    // be involved (but this verification step doesn't influence
-    // inference).
-    VerifyParamBound(ty::ParamTy, SubregionOrigin<'tcx>, Region, Vec<Region>),
+    // VerifyGenericBound(T, _, R, RS): The parameter type `T` (or
+    // associated type) must outlive the region `R`. `T` is known to
+    // outlive `RS`. Therefore verify that `R <= RS[i]` for some
+    // `i`. Inference variables may be involved (but this verification
+    // step doesn't influence inference).
+    VerifyGenericBound(GenericKind<'tcx>, SubregionOrigin<'tcx>, Region, Vec<Region>),
+}
+
+#[deriving(Clone, Show, PartialEq, Eq)]
+pub enum GenericKind<'tcx> {
+    Param(ty::ParamTy),
+    Projection(ty::ProjectionTy<'tcx>),
 }
 
 #[derive(Copy, PartialEq, Eq, Hash)]
@@ -98,12 +104,12 @@ pub enum RegionResolutionError<'tcx> {
     /// `o` requires that `a <= b`, but this does not hold
     ConcreteFailure(SubregionOrigin<'tcx>, Region, Region),
 
-    /// `ParamBoundFailure(p, s, a, bs)
+    /// `GenericBoundFailure(p, s, a, bs)
     ///
-    /// The parameter type `p` must be known to outlive the lifetime
+    /// The parameter/assocated-type `p` must be known to outlive the lifetime
     /// `a`, but it is only known to outlive `bs` (and none of the
     /// regions in `bs` outlive `a`).
-    ParamBoundFailure(SubregionOrigin<'tcx>, ty::ParamTy, Region, Vec<Region>),
+    GenericBoundFailure(SubregionOrigin<'tcx>, GenericKind<'tcx>, Region, Vec<Region>),
 
     /// `SubSupConflict(v, sub_origin, sub_r, sup_origin, sup_r)`:
     ///
@@ -489,12 +495,13 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
         }
     }
 
-    pub fn verify_param_bound(&self,
-                              origin: SubregionOrigin<'tcx>,
-                              param_ty: ty::ParamTy,
-                              sub: Region,
-                              sups: Vec<Region>) {
-        self.add_verify(VerifyParamBound(param_ty, origin, sub, sups));
+    /// See `Verify::VerifyGenericBound`
+    pub fn verify_generic_bound(&self,
+                                origin: SubregionOrigin<'tcx>,
+                                kind: GenericKind<'tcx>,
+                                sub: Region,
+                                sups: Vec<Region>) {
+        self.add_verify(VerifyGenericBound(kind, origin, sub, sups));
     }
 
     pub fn lub_regions(&self,
@@ -660,7 +667,7 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                                     &mut result_set, r,
                                     a, b);
                             }
-                            VerifyParamBound(_, _, a, ref bs) => {
+                            VerifyGenericBound(_, _, a, ref bs) => {
                                 for &b in bs.iter() {
                                     consider_adding_bidirectional_edges(
                                         &mut result_set, r,
@@ -1211,7 +1218,7 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                     errors.push(ConcreteFailure((*origin).clone(), sub, sup));
                 }
 
-                VerifyParamBound(ref param_ty, ref origin, sub, ref sups) => {
+                VerifyGenericBound(ref kind, ref origin, sub, ref sups) => {
                     let sub = normalize(values, sub);
                     if sups.iter()
                            .map(|&sup| normalize(values, sup))
@@ -1223,8 +1230,8 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                     let sups = sups.iter().map(|&sup| normalize(values, sup))
                                           .collect();
                     errors.push(
-                        ParamBoundFailure(
-                            (*origin).clone(), *param_ty, sub, sups));
+                        GenericBoundFailure(
+                            (*origin).clone(), kind.clone(), sub, sups));
                 }
             }
         }
@@ -1584,8 +1591,8 @@ impl<'tcx> Repr<'tcx> for Verify<'tcx> {
             VerifyRegSubReg(_, ref a, ref b) => {
                 format!("VerifyRegSubReg({}, {})", a.repr(tcx), b.repr(tcx))
             }
-            VerifyParamBound(_, ref p, ref a, ref bs) => {
-                format!("VerifyParamBound({}, {}, {})",
+            VerifyGenericBound(_, ref p, ref a, ref bs) => {
+                format!("VerifyGenericBound({}, {}, {})",
                         p.repr(tcx), a.repr(tcx), bs.repr(tcx))
             }
         }
@@ -1624,3 +1631,32 @@ impl<'tcx> Repr<'tcx> for RegionAndOrigin<'tcx> {
                 self.origin.repr(tcx))
     }
 }
+
+impl<'tcx> Repr<'tcx> for GenericKind<'tcx> {
+    fn repr(&self, tcx: &ty::ctxt<'tcx>) -> String {
+        match *self {
+            GenericKind::Param(ref p) => p.repr(tcx),
+            GenericKind::Projection(ref p) => p.repr(tcx),
+        }
+    }
+}
+
+impl<'tcx> UserString<'tcx> for GenericKind<'tcx> {
+    fn user_string(&self, tcx: &ty::ctxt<'tcx>) -> String {
+        match *self {
+            GenericKind::Param(ref p) => p.user_string(tcx),
+            GenericKind::Projection(ref p) => p.user_string(tcx),
+        }
+    }
+}
+
+impl<'tcx> GenericKind<'tcx> {
+    pub fn to_ty(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            GenericKind::Param(ref p) =>
+                p.to_ty(tcx),
+            GenericKind::Projection(ref p) =>
+                ty::mk_projection(tcx, p.trait_ref.clone(), p.item_name),
+        }
+    }
+}