Auto merge of #22512 - nikomatsakis:issue-20300-where-clause-not-bounds, r=nikomatsakis

This is a fix for #20300 though as a side-sweep it fixes a number of stack overflows because it integrates cycle detection into the conversion process. I didn't go through and retest everything.

The tricky part here is that in some cases we have to go find the information we need from the AST -- we can't use the converted form of the where-clauses because we often have to handle something like `T::Item` *while converting the where-clauses themselves*. Since this is also not a fixed-point process we can't just try and keep trying to find the best order. So instead I modified the `AstConv` interface to allow you to request the bounds for a type parameter; we'll then do a secondary scan of the where-clauses to figure out what we need. This may create a cycle in some cases, but we have code to catch that.

Another approach that is NOT taken by this PR would be to "convert" `T::Item` into a form that does not specify what trait it's using. This then kind of defers the problem of picking the trait till later. That might be a good idea, but it would make normalization and everything else much harder, so I'm holding off on that (and hoping to find a better way for handling things like `i32::T`).

This PR also removes "most of" the `bounds` struct from `TypeParameterDef`. Still a little ways to go before `ParamBounds` can be removed entirely -- it's used for supertraits, for example (though those really ought to be converted, I think, to a call to `get_type_parameter_bounds` on `Self` from within the trait definition).

cc @jroesch 

Fixes #20300 

src/librustc/metadata/tydecode.rs

@@ -822,7 +822,6 @@ fn parse_type_param_def_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
     assert_eq!(next(st), '|');
     let index = parse_u32(st);
     assert_eq!(next(st), '|');
-    let bounds = parse_bounds_(st, conv);
     let default = parse_opt(st, |st| parse_ty_(st, conv));
     let object_lifetime_default = parse_object_lifetime_default(st, conv);
 
@@ -831,7 +830,6 @@ fn parse_type_param_def_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
         def_id: def_id,
         space: space,
         index: index,
-        bounds: bounds,
         default: default,
         object_lifetime_default: object_lifetime_default,
     }
@@ -924,18 +922,18 @@ fn parse_bounds_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
 {
     let builtin_bounds = parse_builtin_bounds_(st, conv);
 
+    let region_bounds = parse_region_bounds_(st, conv);
+
     let mut param_bounds = ty::ParamBounds {
-        region_bounds: Vec::new(),
+        region_bounds: region_bounds,
         builtin_bounds: builtin_bounds,
         trait_bounds: Vec::new(),
         projection_bounds: Vec::new(),
     };
+
+
     loop {
         match next(st) {
-            'R' => {
-                param_bounds.region_bounds.push(
-                    parse_region_(st, conv));
-            }
             'I' => {
                 param_bounds.trait_bounds.push(
                     ty::Binder(parse_trait_ref_(st, conv)));
@@ -953,3 +951,18 @@ fn parse_bounds_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
         }
     }
 }
+
+fn parse_region_bounds_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
+                              -> Vec<ty::Region> where
+    F: FnMut(DefIdSource, ast::DefId) -> ast::DefId,
+{
+    let mut region_bounds = Vec::new();
+    loop {
+        match next(st) {
+            'R' => { region_bounds.push(parse_region_(st, conv)); }
+            '.' => { return region_bounds; }
+            c => { panic!("parse_bounds: bad bounds ('{}')", c); }
+        }
+    }
+}
+

src/librustc/metadata/tyencode.rs

@@ -386,10 +386,7 @@ pub fn enc_bounds<'a, 'tcx>(w: &mut SeekableMemWriter, cx: &ctxt<'a, 'tcx>,
                             bs: &ty::ParamBounds<'tcx>) {
     enc_builtin_bounds(w, cx, &bs.builtin_bounds);
 
-    for &r in &bs.region_bounds {
-        mywrite!(w, "R");
-        enc_region(w, cx, r);
-    }
+    enc_region_bounds(w, cx, &bs.region_bounds);
 
     for tp in &bs.trait_bounds {
         mywrite!(w, "I");
@@ -404,12 +401,22 @@ pub fn enc_bounds<'a, 'tcx>(w: &mut SeekableMemWriter, cx: &ctxt<'a, 'tcx>,
     mywrite!(w, ".");
 }
 
+pub fn enc_region_bounds<'a, 'tcx>(w: &mut SeekableMemWriter,
+                            cx: &ctxt<'a, 'tcx>,
+                            rs: &[ty::Region]) {
+    for &r in rs {
+        mywrite!(w, "R");
+        enc_region(w, cx, r);
+    }
+
+    mywrite!(w, ".");
+}
+
 pub fn enc_type_param_def<'a, 'tcx>(w: &mut SeekableMemWriter, cx: &ctxt<'a, 'tcx>,
                                     v: &ty::TypeParameterDef<'tcx>) {
     mywrite!(w, "{}:{}|{}|{}|",
              token::get_name(v.name), (cx.ds)(v.def_id),
              v.space.to_uint(), v.index);
-    enc_bounds(w, cx, &v.bounds);
     enc_opt(w, v.default, |w, t| enc_ty(w, cx, t));
     enc_object_lifetime_default(w, cx, v.object_lifetime_default);
 }

src/librustc/middle/ty.rs

@@ -55,7 +55,7 @@
 use middle::resolve_lifetime;
 use middle::infer;
 use middle::stability;
-use middle::subst::{self, Subst, Substs, VecPerParamSpace};
+use middle::subst::{self, ParamSpace, Subst, Substs, VecPerParamSpace};
 use middle::traits;
 use middle::ty;
 use middle::ty_fold::{self, TypeFoldable, TypeFolder};
@@ -1750,7 +1750,6 @@ pub struct TypeParameterDef<'tcx> {
     pub def_id: ast::DefId,
     pub space: subst::ParamSpace,
     pub index: u32,
-    pub bounds: ParamBounds<'tcx>,
     pub default: Option<Ty<'tcx>>,
     pub object_lifetime_default: Option<ObjectLifetimeDefault>,
 }
@@ -2546,6 +2545,13 @@ pub fn closure_type(&self,
     {
         self.closure_tys.borrow()[def_id].subst(self, substs)
     }
+
+    pub fn type_parameter_def(&self,
+                              node_id: ast::NodeId)
+                              -> TypeParameterDef<'tcx>
+    {
+        self.ty_param_defs.borrow()[node_id].clone()
+    }
 }
 
 // Interns a type/name combination, stores the resulting box in cx.interner,
@@ -2996,6 +3002,13 @@ pub fn as_opt_param_ty(&self) -> Option<ty::ParamTy> {
             _ => None,
         }
     }
+
+    pub fn is_param(&self, space: ParamSpace, index: u32) -> bool {
+        match self.sty {
+            ty::ty_param(ref data) => data.space == space && data.idx == index,
+            _ => false,
+        }
+    }
 }
 
 pub fn walk_ty<'tcx, F>(ty_root: Ty<'tcx>, mut f: F)

src/librustc/middle/ty_fold.rs

@@ -377,7 +377,6 @@ fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> ty::TypeParameterDef
             def_id: self.def_id,
             space: self.space,
             index: self.index,
-            bounds: self.bounds.fold_with(folder),
             default: self.default.fold_with(folder),
             object_lifetime_default: self.object_lifetime_default.fold_with(folder),
         }

src/librustc/session/config.rs

@@ -99,6 +99,7 @@ pub struct Options {
     pub test: bool,
     pub parse_only: bool,
     pub no_trans: bool,
+    pub treat_err_as_bug: bool,
     pub no_analysis: bool,
     pub debugging_opts: DebuggingOptions,
     /// Whether to write dependency files. It's (enabled, optional filename).
@@ -223,6 +224,7 @@ pub fn basic_options() -> Options {
         test: false,
         parse_only: false,
         no_trans: false,
+        treat_err_as_bug: false,
         no_analysis: false,
         debugging_opts: basic_debugging_options(),
         write_dependency_info: (false, None),
@@ -573,6 +575,8 @@ fn parse_passes(slot: &mut Passes, v: Option<&str>) -> bool {
           "Parse only; do not compile, assemble, or link"),
     no_trans: bool = (false, parse_bool,
           "Run all passes except translation; no output"),
+    treat_err_as_bug: bool = (false, parse_bool,
+          "Treat all errors that occur as bugs"),
     no_analysis: bool = (false, parse_bool,
           "Parse and expand the source, but run no analysis"),
     extra_plugins: Vec<String> = (Vec::new(), parse_list,
@@ -843,6 +847,7 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
 
     let parse_only = debugging_opts.parse_only;
     let no_trans = debugging_opts.no_trans;
+    let treat_err_as_bug = debugging_opts.treat_err_as_bug;
     let no_analysis = debugging_opts.no_analysis;
 
     if debugging_opts.debug_llvm {
@@ -1030,6 +1035,7 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         test: test,
         parse_only: parse_only,
         no_trans: no_trans,
+        treat_err_as_bug: treat_err_as_bug,
         no_analysis: no_analysis,
         debugging_opts: debugging_opts,
         write_dependency_info: write_dependency_info,

src/librustc/session/mod.rs

@@ -74,18 +74,27 @@ pub fn fatal(&self, msg: &str) -> ! {
         self.diagnostic().handler().fatal(msg)
     }
     pub fn span_err(&self, sp: Span, msg: &str) {
+        if self.opts.treat_err_as_bug {
+            self.span_bug(sp, msg);
+        }
         match split_msg_into_multilines(msg) {
             Some(msg) => self.diagnostic().span_err(sp, &msg[..]),
             None => self.diagnostic().span_err(sp, msg)
         }
     }
     pub fn span_err_with_code(&self, sp: Span, msg: &str, code: &str) {
+        if self.opts.treat_err_as_bug {
+            self.span_bug(sp, msg);
+        }
         match split_msg_into_multilines(msg) {
             Some(msg) => self.diagnostic().span_err_with_code(sp, &msg[..], code),
             None => self.diagnostic().span_err_with_code(sp, msg, code)
         }
     }
     pub fn err(&self, msg: &str) {
+        if self.opts.treat_err_as_bug {
+            self.bug(msg);
+        }
         self.diagnostic().handler().err(msg)
     }
     pub fn err_count(&self) -> uint {

src/librustc_typeck/astconv.rs

@@ -73,9 +73,14 @@
 pub trait AstConv<'tcx> {
     fn tcx<'a>(&'a self) -> &'a ty::ctxt<'tcx>;
 
-    fn get_item_type_scheme(&self, id: ast::DefId) -> ty::TypeScheme<'tcx>;
+    fn get_item_type_scheme(&self, span: Span, id: ast::DefId)
+                            -> Result<ty::TypeScheme<'tcx>, ErrorReported>;
 
-    fn get_trait_def(&self, id: ast::DefId) -> Rc<ty::TraitDef<'tcx>>;
+    fn get_trait_def(&self, span: Span, id: ast::DefId)
+                     -> Result<Rc<ty::TraitDef<'tcx>>, ErrorReported>;
+
+    fn get_type_parameter_bounds(&self, span: Span, def_id: ast::NodeId)
+                                 -> Result<Vec<ty::PolyTraitRef<'tcx>>, ErrorReported>;
 
     /// Return an (optional) substitution to convert bound type parameters that
     /// are in scope into free ones. This function should only return Some
@@ -683,7 +688,14 @@ fn ast_path_to_trait_ref<'a,'tcx>(
     -> Rc<ty::TraitRef<'tcx>>
 {
     debug!("ast_path_to_trait_ref {:?}", trait_segment);
-    let trait_def = this.get_trait_def(trait_def_id);
+    let trait_def = match this.get_trait_def(span, trait_def_id) {
+        Ok(trait_def) => trait_def,
+        Err(ErrorReported) => {
+            // No convenient way to recover from a cycle here. Just bail. Sorry!
+            this.tcx().sess.abort_if_errors();
+            this.tcx().sess.bug("ErrorReported returned, but no errors reports?")
+        }
+    };
 
     let (regions, types, assoc_bindings) = match trait_segment.parameters {
         ast::AngleBracketedParameters(ref data) => {
@@ -860,10 +872,15 @@ fn ast_path_to_ty<'tcx>(
     item_segment: &ast::PathSegment)
     -> Ty<'tcx>
 {
-    let ty::TypeScheme {
-        generics,
-        ty: decl_ty
-    } = this.get_item_type_scheme(did);
+    let tcx = this.tcx();
+    let (generics, decl_ty) = match this.get_item_type_scheme(span, did) {
+        Ok(ty::TypeScheme { generics,  ty: decl_ty }) => {
+            (generics, decl_ty)
+        }
+        Err(ErrorReported) => {
+            return tcx.types.err;
+        }
+    };
 
     let substs = ast_path_substs_for_ty(this, rscope,
                                         span, param_mode,
@@ -1001,20 +1018,17 @@ fn associated_path_def_to_ty<'tcx>(this: &AstConv<'tcx>,
         return (tcx.types.err, ty_path_def);
     };
 
-    let mut suitable_bounds: Vec<_>;
-    let ty_param_name: ast::Name;
-    { // contain scope of refcell:
-        let ty_param_defs = tcx.ty_param_defs.borrow();
-        let ty_param_def = &ty_param_defs[ty_param_node_id];
-        ty_param_name = ty_param_def.name;
+    let ty_param_name = tcx.ty_param_defs.borrow()[ty_param_node_id].name;
 
+    // FIXME(#20300) -- search where clauses, not bounds
+    let bounds =
+        this.get_type_parameter_bounds(span, ty_param_node_id)
+            .unwrap_or(Vec::new());
 
-        // FIXME(#20300) -- search where clauses, not bounds
-        suitable_bounds =
-            traits::transitive_bounds(tcx, &ty_param_def.bounds.trait_bounds)
-            .filter(|b| trait_defines_associated_type_named(this, b.def_id(), assoc_name))
-            .collect();
-    }
+    let mut suitable_bounds: Vec<_> =
+        traits::transitive_bounds(tcx, &bounds)
+        .filter(|b| trait_defines_associated_type_named(this, b.def_id(), assoc_name))
+        .collect();
 
     if suitable_bounds.len() == 0 {
         span_err!(tcx.sess, span, E0220,

src/librustc_typeck/check/mod.rs

@@ -97,7 +97,7 @@
 use middle::traits;
 use middle::ty::{FnSig, GenericPredicates, VariantInfo, TypeScheme};
 use middle::ty::{Disr, ParamTy, ParameterEnvironment};
-use middle::ty::{self, HasProjectionTypes, RegionEscape, Ty};
+use middle::ty::{self, HasProjectionTypes, RegionEscape, ToPolyTraitRef, Ty};
 use middle::ty::liberate_late_bound_regions;
 use middle::ty::{MethodCall, MethodCallee, MethodMap, ObjectCastMap};
 use middle::ty_fold::{TypeFolder, TypeFoldable};
@@ -106,7 +106,7 @@
 use {CrateCtxt, lookup_full_def, require_same_types};
 use TypeAndSubsts;
 use lint;
-use util::common::{block_query, indenter, loop_query};
+use util::common::{block_query, ErrorReported, indenter, loop_query};
 use util::ppaux::{self, Repr};
 use util::nodemap::{DefIdMap, FnvHashMap, NodeMap};
 use util::lev_distance::lev_distance;
@@ -1206,18 +1206,48 @@ fn check_cast<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
 impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
     fn tcx(&self) -> &ty::ctxt<'tcx> { self.ccx.tcx }
 
-    fn get_item_type_scheme(&self, id: ast::DefId) -> ty::TypeScheme<'tcx> {
-        ty::lookup_item_type(self.tcx(), id)
+    fn get_item_type_scheme(&self, _: Span, id: ast::DefId)
+                            -> Result<ty::TypeScheme<'tcx>, ErrorReported>
+    {
+        Ok(ty::lookup_item_type(self.tcx(), id))
     }
 
-    fn get_trait_def(&self, id: ast::DefId) -> Rc<ty::TraitDef<'tcx>> {
-        ty::lookup_trait_def(self.tcx(), id)
+    fn get_trait_def(&self, _: Span, id: ast::DefId)
+                     -> Result<Rc<ty::TraitDef<'tcx>>, ErrorReported>
+    {
+        Ok(ty::lookup_trait_def(self.tcx(), id))
     }
 
     fn get_free_substs(&self) -> Option<&Substs<'tcx>> {
         Some(&self.inh.param_env.free_substs)
     }
 
+    fn get_type_parameter_bounds(&self,
+                                 _: Span,
+                                 node_id: ast::NodeId)
+                                 -> Result<Vec<ty::PolyTraitRef<'tcx>>, ErrorReported>
+    {
+        let def = self.tcx().type_parameter_def(node_id);
+        let r = self.inh.param_env.caller_bounds
+                                  .iter()
+                                  .filter_map(|predicate| {
+                                      match *predicate {
+                                          ty::Predicate::Trait(ref data) => {
+                                              if data.0.self_ty().is_param(def.space, def.index) {
+                                                  Some(data.to_poly_trait_ref())
+                                              } else {
+                                                  None
+                                              }
+                                          }
+                                          _ => {
+                                              None
+                                          }
+                                      }
+                                  })
+                                  .collect();
+        Ok(r)
+    }
+
     fn ty_infer(&self, _span: Span) -> Ty<'tcx> {
         self.infcx().next_ty_var()
     }
@@ -3607,7 +3637,7 @@ fn check_struct_fields_on_error<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                 }
             } else {
               tcx.sess.span_bug(expr.span,
-                                &format!("unbound path {}", expr.repr(tcx))[])
+                                &format!("unbound path {}", expr.repr(tcx)))
           };
 
           let mut def = path_res.base_def;

src/librustc_typeck/coherence/overlap.rs

@@ -147,7 +147,7 @@ fn visit_item(&mut self, item: &'v ast::Item) {
                             None => {
                                 self.tcx.sess.bug(
                                           &format!("no default implementation recorded for `{:?}`",
-                                          item)[]);
+                                          item));
                             }
                         }
                     }

src/librustdoc/clean/mod.rs

@@ -479,11 +479,10 @@ impl<'tcx> Clean<TyParam> for ty::TypeParameterDef<'tcx> {
     fn clean(&self, cx: &DocContext) -> TyParam {
         cx.external_typarams.borrow_mut().as_mut().unwrap()
           .insert(self.def_id, self.name.clean(cx));
-        let bounds = self.bounds.clean(cx);
         TyParam {
             name: self.name.clean(cx),
             did: self.def_id,
-            bounds: bounds,
+            bounds: vec![], // these are filled in from the where-clauses
             default: self.default.clean(cx),
         }
     }
@@ -892,9 +891,7 @@ fn has_sized_bound(bounds: &[TyParamBound], cx: &DocContext) -> bool {
         // Bounds in the type_params and lifetimes fields are repeated in the predicates
         // field (see rustc_typeck::collect::ty_generics), so remove them.
         let stripped_typarams = gens.types.get_slice(space).iter().map(|tp| {
-            let mut stp = tp.clone();
-            stp.bounds = ty::ParamBounds::empty();
-            stp.clean(cx)
+            tp.clean(cx)
         }).collect::<Vec<_>>();
         let stripped_lifetimes = gens.regions.get_slice(space).iter().map(|rp| {
             let mut srp = rp.clone();

src/test/compile-fail/associated-type-projection-ambig-between-bound-and-where-clause.rs

+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test equality constraints in a where clause where the type being
+// equated appears in a supertrait.
+
+pub trait Vehicle {
+    type Color;
+
+    fn go(&self) {  }
+}
+
+pub trait Box {
+    type Color;
+
+    fn mail(&self) {  }
+}
+
+fn a<C:Vehicle+Box>(_: C::Color) {
+    //~^ ERROR ambiguous associated type `Color` in bounds of `C`
+}
+
+fn b<C>(_: C::Color) where C : Vehicle+Box {
+    //~^ ERROR ambiguous associated type `Color` in bounds of `C`
+}
+
+fn c<C>(_: C::Color) where C : Vehicle, C : Box {
+    //~^ ERROR ambiguous associated type `Color` in bounds of `C`
+}
+
+struct D<X>;
+impl<X> D<X> where X : Vehicle {
+    fn d(&self, _: X::Color) where X : Box { }
+    //~^ ERROR ambiguous associated type `Color` in bounds of `X`
+}
+
+trait E<X:Vehicle> {
+    fn e(&self, _: X::Color) where X : Box;
+    //~^ ERROR ambiguous associated type `Color` in bounds of `X`
+
+    fn f(&self, _: X::Color) where X : Box { }
+    //~^ ERROR ambiguous associated type `Color` in bounds of `X`
+}
+
+pub fn main() { }

src/test/compile-fail/cycle-projection-based-on-where-clause.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Example cycle where a bound on `T` uses a shorthand for `T`. This
+// creates a cycle because we have to know the bounds on `T` to figure
+// out what trait defines `Item`, but we can't know the bounds on `T`
+// without knowing how to handle `T::Item`.
+//
+// Note that in the future cases like this could perhaps become legal,
+// if we got more fine-grained about our cycle detection or changed
+// how we handle `T::Item` resolution.
+
+use std::ops::Add;
+
+// Preamble.
+trait Trait { type Item; }
+
+struct A<T>
+    where T : Trait,
+          T : Add<T::Item>
+    //~^ ERROR illegal recursive type
+{
+    data: T
+}
+
+fn main() {
+}

src/test/compile-fail/cycle-trait-supertrait-direct.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test a supertrait cycle where a trait extends itself.
+
+trait Chromosome: Chromosome {
+    //~^ ERROR unsupported cyclic reference
+}
+
+fn main() { }

src/test/compile-fail/cycle-trait-supertrait-indirect.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test a supertrait cycle where the first trait we find (`A`) is not
+// a direct participant in the cycle.
+
+trait A: B {
+}
+
+trait B: C { }
+
+trait C: B { }
+    //~^ ERROR unsupported cyclic reference
+
+fn main() { }

src/test/run-pass/associated-types-project-from-type-param-via-bound-in-where-clause.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Various uses of `T::Item` syntax where the bound that supplies
+// `Item` originates in a where-clause, not the declaration of
+// `T`. Issue #20300.
+
+use std::marker::{MarkerTrait, PhantomData};
+use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
+use std::sync::atomic::Ordering::SeqCst;
+
+static COUNTER: AtomicUsize = ATOMIC_USIZE_INIT;
+
+// Preamble.
+trait Trait : MarkerTrait { type Item; }
+struct Struct;
+impl Trait for Struct {
+    type Item = u32;
+}
+
+// Where-clause attached on the method which declares `T`.
+struct A;
+impl A {
+    fn foo<T>(_x: T::Item) where T: Trait {
+        COUNTER.fetch_add(1, SeqCst);
+    }
+}
+
+// Where-clause attached on the method to a parameter from the struct.
+struct B<T>(PhantomData<T>);
+impl<T> B<T> {
+    fn foo(_x: T::Item) where T: Trait {
+        COUNTER.fetch_add(10, SeqCst);
+    }
+}
+
+// Where-clause attached to free fn.
+fn c<T>(_: T::Item) where T : Trait {
+    COUNTER.fetch_add(100, SeqCst);
+}
+
+// Where-clause attached to defaulted and non-defaulted trait method.
+trait AnotherTrait {
+    fn method<T>(&self, _: T::Item) where T: Trait;
+    fn default_method<T>(&self, _: T::Item) where T: Trait {
+        COUNTER.fetch_add(1000, SeqCst);
+    }
+}
+struct D;
+impl AnotherTrait for D {
+    fn method<T>(&self, _: T::Item) where T: Trait {
+        COUNTER.fetch_add(10000, SeqCst);
+    }
+}
+
+// Where-clause attached to trait and impl containing the method.
+trait YetAnotherTrait<T>
+    where T : Trait
+{
+    fn method(&self, _: T::Item);
+    fn default_method(&self, _: T::Item) {
+        COUNTER.fetch_add(100000, SeqCst);
+    }
+}
+struct E<T>(PhantomData<T>);
+impl<T> YetAnotherTrait<T> for E<T>
+    where T : Trait
+{
+    fn method(&self, _: T::Item) {
+        COUNTER.fetch_add(1000000, SeqCst);
+    }
+}
+
+// Where-clause attached to inherent impl containing the method.
+struct F<T>(PhantomData<T>);
+impl<T> F<T> where T : Trait {
+    fn method(&self, _: T::Item) {
+        COUNTER.fetch_add(10000000, SeqCst);
+    }
+}
+
+// Where-clause attached to struct.
+#[allow(dead_code)]
+struct G<T> where T : Trait {
+    data: T::Item,
+    phantom: PhantomData<T>,
+}
+
+fn main() {
+    A::foo::<Struct>(22);
+    B::<Struct>::foo(22);
+    c::<Struct>(22);
+    D.method::<Struct>(22);
+    D.default_method::<Struct>(22);
+    E(PhantomData::<Struct>).method(22);
+    E(PhantomData::<Struct>).default_method(22);
+    F(PhantomData::<Struct>).method(22);
+    G::<Struct> { data: 22, phantom: PhantomData };
+    assert_eq!(COUNTER.load(SeqCst), 11111111);
+}

src/test/run-pass/cycle-generic-bound.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #15477. This test just needs to compile.
+
+use std::marker::PhantomFn;
+
+trait Chromosome<X: Chromosome<i32>> : PhantomFn<(Self,X)> {
+}
+
+fn main() { }

src/test/run-pass/cycle-trait-type-trait.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test a case where a supertrait references a type that references
+// the original trait. This poses no problem at the moment.
+
+trait Chromosome: Get<Struct<i32>> {
+}
+
+trait Get<A> {
+    fn get(&self) -> A;
+}
+
+struct Struct<C:Chromosome> { c: C }
+
+fn main() { }