Auto merge of #53588 - tristanburgess:52985_diagnostics_no_concrete_type_behind_existential_type, r=oli-obk

52985 diagnostics no concrete type behind existential type

@oli-obk FYI. See below for new cycle error generated.

```rust
error[E0391]: cycle detected when processing `Foo`
 --> /dev/staging/existential_type_no_concrete_type_nouse_potential.rs:3:1
  |
3 | existential type Foo: Copy;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: ...which requires processing `bar`...
 --> /dev/staging/existential_type_no_concrete_type_nouse_potential.rs:6:23
  |
6 |   fn bar(x: Foo) -> Foo {
  |  _______________________^
7 | |     x
8 | | }
  | |_^
  = note: ...which again requires processing `Foo`, completing the cycle

error: aborting due to previous error

For more information about this error, try `rustc --explain E0391`.
```

4 files changed, 95 insertions, 66 deletions

diff --git a/src/librustc/traits/query/normalize.rs b/src/librustc/traits/query/normalize.rs
index f8b3933e2a1..8b4954cc501 100644
--- a/src/librustc/traits/query/normalize.rs
+++ b/src/librustc/traits/query/normalize.rs
@@ -121,36 +121,14 @@ impl<'cx, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for QueryNormalizer<'cx, 'gcx, 'tcx
                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
                         self.anon_depth += 1;
                         if concrete_ty == ty {
-                            // The type in question can only be inferred in terms of itself. This
-                            // is likely a user code issue, not a compiler issue. Thus, we will
-                            // induce a cycle error by calling the parent query again on the type.
-                            //
-                            // FIXME: Perhaps a better solution would be to have fold_ty()
-                            // itself be a query. Then, a type fold cycle would be detected
-                            // and reported more naturally as part of the query system, rather
-                            // than forcing it here.
-                            //
-                            // FIXME: Need a better span than just one pointing to the type def.
-                            // Should point to a defining use of the type that results in this
-                            // un-normalizable state.
-                            if let Some(param_env_lifted) =
-                                self.tcx().lift_to_global(&self.param_env)
-                            {
-                                if let Some(ty_lifted) = self.tcx().lift_to_global(&concrete_ty) {
-                                    let span = self.tcx().def_span(def_id);
-                                    self.tcx()
-                                        .global_tcx()
-                                        .at(span)
-                                        .normalize_ty_after_erasing_regions(
-                                            param_env_lifted.and(ty_lifted),
-                                        );
-                                    self.tcx().sess.abort_if_errors();
-                                }
-                            }
-                            // If a cycle error can't be emitted, indicate a NoSolution error
-                            // and let the caller handle it.
-                            self.error = true;
-                            return concrete_ty;
+                            bug!(
+                                "infinite recursion generic_ty: {:#?}, substs: {:#?}, \
+                                 concrete_ty: {:#?}, ty: {:#?}",
+                                generic_ty,
+                                substs,
+                                concrete_ty,
+                                ty
+                            );
                         }
                         let folded_ty = self.fold_ty(concrete_ty);
                         self.anon_depth -= 1;
diff --git a/src/librustc_typeck/check/writeback.rs b/src/librustc_typeck/check/writeback.rs
index 1e147b6c263..319de51713b 100644
--- a/src/librustc_typeck/check/writeback.rs
+++ b/src/librustc_typeck/check/writeback.rs
@@ -17,15 +17,15 @@ use rustc::hir;
 use rustc::hir::def_id::{DefId, DefIndex};
 use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc::infer::InferCtxt;
+use rustc::ty::adjustment::{Adjust, Adjustment};
+use rustc::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder};
 use rustc::ty::subst::UnpackedKind;
 use rustc::ty::{self, Ty, TyCtxt};
-use rustc::ty::adjustment::{Adjust, Adjustment};
-use rustc::ty::fold::{TypeFoldable, TypeFolder, BottomUpFolder};
 use rustc::util::nodemap::DefIdSet;
+use rustc_data_structures::sync::Lrc;
+use std::mem;
 use syntax::ast;
 use syntax_pos::Span;
-use std::mem;
-use rustc_data_structures::sync::Lrc;
 
 ///////////////////////////////////////////////////////////////////////////
 // Entry point
@@ -55,8 +55,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         );
         debug!(
             "used_trait_imports({:?}) = {:?}",
-            item_def_id,
-            used_trait_imports
+            item_def_id, used_trait_imports
         );
         wbcx.tables.used_trait_imports = used_trait_imports;
 
@@ -64,8 +63,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
         debug!(
             "writeback: tables for {:?} are {:#?}",
-            item_def_id,
-            wbcx.tables
+            item_def_id, wbcx.tables
         );
 
         self.tcx.alloc_tables(wbcx.tables)
@@ -118,8 +116,8 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
     // operating on scalars, we clear the overload.
     fn fix_scalar_builtin_expr(&mut self, e: &hir::Expr) {
         match e.node {
-            hir::ExprKind::Unary(hir::UnNeg, ref inner) |
-            hir::ExprKind::Unary(hir::UnNot, ref inner) => {
+            hir::ExprKind::Unary(hir::UnNeg, ref inner)
+            | hir::ExprKind::Unary(hir::UnNot, ref inner) => {
                 let inner_ty = self.fcx.node_ty(inner.hir_id);
                 let inner_ty = self.fcx.resolve_type_vars_if_possible(&inner_ty);
 
@@ -178,8 +176,7 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                     let index_ty = tables.expr_ty_adjusted(&index);
                     let index_ty = self.fcx.resolve_type_vars_if_possible(&index_ty);
 
-                    if base_ty.builtin_index().is_some()
-                        && index_ty == self.fcx.tcx.types.usize {
+                    if base_ty.builtin_index().is_some() && index_ty == self.fcx.tcx.types.usize {
                         // Remove the method call record
                         tables.type_dependent_defs_mut().remove(e.hir_id);
                         tables.node_substs_mut().remove(e.hir_id);
@@ -191,24 +188,26 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                                 // of size information - we need to get rid of it
                                 // Since this is "after" the other adjustment to be
                                 // discarded, we do an extra `pop()`
-                                Some(Adjustment { kind: Adjust::Unsize, .. }) => {
+                                Some(Adjustment {
+                                    kind: Adjust::Unsize,
+                                    ..
+                                }) => {
                                     // So the borrow discard actually happens here
                                     a.pop();
-                                },
+                                }
                                 _ => {}
                             }
                         });
                     }
-                },
+                }
                 // Might encounter non-valid indexes at this point, so there
                 // has to be a fall-through
-                _ => {},
+                _ => {}
             }
         }
     }
 }
 
-
 ///////////////////////////////////////////////////////////////////////////
 // Impl of Visitor for Resolver
 //
@@ -262,7 +261,9 @@ impl<'cx, 'gcx, 'tcx> Visitor<'gcx> for WritebackCx<'cx, 'gcx, 'tcx> {
                 if let Some(&bm) = self.fcx.tables.borrow().pat_binding_modes().get(p.hir_id) {
                     self.tables.pat_binding_modes_mut().insert(p.hir_id, bm);
                 } else {
-                    self.tcx().sess.delay_span_bug(p.span, "missing binding mode");
+                    self.tcx()
+                        .sess
+                        .delay_span_bug(p.span, "missing binding mode");
                 }
             }
             hir::PatKind::Struct(_, ref fields, _) => {
@@ -310,8 +311,7 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
             };
             debug!(
                 "Upvar capture for {:?} resolved to {:?}",
-                upvar_id,
-                new_upvar_capture
+                upvar_id, new_upvar_capture
             );
             self.tables
                 .upvar_capture_map
@@ -425,8 +425,7 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                                     if subst == ty {
                                         // found it in the substitution list, replace with the
                                         // parameter from the existential type
-                                        return self
-                                            .tcx()
+                                        return self.tcx()
                                             .global_tcx()
                                             .mk_ty_param(param.index, param.name);
                                     }
@@ -464,14 +463,16 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                                                 name: p.name,
                                             };
                                             trace!("replace {:?} with {:?}", region, reg);
-                                            return self.tcx().global_tcx()
+                                            return self.tcx()
+                                                .global_tcx()
                                                 .mk_region(ty::ReEarlyBound(reg));
                                         }
                                     }
                                 }
                                 trace!("anon_defn: {:#?}", anon_defn);
                                 trace!("generics: {:#?}", generics);
-                                self.tcx().sess
+                                self.tcx()
+                                    .sess
                                     .struct_span_err(
                                         span,
                                         "non-defining existential type use in defining scope",
@@ -480,7 +481,7 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                                         span,
                                         format!(
                                             "lifetime `{}` is part of concrete type but not used \
-                                            in parameter list of existential type",
+                                             in parameter list of existential type",
                                             region,
                                         ),
                                     )
@@ -488,17 +489,27 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                                 self.tcx().global_tcx().mk_region(ty::ReStatic)
                             }
                         }
-                    }
+                    },
                 })
             };
 
-            let old = self.tables.concrete_existential_types.insert(def_id, definition_ty);
+            if let ty::Anon(defin_ty_def_id, _substs) = definition_ty.sty {
+                if def_id == defin_ty_def_id {
+                    // Concrete type resolved to the existential type itself
+                    // Force a cycle error
+                    self.tcx().at(span).type_of(defin_ty_def_id);
+                }
+            }
+
+            let old = self.tables
+                .concrete_existential_types
+                .insert(def_id, definition_ty);
             if let Some(old) = old {
                 if old != definition_ty {
                     span_bug!(
                         span,
                         "visit_anon_types tried to write \
-                        different types for the same existential type: {:?}, {:?}, {:?}",
+                         different types for the same existential type: {:?}, {:?}, {:?}",
                         def_id,
                         definition_ty,
                         old,
@@ -510,7 +521,12 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
 
     fn visit_field_id(&mut self, node_id: ast::NodeId) {
         let hir_id = self.tcx().hir.node_to_hir_id(node_id);
-        if let Some(index) = self.fcx.tables.borrow_mut().field_indices_mut().remove(hir_id) {
+        if let Some(index) = self.fcx
+            .tables
+            .borrow_mut()
+            .field_indices_mut()
+            .remove(hir_id)
+        {
             self.tables.field_indices_mut().insert(hir_id, index);
         }
     }
@@ -559,8 +575,7 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                 let resolved_adjustment = self.resolve(&adjustment, &span);
                 debug!(
                     "Adjustments for node {:?}: {:?}",
-                    hir_id,
-                    resolved_adjustment
+                    hir_id, resolved_adjustment
                 );
                 self.tables
                     .adjustments_mut()
@@ -584,8 +599,7 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                 let resolved_adjustment = self.resolve(&adjustment, &span);
                 debug!(
                     "pat_adjustments for node {:?}: {:?}",
-                    hir_id,
-                    resolved_adjustment
+                    hir_id, resolved_adjustment
                 );
                 self.tables
                     .pat_adjustments_mut()
@@ -701,7 +715,8 @@ impl<'cx, 'gcx, 'tcx> Resolver<'cx, 'gcx, 'tcx> {
     fn report_error(&self, t: Ty<'tcx>) {
         if !self.tcx.sess.has_errors() {
             self.infcx
-                .need_type_info_err(Some(self.body.id()), self.span.to_span(&self.tcx), t).emit();
+                .need_type_info_err(Some(self.body.id()), self.span.to_span(&self.tcx), t)
+                .emit();
         }
     }
 }
diff --git a/src/test/ui/existential_types/nested_existential_types.rs b/src/test/ui/existential_types/nested_existential_types.rs
new file mode 100644
index 00000000000..aac72c71d03
--- /dev/null
+++ b/src/test/ui/existential_types/nested_existential_types.rs
@@ -0,0 +1,31 @@
+// Copyright 2018 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.
+
+#![feature(existential_type)]
+// compile-pass
+mod my_mod {
+  use std::fmt::Debug;
+
+  pub existential type Foo: Debug;
+  pub existential type Foot: Debug;
+
+  pub fn get_foo() -> Foo {
+      5i32
+  }
+
+  pub fn get_foot() -> Foot {
+      get_foo()
+  }
+}
+
+fn main() {
+    let _: my_mod::Foot = my_mod::get_foot();
+}
+
diff --git a/src/test/ui/existential_types/no_inferrable_concrete_type.stderr b/src/test/ui/existential_types/no_inferrable_concrete_type.stderr
index ffb4f4cc144..06e40fd6ab5 100644
--- a/src/test/ui/existential_types/no_inferrable_concrete_type.stderr
+++ b/src/test/ui/existential_types/no_inferrable_concrete_type.stderr
@@ -1,10 +1,15 @@
-error[E0391]: cycle detected when normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: Foo }`
+error[E0391]: cycle detected when processing `Foo`
   --> $DIR/no_inferrable_concrete_type.rs:16:1
    |
 LL | existential type Foo: Copy; //~ cycle detected
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which again requires normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: Foo }`, completing the cycle
+note: ...which requires processing `bar`...
+  --> $DIR/no_inferrable_concrete_type.rs:19:23
+   |
+LL | fn bar(x: Foo) -> Foo { x }
+   |                       ^^^^^
+   = note: ...which again requires processing `Foo`, completing the cycle
 
 error: aborting due to previous error