use approx. bounds to decide whether to add outlives obligations

Before, if we had a projection like `<T as Foo<'0>>::Bar: 'x` and a
where clause like `<T as Foo<'a>>::Bar: 'a`, we considered those to
have nothing to do with one another. Therefore, we would use the
"overconstrained" path of adding `T: 'x` and `'0: 'x` requirements. We
now do a "fuzzy" match where we erase regions first and hence we see
the env bound `'a`.

Author: nikomatsakis

src/librustc/infer/outlives/verify.rs

@@ -89,7 +89,16 @@ impl<'cx, 'gcx, 'tcx> VerifyBoundCx<'cx, 'gcx, 'tcx> {
         &self,
         projection_ty: ty::ProjectionTy<'tcx>,
     ) -> Vec<ty::Region<'tcx>> {
-        self.declared_generic_bounds_from_env(GenericKind::Projection(projection_ty))
+        let projection_ty = GenericKind::Projection(projection_ty).to_ty(self.tcx);
+        let erased_projection_ty = self.tcx.erase_regions(&projection_ty);
+        self.declared_generic_bounds_from_env_with_compare_fn(
+            |ty| if let ty::Projection(..) = ty.sty {
+                let erased_ty = self.tcx.erase_regions(&ty);
+                erased_ty == erased_projection_ty
+            } else {
+                false
+            },
+        )
     }
 
     /// Searches the where clauses in scope for regions that

src/test/ui/nll/ty-outlives/projection-one-region-closure.rs

@@ -85,8 +85,7 @@ where
     // can do better here with a more involved verification step.
 
     with_signature(cell, t, |cell, t| require(cell, t));
-    //~^ ERROR the parameter type `T` may not live long enough
-    //~| ERROR
+    //~^ ERROR
 }
 
 #[rustc_regions]

src/test/ui/nll/ty-outlives/projection-one-region-closure.stderr

@@ -119,8 +119,7 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
                extern "rust-call" fn((std::cell::Cell<&'_#3r ()>, T))
            ]
    = note: number of external vids: 4
-   = note: where T: '_#3r
-   = note: where '_#2r: '_#3r
+   = note: where <T as Anything<ReClosureBound('_#2r)>>::AssocType: '_#3r
 
 note: No external requirements
   --> $DIR/projection-one-region-closure.rs:72:1
@@ -130,7 +129,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | |     T::AssocType: 'a,
 ...  |
-LL | |     //~| ERROR
+LL | |     //~^ ERROR
 LL | | }
    | |_^
    |
@@ -140,27 +139,16 @@ LL | | }
                T
            ]
 
-error[E0309]: the parameter type `T` may not live long enough
+error[E0309]: the associated type `<T as Anything<'_#5r>>::AssocType` may not live long enough
   --> $DIR/projection-one-region-closure.rs:87:29
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: consider adding an explicit lifetime bound `T: ReEarlyBound(0, 'a)`...
-
-error: unsatisfied lifetime constraints
-  --> $DIR/projection-one-region-closure.rs:87:29
-   |
-LL | fn projection_outlives<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
-   |                        --  -- lifetime `'b` defined here
-   |                        |
-   |                        lifetime `'a` defined here
-...
-LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^ closure body requires that `'b` must outlive `'a`
+   = help: consider adding an explicit lifetime bound `<T as Anything<'_#5r>>::AssocType: ReEarlyBound(0, 'a)`...
 
 note: External requirements
-  --> $DIR/projection-one-region-closure.rs:99:29
+  --> $DIR/projection-one-region-closure.rs:98:29
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -177,7 +165,7 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    = note: where '_#2r: '_#3r
 
 note: No external requirements
-  --> $DIR/projection-one-region-closure.rs:93:1
+  --> $DIR/projection-one-region-closure.rs:92:1
    |
 LL | / fn elements_outlive<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
 LL | | where
@@ -194,6 +182,6 @@ LL | | }
                T
            ]
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0309`.

src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.stderr

@@ -101,7 +101,7 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
                extern "rust-call" fn((std::cell::Cell<&'_#3r ()>, T))
            ]
    = note: number of external vids: 4
-   = note: where '_#2r: '_#3r
+   = note: where <T as Anything<ReClosureBound('_#2r)>>::AssocType: '_#3r
 
 note: No external requirements
   --> $DIR/projection-one-region-trait-bound-closure.rs:62:1
@@ -121,16 +121,13 @@ LL | | }
                T
            ]
 
-error: unsatisfied lifetime constraints
+error[E0309]: the associated type `<T as Anything<'_#5r>>::AssocType` may not live long enough
   --> $DIR/projection-one-region-trait-bound-closure.rs:77:29
    |
-LL | fn projection_outlives<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
-   |                        --  -- lifetime `'b` defined here
-   |                        |
-   |                        lifetime `'a` defined here
-...
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^ closure body requires that `'b` must outlive `'a`
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider adding an explicit lifetime bound `<T as Anything<'_#5r>>::AssocType: ReEarlyBound(0, 'a)`...
 
 note: External requirements
   --> $DIR/projection-one-region-trait-bound-closure.rs:87:29
@@ -200,3 +197,4 @@ LL | | }
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0309`.