modify leak-check to track only outgoing edges from placeholders

Also, update the affected tests. This seems strictly better but it is
actually more permissive than I initially intended. In particular it
accepts this

```
forall<'a, 'b> {
  exists<'intersection> {
    'a: 'intersection,
    'b: 'intersection,
  }
}
```

and I'm not sure I want to accept that. It implies that we have a
`'empty` in the new universe intoduced by the `forall`.

src/librustc_infer/infer/region_constraints/leak_check.rs

@@ -42,10 +42,23 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
                 _ => bug!("leak_check: expected placeholder found {:?}", placeholder_region,),
             };
 
-            // Find all regions that are related to this placeholder
-            // in some way. This means any region that either outlives
-            // or is outlived by a placeholder.
-            let mut taint_set = TaintSet::new(TaintDirections::both(), placeholder_region);
+            // Find all regions that this placeholder `!p` must outlive -- i.e.,
+            // any region `r` where `!p: r` must hold. It is an error if any
+            // such region `r` is another placeholder or in a universe that
+            // can't see the placeholder. (This is actually incorrect, because
+            // we don't take into account the possibility of bounds in
+            // environment that tell us that the placeholder may be related to
+            // other regions).
+            //
+            // Note that we *don't* look for cases like `r: !p`. This is
+            // because:
+            //
+            // * If `r` is some other placeholder `!p1`, then we'll find the
+            //   error when we search the regions that `!p1` must outlive.
+            // * If `r` is a variable in some outer universe, then it can
+            //   potentially be assigned to `'static`, so this relation could
+            //   hold.
+            let mut taint_set = TaintSet::new(TaintDirections::outgoing(), placeholder_region);
             taint_set.fixed_point(
                 tcx,
                 self.undo_log.region_constraints(),

src/test/ui/closure-expected-type/expect-fn-supply-fn.rs

@@ -28,14 +28,14 @@ fn expect_free_supply_bound() {
     // Here, we are given a function whose region is bound at closure level,
     // but we expect one bound in the argument. Error results.
     with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
-    //~^ ERROR type mismatch
+    //~^ ERROR mismatched types
 }
 
 fn expect_bound_supply_free_from_fn<'x>(x: &'x u32) {
     // Here, we are given a `fn(&u32)` but we expect a `fn(&'x
     // u32)`. In principle, this could be ok, but we demand equality.
     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
-    //~^ ERROR type mismatch
+    //~^ ERROR mismatched types
 }
 
 fn expect_bound_supply_free_from_closure() {
@@ -44,7 +44,7 @@ fn expect_bound_supply_free_from_closure() {
     // the argument level.
     type Foo<'a> = fn(&'a u32);
     with_closure_expecting_fn_with_bound_region(|x: Foo<'_>, y| {
-    //~^ ERROR type mismatch
+    //~^ ERROR mismatched types
     });
 }
 

src/test/ui/closure-expected-type/expect-fn-supply-fn.stderr

@@ -36,46 +36,33 @@ note: ...does not necessarily outlive the anonymous lifetime #2 defined on the b
 LL |     with_closure_expecting_fn_with_free_region(|x: fn(&'x u32), y| {});
    |                                                ^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0631]: type mismatch in closure arguments
-  --> $DIR/expect-fn-supply-fn.rs:30:5
+error[E0308]: mismatched types
+  --> $DIR/expect-fn-supply-fn.rs:30:52
    |
-LL | fn with_closure_expecting_fn_with_free_region<F>(_: F)
-   |    ------------------------------------------ required by a bound in this
-LL |     where F: for<'a> FnOnce(fn(&'a u32), &i32)
-   |                      ------------------------- required by this bound in `with_closure_expecting_fn_with_free_region`
-...
 LL |     with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ---------------- found signature of `fn(for<'r> fn(&'r u32), _) -> _`
-   |     |
-   |     expected signature of `fn(fn(&'a u32), &i32) -> _`
+   |                                                    ^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `fn(&u32)`
+              found fn pointer `for<'r> fn(&'r u32)`
 
-error[E0631]: type mismatch in closure arguments
-  --> $DIR/expect-fn-supply-fn.rs:37:5
+error[E0308]: mismatched types
+  --> $DIR/expect-fn-supply-fn.rs:37:53
    |
-LL | fn with_closure_expecting_fn_with_bound_region<F>(_: F)
-   |    ------------------------------------------- required by a bound in this
-LL |     where F: FnOnce(fn(&u32), &i32)
-   |              ---------------------- required by this bound in `with_closure_expecting_fn_with_bound_region`
-...
 LL |     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ------------------- found signature of `fn(fn(&'x u32), _) -> _`
-   |     |
-   |     expected signature of `fn(for<'r> fn(&'r u32), &i32) -> _`
+   |                                                     ^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r> fn(&'r u32)`
+              found fn pointer `fn(&'x u32)`
 
-error[E0631]: type mismatch in closure arguments
-  --> $DIR/expect-fn-supply-fn.rs:46:5
+error[E0308]: mismatched types
+  --> $DIR/expect-fn-supply-fn.rs:46:53
    |
-LL | fn with_closure_expecting_fn_with_bound_region<F>(_: F)
-   |    ------------------------------------------- required by a bound in this
-LL |     where F: FnOnce(fn(&u32), &i32)
-   |              ---------------------- required by this bound in `with_closure_expecting_fn_with_bound_region`
-...
 LL |     with_closure_expecting_fn_with_bound_region(|x: Foo<'_>, y| {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ --------------- found signature of `for<'r> fn(fn(&'r u32), _) -> _`
-   |     |
-   |     expected signature of `fn(for<'r> fn(&'r u32), &i32) -> _`
+   |                                                     ^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r> fn(&'r u32)`
+              found fn pointer `fn(&u32)`
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0308, E0631.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/hr-subtype/hr-subtype.bound_a_b_vs_bound_a.stderr

@@ -1,17 +1,14 @@
-error[E0308]: mismatched types
-  --> $DIR/hr-subtype.rs:39:26
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/hr-subtype.rs:96:1
    |
-LL |               gimme::<$t1>(None::<$t2>);
-   |                            ^^^^^^^^^^^ expected concrete lifetime, found bound lifetime parameter 'a
-...
-LL | / check! { bound_a_b_vs_bound_a: (for<'a,'b> fn(&'a u32, &'b u32),
-LL | |                                 for<'a>    fn(&'a u32, &'a u32)) }
-   | |__________________________________________________________________- in this macro invocation
-   |
-   = note: expected enum `std::option::Option<for<'a, 'b> fn(&'a u32, &'b u32)>`
-              found enum `std::option::Option<for<'a> fn(&'a u32, &'a u32)>`
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/hr-subtype/hr-subtype.bound_a_vs_bound_a.stderr

@@ -1,11 +1,11 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/hr-subtype.rs:100:1
+  --> $DIR/hr-subtype.rs:96:1
    |
 LL | / fn main() {
 LL | |
 LL | |
 LL | |
-LL | |
+...  |
 LL | |
 LL | | }
    | |_^

src/test/ui/hr-subtype/hr-subtype.bound_a_vs_bound_b.stderr

@@ -1,11 +1,11 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/hr-subtype.rs:100:1
+  --> $DIR/hr-subtype.rs:96:1
    |
 LL | / fn main() {
 LL | |
 LL | |
 LL | |
-LL | |
+...  |
 LL | |
 LL | | }
    | |_^

src/test/ui/hr-subtype/hr-subtype.bound_a_vs_free_x.stderr

@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/hr-subtype.rs:39:26
    |
 LL |               gimme::<$t1>(None::<$t2>);
-   |                            ^^^^^^^^^^^ expected concrete lifetime, found bound lifetime parameter 'a
+   |                            ^^^^^^^^^^^ one type is more general than the other
 ...
 LL | / check! { bound_a_vs_free_x: (for<'a> fn(&'a u32),
 LL | |                              fn(&'x u32)) }

src/test/ui/hr-subtype/hr-subtype.bound_co_a_b_vs_bound_co_a.stderr

@@ -1,17 +1,14 @@
-error[E0308]: mismatched types
-  --> $DIR/hr-subtype.rs:39:26
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/hr-subtype.rs:96:1
    |
-LL |               gimme::<$t1>(None::<$t2>);
-   |                            ^^^^^^^^^^^ expected concrete lifetime, found bound lifetime parameter 'a
-...
-LL | / check! { bound_co_a_b_vs_bound_co_a: (for<'a,'b> fn(Co<'a>, Co<'b>),
-LL | |                                       for<'a>    fn(Co<'a>, Co<'a>)) }
-   | |______________________________________________________________________- in this macro invocation
-   |
-   = note: expected enum `std::option::Option<for<'a, 'b> fn(Co<'a>, Co<'b>)>`
-              found enum `std::option::Option<for<'a> fn(Co<'a>, Co<'a>)>`
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/hr-subtype/hr-subtype.bound_co_a_co_b_ret_contra_a.stderr

@@ -1,17 +1,14 @@
-error[E0308]: mismatched types
-  --> $DIR/hr-subtype.rs:39:26
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/hr-subtype.rs:96:1
    |
-LL |               gimme::<$t1>(None::<$t2>);
-   |                            ^^^^^^^^^^^ expected concrete lifetime, found bound lifetime parameter 'a
-...
-LL | / check! { bound_co_a_co_b_ret_contra_a: (for<'a,'b> fn(Co<'a>, Co<'b>) -> Contra<'a>,
-LL | |                                         for<'a>    fn(Co<'a>, Co<'a>) -> Contra<'a>) }
-   | |______________________________________________________________________________________- in this macro invocation
-   |
-   = note: expected enum `std::option::Option<for<'a, 'b> fn(Co<'a>, Co<'b>) -> Contra<'a>>`
-              found enum `std::option::Option<for<'a> fn(Co<'a>, Co<'a>) -> Contra<'a>>`
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/hr-subtype/hr-subtype.bound_co_a_vs_bound_co_b.stderr

@@ -1,11 +1,11 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/hr-subtype.rs:100:1
+  --> $DIR/hr-subtype.rs:96:1
    |
 LL | / fn main() {
 LL | |
 LL | |
 LL | |
-LL | |
+...  |
 LL | |
 LL | | }
    | |_^

src/test/ui/hr-subtype/hr-subtype.bound_contra_a_contra_b_ret_co_a.stderr

@@ -1,17 +1,14 @@
-error[E0308]: mismatched types
-  --> $DIR/hr-subtype.rs:39:26
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/hr-subtype.rs:96:1
    |
-LL |               gimme::<$t1>(None::<$t2>);
-   |                            ^^^^^^^^^^^ expected concrete lifetime, found bound lifetime parameter 'a
-...
-LL | / check! { bound_contra_a_contra_b_ret_co_a: (for<'a,'b> fn(Contra<'a>, Contra<'b>) -> Co<'a>,
-LL | |                                             for<'a>    fn(Contra<'a>, Contra<'a>) -> Co<'a>) }
-   | |______________________________________________________________________________________________- in this macro invocation
-   |
-   = note: expected enum `std::option::Option<for<'a, 'b> fn(Contra<'a>, Contra<'b>) -> Co<'a>>`
-              found enum `std::option::Option<for<'a> fn(Contra<'a>, Contra<'a>) -> Co<'a>>`
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/hr-subtype/hr-subtype.bound_inv_a_vs_bound_inv_b.stderr

@@ -1,11 +1,11 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/hr-subtype.rs:100:1
+  --> $DIR/hr-subtype.rs:96:1
    |
 LL | / fn main() {
 LL | |
 LL | |
 LL | |
-LL | |
+...  |
 LL | |
 LL | | }
    | |_^

src/test/ui/hr-subtype/hr-subtype.free_x_vs_free_x.stderr

@@ -1,11 +1,11 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/hr-subtype.rs:100:1
+  --> $DIR/hr-subtype.rs:96:1
    |
 LL | / fn main() {
 LL | |
 LL | |
 LL | |
-LL | |
+...  |
 LL | |
 LL | | }
    | |_^

src/test/ui/hr-subtype/hr-subtype.rs

@@ -42,10 +42,6 @@ macro_rules! check {
             //[bound_inv_a_b_vs_bound_inv_a]~^^^ ERROR
             //[bound_a_b_ret_a_vs_bound_a_ret_a]~^^^^ ERROR
             //[free_inv_x_vs_free_inv_y]~^^^^^ ERROR
-            //[bound_a_b_vs_bound_a]~^^^^^^ ERROR mismatched types
-            //[bound_co_a_co_b_ret_contra_a]~^^^^^^^ ERROR
-            //[bound_contra_a_contra_b_ret_co_a]~^^^^^^^^ ERROR
-            //[bound_co_a_b_vs_bound_co_a]~^^^^^^^^^ ERROR
         }
     }
 }
@@ -103,4 +99,8 @@ fn main() {
 //[bound_inv_a_vs_bound_inv_b]~^^^ ERROR fatal error triggered by #[rustc_error]
 //[bound_co_a_vs_bound_co_b]~^^^^ ERROR fatal error triggered by #[rustc_error]
 //[free_x_vs_free_x]~^^^^^ ERROR fatal error triggered by #[rustc_error]
+//[bound_a_b_vs_bound_a]~^^^^^^ ERROR fatal error triggered by #[rustc_error]
+//[bound_co_a_b_vs_bound_co_a]~^^^^^^^ ERROR fatal error triggered by #[rustc_error]
+//[bound_co_a_co_b_ret_contra_a]~^^^^^^^^ ERROR fatal error triggered by #[rustc_error]
+//[bound_contra_a_contra_b_ret_co_a]~^^^^^^^^^ ERROR fatal error triggered by #[rustc_error]
 }

src/test/ui/hrtb/hrtb-exists-forall-fn.stderr

@@ -2,9 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/hrtb-exists-forall-fn.rs:17:34
    |
 LL |     let _: for<'b> fn(&'b u32) = foo();
-   |            -------------------   ^^^^^ expected concrete lifetime, found bound lifetime parameter 'b
-   |            |
-   |            expected due to this
+   |                                  ^^^^^ one type is more general than the other
    |
    = note: expected fn pointer `for<'b> fn(&'b u32)`
               found fn pointer `fn(&u32)`

src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.rs

@@ -32,5 +32,5 @@ fn main() {
     // NB. *However*, the reinstated leak-check gives an error here.
 
     foo::<()>();
-    //~^ ERROR not satisfied
+    //~^ ERROR implementation of `Trait` is not general enough
 }

src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.stderr

@@ -1,18 +1,14 @@
-error[E0277]: the trait bound `(): Trait<for<'b> fn(&'b u32)>` is not satisfied
-  --> $DIR/hrtb-exists-forall-trait-contravariant.rs:34:11
+error: implementation of `Trait` is not general enough
+  --> $DIR/hrtb-exists-forall-trait-contravariant.rs:34:5
    |
-LL | fn foo<T>()
-   |    --- required by a bound in this
-LL | where
-LL |     T: Trait<for<'b> fn(&'b u32)>,
-   |        -------------------------- required by this bound in `foo`
+LL | trait Trait<T> {}
+   | ----------------- trait `Trait` defined here
 ...
 LL |     foo::<()>();
-   |           ^^ the trait `Trait<for<'b> fn(&'b u32)>` is not implemented for `()`
+   |     ^^^^^^^^^ implementation of `Trait` is not general enough
    |
-   = help: the following implementations were found:
-             <() as Trait<fn(&'a u32)>>
+   = note: `()` must implement `Trait<for<'b> fn(&'b u32)>`
+   = note: ...but `()` actually implements `Trait<fn(&'0 u32)>`, for some specific lifetime `'0`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.