Don't arbitrarily choose one upper bound for hidden captured region error message

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -225,21 +225,26 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
                 // Find something that we can name
                 let upper_bound = self.approx_universal_upper_bound(vid);
-                let upper_bound = &self.definitions[upper_bound];
-                match upper_bound.external_name {
-                    Some(reg) => reg,
-                    None => {
-                        // Nothing exact found, so we pick the first one that we find.
-                        let scc = self.constraint_sccs.scc(vid);
-                        for vid in self.rev_scc_graph.as_ref().unwrap().upper_bounds(scc) {
-                            match self.definitions[vid].external_name {
-                                None => {}
-                                Some(region) if region.is_static() => {}
-                                Some(region) => return region,
-                            }
-                        }
-                        region
-                    }
+                if let Some(universal_region) = self.definitions[upper_bound].external_name {
+                    return universal_region;
+                }
+
+                // Nothing exact found, so we pick a named upper bound, if there's only one.
+                // If there's >1 universal region, then we probably are dealing w/ an intersection
+                // region which cannot be mapped back to a universal.
+                // FIXME: We could probably compute the LUB if there is one.
+                let scc = self.constraint_sccs.scc(vid);
+                let upper_bounds: Vec<_> = self
+                    .rev_scc_graph
+                    .as_ref()
+                    .unwrap()
+                    .upper_bounds(scc)
+                    .filter_map(|vid| self.definitions[vid].external_name)
+                    .filter(|r| !r.is_static())
+                    .collect();
+                match &upper_bounds[..] {
+                    [universal_region] => *universal_region,
+                    _ => region,
                 }
             }
             _ => region,

tests/ui/impl-trait/in-trait/cannot-capture-intersection.rs

@@ -0,0 +1,29 @@
+#![feature(precise_capturing)]
+
+use std::future::Future;
+use std::pin::Pin;
+
+trait MyTrait {
+    fn foo<'a, 'b>(&'a self, x: &'b i32) -> impl Future<Output = i32>;
+}
+
+trait ErasedMyTrait {
+    fn foo<'life0, 'life1, 'dynosaur>(&'life0 self, x: &'life1 i32)
+    -> Pin<Box<dyn Future<Output = i32> + 'dynosaur>>
+    where
+        'life0: 'dynosaur,
+        'life1: 'dynosaur;
+}
+
+struct DynMyTrait<T: ErasedMyTrait> {
+    ptr: T,
+}
+
+impl<T: ErasedMyTrait> MyTrait for DynMyTrait<T> {
+    fn foo<'a, 'b>(&'a self, x: &'b i32) -> impl Future<Output = i32> {
+        self.ptr.foo(x)
+        //~^ ERROR hidden type for `impl Future<Output = i32>` captures lifetime that does not appear in bounds
+    }
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/cannot-capture-intersection.stderr

@@ -0,0 +1,13 @@
+error[E0700]: hidden type for `impl Future<Output = i32>` captures lifetime that does not appear in bounds
+  --> $DIR/cannot-capture-intersection.rs:24:9
+   |
+LL |     fn foo<'a, 'b>(&'a self, x: &'b i32) -> impl Future<Output = i32> {
+   |                                             ------------------------- opaque type defined here
+LL |         self.ptr.foo(x)
+   |         ^^^^^^^^^^^^^^^
+   |
+   = note: hidden type `Pin<Box<dyn Future<Output = i32>>>` captures lifetime `'_`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unrelated.stderr

@@ -2,17 +2,12 @@ error[E0700]: hidden type for `impl Trait<'d, 'e>` captures lifetime that does n
   --> $DIR/ordinary-bounds-unrelated.rs:28:33
    |
 LL | fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'d, 'e>
-   |                     --                                                   ------------------ opaque type defined here
-   |                     |
-   |                     hidden type `Ordinary<'b>` captures the lifetime `'b` as defined here
+   |                                                                          ------------------ opaque type defined here
 ...
 LL |     if condition() { a } else { b }
    |                                 ^
    |
-help: to declare that `impl Trait<'d, 'e>` captures `'b`, you can add an explicit `'b` lifetime bound
-   |
-LL | fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'d, 'e> + 'b
-   |                                                                                             ++++
+   = note: hidden type `Ordinary<'_>` captures lifetime `'_`
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.stderr

@@ -2,17 +2,12 @@ error[E0700]: hidden type for `impl Trait<'a, 'b>` captures lifetime that does n
   --> $DIR/ordinary-bounds-unsuited.rs:31:33
    |
 LL | fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
-   |                     --                                       ------------------ opaque type defined here
-   |                     |
-   |                     hidden type `Ordinary<'b>` captures the lifetime `'b` as defined here
+   |                                                              ------------------ opaque type defined here
 ...
 LL |     if condition() { a } else { b }
    |                                 ^
    |
-help: to declare that `impl Trait<'a, 'b>` captures `'b`, you can add an explicit `'b` lifetime bound
-   |
-LL | fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> + 'b
-   |                                                                                 ++++
+   = note: hidden type `Ordinary<'_>` captures lifetime `'_`
 
 error: aborting due to 1 previous error
 

tests/ui/suggestions/lifetimes/explicit-lifetime-suggestion-in-proper-span-issue-121267.stderr

@@ -2,18 +2,13 @@ error[E0700]: hidden type for `impl Iterator<Item = i32>` captures lifetime that
   --> $DIR/explicit-lifetime-suggestion-in-proper-span-issue-121267.rs:7:5
    |
 LL |   fn bar(src: &crate::Foo) -> impl Iterator<Item = i32> {
-   |                ----------     ------------------------- opaque type defined here
-   |                |
-   |                hidden type `FilterMap<std::slice::Iter<'static, i32>, {closure@$DIR/explicit-lifetime-suggestion-in-proper-span-issue-121267.rs:9:21: 9:24}>` captures the anonymous lifetime defined here
+   |                               ------------------------- opaque type defined here
 LL | /     [0].into_iter()
 LL | |
 LL | |         .filter_map(|_| foo(src))
    | |_________________________________^
    |
-help: to declare that `impl Iterator<Item = i32>` captures `'_`, you can introduce a named lifetime parameter `'a`
-   |
-LL | fn bar<'a>(src: &'a crate::Foo<'a>) -> impl Iterator<Item = i32> + 'a  {
-   |       ++++       ++           ++++                               ++++
+   = note: hidden type `FilterMap<std::slice::Iter<'static, i32>, {closure@$DIR/explicit-lifetime-suggestion-in-proper-span-issue-121267.rs:9:21: 9:24}>` captures lifetime `'_`
 
 error: aborting due to 1 previous error