Handle ReVar in note_and_explain_region.

PR #124918 made this path abort. The added test, from fuzzing,
identified that it is reachable.

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -173,7 +173,10 @@ pub(super) fn note_and_explain_region<'tcx>(
 
         ty::ReError(_) => return,
 
-        ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => {
+        // FIXME(#125431): `ReVar` shouldn't reach here.
+        ty::ReVar(_) => (format!("lifetime `{region}`"), alt_span),
+
+        ty::ReBound(..) | ty::ReErased => {
             bug!("unexpected region for note_and_explain_region: {:?}", region);
         }
     };

tests/ui/inference/note-and-explain-ReVar-124973.rs

@@ -0,0 +1,8 @@
+//@ edition:2018
+
+#![feature(c_variadic)]
+
+async unsafe extern "C" fn multiple_named_lifetimes<'a, 'b>(_: u8, ...) {}
+//~^ ERROR hidden type for `impl Future<Output = ()>` captures lifetime that does not appear in bounds
+
+fn main() {}

tests/ui/inference/note-and-explain-ReVar-124973.stderr

@@ -0,0 +1,13 @@
+error[E0700]: hidden type for `impl Future<Output = ()>` captures lifetime that does not appear in bounds
+  --> $DIR/note-and-explain-ReVar-124973.rs:5:73
+   |
+LL | async unsafe extern "C" fn multiple_named_lifetimes<'a, 'b>(_: u8, ...) {}
+   | ----------------------------------------------------------------------- ^^
+   | |
+   | opaque type defined here
+   |
+   = note: hidden type `{async fn body of multiple_named_lifetimes<'a, 'b>()}` captures lifetime `'_`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0700`.