From 5f4424bfaf6fc9c66df6e19a3f5e96b0fc9a1441 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Mon, 13 May 2024 08:59:02 +1000
Subject: [PATCH] Handle `ReVar` in `note_and_explain_region`.

PR #124918 made this path abort. The added test, from fuzzing,
identified that it is reachable.
---
 .../rustc_infer/src/infer/error_reporting/mod.rs    |  5 ++++-
 tests/ui/inference/note-and-explain-ReVar-124973.rs |  8 ++++++++
 .../inference/note-and-explain-ReVar-124973.stderr  | 13 +++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/inference/note-and-explain-ReVar-124973.rs
 create mode 100644 tests/ui/inference/note-and-explain-ReVar-124973.stderr

diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 46a7e7b2399..e0894ed31bf 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/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);
         }
     };
diff --git a/tests/ui/inference/note-and-explain-ReVar-124973.rs b/tests/ui/inference/note-and-explain-ReVar-124973.rs
new file mode 100644
index 00000000000..f1e24645636
--- /dev/null
+++ b/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() {}
diff --git a/tests/ui/inference/note-and-explain-ReVar-124973.stderr b/tests/ui/inference/note-and-explain-ReVar-124973.stderr
new file mode 100644
index 00000000000..574f6508e4c
--- /dev/null
+++ b/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`.