fix: add CoroutineClosure to describe_field_from_ty for capture names

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -526,7 +526,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 ty::Array(ty, _) | ty::Slice(ty) => {
                     self.describe_field_from_ty(ty, field, variant_index, including_tuple_field)
                 }
-                ty::Closure(def_id, _) | ty::Coroutine(def_id, _) => {
+                ty::Closure(def_id, _) | ty::CoroutineClosure(def_id, _) | ty::Coroutine(def_id, _) => {
                     // We won't be borrowck'ing here if the closure came from another crate,
                     // so it's safe to call `expect_local`.
                     //

tests/ui/async-await/async-closures/borrowck-describe-field.rs

@@ -0,0 +1,19 @@
+// Regression test for #478: describe_field_from_ty missing CoroutineClosure.
+// Before the fix, borrow errors involving async closure captures would show
+// "field 0" instead of the captured variable name, because the CoroutineClosure
+// type was not recognized and fell through to the default arm.
+
+//@ edition:2021
+
+fn main() {
+    let mut x = String::from("hello");
+
+    let c = async || {
+        x.push_str(" world"); //~ ERROR
+    };
+
+    // Immutable borrow of x while async closure holds mutable borrow
+    println!("{}", x); //~ ERROR
+
+    let _ = c;
+}