Fix an ICE in conflict error diagnostics

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1359,7 +1359,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         }
 
         // Get closure's arguments
-        let ty::Closure(_, substs) = typeck_results.expr_ty(closure_expr).kind() else { unreachable!() };
+        let ty::Closure(_, substs) = typeck_results.expr_ty(closure_expr).kind() else { /* hir::Closure can be a generator too */ return };
         let sig = substs.as_closure().sig();
         let tupled_params =
             tcx.erase_late_bound_regions(sig.inputs().iter().next().unwrap().map_bound(|&b| b));

tests/ui/generator/issue-110929-generator-conflict-error-ice.rs

@@ -0,0 +1,12 @@
+// edition:2021
+// compile-flags: -Zdrop-tracking-mir=yes
+#![feature(generators)]
+
+fn main() {
+    let x = &mut ();
+    || {
+        let _c = || yield *&mut *x;
+        || _ = &mut *x;
+        //~^ cannot borrow `*x` as mutable more than once at a time
+    };
+}

tests/ui/generator/issue-110929-generator-conflict-error-ice.stderr

@@ -0,0 +1,18 @@
+error[E0499]: cannot borrow `*x` as mutable more than once at a time
+  --> $DIR/issue-110929-generator-conflict-error-ice.rs:9:9
+   |
+LL |         let _c = || yield *&mut *x;
+   |                  --             -- first borrow occurs due to use of `*x` in generator
+   |                  |
+   |                  first mutable borrow occurs here
+LL |         || _ = &mut *x;
+   |         ^^          -- second borrow occurs due to use of `*x` in closure
+   |         |
+   |         second mutable borrow occurs here
+LL |
+LL |     };
+   |     - first borrow might be used here, when `_c` is dropped and runs the destructor for generator
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0499`.