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test THIR unsafeck too

Co-authored-by: Léo Lanteri Thauvin <leseulartichaut@gmail.com>

Author: pnkfelix

src/test/ui/unsafe/issue-85435-unsafe-op-in-let-under-unsafe-under-closure.rs

@@ -1,4 +1,6 @@
 // check-pass
+// revisions: mir thir
+// [thir]compile-flags: -Z thir-unsafeck
 
 // This is issue #85435. But the real story is reflected in issue #85561, where
 // a bug in the implementation of feature(capture_disjoint_fields) () was