Regression test for issue 85435.

Apply suggestions from code review:

1. (removing confusing comment from my test, since the comment reflects the bad undesirable behavior that is being fixed here.)

2. 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

@@ -0,0 +1,27 @@
+// 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
+// exposed to non-feature-gated code by a diagnostic changing PR that removed
+// the gating in one case.
+
+// This test is double-checking that the case of interest continues to work as
+// expected in the *absence* of that feature gate. At the time of this writing,
+// enabling the feature gate will cause this test to fail. We obviously cannot
+// stabilize that feature until it can correctly handle this test.
+
+fn main() {
+    let val: u8 = 5;
+    let u8_ptr: *const u8 = &val;
+    let _closure = || {
+        unsafe {
+            let tmp = *u8_ptr;
+            tmp
+
+            // Just dereferencing and returning directly compiles fine:
+            // *u8_ptr
+        }
+    };
+}