add test by @eddyb

Author: RalfJung

tests/compile-fail/branchless-select-i128-pointer.rs

@@ -0,0 +1,20 @@
+use std::mem::transmute;
+
+#[cfg(target_pointer_width = "32")]
+type TwoPtrs = i64;
+#[cfg(target_pointer_width = "64")]
+type TwoPtrs = i128;
+
+fn main() {
+    for &my_bool in &[true, false] {
+        let mask = -(my_bool as TwoPtrs); // false -> 0, true -> -1 aka !0
+        // This is branchless code to select one or the other pointer.
+        // For now, Miri brafs on it, but if this code ever passes we better make sure it behaves correctly.
+        let val = unsafe {
+            transmute::<_, &str>(
+                !mask & transmute::<_, TwoPtrs>("false !") | mask & transmute::<_, TwoPtrs>("true !"), //~ERROR encountered (potentially part of) a pointer, but expected plain (non-pointer) bytes
+            )
+        };
+        println!("{}", val);
+    }
+}