Add test for issue-63952

Author: JohnTitor

src/test/ui/consts/issue-63952.rs

@@ -0,0 +1,28 @@
+// Regression test for #63952, shouldn't hang.
+
+use std::usize;
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+struct SliceRepr {
+    ptr: *const u8,
+    len: usize,
+}
+
+union SliceTransmute {
+    repr: SliceRepr,
+    slice: &'static [u8],
+}
+
+// bad slice: length too big to even exist anywhere
+const SLICE_WAY_TOO_LONG: &[u8] = unsafe { //~ ERROR: it is undefined behavior to use this value
+    SliceTransmute {
+        repr: SliceRepr {
+            ptr: &42,
+            len: usize::MAX,
+        },
+    }
+    .slice
+};
+
+fn main() {}

src/test/ui/consts/issue-63952.stderr

@@ -0,0 +1,17 @@
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/issue-63952.rs:18:1
+   |
+LL | / const SLICE_WAY_TOO_LONG: &[u8] = unsafe {
+LL | |     SliceTransmute {
+LL | |         repr: SliceRepr {
+LL | |             ptr: &42,
+...  |
+LL | |     .slice
+LL | | };
+   | |__^ invalid slice: total size is bigger than largest supported object
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.