Auto merge of #54032 - oli-obk:layout_scalar_ranges, r=eddyb

Add forever unstable attribute to allow specifying arbitrary scalar ranges

r? @eddyb for the first commit and @nikomatsakis for the second one

src/test/ui/consts/const-eval/zst_operand_eval.rs

@@ -0,0 +1,5 @@
+// compile-pass
+
+static ASSERT: () = [()][!(std::mem::size_of::<u32>() == 4) as usize];
+
+fn main() {}