explain STACK_THRESHOLD value

Author: tesuji

clippy_lints/src/let_arr_const.rs

@@ -48,6 +48,10 @@ impl<'tcx> LateLintPass<'tcx> for LetArrConst {
             && let Some(init) = local.init
             && !init.span.from_expansion()
         {
+            // LLVM optimizes the load of 16 byte as a single `mov`.
+            // Bigger values make more `mov` instructions generated.
+            // While changing code as this lint suggests, it becomes
+            // a single load (`lea`) of an address in `.rodata`.
             const STACK_THRESHOLD: u64 = 16;
             // lint only `if size_of(init) > STACK_THRESHOLD`
             let ty = cx.typeck_results().expr_ty(init);