Use macro for wrapping += and -=

The current auto-generated += and -= implementation is hard to read, and should be replaced with += where possible. That said, we cannot auto-replace it just yet because Rust behaves differently in debug mode, therefore we should use second best approach - a macro that clearly shows intention without all the boilerplate code.

The only manual code are two macros in the src/enc/mod.rs

Use this replacement file as described in other recent PRs to replace boilerplate code.

<details>
<summary>replacement file content</summary>

```diff
@@
expression cond, expr;
@@
if cond
{
-   let _rhs = 1;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as u32);
+::wrapping_add!(expr, 1);
}

@@
expression expr;
@@
-{
-   let _rhs = 1;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as u32);
+::wrapping_add!(expr, 1);
-}

@@
expression expr;
@@
-{
-   let _rhs = 1u32;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs);
+::wrapping_add!(expr, 1);
-}

@@
expression expr;
@@
-{
-   let _rhs = 1i32;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as u32);
+::wrapping_add!(expr, 1);
-}

@@
expression expr;
@@
-{
-   let _rhs = 1;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as usize);
+::wrapping_add!(expr, 1);
-}

@@
expression inc, expr;
@@
-{
-   let _rhs = inc;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as u32);
+::wrapping_add!(expr, inc as u32);
-}

@@
expression inc, expr;
@@
-{
-   let _rhs = inc;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs);
+::wrapping_add!(expr, inc);
-}

@@
expression inc, expr;
@@
-{
-   let _rhs = inc;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as u32);
+::wrapping_add!(expr, inc as u32);
-}

@@
expression inc, expr;
@@
-{
-   let _rhs = inc;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_add(_rhs as usize);
+::wrapping_add!(expr, inc as usize);
-}

@@
expression expr;
@@
-{
-   let _rhs = 1;
-   let _lhs = &mut expr;
-   *_lhs = (*_lhs).wrapping_sub(_rhs as usize);
+::wrapping_sub!(expr, 1);
-}
```

</details>

Author: nyurik

src/enc/block_splitter.rs

@@ -448,11 +448,7 @@ fn ClusterBlocks<
         i = 0usize;
         while i < length {
             {
-                {
-                    let _rhs = 1;
-                    let _lhs = &mut block_lengths.slice_mut()[block_idx];
-                    *_lhs = (*_lhs).wrapping_add(_rhs as u32);
-                }
+                ::wrapping_add!(block_lengths.slice_mut()[block_idx], 1);
                 if i.wrapping_add(1) == length
                     || block_ids[i] as i32 != block_ids[i.wrapping_add(1)] as i32
                 {

src/enc/cluster.rs

@@ -171,11 +171,10 @@ pub fn BrotliHistogramCombine<
         let best_idx2: u32 = (pairs[0]).idx2;
         HistogramSelfAddHistogram(out, (best_idx1 as usize), (best_idx2 as usize));
         (out[(best_idx1 as usize)]).set_bit_cost((pairs[0]).cost_combo);
-        {
-            let _rhs = cluster_size[(best_idx2 as usize)];
-            let _lhs = &mut cluster_size[(best_idx1 as usize)];
-            *_lhs = (*_lhs).wrapping_add(_rhs);
-        }
+        ::wrapping_add!(
+            cluster_size[(best_idx1 as usize)],
+            cluster_size[(best_idx2 as usize)]
+        );
         for i in 0usize..symbols_size {
             if symbols[i] == best_idx2 {
                 symbols[i] = best_idx1;

src/enc/encode.rs

@@ -1737,16 +1737,8 @@ fn ChooseContextMap(
     i = 0usize;
     while i < 9usize {
         {
-            {
-                let _rhs = bigram_histo[i];
-                let _lhs = &mut monogram_histo[i.wrapping_rem(3)];
-                *_lhs = (*_lhs).wrapping_add(_rhs);
-            }
-            {
-                let _rhs = bigram_histo[i];
-                let _lhs = &mut two_prefix_histo[i.wrapping_rem(6)];
-                *_lhs = (*_lhs).wrapping_add(_rhs);
-            }
+            ::wrapping_add!(monogram_histo[i.wrapping_rem(3)], bigram_histo[i]);
+            ::wrapping_add!(two_prefix_histo[i.wrapping_rem(6)], bigram_histo[i]);
         }
         i = i.wrapping_add(1);
     }

src/enc/metablock.rs

@@ -630,11 +630,10 @@ fn BlockSplitterFinishBlock<
             xself.merge_last_count_ = 0usize;
             xself.target_block_size_ = xself.min_block_size_;
         } else {
-            {
-                let _rhs = xself.block_size_ as u32;
-                let _lhs = &mut split.lengths.slice_mut()[xself.num_blocks_.wrapping_sub(1)];
-                *_lhs = (*_lhs).wrapping_add(_rhs);
-            }
+            ::wrapping_add!(
+                split.lengths.slice_mut()[xself.num_blocks_.wrapping_sub(1)],
+                xself.block_size_ as u32
+            );
             histograms[xself.last_histogram_ix_[0]] = combined_histo[0].clone();
             xself.last_entropy_[0] = combined_entropy[0];
             if split.num_types == 1 {

src/enc/mod.rs

@@ -342,3 +342,21 @@ where
     read_err?;
     Ok(total_out.unwrap())
 }
+
+#[macro_export]
+macro_rules! wrapping_add {
+    ($expr:expr, $increment:expr) => {{
+        let _rhs = $increment;
+        let _lhs = &mut $expr;
+        *_lhs = (*_lhs).wrapping_add(_rhs);
+    }};
+}
+
+#[macro_export]
+macro_rules! wrapping_sub {
+    ($expr:expr, $increment:expr) => {{
+        let _rhs = $increment;
+        let _lhs = &mut $expr;
+        *_lhs = (*_lhs).wrapping_sub(_rhs);
+    }};
+}