[thin_check (rust)] Use vec::with_capacity() to avoid reallocations.

src/pdata/btree.rs

@@ -199,7 +199,7 @@ impl BTreeWalker {
         NV: NodeVisitor<V>,
         V: Unpack,
     {
-        let mut blocks = Vec::new();
+        let mut blocks = Vec::with_capacity(bs.len());
         for b in bs {
             if self.sm_inc(*b)? == 0 {
                 blocks.push(Block::new(*b));

src/pdata/space_map.rs

@@ -159,8 +159,8 @@ impl Unpack for Bitmap {
     fn unpack(data: &[u8]) -> IResult<&[u8], Self> {
         let (mut i, header) = BitmapHeader::unpack(data)?;
 
-        let mut entries = Vec::new();
         let nr_words = (BLOCK_SIZE - BitmapHeader::disk_size() as usize) / 8;
+        let mut entries = Vec::with_capacity(nr_words * 32);
         for _w in 0..nr_words {
             let (tmp, mut word) = le_u64(i)?;