io: use Vec::with_capacity

fish-rust/src/io.rs

@@ -82,7 +82,7 @@ impl SeparatedBuffer {
     }
 
     /// \return the contents size.
-    pub fn size(&self) -> usize {
+    pub fn len(&self) -> usize {
         self.contents_size
     }
 
@@ -94,8 +94,7 @@ impl SeparatedBuffer {
     /// Serialize the contents to a single string, where explicitly separated elements have a
     /// newline appended.
     pub fn newline_serialized(&self) -> Vec<u8> {
-        let mut result = vec![];
-        result.reserve(self.size());
+        let mut result = Vec::with_capacity(self.len());
         for elem in &self.elements {
             result.extend_from_slice(&elem.contents);
             if elem.is_explicitly_separated() {