Pre-allocate output contents during autofix application (#2340)

src/autofix/mod.rs

@@ -27,7 +27,7 @@ fn apply_fixes<'a>(
     fixes: impl Iterator<Item = &'a Fix>,
     locator: &'a Locator<'a>,
 ) -> (String, usize) {
-    let mut output = String::new();
+    let mut output = String::with_capacity(locator.len());
     let mut last_pos: Location = Location::new(1, 0);
     let mut applied: BTreeSet<&Fix> = BTreeSet::default();
     let mut num_fixed: usize = 0;
@@ -68,7 +68,7 @@ fn apply_fixes<'a>(
 
 /// Apply a single fix.
 pub(crate) fn apply_fix(fix: &Fix, locator: &Locator) -> String {
-    let mut output = String::new();
+    let mut output = String::with_capacity(locator.len());
 
     // Add all contents from `last_pos` to `fix.location`.
     let slice = locator.slice_source_code_range(&Range::new(Location::new(1, 0), fix.location));

src/source_code/locator.rs

@@ -139,6 +139,14 @@ impl<'a> Locator<'a> {
             &self.contents[inner_end..outer_end],
         )
     }
+
+    pub fn len(&self) -> usize {
+        self.contents.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.contents.is_empty()
+    }
 }
 
 #[cfg(test)]