use splice to optimize O(m*n) to O(m+n) in StyledBuffer::prepend

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

compiler/rustc_errors/src/styled_buffer.rs

@@ -108,12 +108,8 @@ impl StyledBuffer {
         self.ensure_lines(line);
         let string_len = string.chars().count();
 
-        if !self.lines[line].is_empty() {
-            // Push the old content over to make room for new content
-            for _ in 0..string_len {
-                self.lines[line].insert(0, StyledChar::SPACE);
-            }
-        }
+        // Push the old content over to make room for new content
+        self.lines[line].splice(0..0, std::iter::repeat(StyledChar::SPACE).take(string_len));
 
         self.puts(line, 0, string, style);
     }