add zero allocation indenting function

src/render.rs

@@ -882,7 +882,7 @@ pub fn indent_aware_write(
     }
 
     if let Some(indent) = rc.get_indent_string() {
-        out.write(support::str::with_indent(v, indent).as_ref())?;
+        support::str::write_indented(v, indent, out)?;
     } else {
         out.write(v.as_ref())?;
     }

src/support.rs

@@ -1,6 +1,8 @@
 pub mod str {
     use std::io::{Result, Write};
 
+    use crate::Output;
+
     #[derive(Debug)]
     pub struct StringWriter {
         buf: Vec<u8>,
@@ -70,6 +72,24 @@ pub mod str {
         output
     }
 
+    /// like `with_indent`, but writing straight into the output
+    pub fn write_indented(s: &str, indent: &str, w: &mut dyn Output) -> std::io::Result<()> {
+        let mut i = 0;
+        let len = s.len();
+        loop {
+            let Some(next_newline) = s[i..].find('\n') else {
+                w.write(&s[i..])?;
+                return Ok(());
+            };
+            w.write(&s[i..i + next_newline + 1])?;
+            i += next_newline + 1;
+            if i == len {
+                return Ok(());
+            }
+            w.write(indent)?;
+        }
+    }
+
     #[inline]
     pub(crate) fn whitespace_matcher(c: char) -> bool {
         c == ' ' || c == '\t'