From 2d4bb53e743e0ac64d1d603866f9448decac2a1e Mon Sep 17 00:00:00 2001
From: Aatif Syed <aatif@lo.tech>
Date: Mon, 8 Dec 2025 12:53:56 +0000
Subject: [PATCH] feat: Renderer::render_to(impl Write, ..)

---
 src/renderer/mod.rs           | 11 ++++++++++-
 src/renderer/render.rs        | 28 ++++++++++++++++------------
 src/renderer/styled_buffer.rs | 14 +++++++-------
 3 files changed, 33 insertions(+), 20 deletions(-)

diff --git a/src/renderer/mod.rs b/src/renderer/mod.rs
index eaab6329..b5b7f740 100644
--- a/src/renderer/mod.rs
+++ b/src/renderer/mod.rs
@@ -25,6 +25,8 @@ pub(crate) mod stylesheet;
 mod margin;
 mod styled_buffer;
 
+use std::fmt;
+
 use crate::Report;
 
 pub(crate) use render::normalize_whitespace;
@@ -193,7 +195,14 @@ impl Renderer {
 impl Renderer {
     /// Render a diagnostic [`Report`]
     pub fn render(&self, groups: Report<'_>) -> String {
-        render::render(self, groups)
+        let mut s = String::new();
+        self.render_to(&mut s, groups)
+            .expect("String::write_fmt is infallible");
+        s
+    }
+    /// Render a diagnostic [`Report`] to the given [`Write`](fmt::Write)r
+    pub fn render_to<W: fmt::Write>(&self, to: W, groups: Report<'_>) -> fmt::Result {
+        render::render_to(to, self, groups)
     }
 }
 
diff --git a/src/renderer/render.rs b/src/renderer/render.rs
index 283fe770..71bf8d9b 100644
--- a/src/renderer/render.rs
+++ b/src/renderer/render.rs
@@ -24,9 +24,13 @@ use crate::{
 
 const ANONYMIZED_LINE_NUM: &str = "LL";
 
-pub(crate) fn render(renderer: &Renderer, groups: Report<'_>) -> String {
+pub(crate) fn render_to(
+    mut to: impl fmt::Write,
+    renderer: &Renderer,
+    groups: Report<'_>,
+) -> fmt::Result {
     if renderer.short_message {
-        render_short_message(renderer, groups).unwrap()
+        render_short_message(to, renderer, groups)
     } else {
         let (max_line_num, og_primary_path, groups) = pre_process(groups);
         let max_line_num_len = if renderer.anonymized_line_numbers {
@@ -34,7 +38,6 @@ pub(crate) fn render(renderer: &Renderer, groups: Report<'_>) -> String {
         } else {
             num_decimal_digits(max_line_num)
         };
-        let mut out_string = String::new();
         let group_len = groups.len();
         for (
             g,
@@ -220,19 +223,21 @@ pub(crate) fn render(renderer: &Renderer, groups: Report<'_>) -> String {
                 }
             }
             buffer
-                .render(&level, &renderer.stylesheet, &mut out_string)
+                .render_to(&mut to, &level, &renderer.stylesheet)
                 .unwrap();
             if g != group_len - 1 {
-                use std::fmt::Write;
-
-                writeln!(out_string).unwrap();
+                writeln!(to).unwrap();
             }
         }
-        out_string
+        Ok(())
     }
 }
 
-fn render_short_message(renderer: &Renderer, groups: &[Group<'_>]) -> Result<String, fmt::Error> {
+fn render_short_message(
+    to: impl fmt::Write,
+    renderer: &Renderer,
+    groups: &[Group<'_>],
+) -> fmt::Result {
     let mut buffer = StyledBuffer::new();
     let mut labels = None;
     let group = groups.first().expect("Expected at least one group");
@@ -306,10 +311,9 @@ fn render_short_message(renderer: &Renderer, groups: &[Group<'_>]) -> Result<Str
         buffer.append(0, &format!(": {labels}"), ElementStyle::NoStyle);
     }
 
-    let mut out_string = String::new();
-    buffer.render(&title.level, &renderer.stylesheet, &mut out_string)?;
+    buffer.render_to(to, &title.level, &renderer.stylesheet)?;
 
-    Ok(out_string)
+    Ok(())
 }
 
 #[allow(clippy::too_many_arguments)]
diff --git a/src/renderer/styled_buffer.rs b/src/renderer/styled_buffer.rs
index b64aef9c..2d49946a 100644
--- a/src/renderer/styled_buffer.rs
+++ b/src/renderer/styled_buffer.rs
@@ -39,11 +39,11 @@ impl StyledBuffer {
         }
     }
 
-    pub(crate) fn render(
+    pub(crate) fn render_to(
         &self,
+        mut to: impl Write,
         level: &Level<'_>,
         stylesheet: &Stylesheet,
-        str: &mut String,
     ) -> Result<(), fmt::Error> {
         for (i, line) in self.lines.iter().enumerate() {
             let mut current_style = stylesheet.none;
@@ -51,16 +51,16 @@ impl StyledBuffer {
                 let ch_style = style.color_spec(level, stylesheet);
                 if ch_style != current_style {
                     if !line.is_empty() {
-                        write!(str, "{current_style:#}")?;
+                        write!(to, "{current_style:#}")?;
                     }
                     current_style = ch_style;
-                    write!(str, "{current_style}")?;
+                    write!(to, "{current_style}")?;
                 }
-                write!(str, "{ch}")?;
+                write!(to, "{ch}")?;
             }
-            write!(str, "{current_style:#}")?;
+            write!(to, "{current_style:#}")?;
             if i != self.lines.len() - 1 {
-                writeln!(str)?;
+                writeln!(to)?;
             }
         }
         Ok(())