derive: less string escaping

```text
$ cd rinja_derive_standalone
$ cargo bench

hello_world             time:   [50.420 µs 50.529 µs 50.637 µs]
                        change: [-0.6238% -0.3448% -0.0695%] (p = 0.02 < 0.05)
                        Change within noise threshold.

item_info.html          time:   [58.807 µs 59.108 µs 59.408 µs]
                        change: [-2.4731% -2.0541% -1.6532%] (p = 0.00 < 0.05)
                        Performance has improved.

item_union.html         time:   [182.90 µs 183.97 µs 185.73 µs]
                        change: [-2.6740% -1.8652% -0.7734%] (p = 0.00 < 0.05)
                        Change within noise threshold.

page.html               time:   [767.05 µs 768.05 µs 769.20 µs]
                        change: [-4.5916% -4.2764% -3.9026%] (p = 0.00 < 0.05)
                        Performance has improved.

print_item.html         time:   [154.97 µs 155.26 µs 155.59 µs]
                        change: [-2.0385% -1.7364% -1.4351%] (p = 0.00 < 0.05)
                        Performance has improved.

short_item_info.html    time:   [126.35 µs 126.49 µs 126.67 µs]
                        change: [-6.5708% -6.1935% -5.8314%] (p = 0.00 < 0.05)
                        Performance has improved.

sidebar.html            time:   [196.25 µs 196.68 µs 197.14 µs]
                        change: [-4.9096% -4.4107% -3.9467%] (p = 0.00 < 0.05)
                        Performance has improved.

source.html             time:   [112.69 µs 113.14 µs 113.76 µs]
                        change: [-3.4552% -3.1557% -2.8072%] (p = 0.00 < 0.05)
                        Performance has improved.

type_layout.html        time:   [160.05 µs 160.30 µs 160.59 µs]
                        change: [-11.126% -10.419% -9.2877%] (p = 0.00 < 0.05)
                        Performance has improved.

type_layout_size.html   time:   [66.006 µs 66.340 µs 66.664 µs]
                        change: [-2.3099% -1.3973% +0.0223%] (p = 0.01 < 0.05)
                        Change within noise threshold.
```

rinja_derive/Cargo.toml

@@ -27,6 +27,7 @@ with-warp = []
 [dependencies]
 parser = { package = "rinja_parser", version = "0.2.0", path = "../rinja_parser" }
 basic-toml = { version = "0.1.1", optional = true }
+memchr = "2"
 mime = "0.3"
 mime_guess = "2"
 once_map = "0.4.18"

rinja_derive/src/generator.rs

@@ -1950,19 +1950,19 @@ impl Buffer {
     }
 
     fn write_writer(&mut self, s: &str) -> usize {
-        if self.discard {
-            // nothing to do
-        } else if !self.last_was_write_str {
-            write!(self.buf, "writer.write_str({s:#?})?;").unwrap();
-            self.last_was_write_str = true;
-        } else {
-            // strip trailing `")?;`, leaving an unterminated string
-            let len = self.buf.strip_suffix("\")?;").unwrap().len();
-            self.buf.truncate(len);
-            // append the new string, adding a stray `"` in the mid of the string
-            write!(self.buf, "{s:#?})?;").unwrap();
-            // left shift new string by one to overwrite the stray `"`
-            self.buf.replace_range(len..=len, "");
+        const OPEN: &str = r#"writer.write_str(""#;
+        const CLOSE: &str = r#"")?;"#;
+
+        if !self.discard {
+            if !self.last_was_write_str {
+                self.last_was_write_str = true;
+                self.buf.push_str(OPEN);
+            } else {
+                // strip trailing `")?;`, leaving an unterminated string
+                self.buf.truncate(self.buf.len() - CLOSE.len())
+            }
+            string_escape(&mut self.buf, s);
+            self.buf.push_str(CLOSE);
         }
         s.len()
     }
@@ -2278,3 +2278,26 @@ fn normalize_identifier(ident: &str) -> &str {
     // SAFETY: We know that the input byte slice is pure-ASCII.
     unsafe { std::str::from_utf8_unchecked(&replacement[..ident.len() + 2]) }
 }
+
+/// Similar to `write!(dest, "{src:?}")`, but only escapes the strictly needed characters,
+/// and without the surrounding `"…"` quotation marks.
+pub(crate) fn string_escape(dest: &mut String, src: &str) {
+    // SAFETY: we will only push valid str slices
+    let dest = unsafe { dest.as_mut_vec() };
+    let src = src.as_bytes();
+    let mut last = 0;
+
+    // According to <https://doc.rust-lang.org/reference/tokens.html#string-literals>, every
+    // character is valid except `" \ IsolatedCR`. We don't test if the `\r` is isolated or not,
+    // but always escape it.
+    for x in memchr::memchr3_iter(b'\\', b'"', b'\r', src) {
+        dest.extend(&src[last..x]);
+        dest.extend(match src[x] {
+            b'\\' => br#"\\"#,
+            b'\"' => br#"\""#,
+            _ => br#"\r"#,
+        });
+        last = x + 1;
+    }
+    dest.extend(&src[last..]);
+}

rinja_derive_standalone/Cargo.toml

@@ -26,6 +26,7 @@ with-warp = []
 [dependencies]
 parser = { package = "rinja_parser", version = "0.2.0", path = "../rinja_parser" }
 basic-toml = { version = "0.1.1", optional = true }
+memchr = "2"
 mime = "0.3"
 mime_guess = "2"
 once_map = "0.4.18"