Introduce SourceCodeSlice to reduce the size of FormatElement (#4622)

Introduce `SourceCodeSlice` to reduce the size of `FormatElement`

crates/ruff_formatter/src/source_code.rs

@@ -0,0 +1,81 @@
+use ruff_text_size::TextRange;
+use std::fmt::{Debug, Formatter};
+
+/// The source code of a document that gets formatted
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Default)]
+pub struct SourceCode<'a> {
+    text: &'a str,
+}
+
+impl<'a> SourceCode<'a> {
+    pub fn new(text: &'a str) -> Self {
+        Self { text }
+    }
+
+    pub fn slice(self, range: TextRange) -> SourceCodeSlice {
+        assert!(
+            usize::from(range.end()) <= self.text.len(),
+            "Range end {:?} out of bounds {}.",
+            range.end(),
+            self.text.len()
+        );
+
+        assert!(
+            self.text.is_char_boundary(usize::from(range.start())),
+            "The range start position {:?} is not a char boundary.",
+            range.start()
+        );
+
+        assert!(
+            self.text.is_char_boundary(usize::from(range.end())),
+            "The range end position {:?} is not a char boundary.",
+            range.end()
+        );
+
+        SourceCodeSlice {
+            range,
+            #[cfg(debug_assertions)]
+            text: String::from(&self.text[range]).into_boxed_str(),
+        }
+    }
+}
+
+impl Debug for SourceCode<'_> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.debug_tuple("SourceCode").field(&self.text).finish()
+    }
+}
+
+/// A slice into the source text of a document.
+///
+/// It only stores the range in production builds for a more compact representation, but it
+/// keeps the original text in debug builds for better developer experience.
+#[derive(Clone, Eq, PartialEq)]
+pub struct SourceCodeSlice {
+    range: TextRange,
+    #[cfg(debug_assertions)]
+    text: Box<str>,
+}
+
+impl SourceCodeSlice {
+    /// Returns the slice's text.
+    pub fn text<'a>(&self, code: SourceCode<'a>) -> &'a str {
+        assert!(usize::from(self.range.end()) <= code.text.len(), "The range of this slice is out of bounds. Did you provide the correct source code for this slice?");
+        &code.text[self.range]
+    }
+
+    pub fn range(&self) -> TextRange {
+        self.range
+    }
+}
+
+impl Debug for SourceCodeSlice {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let mut tuple = f.debug_tuple("SourceCodeSlice");
+
+        #[cfg(debug_assertions)]
+        tuple.field(&self.text);
+
+        tuple.field(&self.range).finish()
+    }
+}