Join implicit concatenated strings when they fit on a line

crates/ruff_formatter/src/builders.rs

@@ -1454,7 +1454,7 @@ impl<Context> std::fmt::Debug for Group<'_, Context> {
 /// layout doesn't exceed the line width too, in which case it falls back to the flat layout.
 ///
 /// This IR is identical to the following [`best_fitting`] layout but is implemented as custom IR for
-/// best performance.
+/// better performance.
 ///
 /// ```rust
 /// # use ruff_formatter::prelude::*;

crates/ruff_formatter/src/format_extensions.rs

@@ -139,7 +139,7 @@ where
     /// # Ok(())
     /// # }
     /// ```
-    pub fn inspect(&mut self, f: &mut Formatter<Context>) -> FormatResult<&[FormatElement]> {
+    pub fn inspect(&self, f: &mut Formatter<Context>) -> FormatResult<&[FormatElement]> {
         let result = self.memory.get_or_init(|| f.intern(&self.inner));
 
         match result.as_ref() {

crates/ruff_python_ast/src/expression.rs

@@ -506,7 +506,7 @@ pub enum StringLikePart<'a> {
     FString(&'a ast::FString),
 }
 
-impl StringLikePart<'_> {
+impl<'a> StringLikePart<'a> {
     /// Returns the [`AnyStringFlags`] for the current string-like part.
     pub fn flags(&self) -> AnyStringFlags {
         match self {
@@ -525,6 +525,17 @@ impl StringLikePart<'_> {
         )
     }
 
+    pub const fn is_string_literal(self) -> bool {
+        matches!(self, Self::String(_))
+    }
+
+    pub const fn as_string_literal(self) -> Option<&'a ast::StringLiteral> {
+        match self {
+            StringLikePart::String(value) => Some(value),
+            _ => None,
+        }
+    }
+
     pub const fn is_fstring(self) -> bool {
         matches!(self, Self::FString(_))
     }
@@ -571,6 +582,7 @@ impl Ranged for StringLikePart<'_> {
 /// An iterator over all the [`StringLikePart`] of a string-like expression.
 ///
 /// This is created by the [`StringLike::parts`] method.
+#[derive(Clone)]
 pub enum StringLikePartIter<'a> {
     String(std::slice::Iter<'a, ast::StringLiteral>),
     Bytes(std::slice::Iter<'a, ast::BytesLiteral>),
@@ -607,5 +619,25 @@ impl<'a> Iterator for StringLikePartIter<'a> {
     }
 }
 
+impl DoubleEndedIterator for StringLikePartIter<'_> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        let part = match self {
+            StringLikePartIter::String(inner) => StringLikePart::String(inner.next_back()?),
+            StringLikePartIter::Bytes(inner) => StringLikePart::Bytes(inner.next_back()?),
+            StringLikePartIter::FString(inner) => {
+                let part = inner.next_back()?;
+                match part {
+                    ast::FStringPart::Literal(string_literal) => {
+                        StringLikePart::String(string_literal)
+                    }
+                    ast::FStringPart::FString(f_string) => StringLikePart::FString(f_string),
+                }
+            }
+        };
+
+        Some(part)
+    }
+}
+
 impl FusedIterator for StringLikePartIter<'_> {}
 impl ExactSizeIterator for StringLikePartIter<'_> {}

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/join_implicit_concatenated_string.options.json

@@ -0,0 +1,5 @@
+[
+  {
+    "preview": "enabled"
+  }
+]