All tests are passing

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_python_formatter/src/context.rs

@@ -8,7 +8,6 @@ use ruff_source_file::Locator;
 use std::fmt::{Debug, Formatter};
 use std::ops::{Deref, DerefMut};
 
-#[derive(Clone)]
 pub struct PyFormatContext<'a> {
     options: PyFormatOptions,
     contents: &'a str,
@@ -52,7 +51,6 @@ impl<'a> PyFormatContext<'a> {
         self.contents
     }
 
-    #[allow(unused)]
     pub(crate) fn locator(&self) -> Locator<'a> {
         Locator::new(self.contents)
     }

crates/ruff_python_formatter/src/expression/expr_bytes_literal.rs

@@ -1,3 +1,5 @@
+use ruff_formatter::FormatRuleWithOptions;
+use ruff_formatter::GroupId;
 use ruff_python_ast::ExprBytesLiteral;
 use ruff_python_ast::{AnyNodeRef, StringLike};
 
@@ -8,15 +10,38 @@ use crate::prelude::*;
 use crate::string::{FormatImplicitConcatenatedString, StringLikeExtensions};
 
 #[derive(Default)]
-pub struct FormatExprBytesLiteral;
+pub struct FormatExprBytesLiteral {
+    layout: ExprBytesLiteralLayout,
+}
+
+#[derive(Default)]
+pub struct ExprBytesLiteralLayout {
+    pub implicit_group_id: Option<GroupId>,
+}
+
+impl FormatRuleWithOptions<ExprBytesLiteral, PyFormatContext<'_>> for FormatExprBytesLiteral {
+    type Options = ExprBytesLiteralLayout;
+
+    fn with_options(mut self, options: Self::Options) -> Self {
+        self.layout = options;
+        self
+    }
+}
 
 impl FormatNodeRule<ExprBytesLiteral> for FormatExprBytesLiteral {
     fn fmt_fields(&self, item: &ExprBytesLiteral, f: &mut PyFormatter) -> FormatResult<()> {
         let ExprBytesLiteral { value, .. } = item;
 
         match value.as_slice() {
             [bytes_literal] => bytes_literal.format().fmt(f),
-            _ => in_parentheses_only_group(&FormatImplicitConcatenatedString::new(item)).fmt(f),
+            _ => match self.layout.implicit_group_id {
+                Some(group_id) => group(&FormatImplicitConcatenatedString::new(item))
+                    .with_group_id(Some(group_id))
+                    .fmt(f),
+                None => {
+                    in_parentheses_only_group(&FormatImplicitConcatenatedString::new(item)).fmt(f)
+                }
+            },
         }
     }
 }

crates/ruff_python_formatter/src/expression/expr_f_string.rs

@@ -1,3 +1,4 @@
+use ruff_formatter::{FormatRuleWithOptions, GroupId};
 use ruff_python_ast::{AnyNodeRef, ExprFString, StringLike};
 use ruff_source_file::Locator;
 use ruff_text_size::Ranged;
@@ -10,7 +11,23 @@ use crate::prelude::*;
 use crate::string::{FormatImplicitConcatenatedString, Quoting, StringLikeExtensions};
 
 #[derive(Default)]
-pub struct FormatExprFString;
+pub struct FormatExprFString {
+    layout: ExprFStringLayout,
+}
+
+#[derive(Default)]
+pub struct ExprFStringLayout {
+    pub implicit_group_id: Option<GroupId>,
+}
+
+impl FormatRuleWithOptions<ExprFString, PyFormatContext<'_>> for FormatExprFString {
+    type Options = ExprFStringLayout;
+
+    fn with_options(mut self, options: Self::Options) -> Self {
+        self.layout = options;
+        self
+    }
+}
 
 impl FormatNodeRule<ExprFString> for FormatExprFString {
     fn fmt_fields(&self, item: &ExprFString, f: &mut PyFormatter) -> FormatResult<()> {
@@ -22,7 +39,14 @@ impl FormatNodeRule<ExprFString> for FormatExprFString {
                 f_string_quoting(item, &f.context().locator()),
             )
             .fmt(f),
-            _ => in_parentheses_only_group(&FormatImplicitConcatenatedString::new(item)).fmt(f),
+            _ => match self.layout.implicit_group_id {
+                Some(group_id) => group(&FormatImplicitConcatenatedString::new(item))
+                    .with_group_id(Some(group_id))
+                    .fmt(f),
+                None => {
+                    in_parentheses_only_group(&FormatImplicitConcatenatedString::new(item)).fmt(f)
+                }
+            },
         }
     }
 }
@@ -35,6 +59,7 @@ impl NeedsParentheses for ExprFString {
     ) -> OptionalParentheses {
         if self.value.is_implicit_concatenated() {
             OptionalParentheses::Multiline
+        }
         // TODO(dhruvmanila): Ideally what we want here is a new variant which
         // is something like:
         // - If the expression fits by just adding the parentheses, then add them and
@@ -53,7 +78,7 @@ impl NeedsParentheses for ExprFString {
         //   ```
         // This isn't decided yet, refer to the relevant discussion:
         // https://github.com/astral-sh/ruff/discussions/9785
-        } else if StringLike::FString(self).is_multiline(context.source()) {
+        else if StringLike::FString(self).is_multiline(context.source()) {
             OptionalParentheses::Never
         } else {
             OptionalParentheses::BestFit

crates/ruff_python_formatter/src/expression/expr_string_literal.rs

@@ -1,4 +1,4 @@
-use ruff_formatter::FormatRuleWithOptions;
+use ruff_formatter::{FormatRuleWithOptions, GroupId};
 use ruff_python_ast::{AnyNodeRef, ExprStringLiteral, StringLike};
 
 use crate::expression::parentheses::{
@@ -10,14 +10,29 @@ use crate::string::{FormatImplicitConcatenatedString, StringLikeExtensions};
 
 #[derive(Default)]
 pub struct FormatExprStringLiteral {
-    kind: StringLiteralKind,
+    layout: ExprStringLiteralLayout,
+}
+
+#[derive(Default)]
+pub struct ExprStringLiteralLayout {
+    pub kind: StringLiteralKind,
+    pub implicit_group_id: Option<GroupId>,
+}
+
+impl ExprStringLiteralLayout {
+    pub const fn docstring() -> Self {
+        Self {
+            kind: StringLiteralKind::Docstring,
+            implicit_group_id: None,
+        }
+    }
 }
 
 impl FormatRuleWithOptions<ExprStringLiteral, PyFormatContext<'_>> for FormatExprStringLiteral {
-    type Options = StringLiteralKind;
+    type Options = ExprStringLiteralLayout;
 
     fn with_options(mut self, options: Self::Options) -> Self {
-        self.kind = options;
+        self.layout = options;
         self
     }
 }
@@ -27,15 +42,23 @@ impl FormatNodeRule<ExprStringLiteral> for FormatExprStringLiteral {
         let ExprStringLiteral { value, .. } = item;
 
         match value.as_slice() {
-            [string_literal] => string_literal.format().with_options(self.kind).fmt(f),
+            [string_literal] => string_literal
+                .format()
+                .with_options(self.layout.kind)
+                .fmt(f),
             _ => {
                 // This is just a sanity check because [`DocstringStmt::try_from_statement`]
                 // ensures that the docstring is a *single* string literal.
-                assert!(!self.kind.is_docstring());
+                assert!(!self.layout.kind.is_docstring());
 
-                in_parentheses_only_group(&FormatImplicitConcatenatedString::new(item))
+                match self.layout.implicit_group_id {
+                    Some(group_id) => group(&FormatImplicitConcatenatedString::new(item))
+                        .with_group_id(Some(group_id))
+                        .fmt(f),
+                    None => in_parentheses_only_group(&FormatImplicitConcatenatedString::new(item))
+                        .fmt(f),
+                }
             }
-            .fmt(f),
         }
     }
 }
@@ -48,7 +71,7 @@ impl NeedsParentheses for ExprStringLiteral {
     ) -> OptionalParentheses {
         if self.value.is_implicit_concatenated() {
             OptionalParentheses::Multiline
-        } else if StringLike::String(self).is_multiline(context.source()) {
+        } else if StringLike::from(self).is_multiline(context.source()) {
             OptionalParentheses::Never
         } else {
             OptionalParentheses::BestFit

crates/ruff_python_formatter/src/expression/mod.rs

@@ -768,15 +768,18 @@ impl<'input> CanOmitOptionalParenthesesVisitor<'input> {
             Expr::StringLiteral(ast::ExprStringLiteral { value, .. })
                 if value.is_implicit_concatenated() =>
             {
-                self.update_max_precedence(OperatorPrecedence::String);
+                // FIXME make this a preview only change
+                // self.update_max_precedence(OperatorPrecedence::String);
+                return;
             }
             Expr::BytesLiteral(ast::ExprBytesLiteral { value, .. })
                 if value.is_implicit_concatenated() =>
             {
-                self.update_max_precedence(OperatorPrecedence::String);
+                // self.update_max_precedence(OperatorPrecedence::String);
+                return;
             }
             Expr::FString(ast::ExprFString { value, .. }) if value.is_implicit_concatenated() => {
-                self.update_max_precedence(OperatorPrecedence::String);
+                // self.update_max_precedence(OperatorPrecedence::String);
                 return;
             }
 
@@ -1254,8 +1257,9 @@ pub(crate) fn is_splittable_expression(expr: &Expr, context: &PyFormatContext) -
         }
 
         // String like literals can expand if they are implicit concatenated.
+        // TODO REVIEW
         Expr::FString(fstring) => fstring.value.is_implicit_concatenated(),
-        Expr::StringLiteral(string) => string.value.is_implicit_concatenated(),
+        Expr::StringLiteral(_) => false,
         Expr::BytesLiteral(bytes) => bytes.value.is_implicit_concatenated(),
 
         // Expressions that have no split points per se, but they contain nested sub expressions that might expand.

crates/ruff_python_formatter/src/lib.rs

@@ -160,7 +160,9 @@ mod tests {
     use ruff_python_trivia::CommentRanges;
     use ruff_text_size::{TextRange, TextSize};
 
-    use crate::{format_module_ast, format_module_source, format_range, PyFormatOptions};
+    use crate::{
+        format_module_ast, format_module_source, format_range, PreviewMode, PyFormatOptions,
+    };
 
     /// Very basic test intentionally kept very similar to the CLI
     #[test]
@@ -188,13 +190,11 @@ if True:
     #[test]
     fn quick_test() {
         let source = r#"
-def main() -> None:
-    if True:
-        some_very_long_variable_name_abcdefghijk = Foo()
-        some_very_long_variable_name_abcdefghijk = some_very_long_variable_name_abcdefghijk[
-            some_very_long_variable_name_abcdefghijk.some_very_long_attribute_name
-            == "This is a very long string abcdefghijk"
-        ]
+fstring = (
+    f"We have to remember to escape {braces}."
+    " Like {these}."
+    f" But not {this}."
+)
 
 "#;
         let source_type = PySourceType::Python;
@@ -203,7 +203,8 @@ def main() -> None:
         let source_path = "code_inline.py";
         let parsed = parse(source, source_type.as_mode()).unwrap();
         let comment_ranges = CommentRanges::from(parsed.tokens());
-        let options = PyFormatOptions::from_extension(Path::new(source_path));
+        let options = PyFormatOptions::from_extension(Path::new(source_path))
+            .with_preview(PreviewMode::Enabled);
         let formatted = format_module_ast(&parsed, &comment_ranges, source, options).unwrap();
 
         // Uncomment the `dbg` to print the IR.

crates/ruff_python_formatter/src/other/arguments.rs

@@ -223,6 +223,8 @@ fn is_huggable_string_argument(
     arguments: &Arguments,
     context: &PyFormatContext,
 ) -> bool {
+    // TODO: Implicit concatenated could become regular string. Although not if it is multiline. So that should be fine?
+    //   Double check
     if string.is_implicit_concatenated() || !string.is_multiline(context.source()) {
         return false;
     }

crates/ruff_python_formatter/src/other/f_string.rs

@@ -76,14 +76,9 @@ impl Format<PyFormatContext<'_>> for FormatFString<'_> {
         let quotes = StringQuotes::from(string_kind);
         write!(f, [string_kind.prefix(), quotes])?;
 
-        f.join()
-            .entries(
-                self.value
-                    .elements
-                    .iter()
-                    .map(|element| FormatFStringElement::new(element, context)),
-            )
-            .finish()?;
+        for element in &self.value.elements {
+            FormatFStringElement::new(element, context).fmt(f)?;
+        }
 
         // Ending quote
         quotes.fmt(f)

crates/ruff_python_formatter/src/other/f_string_element.rs

@@ -57,7 +57,7 @@ impl<'a> FormatFStringLiteralElement<'a> {
 impl Format<PyFormatContext<'_>> for FormatFStringLiteralElement<'_> {
     fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
         let literal_content = f.context().locator().slice(self.element.range());
-        let normalized = normalize_string(literal_content, 0, self.context.flags(), true);
+        let normalized = normalize_string(literal_content, 0, self.context.flags(), true, false);
         match &normalized {
             Cow::Borrowed(_) => source_text_slice(self.element.range()).fmt(f),
             Cow::Owned(normalized) => text(normalized).fmt(f),
@@ -231,11 +231,9 @@ impl Format<PyFormatContext<'_>> for FormatFStringExpressionElement<'_> {
                 if let Some(format_spec) = format_spec.as_deref() {
                     token(":").fmt(f)?;
 
-                    f.join()
-                        .entries(format_spec.elements.iter().map(|element| {
-                            FormatFStringElement::new(element, self.context.f_string())
-                        }))
-                        .finish()?;
+                    for element in &format_spec.elements {
+                        FormatFStringElement::new(element, self.context.f_string()).fmt(f)?;
+                    }
 
                     // These trailing comments can only occur if the format specifier is
                     // present. For example,

crates/ruff_python_formatter/src/pattern/mod.rs

@@ -289,18 +289,22 @@ impl<'a> CanOmitOptionalParenthesesVisitor<'a> {
 
             Pattern::MatchValue(value) => match &*value.value {
                 Expr::StringLiteral(string) => {
-                    self.update_max_precedence(OperatorPrecedence::String, string.value.len());
+                    // TODO update?
+
+                    // self.update_max_precedence(OperatorPrecedence::String, string.value.len());
                 }
                 Expr::BytesLiteral(bytes) => {
-                    self.update_max_precedence(OperatorPrecedence::String, bytes.value.len());
+                    // TODO update?
+                    // self.update_max_precedence(OperatorPrecedence::String, bytes.value.len());
                 }
                 // F-strings are allowed according to python's grammar but fail with a syntax error at runtime.
                 // That's why we need to support them for formatting.
                 Expr::FString(string) => {
-                    self.update_max_precedence(
-                        OperatorPrecedence::String,
-                        string.value.as_slice().len(),
-                    );
+                    // TODO update?
+                    // self.update_max_precedence(
+                    //     OperatorPrecedence::String,
+                    //     string.value.as_slice().len(),
+                    // );
                 }
 
                 Expr::NumberLiteral(_) | Expr::Attribute(_) | Expr::UnaryOp(_) => {

crates/ruff_python_formatter/src/preview.rs

@@ -60,3 +60,11 @@ pub(crate) fn is_docstring_code_block_in_docstring_indent_enabled(
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if implicitly concatenated strings should be joined if they all fit on a single line.
+/// See [#9457](https://github.com/astral-sh/ruff/issues/9457)
+/// WARNING: This preview style depends on `is_empty_parameters_no_unnecessary_parentheses_around_return_value_enabled`
+/// because it relies on the new semantic of `IfBreaksParenthesized`.
+pub(crate) fn is_join_implicit_concatenated_string_enabled(context: &PyFormatContext) -> bool {
+    context.is_preview()
+}