Disallow single-line implicit concatenated strings

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/string.py

@@ -138,3 +138,60 @@
 a = """\x1F"""
 a = """\\x1F"""
 a = """\\\x1F"""
+
+
+##############################################################################
+# Implicit concatenated strings
+##############################################################################
+
+# In preview, don't collapse implicit concatenated strings that can't be joined into a single string
+# and that are multiline in the source.
+
+(
+    r"aaaaaaaaa"
+    r"bbbbbbbbbbbbbbbbbbbb"
+)
+
+(
+    r"aaaaaaaaa" r"bbbbbbbbbbbbbbbbbbbb"
+    "cccccccccccccccccccccc"
+)
+
+(
+    """aaaaaaaaa"""
+    """bbbbbbbbbbbbbbbbbbbb"""
+)
+
+(
+    f"""aaaa{
+    10}aaaaa"""
+    fr"""bbbbbbbbbbbbbbbbbbbb"""
+)
+
+if (
+       r"aaaaaaaaa"
+       r"bbbbbbbbbbbbbbbbbbbb"
+   ) + ["aaaaaaaaaa", "bbbbbbbbbbbbbbbb"]: ...
+
+
+
+# In preview, keep implicit concatenated strings on a single line if they fit and are not separate by line breaks in the source
+(
+    r"aaaaaaaaa" r"bbbbbbbbbbbbbbbbbbbb"
+)
+
+r"aaaaaaaaa" r"bbbbbbbbbbbbbbbbbbbb"
+
+(
+    f"aaaa{
+    10}aaaaa" fr"bbbbbbbbbbbbbbbbbbbb"
+)
+
+(
+    r"""aaaaaaaaa""" r"""bbbbbbbbbbbbbbbbbbbb"""
+)
+
+(
+    f"""aaaa{
+    10}aaaaa""" fr"""bbbbbbbbbbbbbbbbbbbb"""
+)

crates/ruff_python_formatter/src/statement/stmt_assign.rs

@@ -20,6 +20,7 @@ use crate::preview::is_join_implicit_concatenated_string_enabled;
 use crate::statement::trailing_semicolon;
 use crate::string::implicit::{
     FormatImplicitConcatenatedStringExpanded, FormatImplicitConcatenatedStringFlat,
+    ImplicitConcatenatedLayout,
 };
 use crate::{has_skip_comment, prelude::*};
 
@@ -375,7 +376,13 @@ impl Format<PyFormatContext<'_>> for FormatStatementsLastExpression<'_> {
                             let f =
                                 &mut WithNodeLevel::new(NodeLevel::Expression(Some(group_id)), f);
 
-                            write!(f, [FormatImplicitConcatenatedStringExpanded::new(string)])
+                            write!(
+                                f,
+                                [FormatImplicitConcatenatedStringExpanded::new(
+                                    string,
+                                    ImplicitConcatenatedLayout::MaybeFlat
+                                )]
+                            )
                         });
 
                         // Join the implicit concatenated string if it fits on a single line
@@ -686,6 +693,7 @@ impl Format<PyFormatContext<'_>> for FormatStatementsLastExpression<'_> {
 
                         FormatImplicitConcatenatedStringExpanded::new(
                             StringLike::try_from(*value).unwrap(),
+                            ImplicitConcatenatedLayout::MaybeFlat,
                         )
                         .fmt(f)
                     })

crates/ruff_python_formatter/src/string/implicit.rs

@@ -1,5 +1,4 @@
-use std::borrow::Cow;
-
+use itertools::Itertools;
 use ruff_formatter::{format_args, write, FormatContext};
 use ruff_python_ast::str::Quote;
 use ruff_python_ast::str_prefix::{
@@ -8,6 +7,7 @@ use ruff_python_ast::str_prefix::{
 use ruff_python_ast::{AnyStringFlags, FStringElement, StringFlags, StringLike, StringLikePart};
 use ruff_source_file::LineRanges;
 use ruff_text_size::{Ranged, TextRange};
+use std::borrow::Cow;
 
 use crate::comments::{leading_comments, trailing_comments};
 use crate::expression::parentheses::in_parentheses_only_soft_line_break_or_space;
@@ -41,12 +41,20 @@ impl<'a> FormatImplicitConcatenatedString<'a> {
 
 impl Format<PyFormatContext<'_>> for FormatImplicitConcatenatedString<'_> {
     fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
-        let expanded = FormatImplicitConcatenatedStringExpanded::new(self.string);
+        let flat = FormatImplicitConcatenatedStringFlat::new(self.string, f.context());
+        let expanded = FormatImplicitConcatenatedStringExpanded::new(
+            self.string,
+            if flat.is_some() {
+                ImplicitConcatenatedLayout::MaybeFlat
+            } else {
+                ImplicitConcatenatedLayout::Multipart
+            },
+        );
 
         // If the string can be joined, try joining the implicit concatenated string into a single string
         // if it fits on the line. Otherwise, parenthesize the string parts and format each part on its
         // own line.
-        if let Some(flat) = FormatImplicitConcatenatedStringFlat::new(self.string, f.context()) {
+        if let Some(flat) = flat {
             write!(
                 f,
                 [if_group_fits_on_line(&flat), if_group_breaks(&expanded)]
@@ -60,13 +68,14 @@ impl Format<PyFormatContext<'_>> for FormatImplicitConcatenatedString<'_> {
 /// Formats an implicit concatenated string where parts are separated by a space or line break.
 pub(crate) struct FormatImplicitConcatenatedStringExpanded<'a> {
     string: StringLike<'a>,
+    layout: ImplicitConcatenatedLayout,
 }
 
 impl<'a> FormatImplicitConcatenatedStringExpanded<'a> {
-    pub(crate) fn new(string: StringLike<'a>) -> Self {
+    pub(crate) fn new(string: StringLike<'a>, layout: ImplicitConcatenatedLayout) -> Self {
         assert!(string.is_implicit_concatenated());
 
-        Self { string }
+        Self { string, layout }
     }
 }
 
@@ -77,6 +86,19 @@ impl Format<PyFormatContext<'_>> for FormatImplicitConcatenatedStringExpanded<'_
 
         let join_implicit_concatenated_string_enabled =
             is_join_implicit_concatenated_string_enabled(f.context());
+
+        // Keep implicit concatenated strings expanded unless they're already written on a single line.
+        if matches!(self.layout, ImplicitConcatenatedLayout::Multipart)
+            && join_implicit_concatenated_string_enabled
+            && self.string.parts().tuple_windows().any(|(a, b)| {
+                f.context()
+                    .source()
+                    .contains_line_break(TextRange::new(a.end(), b.start()))
+            })
+        {
+            expand_parent().fmt(f)?;
+        }
+
         let mut joiner = f.join_with(in_parentheses_only_soft_line_break_or_space());
 
         for part in self.string.parts() {
@@ -108,6 +130,14 @@ impl Format<PyFormatContext<'_>> for FormatImplicitConcatenatedStringExpanded<'_
     }
 }
 
+#[derive(Copy, Clone, Debug)]
+pub(crate) enum ImplicitConcatenatedLayout {
+    /// The string might get joined into a single string if it fits on a single line.
+    MaybeFlat,
+    /// The string will remain a multipart string.
+    Multipart,
+}
+
 /// Formats an implicit concatenated string where parts are joined into a single string if possible.
 pub(crate) struct FormatImplicitConcatenatedStringFlat<'a> {
     string: StringLike<'a>,

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__long_strings_flag_disabled.py.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/black/cases/long_strings_flag_disabled.py
+snapshot_kind: text
 ---
 ## Input
 

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_long_strings.py.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/black/cases/preview_long_strings.py
+snapshot_kind: text
 ---
 ## Input
 

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_long_strings__regression.py.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/black/cases/preview_long_strings__regression.py
+snapshot_kind: text
 ---
 ## Input
 

crates/ruff_python_formatter/tests/snapshots/format@expression__fstring.py.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring.py
+snapshot_kind: text
 ---
 ## Input
 ```python