Improve raw string handling in static_join_to_fstring

Refined logic to better detect when non-raw representation is needed, especially for cases involving backslashes and quote delimiters. Updated test fixtures and snapshots to clarify behavior and expected fixes for edge cases mixing raw and non-raw strings.

crates/ruff_linter/resources/test/fixtures/flynt/FLY002.py

@@ -36,7 +36,7 @@ nok14 = "".join((r"", '"'))  # First is raw, second is not - would break syntax
 nok15 = "".join((r"", "\\"))  # First is raw, second has backslash - would break syntax
 nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
 nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
-nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
 
 # Test that all-raw strings still work (should be OK)
 ok7 = "".join((r"", r"something"))  # Both are raw - OK

crates/ruff_linter/src/rules/flynt/rules/static_join_to_fstring.rs

@@ -109,10 +109,14 @@ fn build_fstring(joiner: &str, joinees: &[Expr], flags: FStringFlags) -> Option<
         // use non-raw representation. This handles cases where raw strings would create invalid
         // syntax or behavior changes.
         if any_raw && !content.is_empty() {
-            let needs_non_raw = content.contains(['\n', '\r', '\0']) || {
+            let needs_non_raw = content.contains(['\r', '\0']) || {
                 // Check if content contains characters that would break raw string syntax
                 let quote_char = flags.quote_str();
-                content.contains(quote_char) || content.contains('\\')
+                // A raw string cannot end with a single backslash if it's immediately
+                // followed by the quote delimiter, as that would be invalid syntax.
+                let ends_with_backslash = content.ends_with('\\')
+                    && (content.len() == 1 || content.chars().nth_back(1) != Some('\\'));
+                content.contains(quote_char) || ends_with_backslash
             };
 
             if needs_non_raw {

crates/ruff_linter/src/rules/flynt/snapshots/ruff_linter__rules__flynt__tests__FLY002_FLY002.py.snap

@@ -125,7 +125,7 @@ help: Replace with `f"{secrets.token_urlsafe()}a{secrets.token_hex()}"`
 13 | nok2 = a.join(["1", "2", "3"])  # Not OK (not a static joiner)
 note: This is an unsafe fix and may change runtime behavior
 
-FLY002 [*] Consider `'line1\nline2'` instead of string join
+FLY002 [*] Consider f-string instead of string join
   --> FLY002.py:20:8
    |
 18 | nok7 = "a".join([f"foo{8}", "bar"])  # Not OK (contains an f-string)
@@ -134,15 +134,16 @@ FLY002 [*] Consider `'line1\nline2'` instead of string join
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 21 | nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote delimiters appear; should bail)
    |
-help: Replace with `'line1\nline2'`
+help: Replace with f-string
 17 | nok6 = "a".join(x for x in "feefoofum")  # Not OK (generator)
 18 | nok7 = "a".join([f"foo{8}", "bar"])  # Not OK (contains an f-string)
 19 | # https://github.com/astral-sh/ruff/issues/19887
    - nok8 = '\n'.join([r'line1','line2'])
-20 + nok8 = 'line1\nline2'
+20 + nok8 = r'''line1
-21 | nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote delimiters appear; should bail)
+21 + line2'''
-22 | 
+22 | nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote delimiters appear; should bail)
-23 | # Regression test for: https://github.com/astral-sh/ruff/issues/7197
+23 | 
+24 | # Regression test for: https://github.com/astral-sh/ruff/issues/7197
 note: This is an unsafe fix and may change runtime behavior
 
 FLY002 [*] Consider `"raw string\n<\"\"\">\n<'''>"` instead of string join
@@ -267,7 +268,7 @@ help: Replace with `"\\"`
 36 + nok15 = "\\"  # First is raw, second has backslash - would break syntax
 37 | nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
 38 | nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
 note: This is an unsafe fix and may change runtime behavior
 
 FLY002 [*] Consider `"\x00"` instead of string join
@@ -278,7 +279,7 @@ FLY002 [*] Consider `"\x00"` instead of string join
 37 | nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
    |         ^^^^^^^^^^^^^^^^^^^^
 38 | nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
    |
 help: Replace with `"\x00"`
 34 | # Mixing raw and non-raw strings can cause syntax errors or behavior changes
@@ -287,7 +288,7 @@ help: Replace with `"\x00"`
    - nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
 37 + nok16 = "\x00"  # First is raw, second has null byte - would introduce null bytes
 38 | nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
 40 | 
 note: This is an unsafe fix and may change runtime behavior
 
@@ -298,7 +299,7 @@ FLY002 [*] Consider `"\r"` instead of string join
 37 | nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
 38 | nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
    |         ^^^^^^^^^^^^^^^^^^^^
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
    |
 help: Replace with `"\r"`
 35 | nok14 = "".join((r"", '"'))  # First is raw, second is not - would break syntax
@@ -306,27 +307,27 @@ help: Replace with `"\r"`
 37 | nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
    - nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
 38 + nok17 = "\r"  # First is raw, second has carriage return - would change behavior
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
 40 | 
 41 | # Test that all-raw strings still work (should be OK)
 note: This is an unsafe fix and may change runtime behavior
 
-FLY002 [*] Consider `"\\r"` instead of string join
+FLY002 [*] Consider `r"\r"` instead of string join
   --> FLY002.py:39:9
    |
 37 | nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
 38 | nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
    |         ^^^^^^^^^^^^^^^^^^^^^
 40 |
 41 | # Test that all-raw strings still work (should be OK)
    |
-help: Replace with `"\\r"`
+help: Replace with `r"\r"`
 36 | nok15 = "".join((r"", "\\"))  # First is raw, second has backslash - would break syntax
 37 | nok16 = "".join((r"", "\0"))  # First is raw, second has null byte - would introduce null bytes
 38 | nok17 = "".join((r"", "\r"))  # First is raw, second has carriage return - would change behavior
-   - nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+   - nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
-39 + nok18 = "\\r"  # First is raw, second has backslash escape followed by r
+39 + nok18 = r"\r"  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
 40 | 
 41 | # Test that all-raw strings still work (should be OK)
 42 | ok7 = "".join((r"", r"something"))  # Both are raw - OK
@@ -341,7 +342,7 @@ FLY002 [*] Consider `r"something"` instead of string join
 43 | ok8 = "\n".join((r"line1", r'line2'))  # Both are raw - OK
    |
 help: Replace with `r"something"`
-39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash escape followed by r
+39 | nok18 = "".join((r"", "\\r"))  # First is raw, second has backslash followed by literal r - OK (no special handling needed)
 40 | 
 41 | # Test that all-raw strings still work (should be OK)
    - ok7 = "".join((r"", r"something"))  # Both are raw - OK
@@ -351,7 +352,7 @@ help: Replace with `r"something"`
 45 | # Test that all-non-raw strings still work (should be OK)
 note: This is an unsafe fix and may change runtime behavior
 
-FLY002 [*] Consider `"line1\nline2"` instead of string join
+FLY002 [*] Consider f-string instead of string join
   --> FLY002.py:43:7
    |
 41 | # Test that all-raw strings still work (should be OK)
@@ -361,15 +362,16 @@ FLY002 [*] Consider `"line1\nline2"` instead of string join
 44 |
 45 | # Test that all-non-raw strings still work (should be OK)
    |
-help: Replace with `"line1\nline2"`
+help: Replace with f-string
 40 | 
 41 | # Test that all-raw strings still work (should be OK)
 42 | ok7 = "".join((r"", r"something"))  # Both are raw - OK
    - ok8 = "\n".join((r"line1", r'line2'))  # Both are raw - OK
-43 + ok8 = "line1\nline2"  # Both are raw - OK
+43 + ok8 = r"""line1
-44 | 
+44 + line2"""  # Both are raw - OK
-45 | # Test that all-non-raw strings still work (should be OK)
+45 | 
-46 | ok9 = "".join(("", '"'))  # Both are non-raw - OK
+46 | # Test that all-non-raw strings still work (should be OK)
+47 | ok9 = "".join(("", '"'))  # Both are non-raw - OK
 note: This is an unsafe fix and may change runtime behavior
 
 FLY002 [*] Consider `'"'` instead of string join