[`flynt`] Use triple quotes for joined raw strings with newlines (`FLY002`) (#20197)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Fixes #19887

- flynt(FLY002): When joining only string constants, upgrade raw
single-quoted strings to raw triple-quoted if the resulting
content contains a newline.
- Choose a safe triple-quote delimiter by switching to the opposite
quote style if the preferred triple appears inside the
content.
- Update FLY002 snapshot to include the `\n'.join([r'line1','line2'])`
case.

## Test Plan

I've added one test case to FLY002.py.

<!-- How was it tested? -->

---------

Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>

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

@@ -16,7 +16,9 @@ nok4 = "a".join([a, a, *a])  # Not OK (not a static length)
 nok5 = "a".join([choice("flarp")])  # Not OK (not a simple call)
 nok6 = "a".join(x for x in "feefoofum")  # Not OK (generator)
 nok7 = "a".join([f"foo{8}", "bar"])  # Not OK (contains an f-string)
-
+# https://github.com/astral-sh/ruff/issues/19887
+nok8 = '\n'.join([r'line1','line2'])
+nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote delimiters appear; should bail)
 
 # Regression test for: https://github.com/astral-sh/ruff/issues/7197
 def create_file_public_url(url, filename):

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

@@ -2,7 +2,7 @@ use ast::FStringFlags;
 use itertools::Itertools;
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
-use ruff_python_ast::{self as ast, Arguments, Expr};
+use ruff_python_ast::{self as ast, Arguments, Expr, StringFlags};
 use ruff_text_size::{Ranged, TextRange};
 
 use crate::checkers::ast::Checker;
@@ -72,24 +72,42 @@ fn is_static_length(elts: &[Expr]) -> bool {
 fn build_fstring(joiner: &str, joinees: &[Expr], flags: FStringFlags) -> Option<Expr> {
     // If all elements are string constants, join them into a single string.
     if joinees.iter().all(Expr::is_string_literal_expr) {
-        let mut flags = None;
+        let mut flags: Option<ast::StringLiteralFlags> = None;
-        let node = ast::StringLiteral {
+        let content = joinees
-            value: joinees
+            .iter()
-                .iter()
+            .filter_map(|expr| {
-                .filter_map(|expr| {
+                if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = expr {
-                    if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = expr {
+                    if flags.is_none() {
-                        if flags.is_none() {
+                        // Take the flags from the first Expr
-                            // take the flags from the first Expr
+                        flags = Some(value.first_literal_flags());
-                            flags = Some(value.first_literal_flags());
-                        }
-                        Some(value.to_str())
-                    } else {
-                        None
                     }
-                })
+                    Some(value.to_str())
-                .join(joiner)
+                } else {
-                .into_boxed_str(),
+                    None
-            flags: flags?,
+                }
+            })
+            .join(joiner);
+
+        let mut flags = flags?;
+
+        // If the result is a raw string and contains a newline, use triple quotes.
+        if flags.prefix().is_raw() && content.contains(['\n', '\r']) {
+            flags = flags.with_triple_quotes(ruff_python_ast::str::TripleQuotes::Yes);
+
+            // Prefer a delimiter that doesn't occur in the content; if both occur, bail.
+            if content.contains(flags.quote_str()) {
+                flags = flags.with_quote_style(flags.quote_style().opposite());
+                if content.contains(flags.quote_str()) {
+                    // Both "'''" and "\"\"\"" are present in content; avoid emitting
+                    // an invalid raw triple-quoted literal (or escaping). Bail on the fix.
+                    return None;
+                }
+            }
+        }
+
+        let node = ast::StringLiteral {
+            value: content.into_boxed_str(),
+            flags,
             range: TextRange::default(),
             node_index: ruff_python_ast::AtomicNodeIndex::NONE,
         };

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

@@ -125,18 +125,39 @@ 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 `f"{url}{filename}"` instead of string join
+FLY002 [*] Consider f-string instead of string join
-  --> FLY002.py:23:11
+  --> FLY002.py:20:8
    |
-21 | # Regression test for: https://github.com/astral-sh/ruff/issues/7197
+18 | nok7 = "a".join([f"foo{8}", "bar"])  # Not OK (contains an f-string)
-22 | def create_file_public_url(url, filename):
+19 | # https://github.com/astral-sh/ruff/issues/19887
-23 |     return''.join([url, filename])
+20 | nok8 = '\n'.join([r'line1','line2'])
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+21 | nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote delimiters appear; should bail)
+   |
+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 = r'''line1
+21 + line2'''
+22 | nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote delimiters appear; should bail)
+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 `f"{url}{filename}"` instead of string join
+  --> FLY002.py:25:11
+   |
+23 | # Regression test for: https://github.com/astral-sh/ruff/issues/7197
+24 | def create_file_public_url(url, filename):
+25 |     return''.join([url, filename])
    |           ^^^^^^^^^^^^^^^^^^^^^^^^
    |
 help: Replace with `f"{url}{filename}"`
-20 | 
+22 | 
-21 | # Regression test for: https://github.com/astral-sh/ruff/issues/7197
+23 | # Regression test for: https://github.com/astral-sh/ruff/issues/7197
-22 | def create_file_public_url(url, filename):
+24 | def create_file_public_url(url, filename):
    -     return''.join([url, filename])
-23 +     return f"{url}{filename}"
+25 +     return f"{url}{filename}"
 note: This is an unsafe fix and may change runtime behavior