Make FLY002 autofix into a constant string instead of an f-string if …

…all `join()` arguments are strings (#4834)

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

@@ -1,5 +1,6 @@
+use itertools::Itertools;
 use ruff_text_size::TextRange;
-use rustpython_parser::ast::{self, Expr, Ranged};
+use rustpython_parser::ast::{self, Constant, Expr, Ranged};
 
 use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Edit, Fix};
 use ruff_macros::{derive_message_formats, violation};
@@ -27,15 +28,48 @@ impl AlwaysAutofixableViolation for StaticJoinToFString {
 }
 
 fn is_static_length(elts: &[Expr]) -> bool {
-    elts.iter().all(|e| !matches!(e, Expr::Starred(_)))
+    elts.iter().all(|e| !e.is_starred_expr())
 }
 
 fn build_fstring(joiner: &str, joinees: &[Expr]) -> Option<Expr> {
+    // If all elements are string constants, join them into a single string.
+    if joinees.iter().all(|expr| {
+        matches!(
+            expr,
+            Expr::Constant(ast::ExprConstant {
+                value: Constant::Str(_),
+                ..
+            })
+        )
+    }) {
+        let node = ast::ExprConstant {
+            value: Constant::Str(
+                joinees
+                    .iter()
+                    .filter_map(|expr| {
+                        if let Expr::Constant(ast::ExprConstant {
+                            value: Constant::Str(string),
+                            ..
+                        }) = expr
+                        {
+                            Some(string.as_str())
+                        } else {
+                            None
+                        }
+                    })
+                    .join(joiner),
+            ),
+            range: TextRange::default(),
+            kind: None,
+        };
+        return Some(node.into());
+    }
+
     let mut fstring_elems = Vec::with_capacity(joinees.len() * 2);
     let mut first = true;
 
     for expr in joinees {
-        if matches!(expr, Expr::JoinedStr(_)) {
+        if expr.is_joined_str_expr() {
             // Oops, already an f-string. We don't know how to handle those
             // gracefully right now.
             return None;
@@ -58,7 +92,7 @@ pub(crate) fn static_join_to_fstring(checker: &mut Checker, expr: &Expr, joiner:
         args,
         keywords,
         ..
-    })= expr else {
+    }) = expr else {
         return;
     };
 

crates/ruff/src/rules/flynt/snapshots/ruff__rules__flynt__tests__FLY002_FLY002.py.snap

@@ -42,7 +42,7 @@ FLY002.py:6:7: FLY002 [*] Consider `f"Finally, {a} World"` instead of string joi
 8 8 | ok4 = "y".join([1, 2, 3])  # Technically OK, though would've been an error originally
 9 9 | ok5 = "a".join([random(), random()])  # OK (simple calls)
 
-FLY002.py:7:7: FLY002 [*] Consider `f"1x2x3"` instead of string join
+FLY002.py:7:7: FLY002 [*] Consider `"1x2x3"` instead of string join
    |
  7 | ok1 = " ".join([a, " World"])  # OK
  8 | ok2 = "".join(["Finally, ", a, " World"])  # OK
@@ -51,14 +51,14 @@ FLY002.py:7:7: FLY002 [*] Consider `f"1x2x3"` instead of string join
 10 | ok4 = "y".join([1, 2, 3])  # Technically OK, though would've been an error originally
 11 | ok5 = "a".join([random(), random()])  # OK (simple calls)
    |
-   = help: Replace with `f"1x2x3"`
+   = help: Replace with `"1x2x3"`
 
 ℹ Suggested fix
 4 4 | a = "Hello"
 5 5 | ok1 = " ".join([a, " World"])  # OK
 6 6 | ok2 = "".join(["Finally, ", a, " World"])  # OK
 7   |-ok3 = "x".join(("1", "2", "3"))  # OK
-  7 |+ok3 = f"1x2x3"  # OK
+  7 |+ok3 = "1x2x3"  # OK
 8 8 | ok4 = "y".join([1, 2, 3])  # Technically OK, though would've been an error originally
 9 9 | ok5 = "a".join([random(), random()])  # OK (simple calls)
 10 10 | ok6 = "a".join([secrets.token_urlsafe(), secrets.token_hex()])  # OK (attr calls)