Make FLY002 emit a string (instead of an f-string) if all join() argu…

…ments are strings

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};
@@ -33,6 +34,7 @@ fn is_static_length(elts: &[Expr]) -> bool {
 fn build_fstring(joiner: &str, joinees: &[Expr]) -> Option<Expr> {
     let mut fstring_elems = Vec::with_capacity(joinees.len() * 2);
     let mut first = true;
+    let mut strings: Vec<&String> = vec![];
 
     for expr in joinees {
         if matches!(expr, Expr::JoinedStr(_)) {
@@ -44,21 +46,39 @@ fn build_fstring(joiner: &str, joinees: &[Expr]) -> Option<Expr> {
             fstring_elems.push(helpers::to_constant_string(joiner));
         }
         fstring_elems.push(helpers::to_fstring_elem(expr)?);
+
+        if let Expr::Constant(ast::ExprConstant {
+            value: Constant::Str(value),
+            ..
+        }) = expr
+        {
+            strings.push(value);
+        }
     }
 
-    let node = ast::ExprJoinedStr {
-        values: fstring_elems,
-        range: TextRange::default(),
+    let node = if strings.len() == joinees.len() {
+        ast::Expr::Constant(ast::ExprConstant {
+            value: Constant::Str(strings.iter().join(joiner)),
+            range: TextRange::default(),
+            kind: None,
+        })
+    } else {
+        ast::ExprJoinedStr {
+            values: fstring_elems,
+            range: TextRange::default(),
+        }
+        .into()
     };
-    Some(node.into())
+
+    Some(node)
 }
 
 pub(crate) fn static_join_to_fstring(checker: &mut Checker, expr: &Expr, joiner: &str) {
     let Expr::Call(ast::ExprCall {
         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)