[refurb] Detect empty f-strings (FURB105) (#21348)

## Summary

Fixes FURB105 (`print-empty-string`) to detect empty f-strings in
addition to regular empty strings. Previously, the rule only flagged
`print("")` but missed `print(f"")`. This fix ensures both cases are
detected and can be automatically fixed.

Fixes #21346

## Problem Analysis

The FURB105 rule checks for unnecessary empty strings passed to
`print()` calls. The `is_empty_string` helper function was only checking
for `Expr::StringLiteral` with empty values, but did not handle
`Expr::FString` (f-strings). As a result, `print(f"")` was not being
flagged as a violation, even though it's semantically equivalent to
`print("")` and should be simplified to `print()`.

The issue occurred because the function used a `matches!` macro that
only checked for string literals:

```rust
fn is_empty_string(expr: &Expr) -> bool {
    matches!(
        expr,
        Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) if value.is_empty()
    )
}
```

## Approach

1. **Import the helper function**: Added `is_empty_f_string` to the
imports from `ruff_python_ast::helpers`, which already provides logic to
detect empty f-strings.

2. **Update `is_empty_string` function**: Changed the implementation
from a `matches!` macro to a `match` expression that handles both string
literals and f-strings:

   ```rust
   fn is_empty_string(expr: &Expr) -> bool {
       match expr {
Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) =>
value.is_empty(),
           Expr::FString(f_string) => is_empty_f_string(f_string),
           _ => false,
       }
   }
   ```

The fix leverages the existing `is_empty_f_string` helper function which
properly handles the complexity of f-strings, including nested f-strings
and interpolated expressions. This ensures the detection is accurate and
consistent with how empty strings are detected elsewhere in the
codebase.

Author: danparizher

crates/ruff_linter/resources/test/fixtures/refurb/FURB105.py

@@ -19,6 +19,9 @@
 print("", **kwargs)
 print(sep="\t")
 print(sep=print(1))
+print(f"")
+print(f"", sep=",")
+print(f"", end="bar")
 
 # OK.
 
@@ -33,3 +36,4 @@
 print("foo", "", "bar", "", sep=",")
 print("", "", **kwargs)
 print(*args, sep=",")
+print(f"foo")

crates/ruff_linter/src/rules/refurb/rules/print_empty_string.rs

@@ -1,5 +1,5 @@
 use ruff_macros::{ViolationMetadata, derive_message_formats};
-use ruff_python_ast::helpers::contains_effect;
+use ruff_python_ast::helpers::{contains_effect, is_empty_f_string};
 use ruff_python_ast::{self as ast, Expr};
 use ruff_python_codegen::Generator;
 use ruff_python_semantic::SemanticModel;
@@ -194,13 +194,11 @@ pub(crate) fn print_empty_string(checker: &Checker, call: &ast::ExprCall) {
 
 /// Check if an expression is a constant empty string.
 fn is_empty_string(expr: &Expr) -> bool {
-    matches!(
-        expr,
-        Expr::StringLiteral(ast::ExprStringLiteral {
-            value,
-            ..
-        }) if value.is_empty()
-    )
+    match expr {
+        Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) => value.is_empty(),
+        Expr::FString(f_string) => is_empty_f_string(f_string),
+        _ => false,
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]

crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB105_FURB105.py.snap

@@ -317,7 +317,7 @@ help: Remove empty string
 19 + print(**kwargs)
 20 | print(sep="\t")
 21 | print(sep=print(1))
-22 | 
+22 | print(f"")
 
 FURB105 [*] Unnecessary separator passed to `print`
   --> FURB105.py:20:1
@@ -327,6 +327,7 @@ FURB105 [*] Unnecessary separator passed to `print`
 20 | print(sep="\t")
    | ^^^^^^^^^^^^^^^
 21 | print(sep=print(1))
+22 | print(f"")
    |
 help: Remove separator
 17 | print("", *args)
@@ -335,8 +336,8 @@ help: Remove separator
    - print(sep="\t")
 20 + print()
 21 | print(sep=print(1))
-22 | 
-23 | # OK.
+22 | print(f"")
+23 | print(f"", sep=",")
 
 FURB105 [*] Unnecessary separator passed to `print`
   --> FURB105.py:21:1
@@ -345,16 +346,75 @@ FURB105 [*] Unnecessary separator passed to `print`
 20 | print(sep="\t")
 21 | print(sep=print(1))
    | ^^^^^^^^^^^^^^^^^^^
-22 |
-23 | # OK.
+22 | print(f"")
+23 | print(f"", sep=",")
    |
 help: Remove separator
 18 | print("", *args, sep="")
 19 | print("", **kwargs)
 20 | print(sep="\t")
    - print(sep=print(1))
 21 + print()
-22 | 
-23 | # OK.
-24 | 
+22 | print(f"")
+23 | print(f"", sep=",")
+24 | print(f"", end="bar")
 note: This is an unsafe fix and may change runtime behavior
+
+FURB105 [*] Unnecessary empty string passed to `print`
+  --> FURB105.py:22:1
+   |
+20 | print(sep="\t")
+21 | print(sep=print(1))
+22 | print(f"")
+   | ^^^^^^^^^^
+23 | print(f"", sep=",")
+24 | print(f"", end="bar")
+   |
+help: Remove empty string
+19 | print("", **kwargs)
+20 | print(sep="\t")
+21 | print(sep=print(1))
+   - print(f"")
+22 + print()
+23 | print(f"", sep=",")
+24 | print(f"", end="bar")
+25 | 
+
+FURB105 [*] Unnecessary empty string and separator passed to `print`
+  --> FURB105.py:23:1
+   |
+21 | print(sep=print(1))
+22 | print(f"")
+23 | print(f"", sep=",")
+   | ^^^^^^^^^^^^^^^^^^^
+24 | print(f"", end="bar")
+   |
+help: Remove empty string and separator
+20 | print(sep="\t")
+21 | print(sep=print(1))
+22 | print(f"")
+   - print(f"", sep=",")
+23 + print()
+24 | print(f"", end="bar")
+25 | 
+26 | # OK.
+
+FURB105 [*] Unnecessary empty string passed to `print`
+  --> FURB105.py:24:1
+   |
+22 | print(f"")
+23 | print(f"", sep=",")
+24 | print(f"", end="bar")
+   | ^^^^^^^^^^^^^^^^^^^^^
+25 |
+26 | # OK.
+   |
+help: Remove empty string
+21 | print(sep=print(1))
+22 | print(f"")
+23 | print(f"", sep=",")
+   - print(f"", end="bar")
+24 + print(end="bar")
+25 | 
+26 | # OK.
+27 |