[`perflint`] Handle tuples in dictionary comprehensions (`PERF403`) (#19934)

This pull request fixes the bug described in issue
[#19153](https://github.com/astral-sh/ruff/issues/19153).

The issue occurred when `PERF403` incorrectly flagged cases involving
tuple unpacking in a for loop. For example:

```python
def f():
    v = {}
    for (o, p), x in [("op", "x")]:
        v[x] = o, p
```

This code was wrongly suggested to be rewritten into a dictionary
comprehension, which changes the semantics.

Changes in this PR:

Updated the `PERF403` rule to correctly handle tuple unpacking in loop
targets.

Added regression tests to ensure this case (and similar ones) are no
longer flagged incorrectly.

Why:
This ensures that `PERF403` only triggers when a dictionary
comprehension is semantically equivalent to the original loop,
preventing false positives.

---------

Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/resources/test/fixtures/perflint/PERF403.py

@@ -192,3 +192,24 @@ def issue_19005_3():
     c = {}
     for a[0], a[1] in ():
         c[a[0]] = a[1]
+
+
+def issue_19153_1():
+    v = {}
+    for o, (x,) in ["ox"]:
+        v[x,] = o
+    return v
+
+
+def issue_19153_2():
+    v = {}
+    for (o, p), x in [("op", "x")]:
+        v[x] = o, p
+    return v
+
+
+def issue_19153_3():
+    v = {}
+    for o, (x,) in ["ox"]: 
+        v[(x,)] = o 
+    return v

crates/ruff_linter/src/rules/perflint/rules/manual_dict_comprehension.rs

@@ -354,21 +354,37 @@ fn convert_to_dict_comprehension(
         "for"
     };
     // Handles the case where `key` has a trailing comma, e.g, `dict[x,] = y`
-    let key_range = if let Expr::Tuple(ast::ExprTuple { elts, .. }) = key {
-        let [expr] = elts.as_slice() else {
+    let key_str = if let Expr::Tuple(ast::ExprTuple {
+        elts,
+        parenthesized,
+        ..
+    }) = key
+    {
+        if elts.len() != 1 {
             return None;
-        };
-        expr.range()
+        }
+        if *parenthesized {
+            locator.slice(key).to_string()
         } else {
-        key.range()
+            format!("({})", locator.slice(key))
+        }
+    } else {
+        locator.slice(key).to_string()
     };
-    let elt_str = format!(
-        "{}: {}",
-        locator.slice(key_range),
-        locator.slice(value.range())
-    );
 
-    let comprehension_str = format!("{{{elt_str} {for_type} {target_str} in {iter_str}{if_str}}}");
+    // If the value is a tuple without parentheses, add them
+    let value_str = if let Expr::Tuple(ast::ExprTuple {
+        parenthesized: false,
+        ..
+    }) = value
+    {
+        format!("({})", locator.slice(value))
+    } else {
+        locator.slice(value).to_string()
+    };
+
+    let comprehension_str =
+        format!("{{{key_str}: {value_str} {for_type} {target_str} in {iter_str}{if_str}}}");
 
     let for_loop_inline_comments = comment_strings_in_range(
         checker,

crates/ruff_linter/src/rules/perflint/snapshots/ruff_linter__rules__perflint__tests__PERF403_PERF403.py.snap

@@ -176,3 +176,36 @@ PERF403 Use a dictionary comprehension instead of a for-loop
     |         ^^^^^^^
     |
 help: Replace for loop with dict comprehension
+
+PERF403 Use a dictionary comprehension instead of a for-loop
+   --> PERF403.py:200:9
+    |
+198 |     v = {}
+199 |     for o, (x,) in ["ox"]:
+200 |         v[x,] = o
+    |         ^^^^^^^^^
+201 |     return v
+    |
+help: Replace for loop with dict comprehension
+
+PERF403 Use a dictionary comprehension instead of a for-loop
+   --> PERF403.py:207:9
+    |
+205 |     v = {}
+206 |     for (o, p), x in [("op", "x")]:
+207 |         v[x] = o, p
+    |         ^^^^^^^^^^^
+208 |     return v
+    |
+help: Replace for loop with dict comprehension
+
+PERF403 Use a dictionary comprehension instead of a for-loop
+   --> PERF403.py:214:9
+    |
+212 |     v = {}
+213 |     for o, (x,) in ["ox"]: 
+214 |         v[(x,)] = o 
+    |         ^^^^^^^^^^^
+215 |     return v
+    |
+help: Replace for loop with dict comprehension

crates/ruff_linter/src/rules/perflint/snapshots/ruff_linter__rules__perflint__tests__preview__PERF403_PERF403.py.snap

@@ -372,8 +372,72 @@ help: Replace for loop with dict comprehension
     -     v = {}
     -     for o,(x,)in():
     -         v[x,]=o
-170 +     v = {x: o for o,(x,) in ()}
+170 +     v = {(x,): o for o,(x,) in ()}
 171 | 
 172 | 
 173 | # https://github.com/astral-sh/ruff/issues/19005
 note: This is an unsafe fix and may change runtime behavior
+
+PERF403 [*] Use a dictionary comprehension instead of a for-loop
+   --> PERF403.py:200:9
+    |
+198 |     v = {}
+199 |     for o, (x,) in ["ox"]:
+200 |         v[x,] = o
+    |         ^^^^^^^^^
+201 |     return v
+    |
+help: Replace for loop with dict comprehension
+195 | 
+196 | 
+197 | def issue_19153_1():
+    -     v = {}
+    -     for o, (x,) in ["ox"]:
+    -         v[x,] = o
+198 +     v = {(x,): o for o, (x,) in ["ox"]}
+199 |     return v
+200 | 
+201 | 
+note: This is an unsafe fix and may change runtime behavior
+
+PERF403 [*] Use a dictionary comprehension instead of a for-loop
+   --> PERF403.py:207:9
+    |
+205 |     v = {}
+206 |     for (o, p), x in [("op", "x")]:
+207 |         v[x] = o, p
+    |         ^^^^^^^^^^^
+208 |     return v
+    |
+help: Replace for loop with dict comprehension
+202 | 
+203 | 
+204 | def issue_19153_2():
+    -     v = {}
+    -     for (o, p), x in [("op", "x")]:
+    -         v[x] = o, p
+205 +     v = {x: (o, p) for (o, p), x in [("op", "x")]}
+206 |     return v
+207 | 
+208 | 
+note: This is an unsafe fix and may change runtime behavior
+
+PERF403 [*] Use a dictionary comprehension instead of a for-loop
+   --> PERF403.py:214:9
+    |
+212 |     v = {}
+213 |     for o, (x,) in ["ox"]: 
+214 |         v[(x,)] = o 
+    |         ^^^^^^^^^^^
+215 |     return v
+    |
+help: Replace for loop with dict comprehension
+209 | 
+210 | 
+211 | def issue_19153_3():
+    -     v = {}
+    -     for o, (x,) in ["ox"]: 
+    -         v[(x,)] = o 
+212 +     v = {(x,): o for o, (x,) in ["ox"]} 
+213 |     return v
+note: This is an unsafe fix and may change runtime behavior