[`flake8-pie`] Remove following comma correctly when the unpacked dictionary is empty (`PIE800`) (#16008)

## Summary

Resolves #15997.

Ruff used to introduce syntax errors while fixing these cases, but no
longer will:

```python
{"a": [], **{},}
#         ^^^^ Removed, leaving two contiguous commas

{"a": [], **({})}
#         ^^^^^ Removed, leaving a stray closing parentheses
```

Previously, the function would take a shortcut if the unpacked
dictionary is empty; now, both cases are handled using the same logic
introduced in #15394. This change slightly modifies that logic to also
remove the first comma following the dictionary, if and only if it is
empty.

## Test Plan

`cargo nextest run` and `cargo insta test`.

crates/ruff_linter/resources/test/fixtures/flake8_pie/PIE800.py

@@ -70,6 +70,32 @@ foo({**foo, **{"bar": True}})  # PIE800
     ,
 })
 
+{
+    "data": [],
+    ** # Foo
+    (  # Comment
+        { "a": b,
+          # Comment
+          }
+    ) ,
+    c: 9,
+}
+
+
+# https://github.com/astral-sh/ruff/issues/15997
+{"a": [], **{},}
+{"a": [], **({}),}
+
+{"a": [], **{}, 6: 3}
+{"a": [], **({}), 6: 3}
+
+{"a": [], **{
+    # Comment
+}, 6: 3}
+{"a": [], **({
+    # Comment
+}), 6: 3}
+
 
 {**foo,    "bar": True  }  # OK
 

crates/ruff_linter/src/rules/flake8_pie/rules/unnecessary_spread.rs

@@ -2,7 +2,7 @@ use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
 use ruff_macros::{derive_message_formats, ViolationMetadata};
 use ruff_python_ast::{self as ast, Expr};
 use ruff_python_parser::{TokenKind, Tokens};
-use ruff_text_size::{Ranged, TextSize};
+use ruff_text_size::{Ranged, TextLen, TextSize};
 
 use crate::checkers::ast::Checker;
 
@@ -75,8 +75,12 @@ fn unnecessary_spread_fix(
         .iter()
         .find(|tok| matches!(tok.kind(), TokenKind::DoubleStar))?;
 
-    if let Some(last) = dict.iter_values().last() {
+    let (empty, last_value_end) = if let Some(last) = dict.iter_values().last() {
-        // Ex) `**{a: 1, b: 2}`
+        (false, last.end())
+    } else {
+        (true, dict.start() + "{".text_len())
+    };
+
     let mut edits = vec![];
     let mut open_parens: u32 = 0;
 
@@ -99,8 +103,10 @@ fn unnecessary_spread_fix(
     }
 
     let mut found_r_curly = false;
-        for tok in tokens.after(last.end()) {
+    let mut found_dict_comma = false;
-            if found_r_curly && open_parens == 0 {
+
+    for tok in tokens.after(last_value_end) {
+        if found_r_curly && open_parens == 0 && (!empty || found_dict_comma) {
             break;
         }
 
@@ -108,6 +114,10 @@ fn unnecessary_spread_fix(
             kind if kind.is_trivia() => {}
             TokenKind::Comma => {
                 edits.push(Edit::range_deletion(tok.range()));
+
+                if found_r_curly {
+                    found_dict_comma = true;
+                }
             }
             TokenKind::Rpar => {
                 if found_r_curly {
@@ -116,6 +126,10 @@ fn unnecessary_spread_fix(
                 }
             }
             TokenKind::Rbrace => {
+                if found_r_curly {
+                    break;
+                }
+
                 edits.push(Edit::range_deletion(tok.range()));
                 found_r_curly = true;
             }
@@ -130,11 +144,4 @@ fn unnecessary_spread_fix(
         Edit::range_deletion(doublestar.range()),
         edits,
     ))
-    } else {
-        // Ex) `**{}`
-        Some(Fix::safe_edit(Edit::deletion(
-            doublestar.start(),
-            dict.end(),
-        )))
-    }
 }

crates/ruff_linter/src/rules/flake8_pie/snapshots/ruff_linter__rules__flake8_pie__tests__PIE800_PIE800.py.snap

@@ -296,3 +296,167 @@ PIE800.py:65:1: PIE800 [*] Unnecessary spread `**`
 70 70 |     ,
 71 71 | })
 72 72 | 
+
+PIE800.py:77:9: PIE800 [*] Unnecessary spread `**`
+   |
+75 |       ** # Foo
+76 |       (  # Comment
+77 | /         { "a": b,
+78 | |           # Comment
+79 | |           }
+   | |___________^ PIE800
+80 |       ) ,
+81 |       c: 9,
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+72 72 | 
+73 73 | {
+74 74 |     "data": [],
+75    |-    ** # Foo
+76    |-    (  # Comment
+77    |-        { "a": b,
+   75 |+     # Foo
+   76 |+      # Comment
+   77 |+         "a": b
+78 78 |           # Comment
+79    |-          }
+80    |-    ) ,
+   79 |+          
+   80 |+     ,
+81 81 |     c: 9,
+82 82 | }
+83 83 | 
+
+PIE800.py:86:13: PIE800 [*] Unnecessary spread `**`
+   |
+85 | # https://github.com/astral-sh/ruff/issues/15997
+86 | {"a": [], **{},}
+   |             ^^ PIE800
+87 | {"a": [], **({}),}
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+83 83 | 
+84 84 | 
+85 85 | # https://github.com/astral-sh/ruff/issues/15997
+86    |-{"a": [], **{},}
+   86 |+{"a": [], }
+87 87 | {"a": [], **({}),}
+88 88 | 
+89 89 | {"a": [], **{}, 6: 3}
+
+PIE800.py:87:14: PIE800 [*] Unnecessary spread `**`
+   |
+85 | # https://github.com/astral-sh/ruff/issues/15997
+86 | {"a": [], **{},}
+87 | {"a": [], **({}),}
+   |              ^^ PIE800
+88 |
+89 | {"a": [], **{}, 6: 3}
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+84 84 | 
+85 85 | # https://github.com/astral-sh/ruff/issues/15997
+86 86 | {"a": [], **{},}
+87    |-{"a": [], **({}),}
+   87 |+{"a": [], }
+88 88 | 
+89 89 | {"a": [], **{}, 6: 3}
+90 90 | {"a": [], **({}), 6: 3}
+
+PIE800.py:89:13: PIE800 [*] Unnecessary spread `**`
+   |
+87 | {"a": [], **({}),}
+88 |
+89 | {"a": [], **{}, 6: 3}
+   |             ^^ PIE800
+90 | {"a": [], **({}), 6: 3}
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+86 86 | {"a": [], **{},}
+87 87 | {"a": [], **({}),}
+88 88 | 
+89    |-{"a": [], **{}, 6: 3}
+   89 |+{"a": [],  6: 3}
+90 90 | {"a": [], **({}), 6: 3}
+91 91 | 
+92 92 | {"a": [], **{
+
+PIE800.py:90:14: PIE800 [*] Unnecessary spread `**`
+   |
+89 | {"a": [], **{}, 6: 3}
+90 | {"a": [], **({}), 6: 3}
+   |              ^^ PIE800
+91 |
+92 | {"a": [], **{
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+87 87 | {"a": [], **({}),}
+88 88 | 
+89 89 | {"a": [], **{}, 6: 3}
+90    |-{"a": [], **({}), 6: 3}
+   90 |+{"a": [],  6: 3}
+91 91 | 
+92 92 | {"a": [], **{
+93 93 |     # Comment
+
+PIE800.py:92:13: PIE800 [*] Unnecessary spread `**`
+   |
+90 |   {"a": [], **({}), 6: 3}
+91 |
+92 |   {"a": [], **{
+   |  _____________^
+93 | |     # Comment
+94 | | }, 6: 3}
+   | |_^ PIE800
+95 |   {"a": [], **({
+96 |       # Comment
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+89 89 | {"a": [], **{}, 6: 3}
+90 90 | {"a": [], **({}), 6: 3}
+91 91 | 
+92    |-{"a": [], **{
+   92 |+{"a": [], 
+93 93 |     # Comment
+94    |-}, 6: 3}
+   94 |+ 6: 3}
+95 95 | {"a": [], **({
+96 96 |     # Comment
+97 97 | }), 6: 3}
+
+PIE800.py:95:14: PIE800 [*] Unnecessary spread `**`
+   |
+93 |       # Comment
+94 |   }, 6: 3}
+95 |   {"a": [], **({
+   |  ______________^
+96 | |     # Comment
+97 | | }), 6: 3}
+   | |_^ PIE800
+   |
+   = help: Remove unnecessary dict
+
+ℹ Safe fix
+92 92 | {"a": [], **{
+93 93 |     # Comment
+94 94 | }, 6: 3}
+95    |-{"a": [], **({
+   95 |+{"a": [], 
+96 96 |     # Comment
+97    |-}), 6: 3}
+   97 |+ 6: 3}
+98 98 | 
+99 99 | 
+100 100 | {**foo,    "bar": True  }  # OK