Retain extra ellipses in protocols and abstract methods (#8769)

## Summary

It turns out that some type checkers rely on the presence of ellipses in
`Protocol` interfaces and abstract methods, in order to differentiate
between default implementations and stubs. This PR modifies the preview
behavior of `PIE790` to avoid flagging "unnecessary" ellipses in such
cases.

Closes https://github.com/astral-sh/ruff/issues/8756.

## Test Plan

`cargo test`

crates/ruff_linter/resources/test/fixtures/flake8_pie/PIE790.py

@@ -177,3 +177,33 @@ for i in range(10):
 for i in range(10):
     ...
     pass
+
+from typing import Protocol
+
+
+class Repro(Protocol):
+    def func(self) -> str:
+        """Docstring"""
+        ...
+
+    def impl(self) -> str:
+        """Docstring"""
+        return self.func()
+
+
+import abc
+
+
+class Repro:
+    @abc.abstractmethod
+    def func(self) -> str:
+        """Docstring"""
+        ...
+
+    def impl(self) -> str:
+        """Docstring"""
+        return self.func()
+
+    def stub(self) -> str:
+        """Docstring"""
+        ...

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

@@ -3,6 +3,7 @@ use ruff_diagnostics::{Diagnostic, Edit, Fix};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::whitespace::trailing_comment_start_offset;
 use ruff_python_ast::Stmt;
+use ruff_python_semantic::{ScopeKind, SemanticModel};
 use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
@@ -93,6 +94,12 @@ pub(crate) fn unnecessary_placeholder(checker: &mut Checker, body: &[Stmt]) {
                 if expr.value.is_ellipsis_literal_expr()
                     && checker.settings.preview.is_enabled() =>
             {
+                // Ellipses are significant in protocol methods and abstract methods. Specifically,
+                // Pyright uses the presence of an ellipsis to indicate that a method is a stub,
+                // rather than a default implementation.
+                if in_protocol_or_abstract_method(checker.semantic()) {
+                    return;
+                }
                 Placeholder::Ellipsis
             }
             _ => continue,
@@ -125,3 +132,21 @@ impl std::fmt::Display for Placeholder {
         }
     }
 }
+
+/// Return `true` if the [`SemanticModel`] is in a `typing.Protocol` subclass or an abstract
+/// method.
+fn in_protocol_or_abstract_method(semantic: &SemanticModel) -> bool {
+    semantic.current_scopes().any(|scope| match scope.kind {
+        ScopeKind::Class(class_def) => class_def
+            .bases()
+            .iter()
+            .any(|base| semantic.match_typing_expr(base, "Protocol")),
+        ScopeKind::Function(function_def) => {
+            ruff_python_semantic::analyze::visibility::is_abstract(
+                &function_def.decorator_list,
+                semantic,
+            )
+        }
+        _ => false,
+    })
+}

crates/ruff_linter/src/rules/flake8_pie/snapshots/ruff_linter__rules__flake8_pie__tests__PIE790_PIE790.py.snap

@@ -473,6 +473,8 @@ PIE790.py:179:5: PIE790 [*] Unnecessary `pass` statement
 178 |     ...
 179 |     pass
     |     ^^^^ PIE790
+180 | 
+181 | from typing import Protocol
     |
     = help: Remove unnecessary `pass`
 
@@ -481,5 +483,8 @@ PIE790.py:179:5: PIE790 [*] Unnecessary `pass` statement
 177 177 | for i in range(10):
 178 178 |     ...
 179     |-    pass
+180 179 | 
+181 180 | from typing import Protocol
+182 181 | 
 
 

crates/ruff_linter/src/rules/flake8_pie/snapshots/ruff_linter__rules__flake8_pie__tests__preview__PIE790_PIE790.py.snap

@@ -634,6 +634,8 @@ PIE790.py:178:5: PIE790 [*] Unnecessary `...` literal
 177 177 | for i in range(10):
 178     |-    ...
 179 178 |     pass
+180 179 | 
+181 180 | from typing import Protocol
 
 PIE790.py:179:5: PIE790 [*] Unnecessary `pass` statement
     |
@@ -641,6 +643,8 @@ PIE790.py:179:5: PIE790 [*] Unnecessary `pass` statement
 178 |     ...
 179 |     pass
     |     ^^^^ PIE790
+180 | 
+181 | from typing import Protocol
     |
     = help: Remove unnecessary `pass`
 
@@ -649,5 +653,23 @@ PIE790.py:179:5: PIE790 [*] Unnecessary `pass` statement
 177 177 | for i in range(10):
 178 178 |     ...
 179     |-    pass
+180 179 | 
+181 180 | from typing import Protocol
+182 181 | 
+
+PIE790.py:209:9: PIE790 [*] Unnecessary `...` literal
+    |
+207 |     def stub(self) -> str:
+208 |         """Docstring"""
+209 |         ...
+    |         ^^^ PIE790
+    |
+    = help: Remove unnecessary `...`
+
+ℹ Safe fix
+206 206 | 
+207 207 |     def stub(self) -> str:
+208 208 |         """Docstring"""
+209     |-        ...