[`flake8-pyi`] Implement `PYI046` (#6098)

## Summary
Checks for the presence of unused private `typing.Protocol` definitions.

ref #848 

## Test Plan

Snapshots and manual runs of flake8.

crates/ruff/resources/test/fixtures/flake8_pyi/PYI046.py

@@ -0,0 +1,18 @@
+import typing
+from typing import Protocol
+
+
+class _Foo(Protocol):
+    bar: int
+
+
+class _Bar(typing.Protocol):
+    bar: int
+
+
+# OK
+class _UsedPrivateProtocol(Protocol):
+    bar: int
+
+
+def uses__UsedPrivateProtocol(arg: _UsedPrivateProtocol) -> None: ...

crates/ruff/resources/test/fixtures/flake8_pyi/PYI046.pyi

@@ -0,0 +1,18 @@
+import typing
+from typing import Protocol
+
+
+class _Foo(object, Protocol):
+    bar: int
+
+
+class _Bar(typing.Protocol):
+    bar: int
+
+
+# OK
+class _UsedPrivateProtocol(Protocol):
+    bar: int
+
+
+def uses__UsedPrivateProtocol(arg: _UsedPrivateProtocol) -> None: ...

crates/ruff/src/checkers/ast/analyze/bindings.rs

@@ -12,6 +12,7 @@ pub(crate) fn bindings(checker: &mut Checker) {
         Rule::UnconventionalImportAlias,
         Rule::UnusedPrivateTypeVar,
         Rule::UnusedVariable,
+        Rule::UnusedPrivateProtocol,
     ]) {
         return;
     }
@@ -71,6 +72,13 @@ pub(crate) fn bindings(checker: &mut Checker) {
                     checker.diagnostics.push(diagnostic);
                 }
             }
+            if checker.enabled(Rule::UnusedPrivateProtocol) {
+                if let Some(diagnostic) =
+                    flake8_pyi::rules::unused_private_protocol(checker, binding)
+                {
+                    checker.diagnostics.push(diagnostic);
+                }
+            }
         }
     }
 }

crates/ruff/src/codes.rs

@@ -650,6 +650,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Pyi, "043") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::TSuffixedTypeAlias),
         (Flake8Pyi, "044") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::FutureAnnotationsInStub),
         (Flake8Pyi, "045") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::IterMethodReturnIterable),
+        (Flake8Pyi, "046") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::UnusedPrivateProtocol),
         (Flake8Pyi, "048") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::StubBodyMultipleStatements),
         (Flake8Pyi, "050") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::NoReturnArgumentAnnotationInStub),
         (Flake8Pyi, "052") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::UnannotatedAssignmentInStub),

crates/ruff/src/rules/flake8_pyi/mod.rs

@@ -95,6 +95,8 @@ mod tests {
     #[test_case(Rule::UnsupportedMethodCallOnAll, Path::new("PYI056.pyi"))]
     #[test_case(Rule::UnusedPrivateTypeVar, Path::new("PYI018.py"))]
     #[test_case(Rule::UnusedPrivateTypeVar, Path::new("PYI018.pyi"))]
+    #[test_case(Rule::UnusedPrivateProtocol, Path::new("PYI046.py"))]
+    #[test_case(Rule::UnusedPrivateProtocol, Path::new("PYI046.pyi"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(

crates/ruff/src/rules/flake8_pyi/rules/unused_private_type_definition.rs

@@ -10,7 +10,7 @@ use crate::checkers::ast::Checker;
 ///
 /// ## Why is this bad?
 /// A private `TypeVar` that is defined but not used is likely a mistake, and
-/// should be removed to avoid confusion.
+/// should either be used, made public, or removed to avoid confusion.
 ///
 /// ## Example
 /// ```python
@@ -31,9 +31,51 @@ impl Violation for UnusedPrivateTypeVar {
     }
 }
 
+/// ## What it does
+/// Checks for the presence of unused private `typing.Protocol` definitions.
+///
+/// ## Why is this bad?
+/// A private `typing.Protocol` that is defined but not used is likely a
+/// mistake, and should either be used, made public, or removed to avoid
+/// confusion.
+///
+/// ## Example
+/// ```python
+/// import typing
+///
+///
+/// class _PrivateProtocol(typing.Protocol):
+///     foo: int
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import typing
+///
+///
+/// class _PrivateProtocol(typing.Protocol):
+///     foo: int
+///
+///
+/// def func(arg: _PrivateProtocol) -> None:
+///     ...
+/// ```
+#[violation]
+pub struct UnusedPrivateProtocol {
+    name: String,
+}
+
+impl Violation for UnusedPrivateProtocol {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let UnusedPrivateProtocol { name } = self;
+        format!("Private protocol `{name}` is never used")
+    }
+}
+
 /// PYI018
 pub(crate) fn unused_private_type_var(checker: &Checker, binding: &Binding) -> Option<Diagnostic> {
-    if !(binding.kind.is_assignment() && binding.is_private_variable()) {
+    if !(binding.kind.is_assignment() && binding.is_private_declaration()) {
         return None;
     }
     if binding.is_used() {
@@ -64,3 +106,35 @@ pub(crate) fn unused_private_type_var(checker: &Checker, binding: &Binding) -> O
         binding.range,
     ))
 }
+
+/// PYI046
+pub(crate) fn unused_private_protocol(checker: &Checker, binding: &Binding) -> Option<Diagnostic> {
+    if !(binding.kind.is_class_definition() && binding.is_private_declaration()) {
+        return None;
+    }
+    if binding.is_used() {
+        return None;
+    }
+
+    let Some(source) = binding.source else {
+        return None;
+    };
+    let Stmt::ClassDef(ast::StmtClassDef { name, bases, .. }) = checker.semantic().stmts[source]
+    else {
+        return None;
+    };
+
+    if !bases
+        .iter()
+        .any(|base| checker.semantic().match_typing_expr(base, "Protocol"))
+    {
+        return None;
+    }
+
+    Some(Diagnostic::new(
+        UnusedPrivateProtocol {
+            name: name.to_string(),
+        },
+        binding.range,
+    ))
+}

crates/ruff/src/rules/flake8_pyi/snapshots/ruff__rules__flake8_pyi__tests__PYI046_PYI046.py.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff/src/rules/flake8_pyi/mod.rs
+---
+

crates/ruff/src/rules/flake8_pyi/snapshots/ruff__rules__flake8_pyi__tests__PYI046_PYI046.pyi.snap

@@ -0,0 +1,18 @@
+---
+source: crates/ruff/src/rules/flake8_pyi/mod.rs
+---
+PYI046.pyi:5:7: PYI046 Private protocol `_Foo` is never used
+  |
+5 | class _Foo(object, Protocol):
+  |       ^^^^ PYI046
+6 |     bar: int
+  |
+
+PYI046.pyi:9:7: PYI046 Private protocol `_Bar` is never used
+   |
+ 9 | class _Bar(typing.Protocol):
+   |       ^^^^ PYI046
+10 |     bar: int
+   |
+
+

crates/ruff_python_semantic/src/binding.rs

@@ -94,9 +94,9 @@ impl<'a> Binding<'a> {
         )
     }
 
-    /// Return `true` if this [`Binding`] represents an private variable
+    /// Return `true` if this [`Binding`] represents an private declaration
     /// (e.g., `_x` in `_x = "private variable"`)
-    pub const fn is_private_variable(&self) -> bool {
+    pub const fn is_private_declaration(&self) -> bool {
         self.flags.contains(BindingFlags::PRIVATE_DECLARATION)
     }
 

ruff.schema.json

@@ -2389,6 +2389,7 @@
         "PYI043",
         "PYI044",
         "PYI045",
+        "PYI046",
         "PYI048",
         "PYI05",
         "PYI050",