[flake8-pyi] Implement PYI018 (#6018)

## Summary

Check for unused private `TypeVar`. See [original
implementation](https://github.com/PyCQA/flake8-pyi/blob/2a86db8271dc500247a8a69419536240334731cf/pyi.py#L1958).

```
$ flake8 --select Y018 crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi

crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi:4:1: Y018 TypeVar "_T" is not used
crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi:5:1: Y018 TypeVar "_P" is not used
```

```
$ ./target/debug/ruff --select PYI018 crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi --no-cache

crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi:4:1: PYI018 TypeVar `_T` is never used
crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi:5:1: PYI018 TypeVar `_P` is never used
Found 2 errors.
```
In the file `unused_private_type_declaration.rs`, I'm planning to add
other rules that are similar to `PYI018` like the `PYI046`, `PYI047` and
`PYI049`.

ref #848

## Test Plan

Snapshots and manual runs of flake8.

crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.py

@@ -0,0 +1,12 @@
+import typing
+from typing import TypeVar
+
+_T = typing.TypeVar("_T")
+_P = TypeVar("_P")
+
+# OK
+_UsedTypeVar = TypeVar("_UsedTypeVar")
+def func(arg: _UsedTypeVar) -> _UsedTypeVar: ...
+
+_A, _B = TypeVar("_A"), TypeVar("_B")
+_C = _D = TypeVar("_C")

crates/ruff/resources/test/fixtures/flake8_pyi/PYI018.pyi

@@ -0,0 +1,12 @@
+import typing
+from typing import TypeVar
+
+_T = typing.TypeVar("_T")
+_P = TypeVar("_P")
+
+# OK
+_UsedTypeVar = TypeVar("_UsedTypeVar")
+def func(arg: _UsedTypeVar) -> _UsedTypeVar: ...
+
+_A, _B = TypeVar("_A"), TypeVar("_B")
+_C = _D = TypeVar("_C")

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

@@ -10,6 +10,7 @@ pub(crate) fn bindings(checker: &mut Checker) {
         Rule::InvalidAllObject,
         Rule::UnaliasedCollectionsAbcSetImport,
         Rule::UnconventionalImportAlias,
+        Rule::UnusedPrivateTypeVar,
         Rule::UnusedVariable,
     ]) {
         return;
@@ -63,6 +64,13 @@ pub(crate) fn bindings(checker: &mut Checker) {
                     checker.diagnostics.push(diagnostic);
                 }
             }
+            if checker.enabled(Rule::UnusedPrivateTypeVar) {
+                if let Some(diagnostic) =
+                    flake8_pyi::rules::unused_private_type_var(checker, binding)
+                {
+                    checker.diagnostics.push(diagnostic);
+                }
+            }
         }
     }
 }

crates/ruff/src/checkers/ast/mod.rs

@@ -1474,6 +1474,11 @@ impl<'a> Checker<'a> {
         // Create the `Binding`.
         let binding_id = self.semantic.push_binding(range, kind, flags);
 
+        // If the name is private, mark is as such.
+        if name.starts_with('_') {
+            self.semantic.bindings[binding_id].flags |= BindingFlags::PRIVATE_DECLARATION;
+        }
+
         // If there's an existing binding in this scope, copy its references.
         if let Some(shadowed_id) = self.semantic.scopes[scope_id].get(name) {
             // If this is an annotation, and we already have an existing value in the same scope,

crates/ruff/src/codes.rs

@@ -632,6 +632,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Pyi, "015") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::AssignmentDefaultInStub),
         (Flake8Pyi, "016") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::DuplicateUnionMember),
         (Flake8Pyi, "017") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::ComplexAssignmentInStub),
+        (Flake8Pyi, "018") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::UnusedPrivateTypeVar),
         (Flake8Pyi, "020") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::QuotedAnnotationInStub),
         (Flake8Pyi, "021") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::DocstringInStub),
         (Flake8Pyi, "024") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::CollectionsNamedTuple),

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

@@ -93,6 +93,8 @@ mod tests {
     #[test_case(Rule::TypeAliasWithoutAnnotation, Path::new("PYI026.pyi"))]
     #[test_case(Rule::UnsupportedMethodCallOnAll, Path::new("PYI056.py"))]
     #[test_case(Rule::UnsupportedMethodCallOnAll, Path::new("PYI056.pyi"))]
+    #[test_case(Rule::UnusedPrivateTypeVar, Path::new("PYI018.py"))]
+    #[test_case(Rule::UnusedPrivateTypeVar, Path::new("PYI018.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/mod.rs

@@ -29,6 +29,7 @@ pub(crate) use unnecessary_literal_union::*;
 pub(crate) use unrecognized_platform::*;
 pub(crate) use unrecognized_version_info::*;
 pub(crate) use unsupported_method_call_on_all::*;
+pub(crate) use unused_private_type_definition::*;
 
 mod any_eq_ne_annotation;
 mod bad_version_info_comparison;
@@ -61,3 +62,4 @@ mod unnecessary_literal_union;
 mod unrecognized_platform;
 mod unrecognized_version_info;
 mod unsupported_method_call_on_all;
+mod unused_private_type_definition;

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

@@ -0,0 +1,66 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_semantic::Binding;
+use rustpython_parser::ast::{self, Expr, Stmt};
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for the presence of unused private `TypeVar` declarations.
+///
+/// ## Why is this bad?
+/// A private `TypeVar` that is defined but not used is likely a mistake, and
+/// should be removed to avoid confusion.
+///
+/// ## Example
+/// ```python
+/// import typing
+///
+/// _T = typing.TypeVar("_T")
+/// ```
+#[violation]
+pub struct UnusedPrivateTypeVar {
+    name: String,
+}
+
+impl Violation for UnusedPrivateTypeVar {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let UnusedPrivateTypeVar { name } = self;
+        format!("Private TypeVar `{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()) {
+        return None;
+    }
+    if binding.is_used() {
+        return None;
+    }
+
+    let Some(source) = binding.source else {
+        return None;
+    };
+    let Stmt::Assign(ast::StmtAssign { targets, value, .. }) = checker.semantic().stmts[source]
+    else {
+        return None;
+    };
+    let [Expr::Name(ast::ExprName { id, .. })] = &targets[..] else {
+        return None;
+    };
+    let Expr::Call(ast::ExprCall { func, .. }) = value.as_ref() else {
+        return None;
+    };
+    if !checker.semantic().match_typing_expr(func, "TypeVar") {
+        return None;
+    }
+
+    Some(Diagnostic::new(
+        UnusedPrivateTypeVar {
+            name: id.to_string(),
+        },
+        binding.range,
+    ))
+}

crates/ruff/src/rules/flake8_pyi/snapshots/ruff__rules__flake8_pyi__tests__PYI018_PYI018.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__PYI018_PYI018.pyi.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff/src/rules/flake8_pyi/mod.rs
+---
+PYI018.pyi:4:1: PYI018 Private TypeVar `_T` is never used
+  |
+2 | from typing import TypeVar
+3 | 
+4 | _T = typing.TypeVar("_T")
+  | ^^ PYI018
+5 | _P = TypeVar("_P")
+  |
+
+PYI018.pyi:5:1: PYI018 Private TypeVar `_P` is never used
+  |
+4 | _T = typing.TypeVar("_T")
+5 | _P = TypeVar("_P")
+  | ^^ PYI018
+6 | 
+7 | # OK
+  |
+
+

crates/ruff_python_semantic/src/binding.rs

@@ -94,6 +94,12 @@ impl<'a> Binding<'a> {
         )
     }
 
+    /// Return `true` if this [`Binding`] represents an private variable
+    /// (e.g., `_x` in `_x = "private variable"`)
+    pub const fn is_private_variable(&self) -> bool {
+        self.flags.contains(BindingFlags::PRIVATE_DECLARATION)
+    }
+
     /// Return `true` if this binding redefines the given binding.
     pub fn redefines(&self, existing: &'a Binding) -> bool {
         match &self.kind {
@@ -264,6 +270,14 @@ bitflags! {
         /// __all__ = [1]
         /// ```
         const INVALID_ALL_OBJECT = 1 << 6;
+
+        /// The binding represents a private declaration.
+        ///
+        /// For example, the binding could be `_T` in:
+        /// ```python
+        /// _T = "This is a private variable"
+        /// ```
+        const PRIVATE_DECLARATION = 1 << 7;
     }
 }