[flake8-pyi] Implement PYI017 (#5895)

## Summary

Implements `PYI017` or `Y017` from `flake8-pyi` plug-in. Mirrors
[upstream
implementation](https://github.com/PyCQA/flake8-pyi/blob/ceab86d16b822d12abae1d8087850d0bc66d2f52/pyi.py#L1039-L1048).
It checks for any assignment with more than 1 target or an assignment to
anything other than a name, and raises a violation for these in stub
files.

Couldn't find a clear and concise explanation for why this is to be
avoided and what is preferred for attribute cases like:

```python
a.b = int
```
So welcome some input there, to learn and to finish up the docs.

## Test Plan

Added test cases from upstream plug-in in a fixture (both `.py` and
`.pyi`). Added a few more.

## Issue link

Refers: #848

crates/ruff/resources/test/fixtures/flake8_pyi/PYI017.py

@@ -0,0 +1,14 @@
+var: int
+a = var  # OK
+
+b = c = int  # OK
+
+a.b = int  # OK
+
+d, e = int, str  # OK
+
+f, g, h = int, str, TypeVar("T")  # OK
+
+i: TypeAlias = int | str  # OK
+
+j: TypeAlias = int  # OK

crates/ruff/resources/test/fixtures/flake8_pyi/PYI017.pyi

@@ -0,0 +1,14 @@
+var: int
+a = var  # OK
+
+b = c = int  # PYI017
+
+a.b = int  # PYI017
+
+d, e = int, str  # PYI017
+
+f, g, h = int, str, TypeVar("T")  # PYI017
+
+i: TypeAlias = int | str  # OK
+
+j: TypeAlias = int  # OK

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

@@ -1492,7 +1492,7 @@ where
                     tryceratops::rules::error_instead_of_exception(self, handlers);
                 }
             }
-            Stmt::Assign(ast::StmtAssign { targets, value, .. }) => {
+            Stmt::Assign(stmt_assign @ ast::StmtAssign { targets, value, .. }) => {
                 if self.enabled(Rule::LambdaAssignment) {
                     if let [target] = &targets[..] {
                         pycodestyle::rules::lambda_assignment(self, target, value, None, stmt);
@@ -1557,6 +1557,7 @@ where
                         Rule::UnprefixedTypeParam,
                         Rule::AssignmentDefaultInStub,
                         Rule::UnannotatedAssignmentInStub,
+                        Rule::ComplexAssignmentInStub,
                         Rule::TypeAliasWithoutAnnotation,
                     ]) {
                         // Ignore assignments in function bodies; those are covered by other rules.
@@ -1576,6 +1577,9 @@ where
                                     self, targets, value,
                                 );
                             }
+                            if self.enabled(Rule::ComplexAssignmentInStub) {
+                                flake8_pyi::rules::complex_assignment_in_stub(self, stmt_assign);
+                            }
                             if self.enabled(Rule::TypeAliasWithoutAnnotation) {
                                 flake8_pyi::rules::type_alias_without_annotation(
                                     self, value, targets,

crates/ruff/src/codes.rs

@@ -629,6 +629,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Pyi, "014") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::ArgumentDefaultInStub),
         (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, "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

@@ -25,6 +25,8 @@ mod tests {
     #[test_case(Rule::BadVersionInfoComparison, Path::new("PYI006.pyi"))]
     #[test_case(Rule::CollectionsNamedTuple, Path::new("PYI024.py"))]
     #[test_case(Rule::CollectionsNamedTuple, Path::new("PYI024.pyi"))]
+    #[test_case(Rule::ComplexAssignmentInStub, Path::new("PYI017.py"))]
+    #[test_case(Rule::ComplexAssignmentInStub, Path::new("PYI017.pyi"))]
     #[test_case(Rule::ComplexIfStatementInStub, Path::new("PYI002.py"))]
     #[test_case(Rule::ComplexIfStatementInStub, Path::new("PYI002.pyi"))]
     #[test_case(Rule::DocstringInStub, Path::new("PYI021.py"))]

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

@@ -0,0 +1,54 @@
+use rustpython_parser::ast::{Expr, StmtAssign};
+
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for assignments with multiple or non-name targets in stub files.
+///
+/// ## Why is this bad?
+/// In general, stub files should be thought of as "data files" for a type
+/// checker, and are not intended to be executed. As such, it's useful to
+/// enforce that only a subset of Python syntax is allowed in a stub file, to
+/// ensure that everything in the stub is unambiguous for the type checker.
+///
+/// The need to perform multi-assignment, or assignment to a non-name target,
+/// likely indicates a misunderstanding of how stub files are intended to be
+/// used.
+///
+/// ## Example
+/// ```python
+/// a = b = int
+/// a.b = int
+/// ```
+///
+/// Use instead:
+/// ```python
+/// a: TypeAlias = int
+/// b: TypeAlias = int
+///
+///
+/// class a:
+///     b: int
+/// ```
+#[violation]
+pub struct ComplexAssignmentInStub;
+
+impl Violation for ComplexAssignmentInStub {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Stubs should not contain assignments to attributes or multiple targets")
+    }
+}
+
+/// PYI017
+pub(crate) fn complex_assignment_in_stub(checker: &mut Checker, stmt: &StmtAssign) {
+    if matches!(stmt.targets.as_slice(), [Expr::Name(_)]) {
+        return;
+    }
+    checker
+        .diagnostics
+        .push(Diagnostic::new(ComplexAssignmentInStub, stmt.range));
+}

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

@@ -1,6 +1,7 @@
 pub(crate) use any_eq_ne_annotation::*;
 pub(crate) use bad_version_info_comparison::*;
 pub(crate) use collections_named_tuple::*;
+pub(crate) use complex_assignment_in_stub::*;
 pub(crate) use complex_if_statement_in_stub::*;
 pub(crate) use docstring_in_stubs::*;
 pub(crate) use duplicate_union_member::*;
@@ -31,6 +32,7 @@ pub(crate) use unrecognized_version_info::*;
 mod any_eq_ne_annotation;
 mod bad_version_info_comparison;
 mod collections_named_tuple;
+mod complex_assignment_in_stub;
 mod complex_if_statement_in_stub;
 mod docstring_in_stubs;
 mod duplicate_union_member;

crates/ruff/src/rules/flake8_pyi/snapshots/ruff__rules__flake8_pyi__tests__PYI017_PYI017.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__PYI017_PYI017.pyi.snap

@@ -0,0 +1,44 @@
+---
+source: crates/ruff/src/rules/flake8_pyi/mod.rs
+---
+PYI017.pyi:4:1: PYI017 Stubs should not contain assignments to attributes or multiple targets
+  |
+2 | a = var  # OK
+3 | 
+4 | b = c = int  # PYI017
+  | ^^^^^^^^^^^ PYI017
+5 | 
+6 | a.b = int  # PYI017
+  |
+
+PYI017.pyi:6:1: PYI017 Stubs should not contain assignments to attributes or multiple targets
+  |
+4 | b = c = int  # PYI017
+5 | 
+6 | a.b = int  # PYI017
+  | ^^^^^^^^^ PYI017
+7 | 
+8 | d, e = int, str  # PYI017
+  |
+
+PYI017.pyi:8:1: PYI017 Stubs should not contain assignments to attributes or multiple targets
+   |
+ 6 | a.b = int  # PYI017
+ 7 | 
+ 8 | d, e = int, str  # PYI017
+   | ^^^^^^^^^^^^^^^ PYI017
+ 9 | 
+10 | f, g, h = int, str, TypeVar("T")  # PYI017
+   |
+
+PYI017.pyi:10:1: PYI017 Stubs should not contain assignments to attributes or multiple targets
+   |
+ 8 | d, e = int, str  # PYI017
+ 9 | 
+10 | f, g, h = int, str, TypeVar("T")  # PYI017
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PYI017
+11 | 
+12 | i: TypeAlias = int | str  # OK
+   |
+
+