Detect unused-asyncio-dangling-task (RUF006) on unused assignments (#9060)

## Summary

Fixes #8863 : Detect asyncio-dangling-task (RUF006) when discarding
return value

## Test Plan

added new two testcases, changed result of an old one that was made more
specific

crates/ruff_linter/resources/test/fixtures/ruff/RUF006.py

@@ -63,11 +63,29 @@ def f():
     tasks = [asyncio.create_task(task) for task in tasks]
 
 
-# OK (false negative)
+# Error
 def f():
     task = asyncio.create_task(coordinator.ws_connect())
 
 
+# Error
+def f():
+    loop = asyncio.get_running_loop()
+    task: asyncio.Task = loop.create_task(coordinator.ws_connect())
+
+
+# OK (potential false negative)
+def f():
+    task = asyncio.create_task(coordinator.ws_connect())
+    background_tasks.add(task)
+
+
+# OK
+async def f():
+    task = asyncio.create_task(coordinator.ws_connect())
+    await task
+
+
 # OK (potential false negative)
 def f():
     do_nothing_with_the_task(asyncio.create_task(coordinator.ws_connect()))
@@ -88,3 +106,19 @@ def f():
 def f():
     loop = asyncio.get_running_loop()
     loop.do_thing(coordinator.ws_connect())
+
+
+# OK
+async def f():
+    task = unused = asyncio.create_task(coordinator.ws_connect())
+    await task
+
+
+# OK (false negative)
+async def f():
+    task = unused = asyncio.create_task(coordinator.ws_connect())
+
+
+# OK
+async def f():
+    task[i] = asyncio.create_task(coordinator.ws_connect())

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

@@ -3,11 +3,12 @@ use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
 use crate::codes::Rule;
-use crate::rules::{flake8_import_conventions, flake8_pyi, pyflakes, pylint};
+use crate::rules::{flake8_import_conventions, flake8_pyi, pyflakes, pylint, ruff};
 
 /// Run lint rules over the [`Binding`]s.
 pub(crate) fn bindings(checker: &mut Checker) {
     if !checker.any_enabled(&[
+        Rule::AsyncioDanglingTask,
         Rule::InvalidAllFormat,
         Rule::InvalidAllObject,
         Rule::NonAsciiName,
@@ -71,5 +72,12 @@ pub(crate) fn bindings(checker: &mut Checker) {
                 checker.diagnostics.push(diagnostic);
             }
         }
+        if checker.enabled(Rule::AsyncioDanglingTask) {
+            if let Some(diagnostic) =
+                ruff::rules::asyncio_dangling_binding(binding, &checker.semantic)
+            {
+                checker.diagnostics.push(diagnostic);
+            }
+        }
     }
 }

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -1571,7 +1571,11 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                 pylint::rules::named_expr_without_context(checker, value);
             }
             if checker.enabled(Rule::AsyncioDanglingTask) {
-                ruff::rules::asyncio_dangling_task(checker, value);
+                if let Some(diagnostic) =
+                    ruff::rules::asyncio_dangling_task(value, checker.semantic())
+                {
+                    checker.diagnostics.push(diagnostic);
+                }
             }
             if checker.enabled(Rule::RepeatedAppend) {
                 refurb::rules::repeated_append(checker, stmt);

crates/ruff_linter/src/rules/ruff/rules/asyncio_dangling_task.rs

@@ -1,14 +1,12 @@
 use std::fmt;
 
-use ruff_python_ast::{self as ast, Expr};
-
+use ast::Stmt;
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
-use ruff_python_semantic::analyze::typing;
+use ruff_python_ast::{self as ast, Expr};
+use ruff_python_semantic::{analyze::typing, Binding, SemanticModel};
 use ruff_text_size::Ranged;
 
-use crate::checkers::ast::Checker;
-
 /// ## What it does
 /// Checks for `asyncio.create_task` and `asyncio.ensure_future` calls
 /// that do not store a reference to the returned result.
@@ -66,35 +64,34 @@ impl Violation for AsyncioDanglingTask {
 }
 
 /// RUF006
-pub(crate) fn asyncio_dangling_task(checker: &mut Checker, expr: &Expr) {
+pub(crate) fn asyncio_dangling_task(expr: &Expr, semantic: &SemanticModel) -> Option<Diagnostic> {
     let Expr::Call(ast::ExprCall { func, .. }) = expr else {
-        return;
+        return None;
     };
 
     // Ex) `asyncio.create_task(...)`
-    if let Some(method) = checker
-        .semantic()
-        .resolve_call_path(func)
-        .and_then(|call_path| match call_path.as_slice() {
-            ["asyncio", "create_task"] => Some(Method::CreateTask),
-            ["asyncio", "ensure_future"] => Some(Method::EnsureFuture),
-            _ => None,
-        })
+    if let Some(method) =
+        semantic
+            .resolve_call_path(func)
+            .and_then(|call_path| match call_path.as_slice() {
+                ["asyncio", "create_task"] => Some(Method::CreateTask),
+                ["asyncio", "ensure_future"] => Some(Method::EnsureFuture),
+                _ => None,
+            })
     {
-        checker.diagnostics.push(Diagnostic::new(
+        return Some(Diagnostic::new(
             AsyncioDanglingTask { method },
             expr.range(),
         ));
-        return;
     }
 
     // Ex) `loop = asyncio.get_running_loop(); loop.create_task(...)`
     if let Expr::Attribute(ast::ExprAttribute { attr, value, .. }) = func.as_ref() {
         if attr == "create_task" {
-            if typing::resolve_assignment(value, checker.semantic()).is_some_and(|call_path| {
+            if typing::resolve_assignment(value, semantic).is_some_and(|call_path| {
                 matches!(call_path.as_slice(), ["asyncio", "get_running_loop"])
             }) {
-                checker.diagnostics.push(Diagnostic::new(
+                return Some(Diagnostic::new(
                     AsyncioDanglingTask {
                         method: Method::CreateTask,
                     },
@@ -103,6 +100,28 @@ pub(crate) fn asyncio_dangling_task(checker: &mut Checker, expr: &Expr) {
             }
         }
     }
+    None
+}
+
+/// RUF006
+pub(crate) fn asyncio_dangling_binding(
+    binding: &Binding,
+    semantic: &SemanticModel,
+) -> Option<Diagnostic> {
+    if binding.is_used() || !binding.kind.is_assignment() {
+        return None;
+    }
+
+    let source = binding.source?;
+    match semantic.statement(source) {
+        Stmt::Assign(ast::StmtAssign { value, targets, .. }) if targets.len() == 1 => {
+            asyncio_dangling_task(value, semantic)
+        }
+        Stmt::AnnAssign(ast::StmtAnnAssign {
+            value: Some(value), ..
+        }) => asyncio_dangling_task(value, semantic),
+        _ => None,
+    }
 }
 
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF006_RUF006.py.snap

@@ -17,11 +17,27 @@ RUF006.py:11:5: RUF006 Store a reference to the return value of `asyncio.ensure_
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ RUF006
    |
 
-RUF006.py:79:5: RUF006 Store a reference to the return value of `asyncio.create_task`
+RUF006.py:68:12: RUF006 Store a reference to the return value of `asyncio.create_task`
    |
-77 | def f():
-78 |     loop = asyncio.get_running_loop()
-79 |     loop.create_task(coordinator.ws_connect())  # Error
+66 | # Error
+67 | def f():
+68 |     task = asyncio.create_task(coordinator.ws_connect())
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ RUF006
+   |
+
+RUF006.py:74:26: RUF006 Store a reference to the return value of `asyncio.create_task`
+   |
+72 | def f():
+73 |     loop = asyncio.get_running_loop()
+74 |     task: asyncio.Task = loop.create_task(coordinator.ws_connect())
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ RUF006
+   |
+
+RUF006.py:97:5: RUF006 Store a reference to the return value of `asyncio.create_task`
+   |
+95 | def f():
+96 |     loop = asyncio.get_running_loop()
+97 |     loop.create_task(coordinator.ws_connect())  # Error
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ RUF006
    |