[flake8-async] Fix false positives with multiple async with items (ASYNC100) (#12643)

## Summary

Please see
https://github.com/astral-sh/ruff/pull/12605#discussion_r1699957443 for
a description of the issue.

They way I fixed it is to get the *last* timeout item in the `with`, and
if it's an `async with` and there are items after it, then don't trigger
the lint.

## Test Plan

Updated the fixture with some more cases.

crates/ruff_linter/resources/test/fixtures/flake8_async/ASYNC100.py

@@ -1,51 +1,63 @@
 import anyio
 import asyncio
 import trio
+from contextlib import nullcontext
 
 
 async def func():
-    async with trio.fail_after():
+    with trio.fail_after():
         ...
 
 
 async def func():
-    async with trio.fail_at():
+    with trio.fail_at():
         await ...
 
 
 async def func():
-    async with trio.move_on_after():
+    with trio.move_on_after():
         ...
 
 
 async def func():
-    async with trio.move_at():
+    with trio.move_at():
         await ...
 
 
 async def func():
-    async with trio.move_at():
+    with trio.move_at():
         async with trio.open_nursery():
             ...
 
 
 async def func():
-    async with anyio.move_on_after(delay=0.2):
+    with trio.move_at():
+        async for x in ...:
             ...
 
 
 async def func():
-    async with anyio.fail_after():
+    with anyio.move_on_after(delay=0.2):
         ...
 
 
 async def func():
-    async with anyio.CancelScope():
+    with anyio.fail_after():
         ...
 
 
 async def func():
-    async with anyio.CancelScope():
+    with anyio.CancelScope():
+        ...
+
+
+async def func():
+    with anyio.CancelScope(), nullcontext():
+        ...
+
+
+async def func():
+    with nullcontext(), anyio.CancelScope():
         ...
 
 
@@ -62,3 +74,18 @@ async def func():
 async def func():
     async with asyncio.timeout(delay=0.2), asyncio.TaskGroup():
         ...
+
+
+async def func():
+    async with asyncio.timeout(delay=0.2), asyncio.TaskGroup(), asyncio.timeout(delay=0.2):
+        ...
+
+
+async def func():
+    async with asyncio.timeout(delay=0.2), asyncio.TaskGroup(), asyncio.timeout(delay=0.2), asyncio.TaskGroup():
+        ...
+
+
+async def func():
+    async with asyncio.timeout(delay=0.2), asyncio.timeout(delay=0.2):
+        ...

crates/ruff_linter/src/rules/flake8_async/rules/cancel_scope_no_checkpoint.rs

@@ -13,9 +13,9 @@ use crate::settings::types::PreviewMode;
 ///
 /// ## Why is this bad?
 /// Some asynchronous context managers, such as `asyncio.timeout` and
-/// `trio.move_on_after`, have no effect unless they contain an `await`
+/// `trio.move_on_after`, have no effect unless they contain a checkpoint.
-/// statement. The use of such context managers without an `await` statement is
+/// The use of such context managers without an `await`, `async with` or
-/// likely a mistake.
+/// `async for` statement is likely a mistake.
 ///
 /// ## Example
 /// ```python
@@ -55,17 +55,29 @@ pub(crate) fn cancel_scope_no_checkpoint(
     with_stmt: &StmtWith,
     with_items: &[WithItem],
 ) {
-    let Some(method_name) = with_items.iter().find_map(|item| {
+    let Some((with_item_pos, method_name)) = with_items
+        .iter()
+        .enumerate()
+        .filter_map(|(pos, item)| {
             let call = item.context_expr.as_call_expr()?;
             let qualified_name = checker
                 .semantic()
                 .resolve_qualified_name(call.func.as_ref())?;
-        MethodName::try_from(&qualified_name)
+            let method_name = MethodName::try_from(&qualified_name)?;
-    }) else {
+            if method_name.is_timeout_context() {
+                Some((pos, method_name))
+            } else {
+                None
+            }
+        })
+        .last()
+    else {
         return;
     };
 
-    if !method_name.is_timeout_context() {
+    // If this is an `async with` and the timeout has items after it, then the
+    // further items are checkpoints.
+    if with_stmt.is_async && with_item_pos < with_items.len() - 1 {
         return;
     }
 
@@ -76,24 +88,6 @@ pub(crate) fn cancel_scope_no_checkpoint(
         return;
     }
 
-    // If there's an `asyncio.TaskGroup()` context manager alongside the timeout, it's fine, as in:
-    // ```python
-    // async with asyncio.timeout(2.0), asyncio.TaskGroup():
-    //     ...
-    // ```
-    if with_items.iter().any(|item| {
-        item.context_expr.as_call_expr().is_some_and(|call| {
-            checker
-                .semantic()
-                .resolve_qualified_name(call.func.as_ref())
-                .is_some_and(|qualified_name| {
-                    matches!(qualified_name.segments(), ["asyncio", "TaskGroup"])
-                })
-        })
-    }) {
-        return;
-    }
-
     if matches!(checker.settings.preview, PreviewMode::Disabled) {
         if matches!(method_name.module(), AsyncModule::Trio) {
             checker.diagnostics.push(Diagnostic::new(

crates/ruff_linter/src/rules/flake8_async/snapshots/ruff_linter__rules__flake8_async__tests__ASYNC100_ASYNC100.py.snap

@@ -1,20 +1,20 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_async/mod.rs
 ---
-ASYNC100.py:7:5: ASYNC100 A `with trio.fail_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:8:5: ASYNC100 A `with trio.fail_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
   |
-6 |   async def func():
+7 |   async def func():
-7 |       async with trio.fail_after():
+8 |       with trio.fail_after():
   |  _____^
-8 | |         ...
+9 | |         ...
   | |___________^ ASYNC100
   |
 
-ASYNC100.py:17:5: ASYNC100 A `with trio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:18:5: ASYNC100 A `with trio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-16 |   async def func():
+17 |   async def func():
-17 |       async with trio.move_on_after():
+18 |       with trio.move_on_after():
    |  _____^
-18 | |         ...
+19 | |         ...
    | |___________^ ASYNC100
    |

crates/ruff_linter/src/rules/flake8_async/snapshots/ruff_linter__rules__flake8_async__tests__preview__ASYNC100_ASYNC100.py.snap

@@ -1,74 +1,101 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_async/mod.rs
 ---
-ASYNC100.py:7:5: ASYNC100 A `with trio.fail_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:8:5: ASYNC100 A `with trio.fail_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
   |
-6 |   async def func():
+7 |   async def func():
-7 |       async with trio.fail_after():
+8 |       with trio.fail_after():
   |  _____^
-8 | |         ...
+9 | |         ...
   | |___________^ ASYNC100
   |
 
-ASYNC100.py:17:5: ASYNC100 A `with trio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:18:5: ASYNC100 A `with trio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-16 |   async def func():
+17 |   async def func():
-17 |       async with trio.move_on_after():
+18 |       with trio.move_on_after():
    |  _____^
-18 | |         ...
+19 | |         ...
    | |___________^ ASYNC100
    |
 
-ASYNC100.py:33:5: ASYNC100 A `with anyio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:40:5: ASYNC100 A `with anyio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-32 |   async def func():
+39 |   async def func():
-33 |       async with anyio.move_on_after(delay=0.2):
+40 |       with anyio.move_on_after(delay=0.2):
    |  _____^
-34 | |         ...
+41 | |         ...
    | |___________^ ASYNC100
    |
 
-ASYNC100.py:38:5: ASYNC100 A `with anyio.fail_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:45:5: ASYNC100 A `with anyio.fail_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-37 |   async def func():
+44 |   async def func():
-38 |       async with anyio.fail_after():
+45 |       with anyio.fail_after():
    |  _____^
-39 | |         ...
+46 | |         ...
    | |___________^ ASYNC100
    |
 
-ASYNC100.py:43:5: ASYNC100 A `with anyio.CancelScope(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:50:5: ASYNC100 A `with anyio.CancelScope(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-42 |   async def func():
+49 |   async def func():
-43 |       async with anyio.CancelScope():
+50 |       with anyio.CancelScope():
    |  _____^
-44 | |         ...
+51 | |         ...
    | |___________^ ASYNC100
    |
 
-ASYNC100.py:48:5: ASYNC100 A `with anyio.CancelScope(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:55:5: ASYNC100 A `with anyio.CancelScope(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-47 |   async def func():
+54 |   async def func():
-48 |       async with anyio.CancelScope():
+55 |       with anyio.CancelScope(), nullcontext():
    |  _____^
-49 | |         ...
+56 | |         ...
    | |___________^ ASYNC100
    |
 
-ASYNC100.py:53:5: ASYNC100 A `with asyncio.timeout(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:60:5: ASYNC100 A `with anyio.CancelScope(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-52 |   async def func():
+59 |   async def func():
-53 |       async with asyncio.timeout(delay=0.2):
+60 |       with nullcontext(), anyio.CancelScope():
    |  _____^
-54 | |         ...
+61 | |         ...
    | |___________^ ASYNC100
    |
 
-ASYNC100.py:58:5: ASYNC100 A `with asyncio.timeout_at(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+ASYNC100.py:65:5: ASYNC100 A `with asyncio.timeout(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
    |
-57 |   async def func():
+64 |   async def func():
-58 |       async with asyncio.timeout_at(when=0.2):
+65 |       async with asyncio.timeout(delay=0.2):
    |  _____^
-59 | |         ...
+66 | |         ...
+   | |___________^ ASYNC100
+   |
+
+ASYNC100.py:70:5: ASYNC100 A `with asyncio.timeout_at(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+   |
+69 |   async def func():
+70 |       async with asyncio.timeout_at(when=0.2):
+   |  _____^
+71 | |         ...
+   | |___________^ ASYNC100
+   |
+
+ASYNC100.py:80:5: ASYNC100 A `with asyncio.timeout(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+   |
+79 |   async def func():
+80 |       async with asyncio.timeout(delay=0.2), asyncio.TaskGroup(), asyncio.timeout(delay=0.2):
+   |  _____^
+81 | |         ...
+   | |___________^ ASYNC100
+   |
+
+ASYNC100.py:90:5: ASYNC100 A `with asyncio.timeout(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
+   |
+89 |   async def func():
+90 |       async with asyncio.timeout(delay=0.2), asyncio.timeout(delay=0.2):
+   |  _____^
+91 | |         ...
    | |___________^ ASYNC100
    |