[ruff] Reduce FastAPI false positives in unused-async (RUF029)

crates/ruff/tests/snapshots/show_settings__display_default_settings.snap

@@ -374,6 +374,7 @@ linter.pylint.max_public_methods = 20
 linter.pylint.max_locals = 15
 linter.pyupgrade.keep_runtime_typing = false
 linter.ruff.parenthesize_tuple_in_subscript = false
+linter.ruff.allow_fastapi_routes_unused_async = false
 
 # Formatter Settings
 formatter.exclude = []

crates/ruff_linter/resources/test/fixtures/ruff/RUF029_fastapi.py

@@ -0,0 +1,8 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+
+@app.post("/count")
+async def fastapi_route():
+    return 1

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

@@ -68,13 +68,46 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn dont_allow_fastapi_routes_unused_async() -> Result<()> {
+        let diagnostics = test_path(
+            Path::new("ruff/RUF029_fastapi.py"),
+            &LinterSettings {
+                ruff: super::settings::Settings {
+                    parenthesize_tuple_in_subscript: false,
+                    allow_fastapi_routes_unused_async: false,
+                },
+                ..LinterSettings::for_rule(Rule::UnusedAsync)
+            },
+        )?;
+        assert_messages!(diagnostics);
+        Ok(())
+    }
+
+    #[test]
+    fn allow_fastapi_routes_unused_async() -> Result<()> {
+        let diagnostics = test_path(
+            Path::new("ruff/RUF029_fastapi.py"),
+            &LinterSettings {
+                ruff: super::settings::Settings {
+                    parenthesize_tuple_in_subscript: false,
+                    allow_fastapi_routes_unused_async: true,
+                },
+                ..LinterSettings::for_rule(Rule::UnusedAsync)
+            },
+        )?;
+        assert_messages!(diagnostics);
+        Ok(())
+    }
+
     #[test]
     fn prefer_parentheses_getitem_tuple() -> Result<()> {
         let diagnostics = test_path(
             Path::new("ruff/RUF031_prefer_parens.py"),
             &LinterSettings {
                 ruff: super::settings::Settings {
                     parenthesize_tuple_in_subscript: true,
+                    allow_fastapi_routes_unused_async: false,
                 },
                 ..LinterSettings::for_rule(Rule::IncorrectlyParenthesizedTupleInSubscript)
             },
@@ -90,6 +123,7 @@ mod tests {
             &LinterSettings {
                 ruff: super::settings::Settings {
                     parenthesize_tuple_in_subscript: false,
+                    allow_fastapi_routes_unused_async: false,
                 },
                 target_version: PythonVersion::Py310,
                 ..LinterSettings::for_rule(Rule::IncorrectlyParenthesizedTupleInSubscript)

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

@@ -1,11 +1,12 @@
+use crate::checkers::ast::Checker;
+use crate::rules::fastapi::rules::is_fastapi_route;
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::identifier::Identifier;
 use ruff_python_ast::visitor::source_order;
 use ruff_python_ast::{self as ast, AnyNodeRef, Expr, Stmt};
 use ruff_python_semantic::analyze::function_type::is_stub;
-
-use crate::checkers::ast::Checker;
+use ruff_python_semantic::Modules;
 
 /// ## What it does
 /// Checks for functions declared `async` that do not await or otherwise use features requiring the
@@ -27,6 +28,9 @@ use crate::checkers::ast::Checker;
 /// def foo():
 ///     bar()
 /// ```
+///
+/// ## Options
+/// - `lint.ruff.allow-fastapi-routes-unused-async`
 #[violation]
 pub struct UnusedAsync {
     name: String,
@@ -173,6 +177,13 @@ pub(crate) fn unused_async(
         return;
     }
 
+    if checker.settings.ruff.allow_fastapi_routes_unused_async
+        && checker.semantic().seen_module(Modules::FASTAPI)
+        && is_fastapi_route(function_def, checker.semantic())
+    {
+        return;
+    }
+
     let found_await_or_async = {
         let mut visitor = AsyncExprVisitor::default();
         source_order::walk_body(&mut visitor, body);

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

@@ -7,6 +7,7 @@ use std::fmt;
 #[derive(Debug, Clone, CacheKey, Default)]
 pub struct Settings {
     pub parenthesize_tuple_in_subscript: bool,
+    pub allow_fastapi_routes_unused_async: bool,
 }
 
 impl fmt::Display for Settings {
@@ -15,7 +16,8 @@ impl fmt::Display for Settings {
             formatter = f,
             namespace = "linter.ruff",
             fields = [
-                self.parenthesize_tuple_in_subscript
+                self.parenthesize_tuple_in_subscript,
+                self.allow_fastapi_routes_unused_async
             ]
         }
         Ok(())

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__allow_fastapi_routes_unused_async.snap

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

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__dont_allow_fastapi_routes_unused_async.snap

@@ -0,0 +1,10 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF029_fastapi.py:7:11: RUF029 Function `fastapi_route` is declared `async`, but doesn't `await` or use `async` features.
+  |
+6 | @app.post("/count")
+7 | async def fastapi_route():
+  |           ^^^^^^^^^^^^^ RUF029
+8 |     return 1
+  |

crates/ruff_workspace/src/options.rs

@@ -2986,6 +2986,18 @@ pub struct RuffOptions {
         "#
     )]
     pub parenthesize_tuple_in_subscript: Option<bool>,
+
+    /// Whether to allow fastapi routes to be defined as async function, although they don't use
+    /// any asynchronous code (see `RUF029` and [FastAPI docs](https://fastapi.tiangolo.com/async/)).
+    #[option(
+        default = r#"false"#,
+        value_type = "bool",
+        example = r#"
+        # Make it a ok to have async functions in FastAPI routes that don't use await
+        parenthesize-tuple-in-subscript = true
+        "#
+    )]
+    pub allow_fastapi_routes_unused_async: Option<bool>,
 }
 
 impl RuffOptions {
@@ -2994,6 +3006,9 @@ impl RuffOptions {
             parenthesize_tuple_in_subscript: self
                 .parenthesize_tuple_in_subscript
                 .unwrap_or_default(),
+            allow_fastapi_routes_unused_async: self
+                .allow_fastapi_routes_unused_async
+                .unwrap_or_default(),
         }
     }
 }