Add unused_async_trait_impl lint

CHANGELOG.md

@@ -7089,6 +7089,7 @@ Released 2018-09-13
 [`unstable_as_mut_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_mut_slice
 [`unstable_as_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_slice
 [`unused_async`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_async
+[`unused_async_trait_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_async_trait_impl
 [`unused_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_collect
 [`unused_enumerate_index`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_enumerate_index
 [`unused_format_specs`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_format_specs

clippy_lints/src/declared_lints.rs

@@ -768,6 +768,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::unnested_or_patterns::UNNESTED_OR_PATTERNS_INFO,
     crate::unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME_INFO,
     crate::unused_async::UNUSED_ASYNC_INFO,
+    crate::unused_async_trait_impl::UNUSED_ASYNC_TRAIT_IMPL_INFO,
     crate::unused_io_amount::UNUSED_IO_AMOUNT_INFO,
     crate::unused_peekable::UNUSED_PEEKABLE_INFO,
     crate::unused_result_ok::UNUSED_RESULT_OK_INFO,

clippy_lints/src/lib.rs

@@ -375,6 +375,7 @@ mod unneeded_struct_pattern;
 mod unnested_or_patterns;
 mod unsafe_removed_from_name;
 mod unused_async;
+mod unused_async_trait_impl;
 mod unused_io_amount;
 mod unused_peekable;
 mod unused_result_ok;
@@ -852,6 +853,7 @@ pub fn register_lint_passes(store: &mut rustc_lint::LintStore, conf: &'static Co
         Box::new(|_| Box::new(volatile_composites::VolatileComposites)),
         Box::new(|_| Box::<replace_box::ReplaceBox>::default()),
         Box::new(move |_| Box::new(manual_ilog2::ManualIlog2::new(conf))),
+        Box::new(|_| Box::new(unused_async_trait_impl::UnusedAsyncTraitImpl)),
         // add late passes here, used by `cargo dev new_lint`
     ];
     store.late_passes.extend(late_lints);

clippy_lints/src/unused_async_trait_impl.rs

@@ -0,0 +1,184 @@
+use clippy_utils::diagnostics::span_lint_hir_and_then;
+use clippy_utils::is_def_id_trait_method;
+use clippy_utils::source::SpanRangeExt;
+use clippy_utils::usage::is_todo_unimplemented_stub;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{FnKind, Visitor, walk_expr, walk_fn};
+use rustc_hir::{
+    Body, Closure, ClosureKind, CoroutineDesugaring, CoroutineKind, Defaultness, Expr, ExprKind, FnDecl, IsAsync, Node,
+    TraitItem, YieldSource,
+};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::hir::nested_filter;
+use rustc_session::impl_lint_pass;
+use rustc_span::Span;
+use rustc_span::def_id::LocalDefId;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for trait function implementations that are declared `async` but have no `.await`s inside of them.
+    ///
+    /// ### Why is this bad?
+    /// Async functions with no async code create computational overhead.
+    /// Even though the trait requires the function to return a future,
+    /// returning a `core::future::ready` with the result is more efficient.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// trait AsyncTrait {
+    ///     async fn get_random_number() -> i64;
+    /// }
+    ///
+    /// impl AsyncTrait for () {
+    ///     async fn get_random_number() -> i64 {
+    ///         4 // Chosen by fair dice roll. Guaranteed to be random.
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Use instead:
+    /// ```no_run
+    /// trait AsyncTrait {
+    ///     async fn get_random_number() -> i64;
+    /// }
+    ///
+    /// impl AsyncTrait for () {
+    ///     fn get_random_number() -> impl Future<Output = i64> {
+    ///         core::future::ready(4) // Chosen by fair dice roll. Guaranteed to be random.
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ### Note
+    /// An `async` block generates code that defers execution until the Future is polled.
+    /// When using `core::future::ready` the code is executed immediately.
+    #[clippy::version = "1.94.0"]
+    pub UNUSED_ASYNC_TRAIT_IMPL,
+    pedantic,
+    "finds async trait impl functions with no await statements"
+}
+
+pub struct UnusedAsyncTraitImpl;
+
+impl_lint_pass!(UnusedAsyncTraitImpl => [UNUSED_ASYNC_TRAIT_IMPL]);
+
+struct AsyncFnVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    found_await: bool,
+    async_depth: usize,
+}
+
+impl<'tcx> Visitor<'tcx> for AsyncFnVisitor<'_, 'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+
+    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
+        if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind
+            && self.async_depth == 1
+        {
+            self.found_await = true;
+        }
+
+        let is_async_block = matches!(
+            ex.kind,
+            ExprKind::Closure(Closure {
+                kind: ClosureKind::Coroutine(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)),
+                ..
+            })
+        );
+
+        if is_async_block {
+            self.async_depth += 1;
+        }
+
+        walk_expr(self, ex);
+
+        if is_async_block {
+            self.async_depth -= 1;
+        }
+    }
+
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for UnusedAsyncTraitImpl {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        fn_kind: FnKind<'tcx>,
+        fn_decl: &'tcx FnDecl<'tcx>,
+        body: &Body<'tcx>,
+        span: Span,
+        def_id: LocalDefId,
+    ) {
+        if let IsAsync::Async(async_span) = fn_kind.asyncness()
+            && !span.from_expansion()
+            && is_def_id_trait_method(cx, def_id)
+            && !is_default_trait_impl(cx, def_id)
+            && !async_fn_contains_todo_unimplemented_macro(cx, body)
+        {
+            let mut visitor = AsyncFnVisitor {
+                cx,
+                found_await: false,
+                async_depth: 0,
+            };
+            walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), def_id);
+            if !visitor.found_await {
+                span_lint_hir_and_then(
+                    cx,
+                    UNUSED_ASYNC_TRAIT_IMPL,
+                    cx.tcx.local_def_id_to_hir_id(def_id),
+                    span,
+                    "unused `async` for async trait impl function with no await statements",
+                    |diag| {
+                        if let Some(output_src) = fn_decl.output.span().get_source_text(cx)
+                            && let Some(body_src) = body.value.span.get_source_text(cx)
+                        {
+                            let output_str = output_src.as_str();
+                            let body_str = body_src.as_str();
+
+                            let sugg = vec![
+                                (async_span, String::new()),
+                                (fn_decl.output.span(), format!("impl Future<Output = {output_str}>")),
+                                (body.value.span, format!("{{ core::future::ready({body_str}) }}")),
+                            ];
+
+                            diag.help("a Future can be constructed from the return value with `core::future::ready`");
+                            diag.multipart_suggestion(
+                                    format!("consider removing the `async` from this function and returning `impl Future<Output = {output_str}>` instead"),
+                                    sugg,
+                                    Applicability::MachineApplicable
+                                );
+                        }
+                    },
+                );
+            }
+        }
+    }
+}
+
+fn is_default_trait_impl(cx: &LateContext<'_>, def_id: LocalDefId) -> bool {
+    matches!(
+        cx.tcx.hir_node_by_def_id(def_id),
+        Node::TraitItem(TraitItem {
+            defaultness: Defaultness::Default { .. },
+            ..
+        })
+    )
+}
+
+fn async_fn_contains_todo_unimplemented_macro(cx: &LateContext<'_>, body: &Body<'_>) -> bool {
+    if let ExprKind::Closure(closure) = body.value.kind
+        && let ClosureKind::Coroutine(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) = closure.kind
+        && let body = cx.tcx.hir_body(closure.body)
+        && let ExprKind::Block(block, _) = body.value.kind
+        && block.stmts.is_empty()
+        && let Some(expr) = block.expr
+        && let ExprKind::DropTemps(inner) = expr.kind
+    {
+        return is_todo_unimplemented_stub(cx, inner);
+    }
+
+    false
+}

tests/ui/unused_async_trait_impl.fixed

@@ -0,0 +1,26 @@
+#![warn(clippy::unused_async_trait_impl)]
+
+trait HasAsyncMethod {
+    async fn do_something() -> u32;
+}
+
+struct Inefficient;
+struct Efficient;
+
+impl HasAsyncMethod for Inefficient {
+     fn do_something() -> impl Future<Output = u32> { core::future::ready({
+        //~^ unused_async_trait_impl
+        1
+    }) }
+}
+
+impl HasAsyncMethod for Efficient {
+    fn do_something() -> impl Future<Output = u32> {
+        core::future::ready(1)
+    }
+}
+
+fn main() {
+    Inefficient::do_something();
+    Efficient::do_something();
+}

tests/ui/unused_async_trait_impl.rs

@@ -0,0 +1,26 @@
+#![warn(clippy::unused_async_trait_impl)]
+
+trait HasAsyncMethod {
+    async fn do_something() -> u32;
+}
+
+struct Inefficient;
+struct Efficient;
+
+impl HasAsyncMethod for Inefficient {
+    async fn do_something() -> u32 {
+        //~^ unused_async_trait_impl
+        1
+    }
+}
+
+impl HasAsyncMethod for Efficient {
+    fn do_something() -> impl Future<Output = u32> {
+        core::future::ready(1)
+    }
+}
+
+fn main() {
+    Inefficient::do_something();
+    Efficient::do_something();
+}

tests/ui/unused_async_trait_impl.stderr

@@ -0,0 +1,22 @@
+error: unused `async` for async trait impl function with no await statements
+  --> tests/ui/unused_async_trait_impl.rs:11:5
+   |
+LL | /     async fn do_something() -> u32 {
+LL | |
+LL | |         1
+LL | |     }
+   | |_____^
+   |
+   = help: a Future can be constructed from the return value with `core::future::ready`
+   = note: `-D clippy::unused-async-trait-impl` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unused_async_trait_impl)]`
+help: consider removing the `async` from this function and returning `impl Future<Output = u32>` instead
+   |
+LL ~      fn do_something() -> impl Future<Output = u32> { core::future::ready({
+LL +
+LL +         1
+LL +     }) }
+   |
+
+error: aborting due to 1 previous error
+