1 use clippy_utils::diagnostics::span_lint_and_then;
2 use clippy_utils::match_function_call_with_def_id;
3 use clippy_utils::source::{position_before_rarrow, snippet_block, snippet_opt};
4 use if_chain::if_chain;
5 use rustc_errors::Applicability;
6 use rustc_hir::intravisit::FnKind;
8 AsyncGeneratorKind, Block, Body, Closure, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound,
9 HirId, ItemKind, LifetimeName, Term, TraitRef, Ty, TyKind, TypeBindingKind,
11 use rustc_lint::{LateContext, LateLintPass};
12 use rustc_session::{declare_lint_pass, declare_tool_lint};
13 use rustc_span::{sym, Span};
15 declare_clippy_lint! {
17 /// It checks for manual implementations of `async` functions.
19 /// ### Why is this bad?
20 /// It's more idiomatic to use the dedicated syntax.
24 /// use std::future::Future;
26 /// fn foo() -> impl Future<Output = i32> { async { 42 } }
30 /// async fn foo() -> i32 { 42 }
32 #[clippy::version = "1.45.0"]
35 "manual implementations of `async` functions can be simplified using the dedicated syntax"
38 declare_lint_pass!(ManualAsyncFn => [MANUAL_ASYNC_FN]);
40 impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
43 cx: &LateContext<'tcx>,
45 decl: &'tcx FnDecl<'_>,
51 if let Some(header) = kind.header();
52 if !header.asyncness.is_async();
53 // Check that this function returns `impl Future`
54 if let FnRetTy::Return(ret_ty) = decl.output;
55 if let Some((trait_ref, output_lifetimes)) = future_trait_ref(cx, ret_ty);
56 if let Some(output) = future_output_ty(trait_ref);
57 if captures_all_lifetimes(decl.inputs, &output_lifetimes);
58 // Check that the body of the function consists of one async block
59 if let ExprKind::Block(block, _) = body.value.kind;
60 if block.stmts.is_empty();
61 if let Some(closure_body) = desugared_async_block(cx, block);
63 let header_span = span.with_hi(ret_ty.span.hi());
69 "this function can be simplified using the `async fn` syntax",
72 if let Some(header_snip) = snippet_opt(cx, header_span);
73 if let Some(ret_pos) = position_before_rarrow(&header_snip);
74 if let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output);
76 let help = format!("make the function `async` and {ret_sugg}");
80 format!("async {}{ret_snip}", &header_snip[..ret_pos]),
81 Applicability::MachineApplicable
84 let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
87 "move the body of the async block to the enclosing function",
89 Applicability::MachineApplicable
100 fn future_trait_ref<'tcx>(
101 cx: &LateContext<'tcx>,
103 ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
105 if let TyKind::OpaqueDef(item_id, bounds, false) = ty.kind;
106 let item = cx.tcx.hir().item(item_id);
107 if let ItemKind::OpaqueTy(opaque) = &item.kind;
108 if let Some(trait_ref) = opaque.bounds.iter().find_map(|bound| {
109 if let GenericBound::Trait(poly, _) = bound {
110 Some(&poly.trait_ref)
115 if trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
117 let output_lifetimes = bounds
119 .filter_map(|bound| {
120 if let GenericArg::Lifetime(lt) = bound {
128 return Some((trait_ref, output_lifetimes));
135 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
137 if let Some(segment) = trait_ref.path.segments.last();
138 if let Some(args) = segment.args;
139 if args.bindings.len() == 1;
140 let binding = &args.bindings[0];
141 if binding.ident.name == sym::Output;
142 if let TypeBindingKind::Equality { term: Term::Ty(output) } = binding.kind;
151 fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
152 let input_lifetimes: Vec<LifetimeName> = inputs
155 if let TyKind::Ref(lt, _) = ty.kind {
163 // The lint should trigger in one of these cases:
164 // - There are no input lifetimes
165 // - There's only one output lifetime bound using `+ '_`
166 // - All input lifetimes are explicitly bound to the output
167 input_lifetimes.is_empty()
168 || (output_lifetimes.len() == 1 && matches!(output_lifetimes[0], LifetimeName::Infer))
171 .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
174 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
176 if let Some(block_expr) = block.expr;
177 if let Some(args) = cx
180 .identity_future_fn()
181 .and_then(|def_id| match_function_call_with_def_id(cx, block_expr, def_id));
184 kind: ExprKind::Closure(&Closure { body, .. }),
187 let closure_body = cx.tcx.hir().body(body);
188 if closure_body.generator_kind == Some(GeneratorKind::Async(AsyncGeneratorKind::Block));
190 return Some(closure_body);
197 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
199 TyKind::Tup(tys) if tys.is_empty() => {
200 let sugg = "remove the return type";
201 Some((sugg, String::new()))
204 let sugg = "return the output of the future directly";
205 snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {snip}")))