1 use clippy_utils::diagnostics::span_lint_and_then;
2 use clippy_utils::source::{position_before_rarrow, snippet_block, snippet_opt};
3 use if_chain::if_chain;
4 use rustc_errors::Applicability;
5 use rustc_hir::intravisit::FnKind;
6 use rustc_hir::{
7 AsyncGeneratorKind, Block, Body, Closure, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound,
8 ImplItem, Item, ItemKind, LifetimeName, Node, Term, TraitRef, Ty, TyKind, TypeBindingKind,
9 };
10 use rustc_lint::{LateContext, LateLintPass};
11 use rustc_session::{declare_lint_pass, declare_tool_lint};
12 use rustc_span::def_id::LocalDefId;
13 use rustc_span::{sym, Span};
14
15 declare_clippy_lint! {
16 /// ### What it does
17 /// It checks for manual implementations of `async` functions.
18 ///
19 /// ### Why is this bad?
20 /// It's more idiomatic to use the dedicated syntax.
21 ///
22 /// ### Example
23 /// ```rust
24 /// use std::future::Future;
25 ///
26 /// fn foo() -> impl Future<Output = i32> { async { 42 } }
27 /// ```
28 /// Use instead:
29 /// ```rust
30 /// async fn foo() -> i32 { 42 }
31 /// ```
32 #[clippy::version = "1.45.0"]
33 pub MANUAL_ASYNC_FN,
34 style,
35 "manual implementations of `async` functions can be simplified using the dedicated syntax"
36 }
37
38 declare_lint_pass!(ManualAsyncFn => [MANUAL_ASYNC_FN]);
39
40 impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
check_fn( &mut self, cx: &LateContext<'tcx>, kind: FnKind<'tcx>, decl: &'tcx FnDecl<'_>, body: &'tcx Body<'_>, span: Span, def_id: LocalDefId, )41 fn check_fn(
42 &mut self,
43 cx: &LateContext<'tcx>,
44 kind: FnKind<'tcx>,
45 decl: &'tcx FnDecl<'_>,
46 body: &'tcx Body<'_>,
47 span: Span,
48 def_id: LocalDefId,
49 ) {
50 if_chain! {
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);
62 if let Node::Item(Item {vis_span, ..}) | Node::ImplItem(ImplItem {vis_span, ..}) =
63 cx.tcx.hir().get_by_def_id(def_id);
64 then {
65 let header_span = span.with_hi(ret_ty.span.hi());
66
67 span_lint_and_then(
68 cx,
69 MANUAL_ASYNC_FN,
70 header_span,
71 "this function can be simplified using the `async fn` syntax",
72 |diag| {
73 if_chain! {
74 if let Some(vis_snip) = snippet_opt(cx, *vis_span);
75 if let Some(header_snip) = snippet_opt(cx, header_span);
76 if let Some(ret_pos) = position_before_rarrow(&header_snip);
77 if let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output);
78 then {
79 let header_snip = if vis_snip.is_empty() {
80 format!("async {}", &header_snip[..ret_pos])
81 } else {
82 format!("{} async {}", vis_snip, &header_snip[vis_snip.len() + 1..ret_pos])
83 };
84
85 let help = format!("make the function `async` and {ret_sugg}");
86 diag.span_suggestion(
87 header_span,
88 help,
89 format!("{header_snip}{ret_snip}"),
90 Applicability::MachineApplicable
91 );
92
93 let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
94 diag.span_suggestion(
95 block.span,
96 "move the body of the async block to the enclosing function",
97 body_snip,
98 Applicability::MachineApplicable
99 );
100 }
101 }
102 },
103 );
104 }
105 }
106 }
107 }
108
future_trait_ref<'tcx>( cx: &LateContext<'tcx>, ty: &'tcx Ty<'tcx>, ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)>109 fn future_trait_ref<'tcx>(
110 cx: &LateContext<'tcx>,
111 ty: &'tcx Ty<'tcx>,
112 ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
113 if_chain! {
114 if let TyKind::OpaqueDef(item_id, bounds, false) = ty.kind;
115 let item = cx.tcx.hir().item(item_id);
116 if let ItemKind::OpaqueTy(opaque) = &item.kind;
117 if let Some(trait_ref) = opaque.bounds.iter().find_map(|bound| {
118 if let GenericBound::Trait(poly, _) = bound {
119 Some(&poly.trait_ref)
120 } else {
121 None
122 }
123 });
124 if trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
125 then {
126 let output_lifetimes = bounds
127 .iter()
128 .filter_map(|bound| {
129 if let GenericArg::Lifetime(lt) = bound {
130 Some(lt.res)
131 } else {
132 None
133 }
134 })
135 .collect();
136
137 return Some((trait_ref, output_lifetimes));
138 }
139 }
140
141 None
142 }
143
future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>>144 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
145 if_chain! {
146 if let Some(segment) = trait_ref.path.segments.last();
147 if let Some(args) = segment.args;
148 if args.bindings.len() == 1;
149 let binding = &args.bindings[0];
150 if binding.ident.name == sym::Output;
151 if let TypeBindingKind::Equality { term: Term::Ty(output) } = binding.kind;
152 then {
153 return Some(output);
154 }
155 }
156
157 None
158 }
159
captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool160 fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
161 let input_lifetimes: Vec<LifetimeName> = inputs
162 .iter()
163 .filter_map(|ty| {
164 if let TyKind::Ref(lt, _) = ty.kind {
165 Some(lt.res)
166 } else {
167 None
168 }
169 })
170 .collect();
171
172 // The lint should trigger in one of these cases:
173 // - There are no input lifetimes
174 // - There's only one output lifetime bound using `+ '_`
175 // - All input lifetimes are explicitly bound to the output
176 input_lifetimes.is_empty()
177 || (output_lifetimes.len() == 1 && matches!(output_lifetimes[0], LifetimeName::Infer))
178 || input_lifetimes
179 .iter()
180 .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
181 }
182
desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>>183 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
184 if_chain! {
185 if let Some(block_expr) = block.expr;
186 if let Expr {
187 kind: ExprKind::Closure(&Closure { body, .. }),
188 ..
189 } = block_expr;
190 let closure_body = cx.tcx.hir().body(body);
191 if closure_body.generator_kind == Some(GeneratorKind::Async(AsyncGeneratorKind::Block));
192 then {
193 return Some(closure_body);
194 }
195 }
196
197 None
198 }
199
suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)>200 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
201 match output.kind {
202 TyKind::Tup(tys) if tys.is_empty() => {
203 let sugg = "remove the return type";
204 Some((sugg, String::new()))
205 },
206 _ => {
207 let sugg = "return the output of the future directly";
208 snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {snip}")))
209 },
210 }
211 }
212