1 use clippy_utils::diagnostics::span_lint_and_then;
2 use clippy_utils::match_function_call;
3 use clippy_utils::paths::FUTURE_FROM_GENERATOR;
4 use clippy_utils::source::{position_before_rarrow, snippet_block, snippet_opt};
5 use if_chain::if_chain;
6 use rustc_errors::Applicability;
7 use rustc_hir::intravisit::FnKind;
9 AsyncGeneratorKind, Block, Body, Closure, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound,
10 HirId, IsAsync, ItemKind, LifetimeName, Term, TraitRef, Ty, TyKind, TypeBindingKind,
12 use rustc_lint::{LateContext, LateLintPass};
13 use rustc_session::{declare_lint_pass, declare_tool_lint};
14 use rustc_span::{sym, Span};
16 declare_clippy_lint! {
18 /// It checks for manual implementations of `async` functions.
20 /// ### Why is this bad?
21 /// It's more idiomatic to use the dedicated syntax.
25 /// use std::future::Future;
27 /// fn foo() -> impl Future<Output = i32> { async { 42 } }
31 /// async fn foo() -> i32 { 42 }
33 #[clippy::version = "1.45.0"]
36 "manual implementations of `async` functions can be simplified using the dedicated syntax"
39 declare_lint_pass!(ManualAsyncFn => [MANUAL_ASYNC_FN]);
41 impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
44 cx: &LateContext<'tcx>,
46 decl: &'tcx FnDecl<'_>,
52 if let Some(header) = kind.header();
53 if header.asyncness == IsAsync::NotAsync;
54 // Check that this function returns `impl Future`
55 if let FnRetTy::Return(ret_ty) = decl.output;
56 if let Some((trait_ref, output_lifetimes)) = future_trait_ref(cx, ret_ty);
57 if let Some(output) = future_output_ty(trait_ref);
58 if captures_all_lifetimes(decl.inputs, &output_lifetimes);
59 // Check that the body of the function consists of one async block
60 if let ExprKind::Block(block, _) = body.value.kind;
61 if block.stmts.is_empty();
62 if let Some(closure_body) = desugared_async_block(cx, block);
64 let header_span = span.with_hi(ret_ty.span.hi());
70 "this function can be simplified using the `async fn` syntax",
73 if let Some(header_snip) = snippet_opt(cx, header_span);
74 if let Some(ret_pos) = position_before_rarrow(&header_snip);
75 if let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output);
77 let help = format!("make the function `async` and {}", ret_sugg);
81 format!("async {}{}", &header_snip[..ret_pos], ret_snip),
82 Applicability::MachineApplicable
85 let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
88 "move the body of the async block to the enclosing function",
90 Applicability::MachineApplicable
101 fn future_trait_ref<'tcx>(
102 cx: &LateContext<'tcx>,
104 ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
106 if let TyKind::OpaqueDef(item_id, bounds) = ty.kind;
107 let item = cx.tcx.hir().item(item_id);
108 if let ItemKind::OpaqueTy(opaque) = &item.kind;
109 if let Some(trait_ref) = opaque.bounds.iter().find_map(|bound| {
110 if let GenericBound::Trait(poly, _) = bound {
111 Some(&poly.trait_ref)
116 if trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
118 let output_lifetimes = bounds
120 .filter_map(|bound| {
121 if let GenericArg::Lifetime(lt) = bound {
129 return Some((trait_ref, output_lifetimes));
136 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
138 if let Some(segment) = trait_ref.path.segments.last();
139 if let Some(args) = segment.args;
140 if args.bindings.len() == 1;
141 let binding = &args.bindings[0];
142 if binding.ident.name == sym::Output;
143 if let TypeBindingKind::Equality{term: Term::Ty(output)} = binding.kind;
152 fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
153 let input_lifetimes: Vec<LifetimeName> = inputs
156 if let TyKind::Rptr(lt, _) = ty.kind {
164 // The lint should trigger in one of these cases:
165 // - There are no input lifetimes
166 // - There's only one output lifetime bound using `+ '_`
167 // - All input lifetimes are explicitly bound to the output
168 input_lifetimes.is_empty()
169 || (output_lifetimes.len() == 1 && matches!(output_lifetimes[0], LifetimeName::Infer))
172 .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
175 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
177 if let Some(block_expr) = block.expr;
178 if let Some(args) = match_function_call(cx, block_expr, &FUTURE_FROM_GENERATOR);
180 if let Expr{kind: ExprKind::Closure(&Closure { body, .. }), ..} = args[0];
181 let closure_body = cx.tcx.hir().body(body);
182 if closure_body.generator_kind == Some(GeneratorKind::Async(AsyncGeneratorKind::Block));
184 return Some(closure_body);
191 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
193 TyKind::Tup(tys) if tys.is_empty() => {
194 let sugg = "remove the return type";
195 Some((sugg, String::new()))
198 let sugg = "return the output of the future directly";
199 snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {}", snip)))