src/tools/clippy/clippy_lints/src/manual_async_fn.rs

   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;
   8 use rustc_hir::{
   9     AsyncGeneratorKind, Block, Body, Closure, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound,
  10     HirId, IsAsync, ItemKind, LifetimeName, Term, TraitRef, Ty, TyKind, TypeBindingKind,
  11 };
  12 use rustc_lint::{LateContext, LateLintPass};
  13 use rustc_session::{declare_lint_pass, declare_tool_lint};
  14 use rustc_span::{sym, Span};
  15
  16 declare_clippy_lint! {
  17     /// ### What it does
  18     /// It checks for manual implementations of `async` functions.
  19     ///
  20     /// ### Why is this bad?
  21     /// It's more idiomatic to use the dedicated syntax.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// use std::future::Future;
  26     ///
  27     /// fn foo() -> impl Future<Output = i32> { async { 42 } }
  28     /// ```
  29     /// Use instead:
  30     /// ```rust
  31     /// async fn foo() -> i32 { 42 }
  32     /// ```
  33     #[clippy::version = "1.45.0"]
  34     pub MANUAL_ASYNC_FN,
  35     style,
  36     "manual implementations of `async` functions can be simplified using the dedicated syntax"
  37 }
  38
  39 declare_lint_pass!(ManualAsyncFn => [MANUAL_ASYNC_FN]);
  40
  41 impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
  42     fn check_fn(
  43         &mut self,
  44         cx: &LateContext<'tcx>,
  45         kind: FnKind<'tcx>,
  46         decl: &'tcx FnDecl<'_>,
  47         body: &'tcx Body<'_>,
  48         span: Span,
  49         _: HirId,
  50     ) {
  51         if_chain! {
  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);
  63             then {
  64                 let header_span = span.with_hi(ret_ty.span.hi());
  65
  66                 span_lint_and_then(
  67                     cx,
  68                     MANUAL_ASYNC_FN,
  69                     header_span,
  70                     "this function can be simplified using the `async fn` syntax",
  71                     |diag| {
  72                         if_chain! {
  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);
  76                             then {
  77                                 let help = format!("make the function `async` and {ret_sugg}");
  78                                 diag.span_suggestion(
  79                                     header_span,
  80                                     &help,
  81                                     format!("async {}{ret_snip}", &header_snip[..ret_pos]),
  82                                     Applicability::MachineApplicable
  83                                 );
  84
  85                                 let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
  86                                 diag.span_suggestion(
  87                                     block.span,
  88                                     "move the body of the async block to the enclosing function",
  89                                     body_snip,
  90                                     Applicability::MachineApplicable
  91                                 );
  92                             }
  93                         }
  94                     },
  95                 );
  96             }
  97         }
  98     }
  99 }
 100
 101 fn future_trait_ref<'tcx>(
 102     cx: &LateContext<'tcx>,
 103     ty: &'tcx Ty<'tcx>,
 104 ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
 105     if_chain! {
 106         if let TyKind::OpaqueDef(item_id, bounds, false) = 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)
 112             } else {
 113                 None
 114             }
 115         });
 116         if trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
 117         then {
 118             let output_lifetimes = bounds
 119                 .iter()
 120                 .filter_map(|bound| {
 121                     if let GenericArg::Lifetime(lt) = bound {
 122                         Some(lt.name)
 123                     } else {
 124                         None
 125                     }
 126                 })
 127                 .collect();
 128
 129             return Some((trait_ref, output_lifetimes));
 130         }
 131     }
 132
 133     None
 134 }
 135
 136 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
 137     if_chain! {
 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;
 144         then {
 145             return Some(output)
 146         }
 147     }
 148
 149     None
 150 }
 151
 152 fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
 153     let input_lifetimes: Vec<LifetimeName> = inputs
 154         .iter()
 155         .filter_map(|ty| {
 156             if let TyKind::Rptr(lt, _) = ty.kind {
 157                 Some(lt.name)
 158             } else {
 159                 None
 160             }
 161         })
 162         .collect();
 163
 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))
 170         || input_lifetimes
 171             .iter()
 172             .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
 173 }
 174
 175 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
 176     if_chain! {
 177         if let Some(block_expr) = block.expr;
 178         if let Some(args) = match_function_call(cx, block_expr, &FUTURE_FROM_GENERATOR);
 179         if args.len() == 1;
 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));
 183         then {
 184             return Some(closure_body);
 185         }
 186     }
 187
 188     None
 189 }
 190
 191 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
 192     match output.kind {
 193         TyKind::Tup(tys) if tys.is_empty() => {
 194             let sugg = "remove the return type";
 195             Some((sugg, String::new()))
 196         },
 197         _ => {
 198             let sugg = "return the output of the future directly";
 199             snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {snip}")))
 200         },
 201     }
 202 }