clippy_lints/src/manual_async_fn.rs

   1 use crate::utils::paths::FUTURE_FROM_GENERATOR;
   2 use crate::utils::{match_function_call, position_before_rarrow, snippet_block, snippet_opt, span_lint_and_then};
   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, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound, HirId,
   8     IsAsync, ItemKind, LifetimeName, 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::Span;
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** It checks for manual implementations of `async` functions.
  16     ///
  17     /// **Why is this bad?** It's more idiomatic to use the dedicated syntax.
  18     ///
  19     /// **Known problems:** None.
  20     ///
  21     /// **Example:**
  22     ///
  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     pub MANUAL_ASYNC_FN,
  33     style,
  34     "manual implementations of `async` functions can be simplified using the dedicated syntax"
  35 }
  36
  37 declare_lint_pass!(ManualAsyncFn => [MANUAL_ASYNC_FN]);
  38
  39 impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
  40     fn check_fn(
  41         &mut self,
  42         cx: &LateContext<'tcx>,
  43         kind: FnKind<'tcx>,
  44         decl: &'tcx FnDecl<'_>,
  45         body: &'tcx Body<'_>,
  46         span: Span,
  47         _: HirId,
  48     ) {
  49         if_chain! {
  50             if let Some(header) = kind.header();
  51             if let IsAsync::NotAsync = header.asyncness;
  52             // Check that this function returns `impl Future`
  53             if let FnRetTy::Return(ret_ty) = decl.output;
  54             if let Some((trait_ref, output_lifetimes)) = future_trait_ref(cx, ret_ty);
  55             if let Some(output) = future_output_ty(trait_ref);
  56             if captures_all_lifetimes(decl.inputs, &output_lifetimes);
  57             // Check that the body of the function consists of one async block
  58             if let ExprKind::Block(block, _) = body.value.kind;
  59             if block.stmts.is_empty();
  60             if let Some(closure_body) = desugared_async_block(cx, block);
  61             then {
  62                 let header_span = span.with_hi(ret_ty.span.hi());
  63
  64                 span_lint_and_then(
  65                     cx,
  66                     MANUAL_ASYNC_FN,
  67                     header_span,
  68                     "this function can be simplified using the `async fn` syntax",
  69                     |diag| {
  70                         if_chain! {
  71                             if let Some(header_snip) = snippet_opt(cx, header_span);
  72                             if let Some(ret_pos) = position_before_rarrow(&header_snip);
  73                             if let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output);
  74                             then {
  75                                 let help = format!("make the function `async` and {}", ret_sugg);
  76                                 diag.span_suggestion(
  77                                     header_span,
  78                                     &help,
  79                                     format!("async {}{}", &header_snip[..ret_pos], ret_snip),
  80                                     Applicability::MachineApplicable
  81                                 );
  82
  83                                 let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
  84                                 diag.span_suggestion(
  85                                     block.span,
  86                                     "move the body of the async block to the enclosing function",
  87                                     body_snip.to_string(),
  88                                     Applicability::MachineApplicable
  89                                 );
  90                             }
  91                         }
  92                     },
  93                 );
  94             }
  95         }
  96     }
  97 }
  98
  99 fn future_trait_ref<'tcx>(
 100     cx: &LateContext<'tcx>,
 101     ty: &'tcx Ty<'tcx>,
 102 ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
 103     if_chain! {
 104         if let TyKind::OpaqueDef(item_id, bounds) = ty.kind;
 105         let item = cx.tcx.hir().item(item_id.id);
 106         if let ItemKind::OpaqueTy(opaque) = &item.kind;
 107         if let Some(trait_ref) = opaque.bounds.iter().find_map(|bound| {
 108             if let GenericBound::Trait(poly, _) = bound {
 109                 Some(&poly.trait_ref)
 110             } else {
 111                 None
 112             }
 113         });
 114         if trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
 115         then {
 116             let output_lifetimes = bounds
 117                 .iter()
 118                 .filter_map(|bound| {
 119                     if let GenericArg::Lifetime(lt) = bound {
 120                         Some(lt.name)
 121                     } else {
 122                         None
 123                     }
 124                 })
 125                 .collect();
 126
 127             return Some((trait_ref, output_lifetimes));
 128         }
 129     }
 130
 131     None
 132 }
 133
 134 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
 135     if_chain! {
 136         if let Some(segment) = trait_ref.path.segments.last();
 137         if let Some(args) = segment.args;
 138         if args.bindings.len() == 1;
 139         let binding = &args.bindings[0];
 140         if binding.ident.as_str() == "Output";
 141         if let TypeBindingKind::Equality{ty: output} = binding.kind;
 142         then {
 143             return Some(output)
 144         }
 145     }
 146
 147     None
 148 }
 149
 150 fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
 151     let input_lifetimes: Vec<LifetimeName> = inputs
 152         .iter()
 153         .filter_map(|ty| {
 154             if let TyKind::Rptr(lt, _) = ty.kind {
 155                 Some(lt.name)
 156             } else {
 157                 None
 158             }
 159         })
 160         .collect();
 161
 162     // The lint should trigger in one of these cases:
 163     // - There are no input lifetimes
 164     // - There's only one output lifetime bound using `+ '_`
 165     // - All input lifetimes are explicitly bound to the output
 166     input_lifetimes.is_empty()
 167         || (output_lifetimes.len() == 1 && matches!(output_lifetimes[0], LifetimeName::Underscore))
 168         || input_lifetimes
 169             .iter()
 170             .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
 171 }
 172
 173 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
 174     if_chain! {
 175         if let Some(block_expr) = block.expr;
 176         if let Some(args) = match_function_call(cx, block_expr, &FUTURE_FROM_GENERATOR);
 177         if args.len() == 1;
 178         if let Expr{kind: ExprKind::Closure(_, _, body_id, ..), ..} = args[0];
 179         let closure_body = cx.tcx.hir().body(body_id);
 180         if let Some(GeneratorKind::Async(AsyncGeneratorKind::Block)) = closure_body.generator_kind;
 181         then {
 182             return Some(closure_body);
 183         }
 184     }
 185
 186     None
 187 }
 188
 189 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
 190     match output.kind {
 191         TyKind::Tup(tys) if tys.is_empty() => {
 192             let sugg = "remove the return type";
 193             Some((sugg, "".into()))
 194         },
 195         _ => {
 196             let sugg = "return the output of the future directly";
 197             snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {}", snip)))
 198         },
 199     }
 200 }