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

   1 use crate::utils::paths::FUTURE_FROM_GENERATOR;
   2 use crate::utils::{match_function_call, 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, GenericBound, HirId, IsAsync,
   8     ItemKind, 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     /// use std::future::Future;
  31     ///
  32     /// async fn foo() -> i32 { 42 }
  33     /// ```
  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 let IsAsync::NotAsync = header.asyncness;
  54             // Check that this function returns `impl Future`
  55             if let FnRetTy::Return(ret_ty) = decl.output;
  56             if let Some(trait_ref) = future_trait_ref(cx, ret_ty);
  57             if let Some(output) = future_output_ty(trait_ref);
  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             then {
  63                 let header_span = span.with_hi(ret_ty.span.hi());
  64
  65                 span_lint_and_then(
  66                     cx,
  67                     MANUAL_ASYNC_FN,
  68                     header_span,
  69                     "this function can be simplified using the `async fn` syntax",
  70                     |diag| {
  71                         if_chain! {
  72                             if let Some(header_snip) = snippet_opt(cx, header_span);
  73                             if let Some(ret_pos) = header_snip.rfind("->");
  74                             if let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output);
  75                             then {
  76                                 let help = format!("make the function `async` and {}", ret_sugg);
  77                                 diag.span_suggestion(
  78                                     header_span,
  79                                     &help,
  80                                     format!("async {}{}", &header_snip[..ret_pos], ret_snip),
  81                                     Applicability::MachineApplicable
  82                                 );
  83
  84                                 let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
  85                                 diag.span_suggestion(
  86                                     block.span,
  87                                     "move the body of the async block to the enclosing function",
  88                                     body_snip.to_string(),
  89                                     Applicability::MachineApplicable
  90                                 );
  91                             }
  92                         }
  93                     },
  94                 );
  95             }
  96         }
  97     }
  98 }
  99
 100 fn future_trait_ref<'tcx>(cx: &LateContext<'tcx>, ty: &'tcx Ty<'tcx>) -> Option<&'tcx TraitRef<'tcx>> {
 101     if_chain! {
 102         if let TyKind::OpaqueDef(item_id, _) = ty.kind;
 103         let item = cx.tcx.hir().item(item_id.id);
 104         if let ItemKind::OpaqueTy(opaque) = &item.kind;
 105         if opaque.bounds.len() == 1;
 106         if let GenericBound::Trait(poly, _) = &opaque.bounds[0];
 107         if poly.trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
 108         then {
 109             return Some(&poly.trait_ref);
 110         }
 111     }
 112
 113     None
 114 }
 115
 116 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
 117     if_chain! {
 118         if let Some(segment) = trait_ref.path.segments.last();
 119         if let Some(args) = segment.args;
 120         if args.bindings.len() == 1;
 121         let binding = &args.bindings[0];
 122         if binding.ident.as_str() == "Output";
 123         if let TypeBindingKind::Equality{ty: output} = binding.kind;
 124         then {
 125             return Some(output)
 126         }
 127     }
 128
 129     None
 130 }
 131
 132 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
 133     if_chain! {
 134         if let Some(block_expr) = block.expr;
 135         if let Some(args) = match_function_call(cx, block_expr, &FUTURE_FROM_GENERATOR);
 136         if args.len() == 1;
 137         if let Expr{kind: ExprKind::Closure(_, _, body_id, ..), ..} = args[0];
 138         let closure_body = cx.tcx.hir().body(body_id);
 139         if let Some(GeneratorKind::Async(AsyncGeneratorKind::Block)) = closure_body.generator_kind;
 140         then {
 141             return Some(closure_body);
 142         }
 143     }
 144
 145     None
 146 }
 147
 148 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
 149     match output.kind {
 150         TyKind::Tup(tys) if tys.is_empty() => {
 151             let sugg = "remove the return type";
 152             Some((sugg, "".into()))
 153         },
 154         _ => {
 155             let sugg = "return the output of the future directly";
 156             snippet_opt(cx, output.span).map(|snip| (sugg, format!("-> {}", snip)))
 157         },
 158     }
 159 }