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, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound, HirId,
  10     IsAsync, ItemKind, LifetimeName, 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     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 {
  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         _: HirId,
  49     ) {
  50         if_chain! {
  51             if let Some(header) = kind.header();
  52             if let IsAsync::NotAsync = header.asyncness;
  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             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) = position_before_rarrow(&header_snip);
  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>(
 101     cx: &LateContext<'tcx>,
 102     ty: &'tcx Ty<'tcx>,
 103 ) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
 104     if_chain! {
 105         if let TyKind::OpaqueDef(item_id, bounds) = ty.kind;
 106         let item = cx.tcx.hir().item(item_id);
 107         if let ItemKind::OpaqueTy(opaque) = &item.kind;
 108         if let Some(trait_ref) = opaque.bounds.iter().find_map(|bound| {
 109             if let GenericBound::Trait(poly, _) = bound {
 110                 Some(&poly.trait_ref)
 111             } else {
 112                 None
 113             }
 114         });
 115         if trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
 116         then {
 117             let output_lifetimes = bounds
 118                 .iter()
 119                 .filter_map(|bound| {
 120                     if let GenericArg::Lifetime(lt) = bound {
 121                         Some(lt.name)
 122                     } else {
 123                         None
 124                     }
 125                 })
 126                 .collect();
 127
 128             return Some((trait_ref, output_lifetimes));
 129         }
 130     }
 131
 132     None
 133 }
 134
 135 fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
 136     if_chain! {
 137         if let Some(segment) = trait_ref.path.segments.last();
 138         if let Some(args) = segment.args;
 139         if args.bindings.len() == 1;
 140         let binding = &args.bindings[0];
 141         if binding.ident.name == sym::Output;
 142         if let TypeBindingKind::Equality{ty: output} = binding.kind;
 143         then {
 144             return Some(output)
 145         }
 146     }
 147
 148     None
 149 }
 150
 151 fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
 152     let input_lifetimes: Vec<LifetimeName> = inputs
 153         .iter()
 154         .filter_map(|ty| {
 155             if let TyKind::Rptr(lt, _) = ty.kind {
 156                 Some(lt.name)
 157             } else {
 158                 None
 159             }
 160         })
 161         .collect();
 162
 163     // The lint should trigger in one of these cases:
 164     // - There are no input lifetimes
 165     // - There's only one output lifetime bound using `+ '_`
 166     // - All input lifetimes are explicitly bound to the output
 167     input_lifetimes.is_empty()
 168         || (output_lifetimes.len() == 1 && matches!(output_lifetimes[0], LifetimeName::Underscore))
 169         || input_lifetimes
 170             .iter()
 171             .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
 172 }
 173
 174 fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
 175     if_chain! {
 176         if let Some(block_expr) = block.expr;
 177         if let Some(args) = match_function_call(cx, block_expr, &FUTURE_FROM_GENERATOR);
 178         if args.len() == 1;
 179         if let Expr{kind: ExprKind::Closure(_, _, body_id, ..), ..} = args[0];
 180         let closure_body = cx.tcx.hir().body(body_id);
 181         if let Some(GeneratorKind::Async(AsyncGeneratorKind::Block)) = closure_body.generator_kind;
 182         then {
 183             return Some(closure_body);
 184         }
 185     }
 186
 187     None
 188 }
 189
 190 fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
 191     match output.kind {
 192         TyKind::Tup(tys) if tys.is_empty() => {
 193             let sugg = "remove the return type";
 194             Some((sugg, "".into()))
 195         },
 196         _ => {
 197             let sugg = "return the output of the future directly";
 198             snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {}", snip)))
 199         },
 200     }
 201 }