src/tools/clippy/clippy_lints/src/future_not_send.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::return_ty;
   3 use rustc_hir::intravisit::FnKind;
   4 use rustc_hir::{Body, FnDecl, HirId};
   5 use rustc_infer::infer::TyCtxtInferExt;
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_middle::ty::{self, AliasTy, Clause, EarlyBinder, PredicateKind};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use rustc_span::{sym, Span};
  10 use rustc_trait_selection::traits::error_reporting::suggestions::TypeErrCtxtExt;
  11 use rustc_trait_selection::traits::{self, FulfillmentError};
  12
  13 declare_clippy_lint! {
  14     /// ### What it does
  15     /// This lint requires Future implementations returned from
  16     /// functions and methods to implement the `Send` marker trait. It is mostly
  17     /// used by library authors (public and internal) that target an audience where
  18     /// multithreaded executors are likely to be used for running these Futures.
  19     ///
  20     /// ### Why is this bad?
  21     /// A Future implementation captures some state that it
  22     /// needs to eventually produce its final value. When targeting a multithreaded
  23     /// executor (which is the norm on non-embedded devices) this means that this
  24     /// state may need to be transported to other threads, in other words the
  25     /// whole Future needs to implement the `Send` marker trait. If it does not,
  26     /// then the resulting Future cannot be submitted to a thread pool in the
  27     /// end user’s code.
  28     ///
  29     /// Especially for generic functions it can be confusing to leave the
  30     /// discovery of this problem to the end user: the reported error location
  31     /// will be far from its cause and can in many cases not even be fixed without
  32     /// modifying the library where the offending Future implementation is
  33     /// produced.
  34     ///
  35     /// ### Example
  36     /// ```rust
  37     /// async fn not_send(bytes: std::rc::Rc<[u8]>) {}
  38     /// ```
  39     /// Use instead:
  40     /// ```rust
  41     /// async fn is_send(bytes: std::sync::Arc<[u8]>) {}
  42     /// ```
  43     #[clippy::version = "1.44.0"]
  44     pub FUTURE_NOT_SEND,
  45     nursery,
  46     "public Futures must be Send"
  47 }
  48
  49 declare_lint_pass!(FutureNotSend => [FUTURE_NOT_SEND]);
  50
  51 impl<'tcx> LateLintPass<'tcx> for FutureNotSend {
  52     fn check_fn(
  53         &mut self,
  54         cx: &LateContext<'tcx>,
  55         kind: FnKind<'tcx>,
  56         decl: &'tcx FnDecl<'tcx>,
  57         _: &'tcx Body<'tcx>,
  58         _: Span,
  59         hir_id: HirId,
  60     ) {
  61         if let FnKind::Closure = kind {
  62             return;
  63         }
  64         let ret_ty = return_ty(cx, hir_id);
  65         if let ty::Alias(ty::Opaque, AliasTy { def_id, substs, .. }) = *ret_ty.kind() {
  66             let preds = cx.tcx.explicit_item_bounds(def_id);
  67             let mut is_future = false;
  68             for &(p, _span) in preds {
  69                 let p = EarlyBinder(p).subst(cx.tcx, substs);
  70                 if let Some(trait_pred) = p.to_opt_poly_trait_pred() {
  71                     if Some(trait_pred.skip_binder().trait_ref.def_id) == cx.tcx.lang_items().future_trait() {
  72                         is_future = true;
  73                         break;
  74                     }
  75                 }
  76             }
  77             if is_future {
  78                 let send_trait = cx.tcx.get_diagnostic_item(sym::Send).unwrap();
  79                 let span = decl.output.span();
  80                 let infcx = cx.tcx.infer_ctxt().build();
  81                 let cause = traits::ObligationCause::misc(span, hir_id);
  82                 let send_errors = traits::fully_solve_bound(&infcx, cause, cx.param_env, ret_ty, send_trait);
  83                 if !send_errors.is_empty() {
  84                     span_lint_and_then(
  85                         cx,
  86                         FUTURE_NOT_SEND,
  87                         span,
  88                         "future cannot be sent between threads safely",
  89                         |db| {
  90                             for FulfillmentError { obligation, .. } in send_errors {
  91                                 infcx
  92                                     .err_ctxt()
  93                                     .maybe_note_obligation_cause_for_async_await(db, &obligation);
  94                                 if let PredicateKind::Clause(Clause::Trait(trait_pred)) =
  95                                     obligation.predicate.kind().skip_binder()
  96                                 {
  97                                     db.note(&format!(
  98                                         "`{}` doesn't implement `{}`",
  99                                         trait_pred.self_ty(),
 100                                         trait_pred.trait_ref.print_only_trait_path(),
 101                                     ));
 102                                 }
 103                             }
 104                         },
 105                     );
 106                 }
 107             }
 108         }
 109     }
 110 }