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::subst::Subst;
   8 use rustc_middle::ty::{Opaque, PredicateKind::Trait};
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10 use rustc_span::{sym, Span};
  11 use rustc_trait_selection::traits::error_reporting::suggestions::InferCtxtExt;
  12 use rustc_trait_selection::traits::{self, FulfillmentError, TraitEngine};
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// This lint requires Future implementations returned from
  17     /// functions and methods to implement the `Send` marker trait. It is mostly
  18     /// used by library authors (public and internal) that target an audience where
  19     /// multithreaded executors are likely to be used for running these Futures.
  20     ///
  21     /// ### Why is this bad?
  22     /// A Future implementation captures some state that it
  23     /// needs to eventually produce its final value. When targeting a multithreaded
  24     /// executor (which is the norm on non-embedded devices) this means that this
  25     /// state may need to be transported to other threads, in other words the
  26     /// whole Future needs to implement the `Send` marker trait. If it does not,
  27     /// then the resulting Future cannot be submitted to a thread pool in the
  28     /// end user’s code.
  29     ///
  30     /// Especially for generic functions it can be confusing to leave the
  31     /// discovery of this problem to the end user: the reported error location
  32     /// will be far from its cause and can in many cases not even be fixed without
  33     /// modifying the library where the offending Future implementation is
  34     /// produced.
  35     ///
  36     /// ### Example
  37     /// ```rust
  38     /// async fn not_send(bytes: std::rc::Rc<[u8]>) {}
  39     /// ```
  40     /// Use instead:
  41     /// ```rust
  42     /// async fn is_send(bytes: std::sync::Arc<[u8]>) {}
  43     /// ```
  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 Opaque(id, subst) = *ret_ty.kind() {
  66             let preds = cx.tcx.explicit_item_bounds(id);
  67             let mut is_future = false;
  68             for &(p, _span) in preds {
  69                 let p = p.subst(cx.tcx, subst);
  70                 if let Some(trait_ref) = p.to_opt_poly_trait_ref() {
  71                     if Some(trait_ref.value.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 send_errors = cx.tcx.infer_ctxt().enter(|infcx| {
  81                     let cause = traits::ObligationCause::misc(span, hir_id);
  82                     let mut fulfillment_cx = traits::FulfillmentContext::new();
  83                     fulfillment_cx.register_bound(&infcx, cx.param_env, ret_ty, send_trait, cause);
  84                     fulfillment_cx.select_all_or_error(&infcx)
  85                 });
  86                 if !send_errors.is_empty() {
  87                     span_lint_and_then(
  88                         cx,
  89                         FUTURE_NOT_SEND,
  90                         span,
  91                         "future cannot be sent between threads safely",
  92                         |db| {
  93                             cx.tcx.infer_ctxt().enter(|infcx| {
  94                                 for FulfillmentError { obligation, .. } in send_errors {
  95                                     infcx.maybe_note_obligation_cause_for_async_await(db, &obligation);
  96                                     if let Trait(trait_pred) = obligation.predicate.kind().skip_binder() {
  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     }
 111 }