1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128

use crate::lints::AsyncFnInTraitDiag;
use crate::LateContext;
use crate::LateLintPass;
use rustc_hir as hir;
use rustc_session::{declare_lint, declare_lint_pass};
use rustc_trait_selection::traits::error_reporting::suggestions::suggest_desugaring_async_fn_to_impl_future_in_trait;

declare_lint! {
    /// The `async_fn_in_trait` lint detects use of `async fn` in the
    /// definition of a publicly-reachable trait.
    ///
    /// ### Example
    ///
    /// ```rust
    /// pub trait Trait {
    ///     async fn method(&self);
    /// }
    /// # fn main() {}
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// When `async fn` is used in a trait definition, the trait does not
    /// promise that the opaque [`Future`] returned by the associated function
    /// or method will implement any [auto traits] such as [`Send`]. This may
    /// be surprising and may make the associated functions or methods on the
    /// trait less useful than intended. On traits exposed publicly from a
    /// crate, this may affect downstream crates whose authors cannot alter
    /// the trait definition.
    ///
    /// For example, this code is invalid:
    ///
    /// ```rust,compile_fail
    /// pub trait Trait {
    ///     async fn method(&self) {}
    /// }
    ///
    /// fn test<T: Trait>(x: T) {
    ///     fn spawn<T: Send>(_: T) {}
    ///     spawn(x.method()); // Not OK.
    /// }
    /// ```
    ///
    /// This lint exists to warn authors of publicly-reachable traits that
    /// they may want to consider desugaring the `async fn` to a normal `fn`
    /// that returns an opaque `impl Future<..> + Send` type.
    ///
    /// For example, instead of:
    ///
    /// ```rust
    /// pub trait Trait {
    ///     async fn method(&self) {}
    /// }
    /// ```
    ///
    /// The author of the trait may want to write:
    ///
    ///
    /// ```rust
    /// use core::future::Future;
    /// pub trait Trait {
    ///     fn method(&self) -> impl Future<Output = ()> + Send { async {} }
    /// }
    /// ```
    ///
    /// This still allows the use of `async fn` within impls of the trait.
    /// However, it also means that the trait will never be compatible with
    /// impls where the returned [`Future`] of the method does not implement
    /// `Send`.
    ///
    /// Conversely, if the trait is used only locally, if it is never used in
    /// generic functions, or if it is only used in single-threaded contexts
    /// that do not care whether the returned [`Future`] implements [`Send`],
    /// then the lint may be suppressed.
    ///
    /// [`Future`]: https://doc.rust-lang.org/core/future/trait.Future.html
    /// [`Send`]: https://doc.rust-lang.org/core/marker/trait.Send.html
    /// [auto traits]: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
    pub ASYNC_FN_IN_TRAIT,
    Warn,
    "use of `async fn` in definition of a publicly-reachable trait"
}

declare_lint_pass!(
    /// Lint for use of `async fn` in the definition of a publicly-reachable
    /// trait.
    AsyncFnInTrait => [ASYNC_FN_IN_TRAIT]
);

impl<'tcx> LateLintPass<'tcx> for AsyncFnInTrait {
    fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'tcx>) {
        if let hir::TraitItemKind::Fn(sig, body) = item.kind
            && let hir::IsAsync::Async(async_span) = sig.header.asyncness
        {
            // RTN can be used to bound `async fn` in traits in a better way than "always"
            if cx.tcx.features().return_type_notation {
                return;
            }

            // Only need to think about library implications of reachable traits
            if !cx.tcx.effective_visibilities(()).is_reachable(item.owner_id.def_id) {
                return;
            }

            let hir::FnRetTy::Return(hir::Ty { kind: hir::TyKind::OpaqueDef(def, ..), .. }) =
                sig.decl.output
            else {
                // This should never happen, but let's not ICE.
                return;
            };
            let sugg = suggest_desugaring_async_fn_to_impl_future_in_trait(
                cx.tcx,
                sig,
                body,
                def.owner_id.def_id,
                " + Send",
            );
            cx.tcx.emit_node_span_lint(
                ASYNC_FN_IN_TRAIT,
                item.hir_id(),
                async_span,
                AsyncFnInTraitDiag { sugg },
            );
        }
    }
}