rustc_lint/
async_closures.rs

1use rustc_hir as hir;
2use rustc_macros::{LintDiagnostic, Subdiagnostic};
3use rustc_session::{declare_lint, declare_lint_pass};
4use rustc_span::Span;
5
6use crate::{LateContext, LateLintPass};
7
8declare_lint! {
9    /// The `closure_returning_async_block` lint detects cases where users
10    /// write a closure that returns an async block.
11    ///
12    /// ### Example
13    ///
14    /// ```rust
15    /// #![warn(closure_returning_async_block)]
16    /// let c = |x: &str| async {};
17    /// ```
18    ///
19    /// {{produces}}
20    ///
21    /// ### Explanation
22    ///
23    /// Using an async closure is preferable over a closure that returns an
24    /// async block, since async closures are less restrictive in how its
25    /// captures are allowed to be used.
26    ///
27    /// For example, this code does not work with a closure returning an async
28    /// block:
29    ///
30    /// ```rust,compile_fail
31    /// async fn callback(x: &str) {}
32    ///
33    /// let captured_str = String::new();
34    /// let c = move || async {
35    ///     callback(&captured_str).await;
36    /// };
37    /// ```
38    ///
39    /// But it does work with async closures:
40    ///
41    /// ```rust
42    /// async fn callback(x: &str) {}
43    ///
44    /// let captured_str = String::new();
45    /// let c = async move || {
46    ///     callback(&captured_str).await;
47    /// };
48    /// ```
49    pub CLOSURE_RETURNING_ASYNC_BLOCK,
50    Allow,
51    "closure that returns `async {}` could be rewritten as an async closure",
52}
53
54declare_lint_pass!(
55    /// Lint for potential usages of async closures and async fn trait bounds.
56    AsyncClosureUsage => [CLOSURE_RETURNING_ASYNC_BLOCK]
57);
58
59impl<'tcx> LateLintPass<'tcx> for AsyncClosureUsage {
60    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
61        let hir::ExprKind::Closure(&hir::Closure {
62            body,
63            kind: hir::ClosureKind::Closure,
64            fn_decl_span,
65            ..
66        }) = expr.kind
67        else {
68            return;
69        };
70
71        let mut body = cx.tcx.hir_body(body).value;
72
73        // Only peel blocks that have no expressions.
74        while let hir::ExprKind::Block(&hir::Block { stmts: [], expr: Some(tail), .. }, None) =
75            body.kind
76        {
77            body = tail;
78        }
79
80        let hir::ExprKind::Closure(&hir::Closure {
81            kind:
82                hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
83                    hir::CoroutineDesugaring::Async,
84                    hir::CoroutineSource::Block,
85                )),
86            fn_decl_span: async_decl_span,
87            ..
88        }) = body.kind
89        else {
90            return;
91        };
92
93        let deletion_span = cx.tcx.sess.source_map().span_extend_while_whitespace(async_decl_span);
94
95        cx.tcx.emit_node_span_lint(
96            CLOSURE_RETURNING_ASYNC_BLOCK,
97            expr.hir_id,
98            fn_decl_span,
99            ClosureReturningAsyncBlock {
100                async_decl_span,
101                sugg: AsyncClosureSugg {
102                    deletion_span,
103                    insertion_span: fn_decl_span.shrink_to_lo(),
104                },
105            },
106        );
107    }
108}
109
110#[derive(LintDiagnostic)]
111#[diag(lint_closure_returning_async_block)]
112struct ClosureReturningAsyncBlock {
113    #[label]
114    async_decl_span: Span,
115    #[subdiagnostic]
116    sugg: AsyncClosureSugg,
117}
118
119#[derive(Subdiagnostic)]
120#[multipart_suggestion(lint_suggestion, applicability = "maybe-incorrect")]
121struct AsyncClosureSugg {
122    #[suggestion_part(code = "")]
123    deletion_span: Span,
124    #[suggestion_part(code = "async ")]
125    insertion_span: Span,
126}
rustc_lint/async_closures.rs

rustc_lint/
async_closures.rs
