rust-clippy/clippy_lints/src/unused_async.rs

use clippy_utils::diagnostics::span_lint_and_then;
use rustc_hir::intravisit::{walk_body, walk_expr, walk_fn, FnKind, Visitor};
use rustc_hir::{Body, Expr, ExprKind, FnDecl, YieldSource};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::nested_filter;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::def_id::LocalDefId;
use rustc_span::Span;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for functions that are declared `async` but have no `.await`s inside of them.
    ///
    /// ### Why is this bad?
    /// Async functions with no async code create overhead, both mentally and computationally.
    /// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
    /// causes runtime overhead and hassle for the caller.
    ///
    /// ### Example
    /// ```rust
    /// async fn get_random_number() -> i64 {
    ///     4 // Chosen by fair dice roll. Guaranteed to be random.
    /// }
    /// let number_future = get_random_number();
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// fn get_random_number_improved() -> i64 {
    ///     4 // Chosen by fair dice roll. Guaranteed to be random.
    /// }
    /// let number_future = async { get_random_number_improved() };
    /// ```
    #[clippy::version = "1.54.0"]
    pub UNUSED_ASYNC,
    pedantic,
    "finds async functions with no await statements"
}

declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);

struct AsyncFnVisitor<'a, 'tcx> {
    cx: &'a LateContext<'tcx>,
    found_await: bool,
    /// Also keep track of `await`s in nested async blocks so we can mention
    /// it in a note
    await_in_async_block: Option<Span>,
    async_depth: usize,
}

impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
    type NestedFilter = nested_filter::OnlyBodies;

    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
        if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
            if self.async_depth == 1 {
                self.found_await = true;
            } else if self.await_in_async_block.is_none() {
                self.await_in_async_block = Some(ex.span);
            }
        }
        walk_expr(self, ex);
    }

    fn nested_visit_map(&mut self) -> Self::Map {
        self.cx.tcx.hir()
    }

    fn visit_body(&mut self, b: &'tcx Body<'tcx>) {
        let is_async_block = matches!(b.generator_kind, Some(rustc_hir::GeneratorKind::Async(_)));

        if is_async_block {
            self.async_depth += 1;
        }

        walk_body(self, b);

        if is_async_block {
            self.async_depth -= 1;
        }
    }
}

impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        fn_kind: FnKind<'tcx>,
        fn_decl: &'tcx FnDecl<'tcx>,
        body: &Body<'tcx>,
        span: Span,
        def_id: LocalDefId,
    ) {
        if !span.from_expansion() && fn_kind.asyncness().is_async() {
            let mut visitor = AsyncFnVisitor {
                cx,
                found_await: false,
                async_depth: 0,
                await_in_async_block: None,
            };
            walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), def_id);
            if !visitor.found_await {
                span_lint_and_then(
                    cx,
                    UNUSED_ASYNC,
                    span,
                    "unused `async` for function with no await statements",
                    |diag| {
                        diag.help("consider removing the `async` from this function");

                        if let Some(span) = visitor.await_in_async_block {
                            diag.span_note(
                                span,
                                "`await` used in an async block, which does not require \
                                the enclosing function to be `async`",
                            );
                        }
                    },
                );
            }
        }
    }
}