rust/clippy_lints/src/manual_async_fn.rs

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::{position_before_rarrow, snippet_block, snippet_opt};
use rustc_errors::Applicability;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{
    Block, Body, Closure, ClosureKind, CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, ExprKind, FnDecl,
    FnRetTy, GenericArg, GenericBound, ImplItem, Item, ItemKind, LifetimeName, Node, Term, TraitRef, Ty, TyKind,
    TypeBindingKind,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;
use rustc_span::def_id::LocalDefId;
use rustc_span::{sym, Span};

declare_clippy_lint! {
    /// ### What it does
    /// It checks for manual implementations of `async` functions.
    ///
    /// ### Why is this bad?
    /// It's more idiomatic to use the dedicated syntax.
    ///
    /// ### Example
    /// ```no_run
    /// use std::future::Future;
    ///
    /// fn foo() -> impl Future<Output = i32> { async { 42 } }
    /// ```
    /// Use instead:
    /// ```no_run
    /// async fn foo() -> i32 { 42 }
    /// ```
    #[clippy::version = "1.45.0"]
    pub MANUAL_ASYNC_FN,
    style,
    "manual implementations of `async` functions can be simplified using the dedicated syntax"
}

declare_lint_pass!(ManualAsyncFn => [MANUAL_ASYNC_FN]);

impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        kind: FnKind<'tcx>,
        decl: &'tcx FnDecl<'_>,
        body: &'tcx Body<'_>,
        span: Span,
        def_id: LocalDefId,
    ) {
        if let Some(header) = kind.header()
            && !header.asyncness.is_async()
            // Check that this function returns `impl Future`
            && let FnRetTy::Return(ret_ty) = decl.output
            && let Some((trait_ref, output_lifetimes)) = future_trait_ref(cx, ret_ty)
            && let Some(output) = future_output_ty(trait_ref)
            && captures_all_lifetimes(decl.inputs, &output_lifetimes)
            // Check that the body of the function consists of one async block
            && let ExprKind::Block(block, _) = body.value.kind
            && block.stmts.is_empty()
            && let Some(closure_body) = desugared_async_block(cx, block)
            && let Node::Item(Item {vis_span, ..}) | Node::ImplItem(ImplItem {vis_span, ..}) =
                cx.tcx.hir_node_by_def_id(def_id)
        {
            let header_span = span.with_hi(ret_ty.span.hi());

            span_lint_and_then(
                cx,
                MANUAL_ASYNC_FN,
                header_span,
                "this function can be simplified using the `async fn` syntax",
                |diag| {
                    if let Some(vis_snip) = snippet_opt(cx, *vis_span)
                        && let Some(header_snip) = snippet_opt(cx, header_span)
                        && let Some(ret_pos) = position_before_rarrow(&header_snip)
                        && let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output)
                    {
                        let header_snip = if vis_snip.is_empty() {
                            format!("async {}", &header_snip[..ret_pos])
                        } else {
                            format!("{} async {}", vis_snip, &header_snip[vis_snip.len() + 1..ret_pos])
                        };

                        let help = format!("make the function `async` and {ret_sugg}");
                        diag.span_suggestion(
                            header_span,
                            help,
                            format!("{header_snip}{ret_snip}"),
                            Applicability::MachineApplicable,
                        );

                        let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
                        diag.span_suggestion(
                            block.span,
                            "move the body of the async block to the enclosing function",
                            body_snip,
                            Applicability::MachineApplicable,
                        );
                    }
                },
            );
        }
    }
}

fn future_trait_ref<'tcx>(
    cx: &LateContext<'tcx>,
    ty: &'tcx Ty<'tcx>,
) -> Option<(&'tcx TraitRef<'tcx>, Vec<LifetimeName>)> {
    if let TyKind::OpaqueDef(item_id, bounds, false) = ty.kind
        && let item = cx.tcx.hir().item(item_id)
        && let ItemKind::OpaqueTy(opaque) = &item.kind
        && let Some(trait_ref) = opaque.bounds.iter().find_map(|bound| {
            if let GenericBound::Trait(poly, _) = bound {
                Some(&poly.trait_ref)
            } else {
                None
            }
        })
        && trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait()
    {
        let output_lifetimes = bounds
            .iter()
            .filter_map(|bound| {
                if let GenericArg::Lifetime(lt) = bound {
                    Some(lt.res)
                } else {
                    None
                }
            })
            .collect();

        return Some((trait_ref, output_lifetimes));
    }

    None
}

fn future_output_ty<'tcx>(trait_ref: &'tcx TraitRef<'tcx>) -> Option<&'tcx Ty<'tcx>> {
    if let Some(segment) = trait_ref.path.segments.last()
        && let Some(args) = segment.args
        && args.bindings.len() == 1
        && let binding = &args.bindings[0]
        && binding.ident.name == sym::Output
        && let TypeBindingKind::Equality { term: Term::Ty(output) } = binding.kind
    {
        return Some(output);
    }

    None
}

fn captures_all_lifetimes(inputs: &[Ty<'_>], output_lifetimes: &[LifetimeName]) -> bool {
    let input_lifetimes: Vec<LifetimeName> = inputs
        .iter()
        .filter_map(|ty| {
            if let TyKind::Ref(lt, _) = ty.kind {
                Some(lt.res)
            } else {
                None
            }
        })
        .collect();

    // The lint should trigger in one of these cases:
    // - There are no input lifetimes
    // - There's only one output lifetime bound using `+ '_`
    // - All input lifetimes are explicitly bound to the output
    input_lifetimes.is_empty()
        || (output_lifetimes.len() == 1 && matches!(output_lifetimes[0], LifetimeName::Infer))
        || input_lifetimes
            .iter()
            .all(|in_lt| output_lifetimes.iter().any(|out_lt| in_lt == out_lt))
}

fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
    if let Some(Expr {
        kind: ExprKind::Closure(&Closure { kind, body, .. }),
        ..
    }) = block.expr
        && let ClosureKind::Coroutine(CoroutineKind::Desugared(CoroutineDesugaring::Async, CoroutineSource::Block)) =
            kind
    {
        return Some(cx.tcx.hir().body(body));
    }

    None
}

fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str, String)> {
    if let TyKind::Tup([]) = output.kind {
        let sugg = "remove the return type";
        Some((sugg, String::new()))
    } else {
        let sugg = "return the output of the future directly";
        snippet_opt(cx, output.span).map(|snip| (sugg, format!(" -> {snip}")))
    }
}