src/tools/clippy/clippy_lints/src/manual_async_fn.rs

use crate::utils::paths::FUTURE_FROM_GENERATOR;
use crate::utils::{match_function_call, snippet_block, snippet_opt, span_lint_and_then};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{
    AsyncGeneratorKind, Block, Body, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericBound, HirId, IsAsync,
    ItemKind, TraitRef, Ty, TyKind, TypeBindingKind,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::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.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    ///
    /// ```rust
    /// use std::future::Future;
    ///
    /// fn foo() -> impl Future<Output = i32> { async { 42 } }
    /// ```
    /// Use instead:
    /// ```rust
    /// use std::future::Future;
    ///
    /// async fn foo() -> i32 { 42 }
    /// ```
    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,
        _: HirId,
    ) {
        if_chain! {
            if let Some(header) = kind.header();
            if let IsAsync::NotAsync = header.asyncness;
            // Check that this function returns `impl Future`
            if let FnRetTy::Return(ret_ty) = decl.output;
            if let Some(trait_ref) = future_trait_ref(cx, ret_ty);
            if let Some(output) = future_output_ty(trait_ref);
            // Check that the body of the function consists of one async block
            if let ExprKind::Block(block, _) = body.value.kind;
            if block.stmts.is_empty();
            if let Some(closure_body) = desugared_async_block(cx, block);
            then {
                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_chain! {
                            if let Some(header_snip) = snippet_opt(cx, header_span);
                            if let Some(ret_pos) = header_snip.rfind("->");
                            if let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output);
                            then {
                                let help = format!("make the function `async` and {}", ret_sugg);
                                diag.span_suggestion(
                                    header_span,
                                    &help,
                                    format!("async {}{}", &header_snip[..ret_pos], 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.to_string(),
                                    Applicability::MachineApplicable
                                );
                            }
                        }
                    },
                );
            }
        }
    }
}

fn future_trait_ref<'tcx>(cx: &LateContext<'tcx>, ty: &'tcx Ty<'tcx>) -> Option<&'tcx TraitRef<'tcx>> {
    if_chain! {
        if let TyKind::OpaqueDef(item_id, _) = ty.kind;
        let item = cx.tcx.hir().item(item_id.id);
        if let ItemKind::OpaqueTy(opaque) = &item.kind;
        if opaque.bounds.len() == 1;
        if let GenericBound::Trait(poly, _) = &opaque.bounds[0];
        if poly.trait_ref.trait_def_id() == cx.tcx.lang_items().future_trait();
        then {
            return Some(&poly.trait_ref);
        }
    }

    None
}

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

    None
}

fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) -> Option<&'tcx Body<'tcx>> {
    if_chain! {
        if let Some(block_expr) = block.expr;
        if let Some(args) = match_function_call(cx, block_expr, &FUTURE_FROM_GENERATOR);
        if args.len() == 1;
        if let Expr{kind: ExprKind::Closure(_, _, body_id, ..), ..} = args[0];
        let closure_body = cx.tcx.hir().body(body_id);
        if let Some(GeneratorKind::Async(AsyncGeneratorKind::Block)) = closure_body.generator_kind;
        then {
            return Some(closure_body);
        }
    }

    None
}

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