src/tools/clippy/clippy_lints/src/eta_reduction.rs

use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
use clippy_utils::higher::VecArgs;
use clippy_utils::source::snippet_opt;
use clippy_utils::ty::type_diagnostic_name;
use clippy_utils::usage::{local_used_after_expr, local_used_in};
use clippy_utils::{higher, is_adjusted, path_to_local, path_to_local_id};
use rustc_errors::Applicability;
use rustc_hir::def_id::DefId;
use rustc_hir::{BindingAnnotation, Expr, ExprKind, FnRetTy, Param, PatKind, QPath, TyKind, Unsafety};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{
    self, Binder, ClosureArgs, ClosureKind, EarlyBinder, FnSig, GenericArg, GenericArgKind, GenericArgsRef,
    ImplPolarity, List, Region, RegionKind, Ty, TypeVisitableExt, TypeckResults,
};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::sym;
use rustc_target::spec::abi::Abi;
use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for closures which just call another function where
    /// the function can be called directly. `unsafe` functions or calls where types
    /// get adjusted are ignored.
    ///
    /// ### Why is this bad?
    /// Needlessly creating a closure adds code for no benefit
    /// and gives the optimizer more work.
    ///
    /// ### Known problems
    /// If creating the closure inside the closure has a side-
    /// effect then moving the closure creation out will change when that side-
    /// effect runs.
    /// See [#1439](https://github.com/rust-lang/rust-clippy/issues/1439) for more details.
    ///
    /// ### Example
    /// ```rust,ignore
    /// xs.map(|x| foo(x))
    /// ```
    ///
    /// Use instead:
    /// ```rust,ignore
    /// // where `foo(_)` is a plain function that takes the exact argument type of `x`.
    /// xs.map(foo)
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub REDUNDANT_CLOSURE,
    style,
    "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for closures which only invoke a method on the closure
    /// argument and can be replaced by referencing the method directly.
    ///
    /// ### Why is this bad?
    /// It's unnecessary to create the closure.
    ///
    /// ### Example
    /// ```rust,ignore
    /// Some('a').map(|s| s.to_uppercase());
    /// ```
    /// may be rewritten as
    /// ```rust,ignore
    /// Some('a').map(char::to_uppercase);
    /// ```
    #[clippy::version = "1.35.0"]
    pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
    pedantic,
    "redundant closures for method calls"
}

declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_FOR_METHOD_CALLS]);

impl<'tcx> LateLintPass<'tcx> for EtaReduction {
    #[allow(clippy::too_many_lines)]
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        let body = if let ExprKind::Closure(c) = expr.kind
            && c.fn_decl.inputs.iter().all(|ty| matches!(ty.kind, TyKind::Infer))
            && matches!(c.fn_decl.output, FnRetTy::DefaultReturn(_))
            && !expr.span.from_expansion()
        {
            cx.tcx.hir().body(c.body)
        } else {
            return;
        };

        if body.value.span.from_expansion() {
            if body.params.is_empty() {
                if let Some(VecArgs::Vec(&[])) = higher::VecArgs::hir(cx, body.value) {
                    // replace `|| vec![]` with `Vec::new`
                    span_lint_and_sugg(
                        cx,
                        REDUNDANT_CLOSURE,
                        expr.span,
                        "redundant closure",
                        "replace the closure with `Vec::new`",
                        "std::vec::Vec::new".into(),
                        Applicability::MachineApplicable,
                    );
                }
            }
            // skip `foo(|| macro!())`
            return;
        }

        let typeck = cx.typeck_results();
        let closure = if let ty::Closure(_, closure_subs) = typeck.expr_ty(expr).kind() {
            closure_subs.as_closure()
        } else {
            return;
        };

        if is_adjusted(cx, body.value) {
            return;
        }

        match body.value.kind {
            ExprKind::Call(callee, args)
                if matches!(callee.kind, ExprKind::Path(QPath::Resolved(..) | QPath::TypeRelative(..))) =>
            {
                let callee_ty = typeck.expr_ty(callee).peel_refs();
                if matches!(
                    type_diagnostic_name(cx, callee_ty),
                    Some(sym::Arc | sym::Rc)
                ) || !check_inputs(typeck, body.params, None, args) {
                    return;
                }
                let callee_ty_adjusted = typeck.expr_adjustments(callee).last().map_or(
                    callee_ty,
                    |a| a.target.peel_refs(),
                );

                let sig = match callee_ty_adjusted.kind() {
                    ty::FnDef(def, _) => cx.tcx.fn_sig(def).skip_binder().skip_binder(),
                    ty::FnPtr(sig) => sig.skip_binder(),
                    ty::Closure(_, subs) => cx
                        .tcx
                        .signature_unclosure(subs.as_closure().sig(), Unsafety::Normal)
                        .skip_binder(),
                    _ => {
                        if typeck.type_dependent_def_id(body.value.hir_id).is_some()
                            && let subs = typeck.node_args(body.value.hir_id)
                            && let output = typeck.expr_ty(body.value)
                            && let ty::Tuple(tys) = *subs.type_at(1).kind()
                        {
                            cx.tcx.mk_fn_sig(tys, output, false, Unsafety::Normal, Abi::Rust)
                        } else {
                            return;
                        }
                    },
                };
                if check_sig(cx, closure, sig)
                    && let generic_args = typeck.node_args(callee.hir_id)
                    // Given some trait fn `fn f() -> ()` and some type `T: Trait`, `T::f` is not
                    // `'static` unless `T: 'static`. The cast `T::f as fn()` will, however, result
                    // in a type which is `'static`.
                    // For now ignore all callee types which reference a type parameter.
                    && !generic_args.types().any(|t| matches!(t.kind(), ty::Param(_)))
                {
                    span_lint_and_then(
                        cx,
                        REDUNDANT_CLOSURE,
                        expr.span,
                        "redundant closure",
                        |diag| {
                            if let Some(mut snippet) = snippet_opt(cx, callee.span) {
                                if let Ok((ClosureKind::FnMut, _))
                                    = cx.tcx.infer_ctxt().build().type_implements_fn_trait(
                                        cx.param_env,
                                        Binder::bind_with_vars(callee_ty_adjusted, List::empty()),
                                        ImplPolarity::Positive,
                                    ) && path_to_local(callee)
                                        .map_or(
                                            false,
                                            |l| local_used_in(cx, l, args) || local_used_after_expr(cx, l, expr),
                                        )
                                {
                                    // Mutable closure is used after current expr; we cannot consume it.
                                    snippet = format!("&mut {snippet}");
                                }
                                diag.span_suggestion(
                                    expr.span,
                                    "replace the closure with the function itself",
                                    snippet,
                                    Applicability::MachineApplicable,
                                );
                            }
                        }
                    );
                }
            },
            ExprKind::MethodCall(path, self_, args, _) if check_inputs(typeck, body.params, Some(self_), args) => {
                if let Some(method_def_id) = typeck.type_dependent_def_id(body.value.hir_id)
                    && check_sig(cx, closure, cx.tcx.fn_sig(method_def_id).skip_binder().skip_binder())
                {
                    span_lint_and_then(
                        cx,
                        REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
                        expr.span,
                        "redundant closure",
                        |diag| {
                            let args = typeck.node_args(body.value.hir_id);
                            let name = get_ufcs_type_name(cx, method_def_id, args);
                            diag.span_suggestion(
                                expr.span,
                                "replace the closure with the method itself",
                                format!("{}::{}", name, path.ident.name),
                                Applicability::MachineApplicable,
                            );
                        },
                    );
                }
            },
            _ => (),
        }
    }
}

fn check_inputs(
    typeck: &TypeckResults<'_>,
    params: &[Param<'_>],
    self_arg: Option<&Expr<'_>>,
    args: &[Expr<'_>],
) -> bool {
    params.len() == self_arg.map_or(0, |_| 1) + args.len()
        && params.iter().zip(self_arg.into_iter().chain(args)).all(|(p, arg)| {
            matches!(
                p.pat.kind,PatKind::Binding(BindingAnnotation::NONE, id, _, None)
                if path_to_local_id(arg, id)
            )
            // Only allow adjustments which change regions (i.e. re-borrowing).
            && typeck
                .expr_adjustments(arg)
                .last()
                .map_or(true, |a| a.target == typeck.expr_ty(arg))
        })
}

fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure: ClosureArgs<'tcx>, call_sig: FnSig<'_>) -> bool {
    call_sig.unsafety == Unsafety::Normal
        && !has_late_bound_to_non_late_bound_regions(
            cx.tcx
                .signature_unclosure(closure.sig(), Unsafety::Normal)
                .skip_binder(),
            call_sig,
        )
}

/// This walks through both signatures and checks for any time a late-bound region is expected by an
/// `impl Fn` type, but the target signature does not have a late-bound region in the same position.
///
/// This is needed because rustc is unable to late bind early-bound regions in a function signature.
fn has_late_bound_to_non_late_bound_regions(from_sig: FnSig<'_>, to_sig: FnSig<'_>) -> bool {
    fn check_region(from_region: Region<'_>, to_region: Region<'_>) -> bool {
        matches!(from_region.kind(), RegionKind::ReLateBound(..))
            && !matches!(to_region.kind(), RegionKind::ReLateBound(..))
    }

    fn check_subs(from_subs: &[GenericArg<'_>], to_subs: &[GenericArg<'_>]) -> bool {
        if from_subs.len() != to_subs.len() {
            return true;
        }
        for (from_arg, to_arg) in to_subs.iter().zip(from_subs) {
            match (from_arg.unpack(), to_arg.unpack()) {
                (GenericArgKind::Lifetime(from_region), GenericArgKind::Lifetime(to_region)) => {
                    if check_region(from_region, to_region) {
                        return true;
                    }
                },
                (GenericArgKind::Type(from_ty), GenericArgKind::Type(to_ty)) => {
                    if check_ty(from_ty, to_ty) {
                        return true;
                    }
                },
                (GenericArgKind::Const(_), GenericArgKind::Const(_)) => (),
                _ => return true,
            }
        }
        false
    }

    fn check_ty(from_ty: Ty<'_>, to_ty: Ty<'_>) -> bool {
        match (from_ty.kind(), to_ty.kind()) {
            (&ty::Adt(_, from_subs), &ty::Adt(_, to_subs)) => check_subs(from_subs, to_subs),
            (&ty::Array(from_ty, _), &ty::Array(to_ty, _)) | (&ty::Slice(from_ty), &ty::Slice(to_ty)) => {
                check_ty(from_ty, to_ty)
            },
            (&ty::Ref(from_region, from_ty, _), &ty::Ref(to_region, to_ty, _)) => {
                check_region(from_region, to_region) || check_ty(from_ty, to_ty)
            },
            (&ty::Tuple(from_tys), &ty::Tuple(to_tys)) => {
                from_tys.len() != to_tys.len()
                    || from_tys
                        .iter()
                        .zip(to_tys)
                        .any(|(from_ty, to_ty)| check_ty(from_ty, to_ty))
            },
            _ => from_ty.has_late_bound_regions(),
        }
    }

    assert!(from_sig.inputs_and_output.len() == to_sig.inputs_and_output.len());
    from_sig
        .inputs_and_output
        .iter()
        .zip(to_sig.inputs_and_output)
        .any(|(from_ty, to_ty)| check_ty(from_ty, to_ty))
}

fn get_ufcs_type_name<'tcx>(cx: &LateContext<'tcx>, method_def_id: DefId, args: GenericArgsRef<'tcx>) -> String {
    let assoc_item = cx.tcx.associated_item(method_def_id);
    let def_id = assoc_item.container_id(cx.tcx);
    match assoc_item.container {
        ty::TraitContainer => cx.tcx.def_path_str(def_id),
        ty::ImplContainer => {
            let ty = cx.tcx.type_of(def_id).instantiate_identity();
            match ty.kind() {
                ty::Adt(adt, _) => cx.tcx.def_path_str(adt.did()),
                ty::Array(..)
                | ty::Dynamic(..)
                | ty::Never
                | ty::RawPtr(_)
                | ty::Ref(..)
                | ty::Slice(_)
                | ty::Tuple(_) => {
                    format!("<{}>", EarlyBinder::bind(ty).instantiate(cx.tcx, args))
                },
                _ => ty.to_string(),
            }
        },
    }
}