rust/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::usage::UsedAfterExprVisitor;
use clippy_utils::{get_enclosing_loop_or_closure, higher, path_to_local_id};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::def_id::DefId;
use rustc_hir::{Expr, ExprKind, Param, PatKind, Unsafety};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow};
use rustc_middle::ty::subst::Subst;
use rustc_middle::ty::{self, ClosureKind, Ty, TypeFoldable};
use rustc_session::{declare_lint_pass, declare_tool_lint};

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
    /// // Bad
    /// xs.map(|x| foo(x))
    ///
    /// // Good
    /// xs.map(foo)
    /// ```
    /// where `foo(_)` is a plain function that takes the exact argument type of
    /// `x`.
    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);
    /// ```
    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 {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if expr.span.from_expansion() {
            return;
        }
        let body = match expr.kind {
            ExprKind::Closure(_, _, id, _, _) => cx.tcx.hir().body(id),
            _ => 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 closure_ty = cx.typeck_results().expr_ty(expr);

        if_chain!(
            if let ExprKind::Call(callee, args) = body.value.kind;
            if let ExprKind::Path(_) = callee.kind;
            if check_inputs(cx, body.params, args);
            let callee_ty = cx.typeck_results().expr_ty_adjusted(callee);
            let call_ty = cx.typeck_results().type_dependent_def_id(body.value.hir_id)
                .map_or(callee_ty, |id| cx.tcx.type_of(id));
            if check_sig(cx, closure_ty, call_ty);
            let substs = cx.typeck_results().node_substs(callee.hir_id);
            // This fixes some false positives that I don't entirely understand
            if substs.is_empty() || !cx.typeck_results().expr_ty(expr).has_late_bound_regions();
            // A type param function ref like `T::f` is not 'static, however
            // it is if cast like `T::f as fn()`. This seems like a rustc bug.
            if !substs.types().any(|t| matches!(t.kind(), ty::Param(_)));
            then {
                span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
                    if let Some(mut snippet) = snippet_opt(cx, callee.span) {
                        if_chain! {
                            if let ty::Closure(_, substs) = callee_ty.peel_refs().kind();
                            if substs.as_closure().kind() == ClosureKind::FnMut;
                            if get_enclosing_loop_or_closure(cx.tcx, expr).is_some()
                                || UsedAfterExprVisitor::is_found(cx, callee);

                            then {
                                // 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,
                        );
                    }
                });
            }
        );

        if_chain!(
            if let ExprKind::MethodCall(path, _, args, _) = body.value.kind;
            if check_inputs(cx, body.params, args);
            let method_def_id = cx.typeck_results().type_dependent_def_id(body.value.hir_id).unwrap();
            let substs = cx.typeck_results().node_substs(body.value.hir_id);
            let call_ty = cx.tcx.type_of(method_def_id).subst(cx.tcx, substs);
            if check_sig(cx, closure_ty, call_ty);
            then {
                span_lint_and_then(cx, REDUNDANT_CLOSURE_FOR_METHOD_CALLS, expr.span, "redundant closure", |diag| {
                    let name = get_ufcs_type_name(cx, method_def_id);
                    diag.span_suggestion(
                        expr.span,
                        "replace the closure with the method itself",
                        format!("{}::{}", name, path.ident.name),
                        Applicability::MachineApplicable,
                    );
                })
            }
        );
    }
}

fn check_inputs(cx: &LateContext<'_>, params: &[Param<'_>], call_args: &[Expr<'_>]) -> bool {
    if params.len() != call_args.len() {
        return false;
    }
    std::iter::zip(params, call_args).all(|(param, arg)| {
        match param.pat.kind {
            PatKind::Binding(_, id, ..) if path_to_local_id(arg, id) => {},
            _ => return false,
        }
        match *cx.typeck_results().expr_adjustments(arg) {
            [] => true,
            [
                Adjustment {
                    kind: Adjust::Deref(None),
                    ..
                },
                Adjustment {
                    kind: Adjust::Borrow(AutoBorrow::Ref(_, mu2)),
                    ..
                },
            ] => {
                // re-borrow with the same mutability is allowed
                let ty = cx.typeck_results().expr_ty(arg);
                matches!(*ty.kind(), ty::Ref(.., mu1) if mu1 == mu2.into())
            },
            _ => false,
        }
    })
}

fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure_ty: Ty<'tcx>, call_ty: Ty<'tcx>) -> bool {
    let call_sig = call_ty.fn_sig(cx.tcx);
    if call_sig.unsafety() == Unsafety::Unsafe {
        return false;
    }
    if !closure_ty.has_late_bound_regions() {
        return true;
    }
    let substs = match closure_ty.kind() {
        ty::Closure(_, substs) => substs,
        _ => return false,
    };
    let closure_sig = cx.tcx.signature_unclosure(substs.as_closure().sig(), Unsafety::Normal);
    cx.tcx.erase_late_bound_regions(closure_sig) == cx.tcx.erase_late_bound_regions(call_sig)
}

fn get_ufcs_type_name(cx: &LateContext<'_>, method_def_id: DefId) -> String {
    match cx.tcx.associated_item(method_def_id).container {
        ty::TraitContainer(def_id) => cx.tcx.def_path_str(def_id),
        ty::ImplContainer(def_id) => {
            let ty = cx.tcx.type_of(def_id);
            match ty.kind() {
                ty::Adt(adt, _) => cx.tcx.def_path_str(adt.did),
                _ => ty.to_string(),
            }
        },
    }
}