rust-clippy/clippy_lints/src/methods/unnecessary_to_owned.rs

use super::implicit_clone::is_clone_like;
use super::unnecessary_iter_cloned::{self, is_into_iter};
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_opt;
use clippy_utils::ty::{
    contains_ty, get_associated_type, get_iterator_item_ty, implements_trait, is_copy, peel_mid_ty_refs,
};
use clippy_utils::{fn_def_id, get_parent_expr, is_diag_item_method, is_diag_trait_item};
use rustc_errors::Applicability;
use rustc_hir::{def_id::DefId, BorrowKind, Expr, ExprKind};
use rustc_lint::LateContext;
use rustc_middle::mir::Mutability;
use rustc_middle::ty::adjustment::{Adjust, Adjustment, OverloadedDeref};
use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
use rustc_middle::ty::{self, PredicateKind, ProjectionPredicate, TraitPredicate, Ty};
use rustc_span::{sym, Symbol};
use std::cmp::max;

use super::UNNECESSARY_TO_OWNED;

pub fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, method_name: Symbol, args: &'tcx [Expr<'tcx>]) {
    if_chain! {
        if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
        if let [receiver] = args;
        then {
            if is_cloned_or_copied(cx, method_name, method_def_id) {
                unnecessary_iter_cloned::check(cx, expr, method_name, receiver);
            } else if is_to_owned_like(cx, method_name, method_def_id) {
                // At this point, we know the call is of a `to_owned`-like function. The functions
                // `check_addr_of_expr` and `check_call_arg` determine whether the call is unnecessary
                // based on its context, that is, whether it is a referent in an `AddrOf` expression, an
                // argument in a `into_iter` call, or an argument in the call of some other function.
                if check_addr_of_expr(cx, expr, method_name, method_def_id, receiver) {
                    return;
                }
                if check_into_iter_call_arg(cx, expr, method_name, receiver) {
                    return;
                }
                check_other_call_arg(cx, expr, method_name, receiver);
            }
        }
    }
}

/// Checks whether `expr` is a referent in an `AddrOf` expression and, if so, determines whether its
/// call of a `to_owned`-like function is unnecessary.
#[allow(clippy::too_many_lines)]
fn check_addr_of_expr(
    cx: &LateContext<'_>,
    expr: &Expr<'_>,
    method_name: Symbol,
    method_def_id: DefId,
    receiver: &Expr<'_>,
) -> bool {
    if_chain! {
        if let Some(parent) = get_parent_expr(cx, expr);
        if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, _) = parent.kind;
        let adjustments = cx.typeck_results().expr_adjustments(parent).iter().collect::<Vec<_>>();
        if let Some(target_ty) = match adjustments[..]
        {
            // For matching uses of `Cow::from`
            [
                Adjustment {
                    kind: Adjust::Deref(None),
                    ..
                },
                Adjustment {
                    kind: Adjust::Borrow(_),
                    target: target_ty,
                },
            ]
            // For matching uses of arrays
            | [
                Adjustment {
                    kind: Adjust::Deref(None),
                    ..
                },
                Adjustment {
                    kind: Adjust::Borrow(_),
                    ..
                },
                Adjustment {
                    kind: Adjust::Pointer(_),
                    target: target_ty,
                },
            ]
            // For matching everything else
            | [
                Adjustment {
                    kind: Adjust::Deref(None),
                    ..
                },
                Adjustment {
                    kind: Adjust::Deref(Some(OverloadedDeref { .. })),
                    ..
                },
                Adjustment {
                    kind: Adjust::Borrow(_),
                    target: target_ty,
                },
            ] => Some(target_ty),
            _ => None,
        };
        let receiver_ty = cx.typeck_results().expr_ty(receiver);
        // Only flag cases where the receiver is copyable or the method is `Cow::into_owned`. This
        // restriction is to ensure there is not overlap between `redundant_clone` and this lint.
        if is_copy(cx, receiver_ty) || is_cow_into_owned(cx, method_name, method_def_id);
        if let Some(receiver_snippet) = snippet_opt(cx, receiver.span);
        then {
            let (target_ty, n_target_refs) = peel_mid_ty_refs(*target_ty);
            let (receiver_ty, n_receiver_refs) = peel_mid_ty_refs(receiver_ty);
            if receiver_ty == target_ty && n_target_refs >= n_receiver_refs {
                span_lint_and_sugg(
                    cx,
                    UNNECESSARY_TO_OWNED,
                    parent.span,
                    &format!("unnecessary use of `{}`", method_name),
                    "use",
                    format!(
                        "{:&>width$}{}",
                        "",
                        receiver_snippet,
                        width = n_target_refs - n_receiver_refs
                    ),
                    Applicability::MachineApplicable,
                );
                return true;
            }
            if_chain! {
                if let Some(deref_trait_id) = cx.tcx.get_diagnostic_item(sym::Deref);
                if implements_trait(cx, receiver_ty, deref_trait_id, &[]);
                if get_associated_type(cx, receiver_ty, deref_trait_id, "Target") == Some(target_ty);
                then {
                    if n_receiver_refs > 0 {
                        span_lint_and_sugg(
                            cx,
                            UNNECESSARY_TO_OWNED,
                            parent.span,
                            &format!("unnecessary use of `{}`", method_name),
                            "use",
                            receiver_snippet,
                            Applicability::MachineApplicable,
                        );
                    } else {
                        span_lint_and_sugg(
                            cx,
                            UNNECESSARY_TO_OWNED,
                            expr.span.with_lo(receiver.span.hi()),
                            &format!("unnecessary use of `{}`", method_name),
                            "remove this",
                            String::new(),
                            Applicability::MachineApplicable,
                        );
                    }
                    return true;
                }
            }
            if_chain! {
                if let Some(as_ref_trait_id) = cx.tcx.get_diagnostic_item(sym::AsRef);
                if implements_trait(cx, receiver_ty, as_ref_trait_id, &[GenericArg::from(target_ty)]);
                then {
                    span_lint_and_sugg(
                        cx,
                        UNNECESSARY_TO_OWNED,
                        parent.span,
                        &format!("unnecessary use of `{}`", method_name),
                        "use",
                        format!("{}.as_ref()", receiver_snippet),
                        Applicability::MachineApplicable,
                    );
                    return true;
                }
            }
        }
    }
    false
}

/// Checks whether `expr` is an argument in an `into_iter` call and, if so, determines whether its
/// call of a `to_owned`-like function is unnecessary.
fn check_into_iter_call_arg(cx: &LateContext<'_>, expr: &Expr<'_>, method_name: Symbol, receiver: &Expr<'_>) -> bool {
    if_chain! {
        if let Some(parent) = get_parent_expr(cx, expr);
        if let Some(callee_def_id) = fn_def_id(cx, parent);
        if is_into_iter(cx, callee_def_id);
        if let Some(iterator_trait_id) = cx.tcx.get_diagnostic_item(sym::Iterator);
        let parent_ty = cx.typeck_results().expr_ty(parent);
        if implements_trait(cx, parent_ty, iterator_trait_id, &[]);
        if let Some(item_ty) = get_iterator_item_ty(cx, parent_ty);
        if let Some(receiver_snippet) = snippet_opt(cx, receiver.span);
        then {
            if unnecessary_iter_cloned::check_for_loop_iter(cx, parent, method_name, receiver, true) {
                return true;
            }
            let cloned_or_copied = if is_copy(cx, item_ty) { "copied" } else { "cloned" };
            // The next suggestion may be incorrect because the removal of the `to_owned`-like
            // function could cause the iterator to hold a reference to a resource that is used
            // mutably. See https://github.com/rust-lang/rust-clippy/issues/8148.
            span_lint_and_sugg(
                cx,
                UNNECESSARY_TO_OWNED,
                parent.span,
                &format!("unnecessary use of `{}`", method_name),
                "use",
                format!("{}.iter().{}()", receiver_snippet, cloned_or_copied),
                Applicability::MaybeIncorrect,
            );
            return true;
        }
    }
    false
}

/// Checks whether `expr` is an argument in a function call and, if so, determines whether its call
/// of a `to_owned`-like function is unnecessary.
fn check_other_call_arg<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'tcx>,
    method_name: Symbol,
    receiver: &'tcx Expr<'tcx>,
) -> bool {
    if_chain! {
        if let Some((maybe_call, maybe_arg)) = skip_addr_of_ancestors(cx, expr);
        if let Some((callee_def_id, call_substs, call_args)) = get_callee_substs_and_args(cx, maybe_call);
        let fn_sig = cx.tcx.fn_sig(callee_def_id).skip_binder();
        if let Some(i) = call_args.iter().position(|arg| arg.hir_id == maybe_arg.hir_id);
        if let Some(input) = fn_sig.inputs().get(i);
        let (input, n_refs) = peel_mid_ty_refs(*input);
        if let (trait_predicates, projection_predicates) = get_input_traits_and_projections(cx, callee_def_id, input);
        if let Some(sized_def_id) = cx.tcx.lang_items().sized_trait();
        if let [trait_predicate] = trait_predicates
            .iter()
            .filter(|trait_predicate| trait_predicate.def_id() != sized_def_id)
            .collect::<Vec<_>>()[..];
        if let Some(deref_trait_id) = cx.tcx.get_diagnostic_item(sym::Deref);
        if let Some(as_ref_trait_id) = cx.tcx.get_diagnostic_item(sym::AsRef);
        let receiver_ty = cx.typeck_results().expr_ty(receiver);
        // If the callee has type parameters, they could appear in `projection_predicate.ty` or the
        // types of `trait_predicate.trait_ref.substs`.
        if if trait_predicate.def_id() == deref_trait_id {
            if let [projection_predicate] = projection_predicates[..] {
                let normalized_ty =
                    cx.tcx
                        .subst_and_normalize_erasing_regions(call_substs, cx.param_env, projection_predicate.term);
                implements_trait(cx, receiver_ty, deref_trait_id, &[])
                    && get_associated_type(cx, receiver_ty, deref_trait_id, "Target")
                        .map_or(false, |ty| ty::Term::Ty(ty) == normalized_ty)
            } else {
                false
            }
        } else if trait_predicate.def_id() == as_ref_trait_id {
            let composed_substs = compose_substs(
                cx,
                &trait_predicate.trait_ref.substs.iter().skip(1).collect::<Vec<_>>()[..],
                call_substs,
            );
            implements_trait(cx, receiver_ty, as_ref_trait_id, &composed_substs)
        } else {
            false
        };
        // We can't add an `&` when the trait is `Deref` because `Target = &T` won't match
        // `Target = T`.
        if n_refs > 0 || is_copy(cx, receiver_ty) || trait_predicate.def_id() != deref_trait_id;
        let n_refs = max(n_refs, if is_copy(cx, receiver_ty) { 0 } else { 1 });
        // If the trait is `AsRef` and the input type variable `T` occurs in the output type, then
        // `T` must not be instantiated with a reference
        // (https://github.com/rust-lang/rust-clippy/issues/8507).
        if (n_refs == 0 && !receiver_ty.is_ref())
            || trait_predicate.def_id() != as_ref_trait_id
            || !contains_ty(fn_sig.output(), input);
        if let Some(receiver_snippet) = snippet_opt(cx, receiver.span);
        then {
            span_lint_and_sugg(
                cx,
                UNNECESSARY_TO_OWNED,
                maybe_arg.span,
                &format!("unnecessary use of `{}`", method_name),
                "use",
                format!("{:&>width$}{}", "", receiver_snippet, width = n_refs),
                Applicability::MachineApplicable,
            );
            return true;
        }
    }
    false
}

/// Walks an expression's ancestors until it finds a non-`AddrOf` expression. Returns the first such
/// expression found (if any) along with the immediately prior expression.
fn skip_addr_of_ancestors<'tcx>(
    cx: &LateContext<'tcx>,
    mut expr: &'tcx Expr<'tcx>,
) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
    while let Some(parent) = get_parent_expr(cx, expr) {
        if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, _) = parent.kind {
            expr = parent;
        } else {
            return Some((parent, expr));
        }
    }
    None
}

/// Checks whether an expression is a function or method call and, if so, returns its `DefId`,
/// `Substs`, and arguments.
fn get_callee_substs_and_args<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'tcx>,
) -> Option<(DefId, SubstsRef<'tcx>, &'tcx [Expr<'tcx>])> {
    if_chain! {
        if let ExprKind::Call(callee, args) = expr.kind;
        let callee_ty = cx.typeck_results().expr_ty(callee);
        if let ty::FnDef(callee_def_id, _) = callee_ty.kind();
        then {
            let substs = cx.typeck_results().node_substs(callee.hir_id);
            return Some((*callee_def_id, substs, args));
        }
    }
    if_chain! {
        if let ExprKind::MethodCall(_, args, _) = expr.kind;
        if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
        then {
            let substs = cx.typeck_results().node_substs(expr.hir_id);
            return Some((method_def_id, substs, args));
        }
    }
    None
}

/// Returns the `TraitPredicate`s and `ProjectionPredicate`s for a function's input type.
fn get_input_traits_and_projections<'tcx>(
    cx: &LateContext<'tcx>,
    callee_def_id: DefId,
    input: Ty<'tcx>,
) -> (Vec<TraitPredicate<'tcx>>, Vec<ProjectionPredicate<'tcx>>) {
    let mut trait_predicates = Vec::new();
    let mut projection_predicates = Vec::new();
    for (predicate, _) in cx.tcx.predicates_of(callee_def_id).predicates.iter() {
        // `substs` should have 1 + n elements. The first is the type on the left hand side of an
        // `as`. The remaining n are trait parameters.
        let is_input_substs = |substs: SubstsRef<'tcx>| {
            if_chain! {
                if let Some(arg) = substs.iter().next();
                if let GenericArgKind::Type(arg_ty) = arg.unpack();
                if arg_ty == input;
                then { true } else { false }
            }
        };
        match predicate.kind().skip_binder() {
            PredicateKind::Trait(trait_predicate) => {
                if is_input_substs(trait_predicate.trait_ref.substs) {
                    trait_predicates.push(trait_predicate);
                }
            },
            PredicateKind::Projection(projection_predicate) => {
                if is_input_substs(projection_predicate.projection_ty.substs) {
                    projection_predicates.push(projection_predicate);
                }
            },
            _ => {},
        }
    }
    (trait_predicates, projection_predicates)
}

/// Composes two substitutions by applying the latter to the types of the former.
fn compose_substs<'tcx>(
    cx: &LateContext<'tcx>,
    left: &[GenericArg<'tcx>],
    right: SubstsRef<'tcx>,
) -> Vec<GenericArg<'tcx>> {
    left.iter()
        .map(|arg| {
            if let GenericArgKind::Type(arg_ty) = arg.unpack() {
                let normalized_ty = cx.tcx.subst_and_normalize_erasing_regions(right, cx.param_env, arg_ty);
                GenericArg::from(normalized_ty)
            } else {
                *arg
            }
        })
        .collect()
}

/// Returns true if the named method is `Iterator::cloned` or `Iterator::copied`.
fn is_cloned_or_copied(cx: &LateContext<'_>, method_name: Symbol, method_def_id: DefId) -> bool {
    (method_name.as_str() == "cloned" || method_name.as_str() == "copied")
        && is_diag_trait_item(cx, method_def_id, sym::Iterator)
}

/// Returns true if the named method can be used to convert the receiver to its "owned"
/// representation.
fn is_to_owned_like(cx: &LateContext<'_>, method_name: Symbol, method_def_id: DefId) -> bool {
    is_clone_like(cx, &*method_name.as_str(), method_def_id)
        || is_cow_into_owned(cx, method_name, method_def_id)
        || is_to_string(cx, method_name, method_def_id)
}

/// Returns true if the named method is `Cow::into_owned`.
fn is_cow_into_owned(cx: &LateContext<'_>, method_name: Symbol, method_def_id: DefId) -> bool {
    method_name.as_str() == "into_owned" && is_diag_item_method(cx, method_def_id, sym::Cow)
}

/// Returns true if the named method is `ToString::to_string`.
fn is_to_string(cx: &LateContext<'_>, method_name: Symbol, method_def_id: DefId) -> bool {
    method_name.as_str() == "to_string" && is_diag_trait_item(cx, method_def_id, sym::ToString)
}