clippy_lints/src/trait_bounds.rs

use clippy_config::Conf;
use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::source::{SpanRangeExt, snippet, snippet_with_applicability};
use clippy_utils::{SpanlessEq, SpanlessHash, is_from_proc_macro};
use core::hash::{Hash, Hasher};
use itertools::Itertools;
use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, IndexEntry};
use rustc_data_structures::unhash::UnhashMap;
use rustc_errors::Applicability;
use rustc_hir::def::Res;
use rustc_hir::{
    AmbigArg, BoundPolarity, GenericBound, Generics, Item, ItemKind, LangItem, Node, Path, PathSegment,
    PredicateOrigin, QPath, TraitBoundModifiers, TraitItem, TraitRef, Ty, TyKind, WherePredicateKind,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::impl_lint_pass;
use rustc_span::Span;

declare_clippy_lint! {
    /// ### What it does
    /// This lint warns about unnecessary type repetitions in trait bounds
    ///
    /// ### Why is this bad?
    /// Repeating the type for every bound makes the code
    /// less readable than combining the bounds
    ///
    /// ### Example
    /// ```no_run
    /// pub fn foo<T>(t: T) where T: Copy, T: Clone {}
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// pub fn foo<T>(t: T) where T: Copy + Clone {}
    /// ```
    #[clippy::version = "1.38.0"]
    pub TYPE_REPETITION_IN_BOUNDS,
    nursery,
    "types are repeated unnecessarily in trait bounds, use `+` instead of using `T: _, T: _`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for cases where generics or trait objects are being used and multiple
    /// syntax specifications for trait bounds are used simultaneously.
    ///
    /// ### Why is this bad?
    /// Duplicate bounds makes the code
    /// less readable than specifying them only once.
    ///
    /// ### Example
    /// ```no_run
    /// fn func<T: Clone + Default>(arg: T) where T: Clone + Default {}
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # mod hidden {
    /// fn func<T: Clone + Default>(arg: T) {}
    /// # }
    ///
    /// // or
    ///
    /// fn func<T>(arg: T) where T: Clone + Default {}
    /// ```
    ///
    /// ```no_run
    /// fn foo<T: Default + Default>(bar: T) {}
    /// ```
    /// Use instead:
    /// ```no_run
    /// fn foo<T: Default>(bar: T) {}
    /// ```
    ///
    /// ```no_run
    /// fn foo<T>(bar: T) where T: Default + Default {}
    /// ```
    /// Use instead:
    /// ```no_run
    /// fn foo<T>(bar: T) where T: Default {}
    /// ```
    #[clippy::version = "1.47.0"]
    pub TRAIT_DUPLICATION_IN_BOUNDS,
    nursery,
    "check if the same trait bounds are specified more than once during a generic declaration"
}

pub struct TraitBounds {
    max_trait_bounds: u64,
    msrv: Msrv,
}

impl TraitBounds {
    pub fn new(conf: &'static Conf) -> Self {
        Self {
            max_trait_bounds: conf.max_trait_bounds,
            msrv: conf.msrv,
        }
    }
}

impl_lint_pass!(TraitBounds => [TYPE_REPETITION_IN_BOUNDS, TRAIT_DUPLICATION_IN_BOUNDS]);

impl<'tcx> LateLintPass<'tcx> for TraitBounds {
    fn check_generics(&mut self, cx: &LateContext<'tcx>, generics: &'tcx Generics<'_>) {
        self.check_type_repetition(cx, generics);
        check_trait_bound_duplication(cx, generics);
    }

    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
        // special handling for self trait bounds as these are not considered generics
        // ie. trait Foo: Display {}
        if let Item {
            kind: ItemKind::Trait(_, _, _, _, _, bounds, ..),
            ..
        } = item
        {
            rollup_traits(cx, bounds, "these bounds contain repeated elements");
        }
    }

    fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'tcx>) {
        let mut self_bounds_map = FxHashMap::default();

        for predicate in item.generics.predicates {
            if let WherePredicateKind::BoundPredicate(bound_predicate) = predicate.kind
                && bound_predicate.origin != PredicateOrigin::ImplTrait
                && !predicate.span.from_expansion()
                && let TyKind::Path(QPath::Resolved(_, Path { segments, .. })) = bound_predicate.bounded_ty.kind
                && let Some(PathSegment {
                    res: Res::SelfTyParam { trait_: def_id },
                    ..
                }) = segments.first()
                && let Some(Node::Item(Item {
                    kind: ItemKind::Trait(_, _, _, _, _, self_bounds, _),
                    ..
                })) = cx.tcx.hir_get_if_local(*def_id)
            {
                if self_bounds_map.is_empty() {
                    for bound in *self_bounds {
                        let Some((self_res, self_segments, _)) = get_trait_info_from_bound(bound) else {
                            continue;
                        };
                        self_bounds_map.insert(self_res, self_segments);
                    }
                }

                bound_predicate
                    .bounds
                    .iter()
                    .filter_map(get_trait_info_from_bound)
                    .for_each(|(trait_item_res, trait_item_segments, span)| {
                        if let Some(self_segments) = self_bounds_map.get(&trait_item_res)
                            && SpanlessEq::new(cx)
                                .paths_by_resolution()
                                .eq_path_segments(self_segments, trait_item_segments)
                        {
                            span_lint_and_help(
                                cx,
                                TRAIT_DUPLICATION_IN_BOUNDS,
                                span,
                                "this trait bound is already specified in trait declaration",
                                None,
                                "consider removing this trait bound",
                            );
                        }
                    });
            }
        }
    }

    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &'tcx Ty<'tcx, AmbigArg>) {
        if let TyKind::Ref(.., mut_ty) = &ty.kind
            && let TyKind::TraitObject(bounds, ..) = mut_ty.ty.kind
            && bounds.len() > 2
        {
            // Build up a hash of every trait we've seen
            // When we see a trait for the first time, add it to unique_traits
            // so we can later use it to build a string of all traits exactly once, without duplicates

            let mut seen_def_ids = FxHashSet::default();
            let mut unique_traits = Vec::new();

            // Iterate the bounds and add them to our seen hash
            // If we haven't yet seen it, add it to the fixed traits
            for bound in bounds {
                let Some(def_id) = bound.trait_ref.trait_def_id() else {
                    continue;
                };

                let new_trait = seen_def_ids.insert(def_id);

                if new_trait {
                    unique_traits.push(bound);
                }
            }

            // If the number of unique traits isn't the same as the number of traits in the bounds,
            // there must be 1 or more duplicates
            if bounds.len() != unique_traits.len() {
                let mut bounds_span = bounds[0].span;

                for bound in bounds.iter().skip(1) {
                    bounds_span = bounds_span.to(bound.span);
                }

                let fixed_trait_snippet = unique_traits
                    .iter()
                    .filter_map(|b| b.span.get_source_text(cx))
                    .join(" + ");

                span_lint_and_sugg(
                    cx,
                    TRAIT_DUPLICATION_IN_BOUNDS,
                    bounds_span,
                    "this trait bound is already specified in trait declaration",
                    "try",
                    fixed_trait_snippet,
                    Applicability::MaybeIncorrect,
                );
            }
        }
    }
}

impl TraitBounds {
    /// Is the given bound a `?Sized` bound, and is combining it (i.e. `T: X + ?Sized`) an error on
    /// this MSRV? See <https://github.com/rust-lang/rust-clippy/issues/8772> for details.
    fn cannot_combine_maybe_bound(&self, cx: &LateContext<'_>, bound: &GenericBound<'_>) -> bool {
        if let GenericBound::Trait(tr) = bound
            && let BoundPolarity::Maybe(_) = tr.modifiers.polarity
            && !self.msrv.meets(cx, msrvs::MAYBE_BOUND_IN_WHERE)
        {
            cx.tcx.lang_items().get(LangItem::Sized) == tr.trait_ref.path.res.opt_def_id()
        } else {
            false
        }
    }

    #[expect(clippy::mutable_key_type)]
    fn check_type_repetition<'tcx>(&self, cx: &LateContext<'tcx>, generics: &'tcx Generics<'_>) {
        struct SpanlessTy<'cx, 'tcx> {
            ty: &'tcx Ty<'tcx>,
            cx: &'cx LateContext<'tcx>,
        }
        impl PartialEq for SpanlessTy<'_, '_> {
            fn eq(&self, other: &Self) -> bool {
                let mut eq = SpanlessEq::new(self.cx);
                eq.inter_expr().eq_ty(self.ty, other.ty)
            }
        }
        impl Hash for SpanlessTy<'_, '_> {
            fn hash<H: Hasher>(&self, h: &mut H) {
                let mut t = SpanlessHash::new(self.cx);
                t.hash_ty(self.ty);
                h.write_u64(t.finish());
            }
        }
        impl Eq for SpanlessTy<'_, '_> {}

        if generics.span.from_expansion() {
            return;
        }
        let mut map: UnhashMap<SpanlessTy<'_, '_>, Vec<&GenericBound<'_>>> = UnhashMap::default();
        let mut applicability = Applicability::MaybeIncorrect;
        for bound in generics.predicates {
            if let WherePredicateKind::BoundPredicate(p) = bound.kind
                && p.origin != PredicateOrigin::ImplTrait
                && p.bounds.len() as u64 <= self.max_trait_bounds
                && !bound.span.from_expansion()
                && let bounds = p
                    .bounds
                    .iter()
                    .filter(|b| !self.cannot_combine_maybe_bound(cx, b))
                    .collect::<Vec<_>>()
                && !bounds.is_empty()
                && let Some(ref v) = map.insert(SpanlessTy { ty: p.bounded_ty, cx }, bounds)
                && !is_from_proc_macro(cx, p.bounded_ty)
            {
                let trait_bounds = v
                    .iter()
                    .copied()
                    .chain(p.bounds.iter())
                    .map(|bound| snippet_with_applicability(cx, bound.span(), "_", &mut applicability))
                    .join(" + ");
                let hint_string = format!(
                    "consider combining the bounds: `{}: {trait_bounds}`",
                    snippet(cx, p.bounded_ty.span, "_"),
                );
                let ty_name = snippet(cx, p.bounded_ty.span, "_");
                span_lint_and_help(
                    cx,
                    TYPE_REPETITION_IN_BOUNDS,
                    bound.span,
                    format!("type `{ty_name}` has already been used as a bound predicate"),
                    None,
                    hint_string,
                );
            }
        }
    }
}

fn check_trait_bound_duplication<'tcx>(cx: &LateContext<'tcx>, generics: &'_ Generics<'tcx>) {
    if generics.span.from_expansion() {
        return;
    }

    // Explanation:
    // fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z)
    // where T: Clone + Default, { unimplemented!(); }
    //       ^^^^^^^^^^^^^^^^^^
    //       |
    // collects each of these where clauses into a set keyed by generic name and comparable trait
    // eg. (T, Clone)
    #[expect(clippy::mutable_key_type)]
    let where_predicates = generics
        .predicates
        .iter()
        .filter_map(|pred| {
            if pred.kind.in_where_clause()
                && let WherePredicateKind::BoundPredicate(bound_predicate) = pred.kind
                && let TyKind::Path(QPath::Resolved(_, path)) = bound_predicate.bounded_ty.kind
            {
                return Some(
                    rollup_traits(
                        cx,
                        bound_predicate.bounds,
                        "these where clauses contain repeated elements",
                    )
                    .into_iter()
                    .map(|(trait_ref, _)| (path.res, trait_ref)),
                );
            }
            None
        })
        .flatten()
        .collect::<FxHashSet<_>>();

    // Explanation:
    // fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z) ...
    //            ^^^^^^^^^^^^^^^^^^  ^^^^^^^
    //            |
    // compare trait bounds keyed by generic name and comparable trait to collected where
    // predicates eg. (T, Clone)
    for predicate in generics.predicates.iter().filter(|pred| !pred.kind.in_where_clause()) {
        if let WherePredicateKind::BoundPredicate(bound_predicate) = predicate.kind
            && bound_predicate.origin != PredicateOrigin::ImplTrait
            && !predicate.span.from_expansion()
            && let TyKind::Path(QPath::Resolved(_, path)) = bound_predicate.bounded_ty.kind
        {
            let traits = rollup_traits(cx, bound_predicate.bounds, "these bounds contain repeated elements");
            for (trait_ref, span) in traits {
                let key = (path.res, trait_ref);
                if where_predicates.contains(&key) {
                    span_lint_and_help(
                        cx,
                        TRAIT_DUPLICATION_IN_BOUNDS,
                        span,
                        "this trait bound is already specified in the where clause",
                        None,
                        "consider removing this trait bound",
                    );
                }
            }
        }
    }
}

struct ComparableTraitRef<'a, 'tcx> {
    cx: &'a LateContext<'tcx>,
    trait_ref: &'tcx TraitRef<'tcx>,
    modifiers: TraitBoundModifiers,
}

impl PartialEq for ComparableTraitRef<'_, '_> {
    fn eq(&self, other: &Self) -> bool {
        SpanlessEq::eq_modifiers(self.modifiers, other.modifiers)
            && SpanlessEq::new(self.cx)
                .paths_by_resolution()
                .eq_path(self.trait_ref.path, other.trait_ref.path)
    }
}
impl Eq for ComparableTraitRef<'_, '_> {}
impl Hash for ComparableTraitRef<'_, '_> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        let mut s = SpanlessHash::new(self.cx).paths_by_resolution();
        s.hash_path(self.trait_ref.path);
        s.hash_modifiers(self.modifiers);
        state.write_u64(s.finish());
    }
}

fn get_trait_info_from_bound<'a>(bound: &'a GenericBound<'_>) -> Option<(Res, &'a [PathSegment<'a>], Span)> {
    if let GenericBound::Trait(t) = bound {
        let trait_path = t.trait_ref.path;
        Some((trait_path.res, trait_path.segments, t.span))
    } else {
        None
    }
}

fn rollup_traits<'cx, 'tcx>(
    cx: &'cx LateContext<'tcx>,
    bounds: &'tcx [GenericBound<'tcx>],
    msg: &'static str,
) -> Vec<(ComparableTraitRef<'cx, 'tcx>, Span)> {
    // Source order is needed for joining spans
    let mut map = FxIndexMap::default();
    let mut repeated_res = false;

    let only_comparable_trait_refs = |bound: &'tcx GenericBound<'tcx>| {
        if let GenericBound::Trait(t) = bound {
            Some((
                ComparableTraitRef {
                    cx,
                    trait_ref: &t.trait_ref,
                    modifiers: t.modifiers,
                },
                t.span,
            ))
        } else {
            None
        }
    };

    for bound in bounds.iter().filter_map(only_comparable_trait_refs) {
        let (comparable_bound, span_direct) = bound;
        match map.entry(comparable_bound) {
            IndexEntry::Occupied(_) => repeated_res = true,
            IndexEntry::Vacant(e) => {
                e.insert(span_direct);
            },
        }
    }

    let comparable_bounds: Vec<_> = map.into_iter().collect();

    if repeated_res && let [first_trait, .., last_trait] = bounds {
        let all_trait_span = first_trait.span().to(last_trait.span());

        let traits = comparable_bounds
            .iter()
            .filter_map(|&(_, span)| span.get_source_text(cx))
            .join(" + ");

        span_lint_and_sugg(
            cx,
            TRAIT_DUPLICATION_IN_BOUNDS,
            all_trait_span,
            msg,
            "try",
            traits,
            Applicability::MachineApplicable,
        );
    }

    comparable_bounds
}