rust-clippy/clippy_lints/src/trait_bounds.rs

use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::source::{snippet, snippet_with_applicability};
use clippy_utils::{in_macro, SpanlessHash};
use if_chain::if_chain;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::unhash::UnhashMap;
use rustc_errors::Applicability;
use rustc_hir::{def::Res, GenericBound, Generics, ParamName, Path, QPath, TyKind, WherePredicate};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_tool_lint, 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
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// pub fn foo<T>(t: T) where T: Copy, T: Clone {}
    /// ```
    ///
    /// Could be written as:
    ///
    /// ```rust
    /// pub fn foo<T>(t: T) where T: Copy + Clone {}
    /// ```
    pub TYPE_REPETITION_IN_BOUNDS,
    pedantic,
    "Types are repeated unnecessary in trait bounds use `+` instead of using `T: _, T: _`"
}

declare_clippy_lint! {
    /// **What it does:** Checks for cases where generics 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 specifing them only once.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// fn func<T: Clone + Default>(arg: T) where T: Clone + Default {}
    /// ```
    ///
    /// Could be written as:
    ///
    /// ```rust
    /// fn func<T: Clone + Default>(arg: T) {}
    /// ```
    /// or
    ///
    /// ```rust
    /// fn func<T>(arg: T) where T: Clone + Default {}
    /// ```
    pub TRAIT_DUPLICATION_IN_BOUNDS,
    pedantic,
    "Check if the same trait bounds are specified twice during a function declaration"
}

#[derive(Copy, Clone)]
pub struct TraitBounds {
    max_trait_bounds: u64,
}

impl TraitBounds {
    #[must_use]
    pub fn new(max_trait_bounds: u64) -> Self {
        Self { max_trait_bounds }
    }
}

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>, gen: &'tcx Generics<'_>) {
        self.check_type_repetition(cx, gen);
        check_trait_bound_duplication(cx, gen);
    }
}

fn get_trait_res_span_from_bound(bound: &GenericBound<'_>) -> Option<(Res, Span)> {
    if let GenericBound::Trait(t, _) = bound {
        Some((t.trait_ref.path.res, t.span))
    } else {
        None
    }
}

impl TraitBounds {
    fn check_type_repetition(self, cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
        if in_macro(gen.span) {
            return;
        }
        let hash = |ty| -> u64 {
            let mut hasher = SpanlessHash::new(cx);
            hasher.hash_ty(ty);
            hasher.finish()
        };
        let mut map: UnhashMap<u64, Vec<&GenericBound<'_>>> = UnhashMap::default();
        let mut applicability = Applicability::MaybeIncorrect;
        for bound in gen.where_clause.predicates {
            if_chain! {
                if let WherePredicate::BoundPredicate(ref p) = bound;
                if p.bounds.len() as u64 <= self.max_trait_bounds;
                if !in_macro(p.span);
                let h = hash(p.bounded_ty);
                if let Some(ref v) = map.insert(h, p.bounds.iter().collect::<Vec<_>>());

                then {
                    let mut hint_string = format!(
                        "consider combining the bounds: `{}:",
                        snippet(cx, p.bounded_ty.span, "_")
                    );
                    for b in v.iter() {
                        if let GenericBound::Trait(ref poly_trait_ref, _) = b {
                            let path = &poly_trait_ref.trait_ref.path;
                            hint_string.push_str(&format!(
                                " {} +",
                                snippet_with_applicability(cx, path.span, "..", &mut applicability)
                            ));
                        }
                    }
                    for b in p.bounds.iter() {
                        if let GenericBound::Trait(ref poly_trait_ref, _) = b {
                            let path = &poly_trait_ref.trait_ref.path;
                            hint_string.push_str(&format!(
                                " {} +",
                                snippet_with_applicability(cx, path.span, "..", &mut applicability)
                            ));
                        }
                    }
                    hint_string.truncate(hint_string.len() - 2);
                    hint_string.push('`');
                    span_lint_and_help(
                        cx,
                        TYPE_REPETITION_IN_BOUNDS,
                        p.span,
                        "this type has already been used as a bound predicate",
                        None,
                        &hint_string,
                    );
                }
            }
        }
    }
}

fn check_trait_bound_duplication(cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
    if in_macro(gen.span) || gen.params.is_empty() || gen.where_clause.predicates.is_empty() {
        return;
    }

    let mut map = FxHashMap::default();
    for param in gen.params {
        if let ParamName::Plain(ref ident) = param.name {
            let res = param
                .bounds
                .iter()
                .filter_map(get_trait_res_span_from_bound)
                .collect::<Vec<_>>();
            map.insert(*ident, res);
        }
    }

    for predicate in gen.where_clause.predicates {
        if_chain! {
            if let WherePredicate::BoundPredicate(ref bound_predicate) = predicate;
            if !in_macro(bound_predicate.span);
            if let TyKind::Path(QPath::Resolved(_, Path { segments, .. })) = bound_predicate.bounded_ty.kind;
            if let Some(segment) = segments.first();
            if let Some(trait_resolutions_direct) = map.get(&segment.ident);
            then {
                for (res_where, _) in bound_predicate.bounds.iter().filter_map(get_trait_res_span_from_bound) {
                    if let Some((_, span_direct)) = trait_resolutions_direct
                                                .iter()
                                                .find(|(res_direct, _)| *res_direct == res_where) {
                        span_lint_and_help(
                            cx,
                            TRAIT_DUPLICATION_IN_BOUNDS,
                            *span_direct,
                            "this trait bound is already specified in the where clause",
                            None,
                            "consider removing this trait bound",
                        );
                    }
                }
            }
        }
    }
}