clippy_lints/src/suspicious_trait_impl.rs

use clippy_utils::diagnostics::span_lint;
use clippy_utils::visitors::for_each_expr_without_closures;
use clippy_utils::{BINOP_TRAITS, OP_ASSIGN_TRAITS, binop_traits, trait_ref_of_method};
use core::ops::ControlFlow;
use rustc_hir as hir;
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;
use rustc_span::Span;

declare_clippy_lint! {
    /// ### What it does
    /// Lints for suspicious operations in impls of arithmetic operators, e.g.
    /// subtracting elements in an Add impl.
    ///
    /// ### Why is this bad?
    /// This is probably a typo or copy-and-paste error and not intended.
    ///
    /// ### Example
    /// ```ignore
    /// impl Add for Foo {
    ///     type Output = Foo;
    ///
    ///     fn add(self, other: Foo) -> Foo {
    ///         Foo(self.0 - other.0)
    ///     }
    /// }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub SUSPICIOUS_ARITHMETIC_IMPL,
    suspicious,
    "suspicious use of operators in impl of arithmetic trait"
}

declare_clippy_lint! {
    /// ### What it does
    /// Lints for suspicious operations in impls of OpAssign, e.g.
    /// subtracting elements in an AddAssign impl.
    ///
    /// ### Why is this bad?
    /// This is probably a typo or copy-and-paste error and not intended.
    ///
    /// ### Example
    /// ```ignore
    /// impl AddAssign for Foo {
    ///     fn add_assign(&mut self, other: Foo) {
    ///         *self = *self - other;
    ///     }
    /// }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub SUSPICIOUS_OP_ASSIGN_IMPL,
    suspicious,
    "suspicious use of operators in impl of OpAssign trait"
}

declare_lint_pass!(SuspiciousImpl => [SUSPICIOUS_ARITHMETIC_IMPL, SUSPICIOUS_OP_ASSIGN_IMPL]);

impl<'tcx> LateLintPass<'tcx> for SuspiciousImpl {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
        match expr.kind {
            hir::ExprKind::Binary(op, _, _) => {
                check_expr_inner(cx, expr, op.node, op.span);
            },
            hir::ExprKind::AssignOp(op, _, _) => {
                check_expr_inner(cx, expr, op.node.into(), op.span);
            },
            _ => {},
        }
    }
}

fn check_expr_inner<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, binop: hir::BinOpKind, span: Span) {
    if let Some((binop_trait_lang, op_assign_trait_lang)) = binop_traits(binop)
            && let Some(binop_trait_id) = cx.tcx.lang_items().get(binop_trait_lang)
            && let Some(op_assign_trait_id) = cx.tcx.lang_items().get(op_assign_trait_lang)

            // Check for more than one binary operation in the implemented function
            // Linting when multiple operations are involved can result in false positives
            && let parent_fn = cx.tcx.hir_get_parent_item(expr.hir_id).def_id
            && let hir::Node::ImplItem(impl_item) = cx.tcx.hir_node_by_def_id(parent_fn)
            && let hir::ImplItemKind::Fn(_, body_id) = impl_item.kind
            && let body = cx.tcx.hir_body(body_id)
            && let parent_fn = cx.tcx.hir_get_parent_item(expr.hir_id)
            && let Some(trait_ref) = trait_ref_of_method(cx, parent_fn)
            && let trait_id = trait_ref.path.res.def_id()
            && ![binop_trait_id, op_assign_trait_id].contains(&trait_id)
            && let Some(&(_, lint)) = [
                (&BINOP_TRAITS, SUSPICIOUS_ARITHMETIC_IMPL),
                (&OP_ASSIGN_TRAITS, SUSPICIOUS_OP_ASSIGN_IMPL),
            ]
                .iter()
                .find(|&(ts, _)| ts.iter().any(|&t| Some(trait_id) == cx.tcx.lang_items().get(t)))
            && count_binops(body.value) == 1
    {
        span_lint(
            cx,
            lint,
            span,
            format!(
                "suspicious use of `{}` in `{}` impl",
                binop.as_str(),
                cx.tcx.item_name(trait_id)
            ),
        );
    }
}

fn count_binops(expr: &hir::Expr<'_>) -> u32 {
    let mut count = 0u32;
    let _: Option<!> = for_each_expr_without_closures(expr, |e| {
        if matches!(
            e.kind,
            hir::ExprKind::Binary(..)
                | hir::ExprKind::Unary(hir::UnOp::Not | hir::UnOp::Neg, _)
                | hir::ExprKind::AssignOp(..)
        ) {
            count += 1;
        }
        ControlFlow::Continue(())
    });
    count
}