rust-clippy/clippy_lints/src/misc.rs

use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg, span_lint_and_then, span_lint_hir_and_then};
use clippy_utils::source::{snippet, snippet_opt};
use clippy_utils::ty::{implements_trait, is_copy};
use if_chain::if_chain;
use rustc_ast::ast::LitKind;
use rustc_errors::Applicability;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{
    self as hir, def, BinOpKind, BindingAnnotation, Body, Expr, ExprKind, FnDecl, HirId, Mutability, PatKind, Stmt,
    StmtKind, TyKind, UnOp,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::{self, Ty};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::hygiene::DesugaringKind;
use rustc_span::source_map::{ExpnKind, Span};
use rustc_span::symbol::sym;

use clippy_utils::consts::{constant, Constant};
use clippy_utils::sugg::Sugg;
use clippy_utils::{
    get_item_name, get_parent_expr, in_constant, is_integer_const, iter_input_pats, last_path_segment,
    match_any_def_paths, path_def_id, paths, unsext, SpanlessEq,
};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for function arguments and let bindings denoted as
    /// `ref`.
    ///
    /// ### Why is this bad?
    /// The `ref` declaration makes the function take an owned
    /// value, but turns the argument into a reference (which means that the value
    /// is destroyed when exiting the function). This adds not much value: either
    /// take a reference type, or take an owned value and create references in the
    /// body.
    ///
    /// For let bindings, `let x = &foo;` is preferred over `let ref x = foo`. The
    /// type of `x` is more obvious with the former.
    ///
    /// ### Known problems
    /// If the argument is dereferenced within the function,
    /// removing the `ref` will lead to errors. This can be fixed by removing the
    /// dereferences, e.g., changing `*x` to `x` within the function.
    ///
    /// ### Example
    /// ```rust
    /// fn foo(ref _x: u8) {}
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// fn foo(_x: &u8) {}
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub TOPLEVEL_REF_ARG,
    style,
    "an entire binding declared as `ref`, in a function argument or a `let` statement"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for comparisons to NaN.
    ///
    /// ### Why is this bad?
    /// NaN does not compare meaningfully to anything – not
    /// even itself – so those comparisons are simply wrong.
    ///
    /// ### Example
    /// ```rust
    /// # let x = 1.0;
    /// if x == f32::NAN { }
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// # let x = 1.0f32;
    /// if x.is_nan() { }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub CMP_NAN,
    correctness,
    "comparisons to `NAN`, which will always return false, probably not intended"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for (in-)equality comparisons on floating-point
    /// values (apart from zero), except in functions called `*eq*` (which probably
    /// implement equality for a type involving floats).
    ///
    /// ### Why is this bad?
    /// Floating point calculations are usually imprecise, so
    /// asking if two values are *exactly* equal is asking for trouble. For a good
    /// guide on what to do, see [the floating point
    /// guide](http://www.floating-point-gui.de/errors/comparison).
    ///
    /// ### Example
    /// ```rust
    /// let x = 1.2331f64;
    /// let y = 1.2332f64;
    ///
    /// if y == 1.23f64 { }
    /// if y != x {} // where both are floats
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// # let x = 1.2331f64;
    /// # let y = 1.2332f64;
    /// let error_margin = f64::EPSILON; // Use an epsilon for comparison
    /// // Or, if Rust <= 1.42, use `std::f64::EPSILON` constant instead.
    /// // let error_margin = std::f64::EPSILON;
    /// if (y - 1.23f64).abs() < error_margin { }
    /// if (y - x).abs() > error_margin { }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub FLOAT_CMP,
    pedantic,
    "using `==` or `!=` on float values instead of comparing difference with an epsilon"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for conversions to owned values just for the sake
    /// of a comparison.
    ///
    /// ### Why is this bad?
    /// The comparison can operate on a reference, so creating
    /// an owned value effectively throws it away directly afterwards, which is
    /// needlessly consuming code and heap space.
    ///
    /// ### Example
    /// ```rust
    /// # let x = "foo";
    /// # let y = String::from("foo");
    /// if x.to_owned() == y {}
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// # let x = "foo";
    /// # let y = String::from("foo");
    /// if x == y {}
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub CMP_OWNED,
    perf,
    "creating owned instances for comparing with others, e.g., `x == \"foo\".to_string()`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for getting the remainder of a division by one or minus
    /// one.
    ///
    /// ### Why is this bad?
    /// The result for a divisor of one can only ever be zero; for
    /// minus one it can cause panic/overflow (if the left operand is the minimal value of
    /// the respective integer type) or results in zero. No one will write such code
    /// deliberately, unless trying to win an Underhanded Rust Contest. Even for that
    /// contest, it's probably a bad idea. Use something more underhanded.
    ///
    /// ### Example
    /// ```rust
    /// # let x = 1;
    /// let a = x % 1;
    /// let a = x % -1;
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub MODULO_ONE,
    correctness,
    "taking a number modulo +/-1, which can either panic/overflow or always returns 0"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for the use of bindings with a single leading
    /// underscore.
    ///
    /// ### Why is this bad?
    /// A single leading underscore is usually used to indicate
    /// that a binding will not be used. Using such a binding breaks this
    /// expectation.
    ///
    /// ### Known problems
    /// The lint does not work properly with desugaring and
    /// macro, it has been allowed in the mean time.
    ///
    /// ### Example
    /// ```rust
    /// let _x = 0;
    /// let y = _x + 1; // Here we are using `_x`, even though it has a leading
    ///                 // underscore. We should rename `_x` to `x`
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub USED_UNDERSCORE_BINDING,
    pedantic,
    "using a binding which is prefixed with an underscore"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for the use of short circuit boolean conditions as
    /// a
    /// statement.
    ///
    /// ### Why is this bad?
    /// Using a short circuit boolean condition as a statement
    /// may hide the fact that the second part is executed or not depending on the
    /// outcome of the first part.
    ///
    /// ### Example
    /// ```rust,ignore
    /// f() && g(); // We should write `if f() { g(); }`.
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub SHORT_CIRCUIT_STATEMENT,
    complexity,
    "using a short circuit boolean condition as a statement"
}

declare_clippy_lint! {
    /// ### What it does
    /// Catch casts from `0` to some pointer type
    ///
    /// ### Why is this bad?
    /// This generally means `null` and is better expressed as
    /// {`std`, `core`}`::ptr::`{`null`, `null_mut`}.
    ///
    /// ### Example
    /// ```rust
    /// let a = 0 as *const u32;
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// let a = std::ptr::null::<u32>();
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub ZERO_PTR,
    style,
    "using `0 as *{const, mut} T`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for (in-)equality comparisons on floating-point
    /// value and constant, except in functions called `*eq*` (which probably
    /// implement equality for a type involving floats).
    ///
    /// ### Why is this bad?
    /// Floating point calculations are usually imprecise, so
    /// asking if two values are *exactly* equal is asking for trouble. For a good
    /// guide on what to do, see [the floating point
    /// guide](http://www.floating-point-gui.de/errors/comparison).
    ///
    /// ### Example
    /// ```rust
    /// let x: f64 = 1.0;
    /// const ONE: f64 = 1.00;
    ///
    /// if x == ONE { } // where both are floats
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// # let x: f64 = 1.0;
    /// # const ONE: f64 = 1.00;
    /// let error_margin = f64::EPSILON; // Use an epsilon for comparison
    /// // Or, if Rust <= 1.42, use `std::f64::EPSILON` constant instead.
    /// // let error_margin = std::f64::EPSILON;
    /// if (x - ONE).abs() < error_margin { }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub FLOAT_CMP_CONST,
    restriction,
    "using `==` or `!=` on float constants instead of comparing difference with an epsilon"
}

declare_lint_pass!(MiscLints => [
    TOPLEVEL_REF_ARG,
    CMP_NAN,
    FLOAT_CMP,
    CMP_OWNED,
    MODULO_ONE,
    USED_UNDERSCORE_BINDING,
    SHORT_CIRCUIT_STATEMENT,
    ZERO_PTR,
    FLOAT_CMP_CONST
]);

impl<'tcx> LateLintPass<'tcx> for MiscLints {
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        k: FnKind<'tcx>,
        decl: &'tcx FnDecl<'_>,
        body: &'tcx Body<'_>,
        span: Span,
        _: HirId,
    ) {
        if let FnKind::Closure = k {
            // Does not apply to closures
            return;
        }
        if in_external_macro(cx.tcx.sess, span) {
            return;
        }
        for arg in iter_input_pats(decl, body) {
            if let PatKind::Binding(BindingAnnotation::Ref | BindingAnnotation::RefMut, ..) = arg.pat.kind {
                span_lint(
                    cx,
                    TOPLEVEL_REF_ARG,
                    arg.pat.span,
                    "`ref` directly on a function argument is ignored. \
                    Consider using a reference type instead",
                );
            }
        }
    }

    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
        if_chain! {
            if !in_external_macro(cx.tcx.sess, stmt.span);
            if let StmtKind::Local(local) = stmt.kind;
            if let PatKind::Binding(an, .., name, None) = local.pat.kind;
            if let Some(init) = local.init;
            if an == BindingAnnotation::Ref || an == BindingAnnotation::RefMut;
            then {
                // use the macro callsite when the init span (but not the whole local span)
                // comes from an expansion like `vec![1, 2, 3]` in `let ref _ = vec![1, 2, 3];`
                let sugg_init = if init.span.from_expansion() && !local.span.from_expansion() {
                    Sugg::hir_with_macro_callsite(cx, init, "..")
                } else {
                    Sugg::hir(cx, init, "..")
                };
                let (mutopt, initref) = if an == BindingAnnotation::RefMut {
                    ("mut ", sugg_init.mut_addr())
                } else {
                    ("", sugg_init.addr())
                };
                let tyopt = if let Some(ty) = local.ty {
                    format!(": &{mutopt}{ty}", mutopt=mutopt, ty=snippet(cx, ty.span, ".."))
                } else {
                    String::new()
                };
                span_lint_hir_and_then(
                    cx,
                    TOPLEVEL_REF_ARG,
                    init.hir_id,
                    local.pat.span,
                    "`ref` on an entire `let` pattern is discouraged, take a reference with `&` instead",
                    |diag| {
                        diag.span_suggestion(
                            stmt.span,
                            "try",
                            format!(
                                "let {name}{tyopt} = {initref};",
                                name=snippet(cx, name.span, ".."),
                                tyopt=tyopt,
                                initref=initref,
                            ),
                            Applicability::MachineApplicable,
                        );
                    }
                );
            }
        };
        if_chain! {
            if let StmtKind::Semi(expr) = stmt.kind;
            if let ExprKind::Binary(ref binop, a, b) = expr.kind;
            if binop.node == BinOpKind::And || binop.node == BinOpKind::Or;
            if let Some(sugg) = Sugg::hir_opt(cx, a);
            then {
                span_lint_hir_and_then(
                    cx,
                    SHORT_CIRCUIT_STATEMENT,
                    expr.hir_id,
                    stmt.span,
                    "boolean short circuit operator in statement may be clearer using an explicit test",
                    |diag| {
                        let sugg = if binop.node == BinOpKind::Or { !sugg } else { sugg };
                        diag.span_suggestion(
                            stmt.span,
                            "replace it with",
                            format!(
                                "if {} {{ {}; }}",
                                sugg,
                                &snippet(cx, b.span, ".."),
                            ),
                            Applicability::MachineApplicable, // snippet
                        );
                    });
            }
        };
    }

    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        match expr.kind {
            ExprKind::Cast(e, ty) => {
                check_cast(cx, expr.span, e, ty);
                return;
            },
            ExprKind::Binary(ref cmp, left, right) => {
                check_binary(cx, expr, cmp, left, right);
                return;
            },
            _ => {},
        }
        if in_attributes_expansion(expr) || expr.span.is_desugaring(DesugaringKind::Await) {
            // Don't lint things expanded by #[derive(...)], etc or `await` desugaring
            return;
        }
        let sym;
        let binding = match expr.kind {
            ExprKind::Path(ref qpath) if !matches!(qpath, hir::QPath::LangItem(..)) => {
                let binding = last_path_segment(qpath).ident.as_str();
                if binding.starts_with('_') &&
                    !binding.starts_with("__") &&
                    binding != "_result" && // FIXME: #944
                    is_used(cx, expr) &&
                    // don't lint if the declaration is in a macro
                    non_macro_local(cx, cx.qpath_res(qpath, expr.hir_id))
                {
                    Some(binding)
                } else {
                    None
                }
            },
            ExprKind::Field(_, ident) => {
                sym = ident.name;
                let name = sym.as_str();
                if name.starts_with('_') && !name.starts_with("__") {
                    Some(name)
                } else {
                    None
                }
            },
            _ => None,
        };
        if let Some(binding) = binding {
            span_lint(
                cx,
                USED_UNDERSCORE_BINDING,
                expr.span,
                &format!(
                    "used binding `{}` which is prefixed with an underscore. A leading \
                     underscore signals that a binding will not be used",
                    binding
                ),
            );
        }
    }
}

fn get_lint_and_message(
    is_comparing_constants: bool,
    is_comparing_arrays: bool,
) -> (&'static rustc_lint::Lint, &'static str) {
    if is_comparing_constants {
        (
            FLOAT_CMP_CONST,
            if is_comparing_arrays {
                "strict comparison of `f32` or `f64` constant arrays"
            } else {
                "strict comparison of `f32` or `f64` constant"
            },
        )
    } else {
        (
            FLOAT_CMP,
            if is_comparing_arrays {
                "strict comparison of `f32` or `f64` arrays"
            } else {
                "strict comparison of `f32` or `f64`"
            },
        )
    }
}

fn check_nan(cx: &LateContext<'_>, expr: &Expr<'_>, cmp_expr: &Expr<'_>) {
    if_chain! {
        if !in_constant(cx, cmp_expr.hir_id);
        if let Some((value, _)) = constant(cx, cx.typeck_results(), expr);
        if match value {
            Constant::F32(num) => num.is_nan(),
            Constant::F64(num) => num.is_nan(),
            _ => false,
        };
        then {
            span_lint(
                cx,
                CMP_NAN,
                cmp_expr.span,
                "doomed comparison with `NAN`, use `{f32,f64}::is_nan()` instead",
            );
        }
    }
}

fn is_named_constant<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
    if let Some((_, res)) = constant(cx, cx.typeck_results(), expr) {
        res
    } else {
        false
    }
}

fn is_allowed<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
    match constant(cx, cx.typeck_results(), expr) {
        Some((Constant::F32(f), _)) => f == 0.0 || f.is_infinite(),
        Some((Constant::F64(f), _)) => f == 0.0 || f.is_infinite(),
        Some((Constant::Vec(vec), _)) => vec.iter().all(|f| match f {
            Constant::F32(f) => *f == 0.0 || (*f).is_infinite(),
            Constant::F64(f) => *f == 0.0 || (*f).is_infinite(),
            _ => false,
        }),
        _ => false,
    }
}

// Return true if `expr` is the result of `signum()` invoked on a float value.
fn is_signum(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    // The negation of a signum is still a signum
    if let ExprKind::Unary(UnOp::Neg, child_expr) = expr.kind {
        return is_signum(cx, child_expr);
    }

    if_chain! {
        if let ExprKind::MethodCall(method_name, [ref self_arg, ..], _) = expr.kind;
        if sym!(signum) == method_name.ident.name;
        // Check that the receiver of the signum() is a float (expressions[0] is the receiver of
        // the method call)
        then {
            return is_float(cx, self_arg);
        }
    }
    false
}

fn is_float(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    let value = &cx.typeck_results().expr_ty(expr).peel_refs().kind();

    if let ty::Array(arr_ty, _) = value {
        return matches!(arr_ty.kind(), ty::Float(_));
    };

    matches!(value, ty::Float(_))
}

fn is_array(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    matches!(&cx.typeck_results().expr_ty(expr).peel_refs().kind(), ty::Array(_, _))
}

#[expect(clippy::too_many_lines)]
fn check_to_owned(cx: &LateContext<'_>, expr: &Expr<'_>, other: &Expr<'_>, left: bool) {
    #[derive(Default)]
    struct EqImpl {
        ty_eq_other: bool,
        other_eq_ty: bool,
    }

    impl EqImpl {
        fn is_implemented(&self) -> bool {
            self.ty_eq_other || self.other_eq_ty
        }
    }

    fn symmetric_partial_eq<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, other: Ty<'tcx>) -> Option<EqImpl> {
        cx.tcx.lang_items().eq_trait().map(|def_id| EqImpl {
            ty_eq_other: implements_trait(cx, ty, def_id, &[other.into()]),
            other_eq_ty: implements_trait(cx, other, def_id, &[ty.into()]),
        })
    }

    let typeck = cx.typeck_results();
    let (arg, arg_span) = match expr.kind {
        ExprKind::MethodCall(.., [arg], _)
            if typeck
                .type_dependent_def_id(expr.hir_id)
                .and_then(|id| cx.tcx.trait_of_item(id))
                .map_or(false, |id| {
                    matches!(cx.tcx.get_diagnostic_name(id), Some(sym::ToString | sym::ToOwned))
                }) =>
        {
            (arg, arg.span)
        },
        ExprKind::Call(path, [arg])
            if path_def_id(cx, path)
                .and_then(|id| match_any_def_paths(cx, id, &[&paths::FROM_STR_METHOD, &paths::FROM_FROM]))
                .map_or(false, |idx| match idx {
                    0 => true,
                    1 => !is_copy(cx, typeck.expr_ty(expr)),
                    _ => false,
                }) =>
        {
            (arg, arg.span)
        },
        _ => return,
    };

    let arg_ty = typeck.expr_ty(arg);
    let other_ty = typeck.expr_ty(other);

    let without_deref = symmetric_partial_eq(cx, arg_ty, other_ty).unwrap_or_default();
    let with_deref = arg_ty
        .builtin_deref(true)
        .and_then(|tam| symmetric_partial_eq(cx, tam.ty, other_ty))
        .unwrap_or_default();

    if !with_deref.is_implemented() && !without_deref.is_implemented() {
        return;
    }

    let other_gets_derefed = matches!(other.kind, ExprKind::Unary(UnOp::Deref, _));

    let lint_span = if other_gets_derefed {
        expr.span.to(other.span)
    } else {
        expr.span
    };

    span_lint_and_then(
        cx,
        CMP_OWNED,
        lint_span,
        "this creates an owned instance just for comparison",
        |diag| {
            // This also catches `PartialEq` implementations that call `to_owned`.
            if other_gets_derefed {
                diag.span_label(lint_span, "try implementing the comparison without allocating");
                return;
            }

            let arg_snip = snippet(cx, arg_span, "..");
            let expr_snip;
            let eq_impl;
            if with_deref.is_implemented() {
                expr_snip = format!("*{}", arg_snip);
                eq_impl = with_deref;
            } else {
                expr_snip = arg_snip.to_string();
                eq_impl = without_deref;
            };

            let span;
            let hint;
            if (eq_impl.ty_eq_other && left) || (eq_impl.other_eq_ty && !left) {
                span = expr.span;
                hint = expr_snip;
            } else {
                span = expr.span.to(other.span);

                let cmp_span = if other.span < expr.span {
                    other.span.between(expr.span)
                } else {
                    expr.span.between(other.span)
                };
                if eq_impl.ty_eq_other {
                    hint = format!(
                        "{}{}{}",
                        expr_snip,
                        snippet(cx, cmp_span, ".."),
                        snippet(cx, other.span, "..")
                    );
                } else {
                    hint = format!(
                        "{}{}{}",
                        snippet(cx, other.span, ".."),
                        snippet(cx, cmp_span, ".."),
                        expr_snip
                    );
                }
            }

            diag.span_suggestion(
                span,
                "try",
                hint,
                Applicability::MachineApplicable, // snippet
            );
        },
    );
}

/// Heuristic to see if an expression is used. Should be compatible with
/// `unused_variables`'s idea
/// of what it means for an expression to be "used".
fn is_used(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    get_parent_expr(cx, expr).map_or(true, |parent| match parent.kind {
        ExprKind::Assign(_, rhs, _) | ExprKind::AssignOp(_, _, rhs) => SpanlessEq::new(cx).eq_expr(rhs, expr),
        _ => is_used(cx, parent),
    })
}

/// Tests whether an expression is in a macro expansion (e.g., something
/// generated by `#[derive(...)]` or the like).
fn in_attributes_expansion(expr: &Expr<'_>) -> bool {
    use rustc_span::hygiene::MacroKind;
    if expr.span.from_expansion() {
        let data = expr.span.ctxt().outer_expn_data();
        matches!(data.kind, ExpnKind::Macro(MacroKind::Attr, _))
    } else {
        false
    }
}

/// Tests whether `res` is a variable defined outside a macro.
fn non_macro_local(cx: &LateContext<'_>, res: def::Res) -> bool {
    if let def::Res::Local(id) = res {
        !cx.tcx.hir().span(id).from_expansion()
    } else {
        false
    }
}

fn check_cast(cx: &LateContext<'_>, span: Span, e: &Expr<'_>, ty: &hir::Ty<'_>) {
    if_chain! {
        if let TyKind::Ptr(ref mut_ty) = ty.kind;
        if let ExprKind::Lit(ref lit) = e.kind;
        if let LitKind::Int(0, _) = lit.node;
        if !in_constant(cx, e.hir_id);
        then {
            let (msg, sugg_fn) = match mut_ty.mutbl {
                Mutability::Mut => ("`0 as *mut _` detected", "std::ptr::null_mut"),
                Mutability::Not => ("`0 as *const _` detected", "std::ptr::null"),
            };

            let (sugg, appl) = if let TyKind::Infer = mut_ty.ty.kind {
                (format!("{}()", sugg_fn), Applicability::MachineApplicable)
            } else if let Some(mut_ty_snip) = snippet_opt(cx, mut_ty.ty.span) {
                (format!("{}::<{}>()", sugg_fn, mut_ty_snip), Applicability::MachineApplicable)
            } else {
                // `MaybeIncorrect` as type inference may not work with the suggested code
                (format!("{}()", sugg_fn), Applicability::MaybeIncorrect)
            };
            span_lint_and_sugg(cx, ZERO_PTR, span, msg, "try", sugg, appl);
        }
    }
}

fn check_binary<'a>(
    cx: &LateContext<'a>,
    expr: &Expr<'_>,
    cmp: &rustc_span::source_map::Spanned<rustc_hir::BinOpKind>,
    left: &'a Expr<'_>,
    right: &'a Expr<'_>,
) {
    let op = cmp.node;
    if op.is_comparison() {
        check_nan(cx, left, expr);
        check_nan(cx, right, expr);
        check_to_owned(cx, left, right, true);
        check_to_owned(cx, right, left, false);
    }
    if (op == BinOpKind::Eq || op == BinOpKind::Ne) && (is_float(cx, left) || is_float(cx, right)) {
        if is_allowed(cx, left) || is_allowed(cx, right) {
            return;
        }

        // Allow comparing the results of signum()
        if is_signum(cx, left) && is_signum(cx, right) {
            return;
        }

        if let Some(name) = get_item_name(cx, expr) {
            let name = name.as_str();
            if name == "eq" || name == "ne" || name == "is_nan" || name.starts_with("eq_") || name.ends_with("_eq") {
                return;
            }
        }
        let is_comparing_arrays = is_array(cx, left) || is_array(cx, right);
        let (lint, msg) = get_lint_and_message(
            is_named_constant(cx, left) || is_named_constant(cx, right),
            is_comparing_arrays,
        );
        span_lint_and_then(cx, lint, expr.span, msg, |diag| {
            let lhs = Sugg::hir(cx, left, "..");
            let rhs = Sugg::hir(cx, right, "..");

            if !is_comparing_arrays {
                diag.span_suggestion(
                    expr.span,
                    "consider comparing them within some margin of error",
                    format!(
                        "({}).abs() {} error_margin",
                        lhs - rhs,
                        if op == BinOpKind::Eq { '<' } else { '>' }
                    ),
                    Applicability::HasPlaceholders, // snippet
                );
            }
            diag.note("`f32::EPSILON` and `f64::EPSILON` are available for the `error_margin`");
        });
    } else if op == BinOpKind::Rem {
        if is_integer_const(cx, right, 1) {
            span_lint(cx, MODULO_ONE, expr.span, "any number modulo 1 will be 0");
        }

        if let ty::Int(ity) = cx.typeck_results().expr_ty(right).kind() {
            if is_integer_const(cx, right, unsext(cx.tcx, -1, *ity)) {
                span_lint(
                    cx,
                    MODULO_ONE,
                    expr.span,
                    "any number modulo -1 will panic/overflow or result in 0",
                );
            }
        };
    }
}