rust-clippy/clippy_lints/src/identity_op.rs

use clippy_utils::consts::{constant_full_int, constant_simple, Constant, FullInt};
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::{clip, unsext};
use rustc_errors::Applicability;
use rustc_hir::{BinOp, BinOpKind, Expr, ExprKind, Node};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for identity operations, e.g., `x + 0`.
    ///
    /// ### Why is this bad?
    /// This code can be removed without changing the
    /// meaning. So it just obscures what's going on. Delete it mercilessly.
    ///
    /// ### Example
    /// ```rust
    /// # let x = 1;
    /// x / 1 + 0 * 1 - 0 | 0;
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub IDENTITY_OP,
    complexity,
    "using identity operations, e.g., `x + 0` or `y / 1`"
}

declare_lint_pass!(IdentityOp => [IDENTITY_OP]);

impl<'tcx> LateLintPass<'tcx> for IdentityOp {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if expr.span.from_expansion() {
            return;
        }
        if let ExprKind::Binary(cmp, left, right) = &expr.kind {
            if !is_allowed(cx, *cmp, left, right) {
                match cmp.node {
                    BinOpKind::Add | BinOpKind::BitOr | BinOpKind::BitXor => {
                        check(cx, left, 0, expr.span, right.span, needs_parenthesis(cx, expr, right));
                        check(cx, right, 0, expr.span, left.span, Parens::Unneeded);
                    },
                    BinOpKind::Shl | BinOpKind::Shr | BinOpKind::Sub => {
                        check(cx, right, 0, expr.span, left.span, Parens::Unneeded);
                    },
                    BinOpKind::Mul => {
                        check(cx, left, 1, expr.span, right.span, needs_parenthesis(cx, expr, right));
                        check(cx, right, 1, expr.span, left.span, Parens::Unneeded);
                    },
                    BinOpKind::Div => check(cx, right, 1, expr.span, left.span, Parens::Unneeded),
                    BinOpKind::BitAnd => {
                        check(cx, left, -1, expr.span, right.span, needs_parenthesis(cx, expr, right));
                        check(cx, right, -1, expr.span, left.span, Parens::Unneeded);
                    },
                    BinOpKind::Rem => check_remainder(cx, left, right, expr.span, left.span),
                    _ => (),
                }
            }
        }
    }
}

#[derive(Copy, Clone)]
enum Parens {
    Needed,
    Unneeded,
}

/// Checks if `left op right` needs parenthesis when reduced to `right`
/// e.g. `0 + if b { 1 } else { 2 } + if b { 3 } else { 4 }` cannot be reduced
/// to `if b { 1 } else { 2 } + if b { 3 } else { 4 }` where the `if` could be
/// interpreted as a statement
///
/// See #8724
fn needs_parenthesis(cx: &LateContext<'_>, binary: &Expr<'_>, right: &Expr<'_>) -> Parens {
    match right.kind {
        ExprKind::Binary(_, lhs, _) | ExprKind::Cast(lhs, _) => {
            // ensure we're checking against the leftmost expression of `right`
            //
            //     ~~~ `lhs`
            // 0 + {4} * 2
            //     ~~~~~~~ `right`
            return needs_parenthesis(cx, binary, lhs);
        },
        ExprKind::If(..) | ExprKind::Match(..) | ExprKind::Block(..) | ExprKind::Loop(..) => {},
        _ => return Parens::Unneeded,
    }

    let mut prev_id = binary.hir_id;
    for (_, node) in cx.tcx.hir().parent_iter(binary.hir_id) {
        if let Node::Expr(expr) = node
            && let ExprKind::Binary(_, lhs, _) | ExprKind::Cast(lhs, _) = expr.kind
            && lhs.hir_id == prev_id
        {
            // keep going until we find a node that encompasses left of `binary`
            prev_id = expr.hir_id;
            continue;
        }

        match node {
            Node::Block(_) | Node::Stmt(_) => break,
            _ => return Parens::Unneeded,
        };
    }

    Parens::Needed
}

fn is_allowed(cx: &LateContext<'_>, cmp: BinOp, left: &Expr<'_>, right: &Expr<'_>) -> bool {
    // This lint applies to integers
    !cx.typeck_results().expr_ty(left).peel_refs().is_integral()
        || !cx.typeck_results().expr_ty(right).peel_refs().is_integral()
        // `1 << 0` is a common pattern in bit manipulation code
        || (cmp.node == BinOpKind::Shl
            && constant_simple(cx, cx.typeck_results(), right) == Some(Constant::Int(0))
            && constant_simple(cx, cx.typeck_results(), left) == Some(Constant::Int(1)))
}

fn check_remainder(cx: &LateContext<'_>, left: &Expr<'_>, right: &Expr<'_>, span: Span, arg: Span) {
    let lhs_const = constant_full_int(cx, cx.typeck_results(), left);
    let rhs_const = constant_full_int(cx, cx.typeck_results(), right);
    if match (lhs_const, rhs_const) {
        (Some(FullInt::S(lv)), Some(FullInt::S(rv))) => lv.abs() < rv.abs(),
        (Some(FullInt::U(lv)), Some(FullInt::U(rv))) => lv < rv,
        _ => return,
    } {
        span_ineffective_operation(cx, span, arg, Parens::Unneeded);
    }
}

fn check(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span, parens: Parens) {
    if let Some(Constant::Int(v)) = constant_simple(cx, cx.typeck_results(), e).map(Constant::peel_refs) {
        let check = match *cx.typeck_results().expr_ty(e).peel_refs().kind() {
            ty::Int(ity) => unsext(cx.tcx, -1_i128, ity),
            ty::Uint(uty) => clip(cx.tcx, !0, uty),
            _ => return,
        };
        if match m {
            0 => v == 0,
            -1 => v == check,
            1 => v == 1,
            _ => unreachable!(),
        } {
            span_ineffective_operation(cx, span, arg, parens);
        }
    }
}

fn span_ineffective_operation(cx: &LateContext<'_>, span: Span, arg: Span, parens: Parens) {
    let mut applicability = Applicability::MachineApplicable;
    let expr_snippet = snippet_with_applicability(cx, arg, "..", &mut applicability);

    let suggestion = match parens {
        Parens::Needed => format!("({expr_snippet})"),
        Parens::Unneeded => expr_snippet.into_owned(),
    };

    span_lint_and_sugg(
        cx,
        IDENTITY_OP,
        span,
        "this operation has no effect",
        "consider reducing it to",
        suggestion,
        applicability,
    );
}