clippy_lints/src/manual_rotate.rs

use std::fmt::Display;

use clippy_utils::consts::{ConstEvalCtxt, Constant};
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::sugg;
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::declare_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    ///
    /// It detects manual bit rotations that could be rewritten using standard
    /// functions `rotate_left` or `rotate_right`.
    ///
    /// ### Why is this bad?
    ///
    /// Calling the function better conveys the intent.
    ///
    /// ### Known issues
    ///
    /// Currently, the lint only catches shifts by constant amount.
    ///
    /// ### Example
    /// ```no_run
    /// let x = 12345678_u32;
    /// let _ = (x >> 8) | (x << 24);
    /// ```
    /// Use instead:
    /// ```no_run
    /// let x = 12345678_u32;
    /// let _ = x.rotate_right(8);
    /// ```
    #[clippy::version = "1.81.0"]
    pub MANUAL_ROTATE,
    style,
    "using bit shifts to rotate integers"
}

declare_lint_pass!(ManualRotate => [MANUAL_ROTATE]);

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum ShiftDirection {
    Left,
    Right,
}

impl Display for ShiftDirection {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(match self {
            Self::Left => "rotate_left",
            Self::Right => "rotate_right",
        })
    }
}

fn parse_shift<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'tcx>,
) -> Option<(ShiftDirection, u128, &'tcx Expr<'tcx>)> {
    if let ExprKind::Binary(op, l, r) = expr.kind {
        let dir = match op.node {
            BinOpKind::Shl => ShiftDirection::Left,
            BinOpKind::Shr => ShiftDirection::Right,
            _ => return None,
        };
        let const_expr = ConstEvalCtxt::new(cx).eval(r)?;
        if let Constant::Int(shift) = const_expr {
            return Some((dir, shift, l));
        }
    }
    None
}

impl LateLintPass<'_> for ManualRotate {
    fn check_expr<'tcx>(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
        if let ExprKind::Binary(op, l, r) = expr.kind
            && let BinOpKind::Add | BinOpKind::BitOr = op.node
            && let Some((l_shift_dir, l_amount, l_expr)) = parse_shift(cx, l)
            && let Some((r_shift_dir, r_amount, r_expr)) = parse_shift(cx, r)
        {
            if l_shift_dir == r_shift_dir {
                return;
            }
            if !clippy_utils::eq_expr_value(cx, l_expr, r_expr) {
                return;
            }
            let Some(bit_width) = (match cx.typeck_results().expr_ty(expr).kind() {
                ty::Int(itype) => itype.bit_width(),
                ty::Uint(itype) => itype.bit_width(),
                _ => return,
            }) else {
                return;
            };
            if l_amount + r_amount == u128::from(bit_width) {
                let (shift_function, amount) = if l_amount < r_amount {
                    (l_shift_dir, l_amount)
                } else {
                    (r_shift_dir, r_amount)
                };
                let mut applicability = Applicability::MachineApplicable;
                let expr_sugg = sugg::Sugg::hir_with_applicability(cx, l_expr, "_", &mut applicability).maybe_paren();
                span_lint_and_sugg(
                    cx,
                    MANUAL_ROTATE,
                    expr.span,
                    "there is no need to manually implement bit rotation",
                    "this expression can be rewritten as",
                    format!("{expr_sugg}.{shift_function}({amount})"),
                    Applicability::MachineApplicable,
                );
            }
        }
    }
}