manual_saturating_arithmetic.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::{match_def_path, path_def_id};
use rustc_ast::ast;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_middle::ty::layout::LayoutOf;

pub fn check(
    cx: &LateContext<'_>,
    expr: &hir::Expr<'_>,
    arith_lhs: &hir::Expr<'_>,
    arith_rhs: &hir::Expr<'_>,
    unwrap_arg: &hir::Expr<'_>,
    arith: &str,
) {
    let ty = cx.typeck_results().expr_ty(arith_lhs);
    if !ty.is_integral() {
        return;
    }

    let Some(mm) = is_min_or_max(cx, unwrap_arg) else {
        return;
    };

    if ty.is_signed() {
        use self::MinMax::{Max, Min};
        use self::Sign::{Neg, Pos};

        let Some(sign) = lit_sign(arith_rhs) else {
            return;
        };

        match (arith, sign, mm) {
            ("add", Pos, Max) | ("add", Neg, Min) | ("sub", Neg, Max) | ("sub", Pos, Min) => (),
            // "mul" is omitted because lhs can be negative.
            _ => return,
        }
    } else {
        match (mm, arith) {
            (MinMax::Max, "add" | "mul") | (MinMax::Min, "sub") => (),
            _ => return,
        }
    }

    let mut applicability = Applicability::MachineApplicable;
    span_lint_and_sugg(
        cx,
        super::MANUAL_SATURATING_ARITHMETIC,
        expr.span,
        "manual saturating arithmetic",
        format!("consider using `saturating_{arith}`"),
        format!(
            "{}.saturating_{arith}({})",
            snippet_with_applicability(cx, arith_lhs.span, "..", &mut applicability),
            snippet_with_applicability(cx, arith_rhs.span, "..", &mut applicability),
        ),
        applicability,
    );
}

#[derive(PartialEq, Eq)]
enum MinMax {
    Min,
    Max,
}

fn is_min_or_max(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> Option<MinMax> {
    // `T::max_value()` `T::min_value()` inherent methods
    if let hir::ExprKind::Call(func, []) = &expr.kind
        && let hir::ExprKind::Path(hir::QPath::TypeRelative(_, segment)) = &func.kind
    {
        match segment.ident.as_str() {
            "max_value" => return Some(MinMax::Max),
            "min_value" => return Some(MinMax::Min),
            _ => {},
        }
    }

    let ty = cx.typeck_results().expr_ty(expr);
    let ty_str = ty.to_string();

    // `std::T::MAX` `std::T::MIN` constants
    if let Some(id) = path_def_id(cx, expr) {
        if match_def_path(cx, id, &["core", &ty_str, "MAX"]) {
            return Some(MinMax::Max);
        }

        if match_def_path(cx, id, &["core", &ty_str, "MIN"]) {
            return Some(MinMax::Min);
        }
    }

    // Literals
    let bits = cx.layout_of(ty).unwrap().size.bits();
    let (minval, maxval): (u128, u128) = if ty.is_signed() {
        let minval = 1 << (bits - 1);
        let mut maxval = !(1 << (bits - 1));
        if bits != 128 {
            maxval &= (1 << bits) - 1;
        }
        (minval, maxval)
    } else {
        (0, if bits == 128 { !0 } else { (1 << bits) - 1 })
    };

    let check_lit = |expr: &hir::Expr<'_>, check_min: bool| {
        if let hir::ExprKind::Lit(lit) = &expr.kind {
            if let ast::LitKind::Int(value, _) = lit.node {
                if value == maxval {
                    return Some(MinMax::Max);
                }

                if check_min && value == minval {
                    return Some(MinMax::Min);
                }
            }
        }

        None
    };

    if let r @ Some(_) = check_lit(expr, !ty.is_signed()) {
        return r;
    }

    if ty.is_signed() {
        if let hir::ExprKind::Unary(hir::UnOp::Neg, val) = &expr.kind {
            return check_lit(val, true);
        }
    }

    None
}

#[derive(PartialEq, Eq)]
enum Sign {
    Pos,
    Neg,
}

fn lit_sign(expr: &hir::Expr<'_>) -> Option<Sign> {
    if let hir::ExprKind::Unary(hir::UnOp::Neg, inner) = &expr.kind {
        if let hir::ExprKind::Lit(..) = &inner.kind {
            return Some(Sign::Neg);
        }
    } else if let hir::ExprKind::Lit(..) = &expr.kind {
        return Some(Sign::Pos);
    }

    None
}