rust/clippy_lints/src/implicit_saturating_sub.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::{higher, is_integer_literal, peel_blocks_with_stmt, SpanlessEq};
use rustc_ast::ast::LitKind;
use rustc_data_structures::packed::Pu128;
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Expr, ExprKind, QPath};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for implicit saturating subtraction.
    ///
    /// ### Why is this bad?
    /// Simplicity and readability. Instead we can easily use an builtin function.
    ///
    /// ### Example
    /// ```no_run
    /// # let end: u32 = 10;
    /// # let start: u32 = 5;
    /// let mut i: u32 = end - start;
    ///
    /// if i != 0 {
    ///     i -= 1;
    /// }
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # let end: u32 = 10;
    /// # let start: u32 = 5;
    /// let mut i: u32 = end - start;
    ///
    /// i = i.saturating_sub(1);
    /// ```
    #[clippy::version = "1.44.0"]
    pub IMPLICIT_SATURATING_SUB,
    style,
    "Perform saturating subtraction instead of implicitly checking lower bound of data type"
}

declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);

impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
        if expr.span.from_expansion() {
            return;
        }
        if let Some(higher::If { cond, then, r#else: None }) = higher::If::hir(expr)

            // Check if the conditional expression is a binary operation
            && let ExprKind::Binary(ref cond_op, cond_left, cond_right) = cond.kind

            // Ensure that the binary operator is >, !=, or <
            && (BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node)

            // Check if assign operation is done
            && let Some(target) = subtracts_one(cx, then)

            // Extracting out the variable name
            && let ExprKind::Path(QPath::Resolved(_, ares_path)) = target.kind
        {
            // Handle symmetric conditions in the if statement
            let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
                if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
                    (cond_left, cond_right)
                } else {
                    return;
                }
            } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
                if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
                    (cond_right, cond_left)
                } else {
                    return;
                }
            } else {
                return;
            };

            // Check if the variable in the condition statement is an integer
            if !cx.typeck_results().expr_ty(cond_var).is_integral() {
                return;
            }

            // Get the variable name
            let var_name = ares_path.segments[0].ident.name.as_str();
            match cond_num_val.kind {
                ExprKind::Lit(cond_lit) => {
                    // Check if the constant is zero
                    if let LitKind::Int(Pu128(0), _) = cond_lit.node {
                        if cx.typeck_results().expr_ty(cond_left).is_signed() {
                        } else {
                            print_lint_and_sugg(cx, var_name, expr);
                        };
                    }
                },
                ExprKind::Path(QPath::TypeRelative(_, name)) => {
                    if name.ident.as_str() == "MIN"
                        && let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id)
                        && let Some(impl_id) = cx.tcx.impl_of_method(const_id)
                        && let None = cx.tcx.impl_trait_ref(impl_id) // An inherent impl
                        && cx.tcx.type_of(impl_id).instantiate_identity().is_integral()
                    {
                        print_lint_and_sugg(cx, var_name, expr);
                    }
                },
                ExprKind::Call(func, []) => {
                    if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind
                        && name.ident.as_str() == "min_value"
                        && let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id)
                        && let Some(impl_id) = cx.tcx.impl_of_method(func_id)
                        && let None = cx.tcx.impl_trait_ref(impl_id) // An inherent impl
                        && cx.tcx.type_of(impl_id).instantiate_identity().is_integral()
                    {
                        print_lint_and_sugg(cx, var_name, expr);
                    }
                },
                _ => (),
            }
        }
    }
}

fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
    match peel_blocks_with_stmt(expr).kind {
        ExprKind::AssignOp(ref op1, target, value) => {
            // Check if literal being subtracted is one
            (BinOpKind::Sub == op1.node && is_integer_literal(value, 1)).then_some(target)
        },
        ExprKind::Assign(target, value, _) => {
            if let ExprKind::Binary(ref op1, left1, right1) = value.kind
                && BinOpKind::Sub == op1.node
                && SpanlessEq::new(cx).eq_expr(left1, target)
                && is_integer_literal(right1, 1)
            {
                Some(target)
            } else {
                None
            }
        },
        _ => None,
    }
}

fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
    span_lint_and_sugg(
        cx,
        IMPLICIT_SATURATING_SUB,
        expr.span,
        "implicitly performing saturating subtraction",
        "try",
        format!("{var_name} = {var_name}.saturating_sub({});", '1'),
        Applicability::MachineApplicable,
    );
}