rust/clippy_lints/src/ranges.rs

use clippy_config::Conf;
use clippy_utils::consts::{ConstEvalCtxt, Constant};
use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg, span_lint_and_then};
use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::res::MaybeResPath;
use clippy_utils::source::{SpanRangeExt, snippet, snippet_with_applicability};
use clippy_utils::sugg::Sugg;
use clippy_utils::ty::implements_trait;
use clippy_utils::{expr_use_ctxt, fn_def_id, get_parent_expr, higher, is_in_const_context, is_integer_const, sym};
use rustc_ast::Mutability;
use rustc_ast::ast::RangeLimits;
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Expr, ExprKind, HirId, LangItem, Node};
use rustc_lint::{LateContext, LateLintPass, Lint};
use rustc_middle::ty::{self, ClauseKind, GenericArgKind, PredicatePolarity, Ty};
use rustc_session::impl_lint_pass;
use rustc_span::{DesugaringKind, Span, Spanned};
use std::cmp::Ordering;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for expressions like `x >= 3 && x < 8` that could
    /// be more readably expressed as `(3..8).contains(x)`.
    ///
    /// ### Why is this bad?
    /// `contains` expresses the intent better and has less
    /// failure modes (such as fencepost errors or using `||` instead of `&&`).
    ///
    /// ### Example
    /// ```no_run
    /// // given
    /// let x = 6;
    ///
    /// assert!(x >= 3 && x < 8);
    /// ```
    /// Use instead:
    /// ```no_run
    ///# let x = 6;
    /// assert!((3..8).contains(&x));
    /// ```
    #[clippy::version = "1.49.0"]
    pub MANUAL_RANGE_CONTAINS,
    style,
    "manually reimplementing {`Range`, `RangeInclusive`}`::contains`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for inclusive ranges where 1 is subtracted from
    /// the upper bound, e.g., `x..=(y-1)`.
    ///
    /// ### Why is this bad?
    /// The code is more readable with an exclusive range
    /// like `x..y`.
    ///
    /// ### Limitations
    /// The lint is conservative and will trigger only when switching
    /// from an inclusive to an exclusive range is provably safe from
    /// a typing point of view. This corresponds to situations where
    /// the range is used as an iterator, or for indexing.
    ///
    /// ### Example
    /// ```no_run
    /// # let x = 0;
    /// # let y = 1;
    /// for i in x..=(y-1) {
    ///     // ..
    /// }
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # let x = 0;
    /// # let y = 1;
    /// for i in x..y {
    ///     // ..
    /// }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub RANGE_MINUS_ONE,
    pedantic,
    "`x..=(y-1)` reads better as `x..y`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for exclusive ranges where 1 is added to the
    /// upper bound, e.g., `x..(y+1)`.
    ///
    /// ### Why is this bad?
    /// The code is more readable with an inclusive range
    /// like `x..=y`.
    ///
    /// ### Limitations
    /// The lint is conservative and will trigger only when switching
    /// from an exclusive to an inclusive range is provably safe from
    /// a typing point of view. This corresponds to situations where
    /// the range is used as an iterator, or for indexing.
    ///
    /// ### Known problems
    /// Will add unnecessary pair of parentheses when the
    /// expression is not wrapped in a pair but starts with an opening parenthesis
    /// and ends with a closing one.
    /// I.e., `let _ = (f()+1)..(f()+1)` results in `let _ = ((f()+1)..=f())`.
    ///
    /// Also in many cases, inclusive ranges are still slower to run than
    /// exclusive ranges, because they essentially add an extra branch that
    /// LLVM may fail to hoist out of the loop.
    ///
    /// ### Example
    /// ```no_run
    /// # let x = 0;
    /// # let y = 1;
    /// for i in x..(y+1) {
    ///     // ..
    /// }
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # let x = 0;
    /// # let y = 1;
    /// for i in x..=y {
    ///     // ..
    /// }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub RANGE_PLUS_ONE,
    pedantic,
    "`x..(y+1)` reads better as `x..=y`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for range expressions `x..y` where both `x` and `y`
    /// are constant and `x` is greater to `y`. Also triggers if `x` is equal to `y` when they are conditions to a `for` loop.
    ///
    /// ### Why is this bad?
    /// Empty ranges yield no values so iterating them is a no-op.
    /// Moreover, trying to use a reversed range to index a slice will panic at run-time.
    ///
    /// ### Example
    /// ```rust,no_run
    /// fn main() {
    ///     (10..=0).for_each(|x| println!("{}", x));
    ///
    ///     let arr = [1, 2, 3, 4, 5];
    ///     let sub = &arr[3..1];
    /// }
    /// ```
    /// Use instead:
    /// ```no_run
    /// fn main() {
    ///     (0..=10).rev().for_each(|x| println!("{}", x));
    ///
    ///     let arr = [1, 2, 3, 4, 5];
    ///     let sub = &arr[1..3];
    /// }
    /// ```
    #[clippy::version = "1.45.0"]
    pub REVERSED_EMPTY_RANGES,
    correctness,
    "reversing the limits of range expressions, resulting in empty ranges"
}

impl_lint_pass!(Ranges => [
    MANUAL_RANGE_CONTAINS,
    RANGE_MINUS_ONE,
    RANGE_PLUS_ONE,
    REVERSED_EMPTY_RANGES,
]);

pub struct Ranges {
    msrv: Msrv,
}

impl Ranges {
    pub fn new(conf: &'static Conf) -> Self {
        Self { msrv: conf.msrv }
    }
}

impl<'tcx> LateLintPass<'tcx> for Ranges {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if let ExprKind::Binary(ref op, l, r) = expr.kind
            && self.msrv.meets(cx, msrvs::RANGE_CONTAINS)
        {
            check_possible_range_contains(cx, op.node, l, r, expr, expr.span);
        }

        check_exclusive_range_plus_one(cx, expr);
        check_inclusive_range_minus_one(cx, expr);
        check_reversed_empty_range(cx, expr);
    }
}

fn check_possible_range_contains(
    cx: &LateContext<'_>,
    op: BinOpKind,
    left: &Expr<'_>,
    right: &Expr<'_>,
    expr: &Expr<'_>,
    span: Span,
) {
    if is_in_const_context(cx) {
        return;
    }

    let combine_and = match op {
        BinOpKind::And | BinOpKind::BitAnd => true,
        BinOpKind::Or | BinOpKind::BitOr => false,
        _ => return,
    };
    // value, name, order (higher/lower), inclusiveness
    if let (Some(l), Some(r)) = (check_range_bounds(cx, left), check_range_bounds(cx, right)) {
        // we only lint comparisons on the same name and with different
        // direction
        if l.id != r.id || l.ord == r.ord {
            return;
        }
        let ord = Constant::partial_cmp(cx.tcx, cx.typeck_results().expr_ty(l.expr), &l.val, &r.val);
        if combine_and && ord == Some(r.ord) {
            // order lower bound and upper bound
            let (l_span, u_span, l_inc, u_inc) = if r.ord == Ordering::Less {
                (l.val_span, r.val_span, l.inc, r.inc)
            } else {
                (r.val_span, l.val_span, r.inc, l.inc)
            };
            // we only lint inclusive lower bounds
            if !l_inc {
                return;
            }
            let (range_type, range_op) = if u_inc {
                ("RangeInclusive", "..=")
            } else {
                ("Range", "..")
            };
            let mut applicability = Applicability::MachineApplicable;
            let name = snippet_with_applicability(cx, l.name_span, "_", &mut applicability);
            let lo = snippet_with_applicability(cx, l_span, "_", &mut applicability);
            let hi = snippet_with_applicability(cx, u_span, "_", &mut applicability);
            let space = if lo.ends_with('.') { " " } else { "" };
            span_lint_and_sugg(
                cx,
                MANUAL_RANGE_CONTAINS,
                span,
                format!("manual `{range_type}::contains` implementation"),
                "use",
                format!("({lo}{space}{range_op}{hi}).contains(&{name})"),
                applicability,
            );
        } else if !combine_and && ord == Some(l.ord) {
            // `!_.contains(_)`
            // order lower bound and upper bound
            let (l_span, u_span, l_inc, u_inc) = if l.ord == Ordering::Less {
                (l.val_span, r.val_span, l.inc, r.inc)
            } else {
                (r.val_span, l.val_span, r.inc, l.inc)
            };
            if l_inc {
                return;
            }
            let (range_type, range_op) = if u_inc {
                ("Range", "..")
            } else {
                ("RangeInclusive", "..=")
            };
            let mut applicability = Applicability::MachineApplicable;
            let name = snippet_with_applicability(cx, l.name_span, "_", &mut applicability);
            let lo = snippet_with_applicability(cx, l_span, "_", &mut applicability);
            let hi = snippet_with_applicability(cx, u_span, "_", &mut applicability);
            let space = if lo.ends_with('.') { " " } else { "" };
            span_lint_and_sugg(
                cx,
                MANUAL_RANGE_CONTAINS,
                span,
                format!("manual `!{range_type}::contains` implementation"),
                "use",
                format!("!({lo}{space}{range_op}{hi}).contains(&{name})"),
                applicability,
            );
        }
    }

    // If the LHS is the same operator, we have to recurse to get the "real" RHS, since they have
    // the same operator precedence
    if let ExprKind::Binary(ref lhs_op, _left, new_lhs) = left.kind
        && op == lhs_op.node
        && let new_span = Span::new(new_lhs.span.lo(), right.span.hi(), expr.span.ctxt(), expr.span.parent())
        && new_span.check_source_text(cx, |src| {
            // Do not continue if we have mismatched number of parens, otherwise the suggestion is wrong
            src.matches('(').count() == src.matches(')').count()
        })
    {
        check_possible_range_contains(cx, op, new_lhs, right, expr, new_span);
    }
}

struct RangeBounds<'a> {
    val: Constant,
    expr: &'a Expr<'a>,
    id: HirId,
    name_span: Span,
    val_span: Span,
    ord: Ordering,
    inc: bool,
}

// Takes a binary expression such as x <= 2 as input
// Breaks apart into various pieces, such as the value of the number,
// hir id of the variable, and direction/inclusiveness of the operator
fn check_range_bounds<'a>(cx: &'a LateContext<'_>, ex: &'a Expr<'_>) -> Option<RangeBounds<'a>> {
    if let ExprKind::Binary(ref op, l, r) = ex.kind {
        let (inclusive, ordering) = match op.node {
            BinOpKind::Gt => (false, Ordering::Greater),
            BinOpKind::Ge => (true, Ordering::Greater),
            BinOpKind::Lt => (false, Ordering::Less),
            BinOpKind::Le => (true, Ordering::Less),
            _ => return None,
        };
        if let Some(id) = l.res_local_id() {
            if let Some(c) = ConstEvalCtxt::new(cx).eval(r) {
                return Some(RangeBounds {
                    val: c,
                    expr: r,
                    id,
                    name_span: l.span,
                    val_span: r.span,
                    ord: ordering,
                    inc: inclusive,
                });
            }
        } else if let Some(id) = r.res_local_id()
            && let Some(c) = ConstEvalCtxt::new(cx).eval(l)
        {
            return Some(RangeBounds {
                val: c,
                expr: l,
                id,
                name_span: r.span,
                val_span: l.span,
                ord: ordering.reverse(),
                inc: inclusive,
            });
        }
    }
    None
}

/// Check whether `expr` could switch range types without breaking the typing requirements. This is
/// generally the case when `expr` is used as an iterator for example, or as a slice or `&str`
/// index.
///
/// FIXME: Note that the current implementation may still return false positives. A proper fix would
/// check that the obligations are still satisfied after switching the range type.
fn can_switch_ranges<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'_>,
    original: RangeLimits,
    inner_ty: Ty<'tcx>,
) -> bool {
    let use_ctxt = expr_use_ctxt(cx, expr);
    let (Node::Expr(parent_expr), false) = (use_ctxt.node, use_ctxt.is_ty_unified) else {
        return false;
    };

    // Check if `expr` is the argument of a compiler-generated `IntoIter::into_iter(expr)`
    if let ExprKind::Call(func, [arg]) = parent_expr.kind
        && arg.hir_id == use_ctxt.child_id
        && let ExprKind::Path(qpath) = func.kind
        && cx.tcx.qpath_is_lang_item(qpath, LangItem::IntoIterIntoIter)
        && parent_expr.span.is_desugaring(DesugaringKind::ForLoop)
    {
        return true;
    }

    // Check if `expr` is used as the receiver of a method of the `Iterator`, `IntoIterator`,
    // or `RangeBounds` traits.
    if let ExprKind::MethodCall(_, receiver, _, _) = parent_expr.kind
        && receiver.hir_id == use_ctxt.child_id
        && let Some(method_did) = cx.typeck_results().type_dependent_def_id(parent_expr.hir_id)
        && let Some(trait_did) = cx.tcx.trait_of_assoc(method_did)
        && matches!(
            cx.tcx.get_diagnostic_name(trait_did),
            Some(sym::Iterator | sym::IntoIterator | sym::RangeBounds)
        )
    {
        return true;
    }

    // Check if `expr` is an argument of a call which requires an `Iterator`, `IntoIterator`,
    // or `RangeBounds` trait.
    if let ExprKind::Call(_, args) | ExprKind::MethodCall(_, _, args, _) = parent_expr.kind
        && let Some(id) = fn_def_id(cx, parent_expr)
        && let Some(arg_idx) = args.iter().position(|e| e.hir_id == use_ctxt.child_id)
    {
        let input_idx = if matches!(parent_expr.kind, ExprKind::MethodCall(..)) {
            arg_idx + 1
        } else {
            arg_idx
        };
        let inputs = cx
            .tcx
            .liberate_late_bound_regions(id, cx.tcx.fn_sig(id).instantiate_identity())
            .inputs();
        let expr_ty = inputs[input_idx];
        // Check that the `expr` type is present only once, otherwise modifying just one of them might be
        // risky if they are referenced using the same generic type for example.
        if inputs.iter().enumerate().all(|(n, ty)|
                                         n == input_idx
                                         || !ty.walk().any(|arg| matches!(arg.kind(),
                                                                          GenericArgKind::Type(ty) if ty == expr_ty)))
            // Look for a clause requiring `Iterator`, `IntoIterator`, or `RangeBounds`, and resolving to `expr_type`.
            && cx
                .tcx
                .param_env(id)
                .caller_bounds()
                .into_iter()
                .any(|p| {
                    if let ClauseKind::Trait(t) = p.kind().skip_binder()
                        && t.polarity == PredicatePolarity::Positive
                        && matches!(
                            cx.tcx.get_diagnostic_name(t.trait_ref.def_id),
                            Some(sym::Iterator | sym::IntoIterator | sym::RangeBounds)
                        )
                    {
                        t.self_ty() == expr_ty
                    } else {
                        false
                    }
                })
        {
            return true;
        }
    }

    // Check if `expr` is used for indexing, and if the switched range type could be used
    // as well.
    if let ExprKind::Index(outer_expr, index, _) = parent_expr.kind
        && index.hir_id == expr.hir_id
        // Build the switched range type (for example `RangeInclusive<usize>`).
        && let Some(switched_range_def_id) = match original {
            RangeLimits::HalfOpen => cx.tcx.lang_items().range_inclusive_struct(),
            RangeLimits::Closed => cx.tcx.lang_items().range_struct(),
        }
        && let switched_range_ty = cx
            .tcx
            .type_of(switched_range_def_id)
            .instantiate(cx.tcx, &[inner_ty.into()])
        // Check that the switched range type can be used for indexing the original expression
        // through the `Index` or `IndexMut` trait.
        && let ty::Ref(_, outer_ty, mutability) = cx.typeck_results().expr_ty_adjusted(outer_expr).kind()
        && let Some(index_def_id) = match mutability {
            Mutability::Not => cx.tcx.lang_items().index_trait(),
            Mutability::Mut => cx.tcx.lang_items().index_mut_trait(),
        }
       && implements_trait(cx, *outer_ty, index_def_id, &[switched_range_ty.into()])
    // We could also check that the associated item of the `index_def_id` trait with the switched range type
    // return the same type, but it is reasonable to expect so. We can't check that the result is identical
    // in both `Index<Range<…>>` and `Index<RangeInclusive<…>>` anyway.
    {
        return true;
    }

    false
}

// exclusive range plus one: `x..(y+1)`
fn check_exclusive_range_plus_one<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
    check_range_switch(
        cx,
        expr,
        RangeLimits::HalfOpen,
        y_plus_one,
        RANGE_PLUS_ONE,
        "an inclusive range would be more readable",
        "..=",
    );
}

// inclusive range minus one: `x..=(y-1)`
fn check_inclusive_range_minus_one<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
    check_range_switch(
        cx,
        expr,
        RangeLimits::Closed,
        y_minus_one,
        RANGE_MINUS_ONE,
        "an exclusive range would be more readable",
        "..",
    );
}

/// Check for a `kind` of range in `expr`, check for `predicate` on the end,
/// and emit the `lint` with `msg` and the `operator`.
fn check_range_switch<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'_>,
    kind: RangeLimits,
    predicate: impl for<'hir> FnOnce(&LateContext<'_>, &Expr<'hir>) -> Option<&'hir Expr<'hir>>,
    lint: &'static Lint,
    msg: &'static str,
    operator: &str,
) {
    if let Some(range) = higher::Range::hir(cx, expr)
        && let higher::Range {
            start,
            end: Some(end),
            limits,
            span,
        } = range
        && span.can_be_used_for_suggestions()
        && limits == kind
        && let Some(y) = predicate(cx, end)
        && can_switch_ranges(cx, expr, kind, cx.typeck_results().expr_ty(y))
    {
        span_lint_and_then(cx, lint, span, msg, |diag| {
            let mut app = Applicability::MachineApplicable;
            let start = start.map_or(String::new(), |x| {
                Sugg::hir_with_applicability(cx, x, "<x>", &mut app)
                    .maybe_paren()
                    .to_string()
            });
            let end = Sugg::hir_with_applicability(cx, y, "<y>", &mut app).maybe_paren();
            match span.with_source_text(cx, |src| src.starts_with('(') && src.ends_with(')')) {
                Some(true) => {
                    diag.span_suggestion(span, "use", format!("({start}{operator}{end})"), app);
                },
                Some(false) => {
                    diag.span_suggestion(span, "use", format!("{start}{operator}{end}"), app);
                },
                None => {},
            }
        });
    }
}

fn check_reversed_empty_range(cx: &LateContext<'_>, expr: &Expr<'_>) {
    fn inside_indexing_expr(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
        matches!(
            get_parent_expr(cx, expr),
            Some(Expr {
                kind: ExprKind::Index(..),
                ..
            })
        )
    }

    fn is_for_loop_arg(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
        let mut cur_expr = expr;
        while let Some(parent_expr) = get_parent_expr(cx, cur_expr) {
            match higher::ForLoop::hir(parent_expr) {
                Some(higher::ForLoop { arg, .. }) if arg.hir_id == expr.hir_id => return true,
                _ => cur_expr = parent_expr,
            }
        }

        false
    }

    fn is_empty_range(limits: RangeLimits, ordering: Ordering) -> bool {
        match limits {
            RangeLimits::HalfOpen => ordering != Ordering::Less,
            RangeLimits::Closed => ordering == Ordering::Greater,
        }
    }

    if let Some(higher::Range {
        start: Some(start),
        end: Some(end),
        limits,
        span,
    }) = higher::Range::hir(cx, expr)
        && let ty = cx.typeck_results().expr_ty(start)
        && let ty::Int(_) | ty::Uint(_) = ty.kind()
        && let ecx = ConstEvalCtxt::new(cx)
        && let Some(start_idx) = ecx.eval(start)
        && let Some(end_idx) = ecx.eval(end)
        && let Some(ordering) = Constant::partial_cmp(cx.tcx, ty, &start_idx, &end_idx)
        && is_empty_range(limits, ordering)
    {
        if inside_indexing_expr(cx, expr) {
            // Avoid linting `N..N` as it has proven to be useful, see #5689 and #5628 ...
            if ordering != Ordering::Equal {
                span_lint(
                    cx,
                    REVERSED_EMPTY_RANGES,
                    span,
                    "this range is reversed and using it to index a slice will panic at run-time",
                );
            }
        // ... except in for loop arguments for backwards compatibility with `reverse_range_loop`
        } else if ordering != Ordering::Equal || is_for_loop_arg(cx, expr) {
            span_lint_and_then(
                cx,
                REVERSED_EMPTY_RANGES,
                span,
                "this range is empty so it will yield no values",
                |diag| {
                    if ordering != Ordering::Equal {
                        let start_snippet = snippet(cx, start.span, "_");
                        let end_snippet = snippet(cx, end.span, "_");
                        let dots = match limits {
                            RangeLimits::HalfOpen => "..",
                            RangeLimits::Closed => "..=",
                        };

                        diag.span_suggestion(
                            span,
                            "consider using the following if you are attempting to iterate over this \
                             range in reverse",
                            format!("({end_snippet}{dots}{start_snippet}).rev()"),
                            Applicability::MaybeIncorrect,
                        );
                    }
                },
            );
        }
    }
}

fn y_plus_one<'tcx>(cx: &LateContext<'_>, expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
    match expr.kind {
        ExprKind::Binary(
            Spanned {
                node: BinOpKind::Add, ..
            },
            lhs,
            rhs,
        ) => {
            if is_integer_const(cx, lhs, 1) {
                Some(rhs)
            } else if is_integer_const(cx, rhs, 1) {
                Some(lhs)
            } else {
                None
            }
        },
        _ => None,
    }
}

fn y_minus_one<'tcx>(cx: &LateContext<'_>, expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
    match expr.kind {
        ExprKind::Binary(
            Spanned {
                node: BinOpKind::Sub, ..
            },
            lhs,
            rhs,
        ) if is_integer_const(cx, rhs, 1) => Some(lhs),
        _ => None,
    }
}