rust-clippy/clippy_lints/src/ranges.rs

use crate::consts::{constant, Constant};
use if_chain::if_chain;
use rustc_ast::ast::RangeLimits;
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Expr, ExprKind, QPath};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Spanned;
use std::cmp::Ordering;

use crate::utils::sugg::Sugg;
use crate::utils::{get_parent_expr, is_integer_const, snippet, snippet_opt, span_lint, span_lint_and_then};
use crate::utils::{higher, SpanlessEq};

declare_clippy_lint! {
    /// **What it does:** Checks for zipping a collection with the range of
    /// `0.._.len()`.
    ///
    /// **Why is this bad?** The code is better expressed with `.enumerate()`.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// # let x = vec![1];
    /// x.iter().zip(0..x.len());
    /// ```
    /// Could be written as
    /// ```rust
    /// # let x = vec![1];
    /// x.iter().enumerate();
    /// ```
    pub RANGE_ZIP_WITH_LEN,
    complexity,
    "zipping iterator with a range when `enumerate()` would do"
}

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`.
    ///
    /// **Known problems:** Will add unnecessary pair of parentheses when the
    /// expression is not wrapped in a pair but starts with a 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:**
    /// ```rust,ignore
    /// for x..(y+1) { .. }
    /// ```
    /// Could be written as
    /// ```rust,ignore
    /// for x..=y { .. }
    /// ```
    pub RANGE_PLUS_ONE,
    pedantic,
    "`x..(y+1)` reads better as `x..=y`"
}

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`.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust,ignore
    /// for x..=(y-1) { .. }
    /// ```
    /// Could be written as
    /// ```rust,ignore
    /// for x..y { .. }
    /// ```
    pub RANGE_MINUS_ONE,
    complexity,
    "`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 or equal to `y`.
    ///
    /// **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.
    ///
    /// **Known problems:** None.
    ///
    /// **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:
    /// ```rust
    /// fn main() {
    ///     (0..=10).rev().for_each(|x| println!("{}", x));
    ///
    ///     let arr = [1, 2, 3, 4, 5];
    ///     let sub = &arr[1..3];
    /// }
    /// ```
    pub REVERSED_EMPTY_RANGES,
    correctness,
    "reversing the limits of range expressions, resulting in empty ranges"
}

declare_lint_pass!(Ranges => [
    RANGE_ZIP_WITH_LEN,
    RANGE_PLUS_ONE,
    RANGE_MINUS_ONE,
    REVERSED_EMPTY_RANGES,
]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Ranges {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
        if let ExprKind::MethodCall(ref path, _, ref args) = expr.kind {
            let name = path.ident.as_str();
            if name == "zip" && args.len() == 2 {
                let iter = &args[0].kind;
                let zip_arg = &args[1];
                if_chain! {
                    // `.iter()` call
                    if let ExprKind::MethodCall(ref iter_path, _, ref iter_args ) = *iter;
                    if iter_path.ident.name == sym!(iter);
                    // range expression in `.zip()` call: `0..x.len()`
                    if let Some(higher::Range { start: Some(start), end: Some(end), .. }) = higher::range(cx, zip_arg);
                    if is_integer_const(cx, start, 0);
                    // `.len()` call
                    if let ExprKind::MethodCall(ref len_path, _, ref len_args) = end.kind;
                    if len_path.ident.name == sym!(len) && len_args.len() == 1;
                    // `.iter()` and `.len()` called on same `Path`
                    if let ExprKind::Path(QPath::Resolved(_, ref iter_path)) = iter_args[0].kind;
                    if let ExprKind::Path(QPath::Resolved(_, ref len_path)) = len_args[0].kind;
                    if SpanlessEq::new(cx).eq_path_segments(&iter_path.segments, &len_path.segments);
                     then {
                         span_lint(cx,
                                   RANGE_ZIP_WITH_LEN,
                                   expr.span,
                                   &format!("It is more idiomatic to use `{}.iter().enumerate()`",
                                            snippet(cx, iter_args[0].span, "_")));
                    }
                }
            }
        }

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

// exclusive range plus one: `x..(y+1)`
fn check_exclusive_range_plus_one(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
    if_chain! {
        if let Some(higher::Range {
            start,
            end: Some(end),
            limits: RangeLimits::HalfOpen
        }) = higher::range(cx, expr);
        if let Some(y) = y_plus_one(cx, end);
        then {
            let span = if expr.span.from_expansion() {
                expr.span
                    .ctxt()
                    .outer_expn_data()
                    .call_site
            } else {
                expr.span
            };
            span_lint_and_then(
                cx,
                RANGE_PLUS_ONE,
                span,
                "an inclusive range would be more readable",
                |diag| {
                    let start = start.map_or(String::new(), |x| Sugg::hir(cx, x, "x").to_string());
                    let end = Sugg::hir(cx, y, "y");
                    if let Some(is_wrapped) = &snippet_opt(cx, span) {
                        if is_wrapped.starts_with('(') && is_wrapped.ends_with(')') {
                            diag.span_suggestion(
                                span,
                                "use",
                                format!("({}..={})", start, end),
                                Applicability::MaybeIncorrect,
                            );
                        } else {
                            diag.span_suggestion(
                                span,
                                "use",
                                format!("{}..={}", start, end),
                                Applicability::MachineApplicable, // snippet
                            );
                        }
                    }
                },
            );
        }
    }
}

// inclusive range minus one: `x..=(y-1)`
fn check_inclusive_range_minus_one(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
    if_chain! {
        if let Some(higher::Range { start, end: Some(end), limits: RangeLimits::Closed }) = higher::range(cx, expr);
        if let Some(y) = y_minus_one(cx, end);
        then {
            span_lint_and_then(
                cx,
                RANGE_MINUS_ONE,
                expr.span,
                "an exclusive range would be more readable",
                |diag| {
                    let start = start.map_or(String::new(), |x| Sugg::hir(cx, x, "x").to_string());
                    let end = Sugg::hir(cx, y, "y");
                    diag.span_suggestion(
                        expr.span,
                        "use",
                        format!("{}..{}", start, end),
                        Applicability::MachineApplicable, // snippet
                    );
                },
            );
        }
    }
}

fn check_reversed_empty_range(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
    fn inside_indexing_expr<'a>(cx: &'a LateContext<'_, '_>, expr: &Expr<'_>) -> Option<&'a Expr<'a>> {
        match get_parent_expr(cx, expr) {
            parent_expr @ Some(Expr {
                kind: ExprKind::Index(..),
                ..
            }) => parent_expr,
            _ => None,
        }
    }

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

    if_chain! {
        if let Some(higher::Range { start: Some(start), end: Some(end), limits }) = higher::range(cx, expr);
        let ty = cx.tables.expr_ty(start);
        if let ty::Int(_) | ty::Uint(_) = ty.kind;
        if let Some((start_idx, _)) = constant(cx, cx.tables, start);
        if let Some((end_idx, _)) = constant(cx, cx.tables, end);
        if let Some(ordering) = Constant::partial_cmp(cx.tcx, ty, &start_idx, &end_idx);
        if is_empty_range(limits, ordering);
        then {
            if let Some(parent_expr) = inside_indexing_expr(cx, expr) {
                let (reason, outcome) = if ordering == Ordering::Equal {
                    ("empty", "always yield an empty slice")
                } else {
                    ("reversed", "panic at run-time")
                };

                span_lint_and_then(
                    cx,
                    REVERSED_EMPTY_RANGES,
                    expr.span,
                    &format!("this range is {} and using it to index a slice will {}", reason, outcome),
                    |diag| {
                        if_chain! {
                            if ordering == Ordering::Equal;
                            if let ty::Slice(slice_ty) = cx.tables.expr_ty(parent_expr).kind;
                            then {
                                diag.span_suggestion(
                                    parent_expr.span,
                                    "if you want an empty slice, use",
                                    format!("[] as &[{}]", slice_ty),
                                    Applicability::MaybeIncorrect
                                );
                            }
                        }
                    }
                );
            } else {
                span_lint_and_then(
                    cx,
                    REVERSED_EMPTY_RANGES,
                    expr.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(
                                expr.span,
                                "consider using the following if you are attempting to iterate over this \
                                 range in reverse",
                                format!("({}{}{}).rev()", end_snippet, dots, start_snippet),
                                Applicability::MaybeIncorrect,
                            );
                        }
                    },
                );
            }
        }
    }
}

fn y_plus_one<'t>(cx: &LateContext<'_, '_>, expr: &'t Expr<'_>) -> Option<&'t Expr<'t>> {
    match expr.kind {
        ExprKind::Binary(
            Spanned {
                node: BinOpKind::Add, ..
            },
            ref lhs,
            ref 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<'t>(cx: &LateContext<'_, '_>, expr: &'t Expr<'_>) -> Option<&'t Expr<'t>> {
    match expr.kind {
        ExprKind::Binary(
            Spanned {
                node: BinOpKind::Sub, ..
            },
            ref lhs,
            ref rhs,
        ) if is_integer_const(cx, rhs, 1) => Some(lhs),
        _ => None,
    }
}