rust/clippy_lints/src/ranges.rs

use rustc::lint::*;
use rustc::hir::*;
use syntax::codemap::Spanned;
use utils::{is_integer_literal, match_type, paths, snippet, span_lint, unsugar_range, UnsugaredRange};

/// **What it does:** This lint checks for iterating over ranges with a `.step_by(0)`, which never terminates.
///
/// **Why is this bad?** This very much looks like an oversight, since with `loop { .. }` there is an obvious better way to endlessly loop.
///
/// **Known problems:** None
///
/// **Example:** `for x in (5..5).step_by(0) { .. }`
declare_lint! {
    pub RANGE_STEP_BY_ZERO, Warn,
    "using Range::step_by(0), which produces an infinite iterator"
}
/// **What it does:** This lint 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:** `x.iter().zip(0..x.len())`
declare_lint! {
    pub RANGE_ZIP_WITH_LEN, Warn,
    "zipping iterator with a range when enumerate() would do"
}

#[derive(Copy,Clone)]
pub struct StepByZero;

impl LintPass for StepByZero {
    fn get_lints(&self) -> LintArray {
        lint_array!(RANGE_STEP_BY_ZERO, RANGE_ZIP_WITH_LEN)
    }
}

impl LateLintPass for StepByZero {
    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
        if let ExprMethodCall(Spanned { node: ref name, .. }, _, ref args) = expr.node {
            // Range with step_by(0).
            if name.as_str() == "step_by" && args.len() == 2 && has_step_by(cx, &args[0]) &&
               is_integer_literal(&args[1], 0) {
                span_lint(cx,
                          RANGE_STEP_BY_ZERO,
                          expr.span,
                          "Range::step_by(0) produces an infinite iterator. Consider using `std::iter::repeat()` \
                           instead");
            } else if name.as_str() == "zip" && args.len() == 2 {
                let iter = &args[0].node;
                let zip_arg = &args[1];
                if_let_chain! {
                    [
                        // .iter() call
                        let ExprMethodCall( Spanned { node: ref iter_name, .. }, _, ref iter_args ) = *iter,
                        iter_name.as_str() == "iter",
                        // range expression in .zip() call: 0..x.len()
                        let Some(UnsugaredRange { start: Some(ref start), end: Some(ref end), .. }) = unsugar_range(zip_arg),
                        is_integer_literal(start, 0),
                        // .len() call
                        let ExprMethodCall(Spanned { node: ref len_name, .. }, _, ref len_args) = end.node,
                        len_name.as_str() == "len" && len_args.len() == 1,
                        // .iter() and .len() called on same Path
                        let ExprPath(_, Path { segments: ref iter_path, .. }) = iter_args[0].node,
                        let ExprPath(_, Path { segments: ref len_path, .. }) = len_args[0].node,
                        iter_path == len_path
                     ], {
                        span_lint(cx,
                                  RANGE_ZIP_WITH_LEN,
                                  expr.span,
                                  &format!("It is more idiomatic to use {}.iter().enumerate()",
                                           snippet(cx, iter_args[0].span, "_")));
                    }
                }
            }
        }
    }
}

fn has_step_by(cx: &LateContext, expr: &Expr) -> bool {
    // No need for walk_ptrs_ty here because step_by moves self, so it
    // can't be called on a borrowed range.
    let ty = cx.tcx.expr_ty(expr);

    // Note: `RangeTo`, `RangeToInclusive` and `RangeFull` don't have step_by
    match_type(cx, ty, &paths::RANGE)
        || match_type(cx, ty, &paths::RANGE_FROM)
        || match_type(cx, ty, &paths::RANGE_INCLUSIVE)
}