rust/clippy_lints/src/bytecount.rs

use rustc::hir::*;
use rustc::lint::*;
use rustc::ty;
use syntax::ast::{Name, UintTy};
use utils::{contains_name, get_pat_name, match_type, paths, single_segment_path,
            snippet, span_lint_and_sugg, walk_ptrs_ty};

/// **What it does:** Checks for naive byte counts
///
/// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
/// crate has methods to count your bytes faster, especially for large slices.
///
/// **Known problems:** If you have predominantly small slices, the
/// `bytecount::count(..)` method may actually be slower. However, if you can
/// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
/// faster in those cases.
///
/// **Example:**
///
/// ```rust
/// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
/// ```
declare_lint! {
    pub NAIVE_BYTECOUNT,
    Warn,
    "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
}

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

impl LintPass for ByteCount {
    fn get_lints(&self) -> LintArray {
        lint_array!(NAIVE_BYTECOUNT)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
        if_let_chain!([
            let ExprMethodCall(ref count, _, ref count_args) = expr.node,
            count.name == "count",
            count_args.len() == 1,
            let ExprMethodCall(ref filter, _, ref filter_args) = count_args[0].node,
            filter.name == "filter",
            filter_args.len() == 2,
            let ExprClosure(_, _, body_id, _, _) = filter_args[1].node,
        ], {
            let body = cx.tcx.hir.body(body_id);
            if_let_chain!([
                body.arguments.len() == 1,
                let Some(argname) = get_pat_name(&body.arguments[0].pat),
                let ExprBinary(ref op, ref l, ref r) = body.value.node,
                op.node == BiEq,
                match_type(cx,
                           walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
                           &paths::SLICE_ITER),
            ], {
                let needle = match get_path_name(l) {
                    Some(name) if check_arg(name, argname, r) => r,
                    _ => match get_path_name(r) {
                        Some(name) if check_arg(name, argname, l) => l,
                        _ => { return; }
                    }
                };
                if ty::TyUint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
                    return;
                }
                let haystack = if let ExprMethodCall(ref path, _, ref args) =
                        filter_args[0].node {
                    let p = path.name;
                    if (p == "iter" || p == "iter_mut") && args.len() == 1 {
                        &args[0]
                    } else {
                        &filter_args[0]
                    }
                } else {
                    &filter_args[0]
                };
                span_lint_and_sugg(cx,
                                   NAIVE_BYTECOUNT,
                                   expr.span,
                                   "You appear to be counting bytes the naive way",
                                   "Consider using the bytecount crate",
                                   format!("bytecount::count({}, {})",
                                            snippet(cx, haystack.span, ".."),
                                            snippet(cx, needle.span, "..")));
            });
        });
    }
}

fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
    name == arg && !contains_name(name, needle)
}

fn get_path_name(expr: &Expr) -> Option<Name> {
    match expr.node {
        ExprBox(ref e) | ExprAddrOf(_, ref e) | ExprUnary(UnOp::UnDeref, ref e) => get_path_name(e),
        ExprBlock(ref b) => if b.stmts.is_empty() {
            b.expr.as_ref().and_then(|p| get_path_name(p))
        } else {
            None
        },
        ExprPath(ref qpath) => single_segment_path(qpath).map(|ps| ps.name),
        _ => None,
    }
}