use rustc::hir::*; use rustc::lint::*; use rustc::ty; use syntax::ast::{Name, UintTy}; use utils::{contains_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_pat_name(pat: &Pat) -> Option<Name> { match pat.node { PatKind::Binding(_, _, ref spname, _) => Some(spname.node), PatKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.name), PatKind::Box(ref p) | PatKind::Ref(ref p, _) => get_pat_name(&*p), _ => None, } } 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, } }