mirror of
https://github.com/rust-lang/rust-clippy
synced 2024-12-22 19:13:29 +00:00
660 lines
24 KiB
Rust
660 lines
24 KiB
Rust
use std::collections::VecDeque;
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use clippy_utils::diagnostics::span_lint_and_sugg;
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use clippy_utils::is_lint_allowed;
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use itertools::{izip, Itertools};
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use rustc_ast::{walk_list, Label, Mutability};
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use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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use rustc_errors::Applicability;
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use rustc_hir::def::{DefKind, Res};
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use rustc_hir::def_id::DefId;
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use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
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use rustc_hir::intravisit::{walk_expr, walk_stmt, FnKind, Visitor};
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use rustc_hir::{
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Arm, Block, Body, Closure, Expr, ExprKind, Guard, HirId, ImplicitSelfKind, Let, Local, Pat, PatKind, Path,
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PathSegment, QPath, Stmt, StmtKind, TyKind, UnOp,
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};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::ty;
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use rustc_middle::ty::{Ty, TyCtxt, TypeckResults};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::symbol::kw;
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use rustc_span::symbol::Ident;
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use rustc_span::Span;
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declare_clippy_lint! {
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/// ### What it does
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/// Checks for arguments that are only used in recursion with no side-effects.
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///
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/// ### Why is this bad?
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/// It could contain a useless calculation and can make function simpler.
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///
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/// The arguments can be involved in calculations and assignments but as long as
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/// the calculations have no side-effects (function calls or mutating dereference)
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/// and the assigned variables are also only in recursion, it is useless.
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///
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/// ### Known problems
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/// Too many code paths in the linting code are currently untested and prone to produce false
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/// positives or are prone to have performance implications.
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///
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/// In some cases, this would not catch all useless arguments.
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///
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/// ```rust
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/// fn foo(a: usize, b: usize) -> usize {
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/// let f = |x| x + 1;
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///
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/// if a == 0 {
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/// 1
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/// } else {
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/// foo(a - 1, f(b))
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/// }
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/// }
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/// ```
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///
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/// For example, the argument `b` is only used in recursion, but the lint would not catch it.
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///
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/// List of some examples that can not be caught:
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/// - binary operation of non-primitive types
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/// - closure usage
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/// - some `break` relative operations
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/// - struct pattern binding
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///
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/// Also, when you recurse the function name with path segments, it is not possible to detect.
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///
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/// ### Example
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/// ```rust
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/// fn f(a: usize, b: usize) -> usize {
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/// if a == 0 {
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/// 1
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/// } else {
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/// f(a - 1, b + 1)
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/// }
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/// }
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/// # fn main() {
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/// # print!("{}", f(1, 1));
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/// # }
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/// ```
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/// Use instead:
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/// ```rust
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/// fn f(a: usize) -> usize {
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/// if a == 0 {
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/// 1
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/// } else {
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/// f(a - 1)
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/// }
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/// }
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/// # fn main() {
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/// # print!("{}", f(1));
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/// # }
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/// ```
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#[clippy::version = "1.61.0"]
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pub ONLY_USED_IN_RECURSION,
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nursery,
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"arguments that is only used in recursion can be removed"
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}
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declare_lint_pass!(OnlyUsedInRecursion => [ONLY_USED_IN_RECURSION]);
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impl<'tcx> LateLintPass<'tcx> for OnlyUsedInRecursion {
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fn check_fn(
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&mut self,
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cx: &LateContext<'tcx>,
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kind: FnKind<'tcx>,
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decl: &'tcx rustc_hir::FnDecl<'tcx>,
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body: &'tcx Body<'tcx>,
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_: Span,
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id: HirId,
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) {
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if is_lint_allowed(cx, ONLY_USED_IN_RECURSION, id) {
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return;
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}
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if let FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) = kind {
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let def_id = id.owner.to_def_id();
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let data = cx.tcx.def_path(def_id).data;
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if data.len() > 1 {
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match data.get(data.len() - 2) {
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Some(DisambiguatedDefPathData {
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data: DefPathData::Impl,
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disambiguator,
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}) if *disambiguator != 0 => return,
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_ => {},
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}
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}
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let has_self = !matches!(decl.implicit_self, ImplicitSelfKind::None);
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let ty_res = cx.typeck_results();
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let param_span = body
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.params
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.iter()
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.flat_map(|param| {
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let mut v = Vec::new();
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param.pat.each_binding(|_, hir_id, span, ident| {
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v.push((hir_id, span, ident));
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});
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v
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})
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.skip(if has_self { 1 } else { 0 })
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.filter(|(_, _, ident)| !ident.name.as_str().starts_with('_'))
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.collect_vec();
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let params = body.params.iter().map(|param| param.pat).collect();
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let mut visitor = SideEffectVisit {
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graph: FxHashMap::default(),
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has_side_effect: FxHashSet::default(),
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ret_vars: Vec::new(),
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contains_side_effect: false,
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break_vars: FxHashMap::default(),
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params,
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fn_ident: ident,
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fn_def_id: def_id,
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is_method: matches!(kind, FnKind::Method(..)),
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has_self,
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ty_res,
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tcx: cx.tcx,
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visited_exprs: FxHashSet::default(),
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};
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visitor.visit_expr(&body.value);
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let vars = std::mem::take(&mut visitor.ret_vars);
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// this would set the return variables to side effect
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visitor.add_side_effect(vars);
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let mut queue = visitor.has_side_effect.iter().copied().collect::<VecDeque<_>>();
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// a simple BFS to check all the variables that have side effect
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while let Some(id) = queue.pop_front() {
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if let Some(next) = visitor.graph.get(&id) {
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for i in next {
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if !visitor.has_side_effect.contains(i) {
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visitor.has_side_effect.insert(*i);
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queue.push_back(*i);
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}
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}
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}
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}
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for (id, span, ident) in param_span {
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// if the variable is not used in recursion, it would be marked as unused
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if !visitor.has_side_effect.contains(&id) {
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let mut queue = VecDeque::new();
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let mut visited = FxHashSet::default();
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queue.push_back(id);
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// a simple BFS to check the graph can reach to itself
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// if it can't, it means the variable is never used in recursion
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while let Some(id) = queue.pop_front() {
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if let Some(next) = visitor.graph.get(&id) {
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for i in next {
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if !visited.contains(i) {
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visited.insert(id);
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queue.push_back(*i);
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}
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}
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}
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}
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if visited.contains(&id) {
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span_lint_and_sugg(
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cx,
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ONLY_USED_IN_RECURSION,
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span,
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"parameter is only used in recursion",
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"if this is intentional, prefix with an underscore",
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format!("_{}", ident.name.as_str()),
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Applicability::MaybeIncorrect,
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);
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}
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}
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}
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}
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}
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}
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pub fn is_primitive(ty: Ty<'_>) -> bool {
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let ty = ty.peel_refs();
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ty.is_primitive() || ty.is_str()
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}
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pub fn is_array(ty: Ty<'_>) -> bool {
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let ty = ty.peel_refs();
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ty.is_array() || ty.is_array_slice()
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}
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/// This builds the graph of side effect.
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/// The edge `a -> b` means if `a` has side effect, `b` will have side effect.
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///
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/// There are some example in following code:
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/// ```rust, ignore
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/// let b = 1;
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/// let a = b; // a -> b
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/// let (c, d) = (a, b); // c -> b, d -> b
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///
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/// let e = if a == 0 { // e -> a
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/// c // e -> c
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/// } else {
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/// d // e -> d
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/// };
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/// ```
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pub struct SideEffectVisit<'tcx> {
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graph: FxHashMap<HirId, FxHashSet<HirId>>,
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has_side_effect: FxHashSet<HirId>,
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// bool for if the variable was dereferenced from mutable reference
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ret_vars: Vec<(HirId, bool)>,
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contains_side_effect: bool,
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// break label
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break_vars: FxHashMap<Ident, Vec<(HirId, bool)>>,
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params: Vec<&'tcx Pat<'tcx>>,
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fn_ident: Ident,
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fn_def_id: DefId,
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is_method: bool,
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has_self: bool,
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ty_res: &'tcx TypeckResults<'tcx>,
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tcx: TyCtxt<'tcx>,
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visited_exprs: FxHashSet<HirId>,
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}
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impl<'tcx> Visitor<'tcx> for SideEffectVisit<'tcx> {
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fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
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match s.kind {
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StmtKind::Local(Local {
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pat, init: Some(init), ..
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}) => {
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self.visit_pat_expr(pat, init, false);
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},
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StmtKind::Item(_) | StmtKind::Expr(_) | StmtKind::Semi(_) => {
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walk_stmt(self, s);
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},
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StmtKind::Local(_) => {},
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}
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self.ret_vars.clear();
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}
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fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
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if !self.visited_exprs.insert(ex.hir_id) {
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return;
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}
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match ex.kind {
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ExprKind::Array(exprs) | ExprKind::Tup(exprs) => {
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self.ret_vars = exprs
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.iter()
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.flat_map(|expr| {
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self.visit_expr(expr);
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std::mem::take(&mut self.ret_vars)
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})
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.collect();
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},
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ExprKind::Call(callee, args) => self.visit_fn(callee, args),
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ExprKind::MethodCall(path, args, _) => self.visit_method_call(path, args),
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ExprKind::Binary(_, lhs, rhs) => {
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self.visit_bin_op(lhs, rhs);
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},
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ExprKind::Unary(op, expr) => self.visit_un_op(op, expr),
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ExprKind::Let(Let { pat, init, .. }) => self.visit_pat_expr(pat, init, false),
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ExprKind::If(bind, then_expr, else_expr) => {
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self.visit_if(bind, then_expr, else_expr);
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},
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ExprKind::Match(expr, arms, _) => self.visit_match(expr, arms),
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// since analysing the closure is not easy, just set all variables in it to side-effect
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ExprKind::Closure(&Closure { body, .. }) => {
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let body = self.tcx.hir().body(body);
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self.visit_body(body);
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let vars = std::mem::take(&mut self.ret_vars);
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self.add_side_effect(vars);
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},
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ExprKind::Loop(block, label, _, _) | ExprKind::Block(block, label) => {
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self.visit_block_label(block, label);
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},
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ExprKind::Assign(bind, expr, _) => {
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self.visit_assign(bind, expr);
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},
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ExprKind::AssignOp(_, bind, expr) => {
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self.visit_assign(bind, expr);
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self.visit_bin_op(bind, expr);
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},
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ExprKind::Field(expr, _) => {
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self.visit_expr(expr);
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if matches!(self.ty_res.expr_ty(expr).kind(), ty::Ref(_, _, Mutability::Mut)) {
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self.ret_vars.iter_mut().for_each(|(_, b)| *b = true);
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}
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},
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ExprKind::Index(expr, index) => {
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self.visit_expr(expr);
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let mut vars = std::mem::take(&mut self.ret_vars);
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self.visit_expr(index);
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self.ret_vars.append(&mut vars);
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if !is_array(self.ty_res.expr_ty(expr)) {
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self.add_side_effect(self.ret_vars.clone());
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} else if matches!(self.ty_res.expr_ty(expr).kind(), ty::Ref(_, _, Mutability::Mut)) {
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self.ret_vars.iter_mut().for_each(|(_, b)| *b = true);
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}
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},
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ExprKind::Break(dest, Some(expr)) => {
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self.visit_expr(expr);
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if let Some(label) = dest.label {
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self.break_vars
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.entry(label.ident)
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.or_insert(Vec::new())
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.append(&mut self.ret_vars);
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}
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self.contains_side_effect = true;
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},
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ExprKind::Ret(Some(expr)) => {
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self.visit_expr(expr);
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let vars = std::mem::take(&mut self.ret_vars);
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self.add_side_effect(vars);
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self.contains_side_effect = true;
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},
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ExprKind::Break(_, None) | ExprKind::Continue(_) | ExprKind::Ret(None) => {
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self.contains_side_effect = true;
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},
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ExprKind::Struct(_, exprs, expr) => {
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let mut ret_vars = exprs
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.iter()
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.flat_map(|field| {
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self.visit_expr(field.expr);
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std::mem::take(&mut self.ret_vars)
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})
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.collect();
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walk_list!(self, visit_expr, expr);
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self.ret_vars.append(&mut ret_vars);
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},
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_ => walk_expr(self, ex),
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}
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}
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fn visit_path(&mut self, path: &'tcx Path<'tcx>, _id: HirId) {
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if let Res::Local(id) = path.res {
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self.ret_vars.push((id, false));
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}
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}
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}
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impl<'tcx> SideEffectVisit<'tcx> {
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fn visit_assign(&mut self, lhs: &'tcx Expr<'tcx>, rhs: &'tcx Expr<'tcx>) {
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// Just support array and tuple unwrapping for now.
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//
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// ex) `(a, b) = (c, d);`
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// The graph would look like this:
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// a -> c
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// b -> d
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//
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// This would minimize the connection of the side-effect graph.
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match (&lhs.kind, &rhs.kind) {
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(ExprKind::Array(lhs), ExprKind::Array(rhs)) | (ExprKind::Tup(lhs), ExprKind::Tup(rhs)) => {
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// if not, it is a compile error
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debug_assert!(lhs.len() == rhs.len());
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izip!(*lhs, *rhs).for_each(|(lhs, rhs)| self.visit_assign(lhs, rhs));
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},
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// in other assigns, we have to connect all each other
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// because they can be connected somehow
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_ => {
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self.visit_expr(lhs);
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let lhs_vars = std::mem::take(&mut self.ret_vars);
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self.visit_expr(rhs);
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let rhs_vars = std::mem::take(&mut self.ret_vars);
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self.connect_assign(&lhs_vars, &rhs_vars, false);
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},
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}
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}
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fn visit_block_label(&mut self, block: &'tcx Block<'tcx>, label: Option<Label>) {
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self.visit_block(block);
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let _ = label.and_then(|label| {
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self.break_vars
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.remove(&label.ident)
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.map(|mut break_vars| self.ret_vars.append(&mut break_vars))
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});
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}
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fn visit_bin_op(&mut self, lhs: &'tcx Expr<'tcx>, rhs: &'tcx Expr<'tcx>) {
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self.visit_expr(lhs);
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let mut ret_vars = std::mem::take(&mut self.ret_vars);
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self.visit_expr(rhs);
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self.ret_vars.append(&mut ret_vars);
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// the binary operation between non primitive values are overloaded operators
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// so they can have side-effects
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if !is_primitive(self.ty_res.expr_ty(lhs)) || !is_primitive(self.ty_res.expr_ty(rhs)) {
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self.ret_vars.iter().for_each(|id| {
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self.has_side_effect.insert(id.0);
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});
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self.contains_side_effect = true;
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}
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}
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fn visit_un_op(&mut self, op: UnOp, expr: &'tcx Expr<'tcx>) {
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self.visit_expr(expr);
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let ty = self.ty_res.expr_ty(expr);
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// dereferencing a reference has no side-effect
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if !is_primitive(ty) && !matches!((op, ty.kind()), (UnOp::Deref, ty::Ref(..))) {
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self.add_side_effect(self.ret_vars.clone());
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}
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if matches!((op, ty.kind()), (UnOp::Deref, ty::Ref(_, _, Mutability::Mut))) {
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self.ret_vars.iter_mut().for_each(|(_, b)| *b = true);
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}
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}
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fn visit_pat_expr(&mut self, pat: &'tcx Pat<'tcx>, expr: &'tcx Expr<'tcx>, connect_self: bool) {
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match (&pat.kind, &expr.kind) {
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(PatKind::Tuple(pats, _), ExprKind::Tup(exprs)) => {
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self.ret_vars = izip!(*pats, *exprs)
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.flat_map(|(pat, expr)| {
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self.visit_pat_expr(pat, expr, connect_self);
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std::mem::take(&mut self.ret_vars)
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})
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.collect();
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},
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(PatKind::Slice(front_exprs, _, back_exprs), ExprKind::Array(exprs)) => {
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let mut vars = izip!(*front_exprs, *exprs)
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.flat_map(|(pat, expr)| {
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self.visit_pat_expr(pat, expr, connect_self);
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std::mem::take(&mut self.ret_vars)
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})
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.collect();
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self.ret_vars = izip!(back_exprs.iter().rev(), exprs.iter().rev())
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.flat_map(|(pat, expr)| {
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self.visit_pat_expr(pat, expr, connect_self);
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std::mem::take(&mut self.ret_vars)
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})
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.collect();
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self.ret_vars.append(&mut vars);
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},
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_ => {
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let mut lhs_vars = Vec::new();
|
|
pat.each_binding(|_, id, _, _| lhs_vars.push((id, false)));
|
|
self.visit_expr(expr);
|
|
let rhs_vars = std::mem::take(&mut self.ret_vars);
|
|
self.connect_assign(&lhs_vars, &rhs_vars, connect_self);
|
|
self.ret_vars = rhs_vars;
|
|
},
|
|
}
|
|
}
|
|
|
|
fn visit_fn(&mut self, callee: &'tcx Expr<'tcx>, args: &'tcx [Expr<'tcx>]) {
|
|
self.visit_expr(callee);
|
|
let mut ret_vars = std::mem::take(&mut self.ret_vars);
|
|
self.add_side_effect(ret_vars.clone());
|
|
|
|
let mut is_recursive = false;
|
|
|
|
if_chain! {
|
|
if !self.has_self;
|
|
if let ExprKind::Path(QPath::Resolved(_, path)) = callee.kind;
|
|
if let Res::Def(DefKind::Fn, def_id) = path.res;
|
|
if self.fn_def_id == def_id;
|
|
then {
|
|
is_recursive = true;
|
|
}
|
|
}
|
|
|
|
if_chain! {
|
|
if !self.has_self && self.is_method;
|
|
if let ExprKind::Path(QPath::TypeRelative(ty, segment)) = callee.kind;
|
|
if segment.ident == self.fn_ident;
|
|
if let TyKind::Path(QPath::Resolved(_, path)) = ty.kind;
|
|
if let Res::SelfTy{ .. } = path.res;
|
|
then {
|
|
is_recursive = true;
|
|
}
|
|
}
|
|
|
|
if is_recursive {
|
|
izip!(self.params.clone(), args).for_each(|(pat, expr)| {
|
|
self.visit_pat_expr(pat, expr, true);
|
|
self.ret_vars.clear();
|
|
});
|
|
} else {
|
|
// This would set arguments used in closure that does not have side-effect.
|
|
// Closure itself can be detected whether there is a side-effect, but the
|
|
// value of variable that is holding closure can change.
|
|
// So, we just check the variables.
|
|
self.ret_vars = args
|
|
.iter()
|
|
.flat_map(|expr| {
|
|
self.visit_expr(expr);
|
|
std::mem::take(&mut self.ret_vars)
|
|
})
|
|
.collect_vec()
|
|
.into_iter()
|
|
.map(|id| {
|
|
self.has_side_effect.insert(id.0);
|
|
id
|
|
})
|
|
.collect();
|
|
self.contains_side_effect = true;
|
|
}
|
|
|
|
self.ret_vars.append(&mut ret_vars);
|
|
}
|
|
|
|
fn visit_method_call(&mut self, path: &'tcx PathSegment<'tcx>, args: &'tcx [Expr<'tcx>]) {
|
|
if_chain! {
|
|
if self.is_method;
|
|
if path.ident == self.fn_ident;
|
|
if let ExprKind::Path(QPath::Resolved(_, path)) = args.first().unwrap().kind;
|
|
if let Res::Local(..) = path.res;
|
|
let ident = path.segments.last().unwrap().ident;
|
|
if ident.name == kw::SelfLower;
|
|
then {
|
|
izip!(self.params.clone(), args.iter())
|
|
.for_each(|(pat, expr)| {
|
|
self.visit_pat_expr(pat, expr, true);
|
|
self.ret_vars.clear();
|
|
});
|
|
} else {
|
|
self.ret_vars = args
|
|
.iter()
|
|
.flat_map(|expr| {
|
|
self.visit_expr(expr);
|
|
std::mem::take(&mut self.ret_vars)
|
|
})
|
|
.collect_vec()
|
|
.into_iter()
|
|
.map(|a| {
|
|
self.has_side_effect.insert(a.0);
|
|
a
|
|
})
|
|
.collect();
|
|
self.contains_side_effect = true;
|
|
}
|
|
}
|
|
}
|
|
|
|
fn visit_if(&mut self, bind: &'tcx Expr<'tcx>, then_expr: &'tcx Expr<'tcx>, else_expr: Option<&'tcx Expr<'tcx>>) {
|
|
let contains_side_effect = self.contains_side_effect;
|
|
self.contains_side_effect = false;
|
|
self.visit_expr(bind);
|
|
let mut vars = std::mem::take(&mut self.ret_vars);
|
|
self.visit_expr(then_expr);
|
|
let mut then_vars = std::mem::take(&mut self.ret_vars);
|
|
walk_list!(self, visit_expr, else_expr);
|
|
if self.contains_side_effect {
|
|
self.add_side_effect(vars.clone());
|
|
}
|
|
self.contains_side_effect |= contains_side_effect;
|
|
self.ret_vars.append(&mut vars);
|
|
self.ret_vars.append(&mut then_vars);
|
|
}
|
|
|
|
fn visit_match(&mut self, expr: &'tcx Expr<'tcx>, arms: &'tcx [Arm<'tcx>]) {
|
|
self.visit_expr(expr);
|
|
let mut expr_vars = std::mem::take(&mut self.ret_vars);
|
|
self.ret_vars = arms
|
|
.iter()
|
|
.flat_map(|arm| {
|
|
let contains_side_effect = self.contains_side_effect;
|
|
self.contains_side_effect = false;
|
|
// this would visit `expr` multiple times
|
|
// but couldn't think of a better way
|
|
self.visit_pat_expr(arm.pat, expr, false);
|
|
let mut vars = std::mem::take(&mut self.ret_vars);
|
|
let _ = arm.guard.as_ref().map(|guard| {
|
|
self.visit_expr(match guard {
|
|
Guard::If(expr) | Guard::IfLet(Let { init: expr, .. }) => expr,
|
|
});
|
|
vars.append(&mut self.ret_vars);
|
|
});
|
|
self.visit_expr(arm.body);
|
|
if self.contains_side_effect {
|
|
self.add_side_effect(vars.clone());
|
|
self.add_side_effect(expr_vars.clone());
|
|
}
|
|
self.contains_side_effect |= contains_side_effect;
|
|
vars.append(&mut self.ret_vars);
|
|
vars
|
|
})
|
|
.collect();
|
|
self.ret_vars.append(&mut expr_vars);
|
|
}
|
|
|
|
fn connect_assign(&mut self, lhs: &[(HirId, bool)], rhs: &[(HirId, bool)], connect_self: bool) {
|
|
// if mutable dereference is on assignment it can have side-effect
|
|
// (this can lead to parameter mutable dereference and change the original value)
|
|
// too hard to detect whether this value is from parameter, so this would all
|
|
// check mutable dereference assignment to side effect
|
|
lhs.iter().filter(|(_, b)| *b).for_each(|(id, _)| {
|
|
self.has_side_effect.insert(*id);
|
|
self.contains_side_effect = true;
|
|
});
|
|
|
|
// there is no connection
|
|
if lhs.is_empty() || rhs.is_empty() {
|
|
return;
|
|
}
|
|
|
|
// by connected rhs in cycle, the connections would decrease
|
|
// from `n * m` to `n + m`
|
|
// where `n` and `m` are length of `lhs` and `rhs`.
|
|
|
|
// unwrap is possible since rhs is not empty
|
|
let rhs_first = rhs.first().unwrap();
|
|
for (id, _) in lhs.iter() {
|
|
if connect_self || *id != rhs_first.0 {
|
|
self.graph
|
|
.entry(*id)
|
|
.or_insert_with(FxHashSet::default)
|
|
.insert(rhs_first.0);
|
|
}
|
|
}
|
|
|
|
let rhs = rhs.iter();
|
|
izip!(rhs.clone().cycle().skip(1), rhs).for_each(|(from, to)| {
|
|
if connect_self || from.0 != to.0 {
|
|
self.graph.entry(from.0).or_insert_with(FxHashSet::default).insert(to.0);
|
|
}
|
|
});
|
|
}
|
|
|
|
fn add_side_effect(&mut self, v: Vec<(HirId, bool)>) {
|
|
for (id, _) in v {
|
|
self.has_side_effect.insert(id);
|
|
self.contains_side_effect = true;
|
|
}
|
|
}
|
|
}
|