2 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
4 use rustc_ast::util::{classify, parser};
5 use rustc_ast::{ExprKind, StmtKind};
6 use rustc_errors::{fluent, pluralize, Applicability, MultiSpan};
8 use rustc_hir::def::{DefKind, Res};
9 use rustc_hir::def_id::DefId;
10 use rustc_infer::traits::util::elaborate_predicates_with_span;
11 use rustc_middle::ty::adjustment;
12 use rustc_middle::ty::{self, DefIdTree, Ty};
13 use rustc_span::symbol::Symbol;
14 use rustc_span::symbol::{kw, sym};
15 use rustc_span::{BytePos, Span};
19 /// The `unused_must_use` lint detects unused result of a type flagged as
25 /// fn returns_result() -> Result<(), ()> {
38 /// The `#[must_use]` attribute is an indicator that it is a mistake to
39 /// ignore the value. See [the reference] for more details.
41 /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
44 "unused result of a type flagged as `#[must_use]`",
45 report_in_external_macro
49 /// The `unused_results` lint checks for the unused result of an
50 /// expression in a statement.
54 /// ```rust,compile_fail
55 /// #![deny(unused_results)]
56 /// fn foo<T>() -> T { panic!() }
67 /// Ignoring the return value of a function may indicate a mistake. In
68 /// cases were it is almost certain that the result should be used, it is
69 /// recommended to annotate the function with the [`must_use` attribute].
70 /// Failure to use such a return value will trigger the [`unused_must_use`
71 /// lint] which is warn-by-default. The `unused_results` lint is
72 /// essentially the same, but triggers for *all* return values.
74 /// This lint is "allow" by default because it can be noisy, and may not be
75 /// an actual problem. For example, calling the `remove` method of a `Vec`
76 /// or `HashMap` returns the previous value, which you may not care about.
77 /// Using this lint would require explicitly ignoring or discarding such
80 /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
81 /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
84 "unused result of an expression in a statement"
87 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
89 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
90 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
91 let hir::StmtKind::Semi(expr) = s.kind else { return; };
93 if let hir::ExprKind::Ret(..) = expr.kind {
97 if let hir::ExprKind::Match(await_expr, _arms, hir::MatchSource::AwaitDesugar) = expr.kind
98 && let ty = cx.typeck_results().expr_ty(&await_expr)
99 && let ty::Opaque(future_def_id, _) = ty.kind()
100 && cx.tcx.ty_is_opaque_future(ty)
101 // FIXME: This also includes non-async fns that return `impl Future`.
102 && let async_fn_def_id = cx.tcx.parent(*future_def_id)
103 && check_must_use_def(
107 "output of future returned by ",
111 // We have a bare `foo().await;` on an opaque type from an async function that was
112 // annotated with `#[must_use]`.
116 let ty = cx.typeck_results().expr_ty(&expr);
118 let must_use_result = is_ty_must_use(cx, ty, &expr, expr.span);
119 let type_lint_emitted_or_suppressed = match must_use_result {
121 emit_must_use_untranslated(cx, &path, "", "", 1);
127 let fn_warned = check_fn_must_use(cx, expr);
129 if !fn_warned && type_lint_emitted_or_suppressed {
130 // We don't warn about unused unit or uninhabited types.
131 // (See https://github.com/rust-lang/rust/issues/43806 for details.)
135 let must_use_op = match expr.kind {
136 // Hardcoding operators here seemed more expedient than the
137 // refactoring that would be needed to look up the `#[must_use]`
138 // attribute which does exist on the comparison trait methods
139 hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
145 | hir::BinOpKind::Gt => Some("comparison"),
147 | hir::BinOpKind::Sub
148 | hir::BinOpKind::Div
149 | hir::BinOpKind::Mul
150 | hir::BinOpKind::Rem => Some("arithmetic operation"),
151 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
152 hir::BinOpKind::BitXor
153 | hir::BinOpKind::BitAnd
154 | hir::BinOpKind::BitOr
155 | hir::BinOpKind::Shl
156 | hir::BinOpKind::Shr => Some("bitwise operation"),
158 hir::ExprKind::AddrOf(..) => Some("borrow"),
159 hir::ExprKind::Unary(..) => Some("unary operation"),
163 let mut op_warned = false;
165 if let Some(must_use_op) = must_use_op {
166 cx.struct_span_lint(UNUSED_MUST_USE, expr.span, fluent::lint_unused_op, |lint| {
167 lint.set_arg("op", must_use_op)
168 .span_label(expr.span, fluent::label)
169 .span_suggestion_verbose(
170 expr.span.shrink_to_lo(),
173 Applicability::MachineApplicable,
179 if !(type_lint_emitted_or_suppressed || fn_warned || op_warned) {
180 cx.struct_span_lint(UNUSED_RESULTS, s.span, fluent::lint_unused_result, |lint| {
181 lint.set_arg("ty", ty)
185 fn check_fn_must_use(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
186 let maybe_def_id = match expr.kind {
187 hir::ExprKind::Call(ref callee, _) => {
189 hir::ExprKind::Path(ref qpath) => {
190 match cx.qpath_res(qpath, callee.hir_id) {
191 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
192 // `Res::Local` if it was a closure, for which we
193 // do not currently support must-use linting
200 hir::ExprKind::MethodCall(..) => {
201 cx.typeck_results().type_dependent_def_id(expr.hir_id)
205 if let Some(def_id) = maybe_def_id {
206 check_must_use_def(cx, def_id, expr.span, "return value of ", "")
212 /// A path through a type to a must_use source. Contains useful info for the lint.
215 /// Suppress must_use checking.
217 /// The root of the normal must_use lint with an optional message.
218 Def(Span, DefId, Option<Symbol>),
221 TraitObject(Box<Self>),
222 TupleElement(Vec<(usize, Self)>),
223 Array(Box<Self>, u64),
224 /// The root of the unused_closures lint.
226 /// The root of the unused_generators lint.
230 #[instrument(skip(cx, expr), level = "debug", ret)]
231 fn is_ty_must_use<'tcx>(
232 cx: &LateContext<'tcx>,
234 expr: &hir::Expr<'_>,
236 ) -> Option<MustUsePath> {
238 || !ty.is_inhabited_from(
240 cx.tcx.parent_module(expr.hir_id).to_def_id(),
244 return Some(MustUsePath::Suppressed);
248 ty::Adt(..) if ty.is_box() => {
249 let boxed_ty = ty.boxed_ty();
250 is_ty_must_use(cx, boxed_ty, expr, span)
251 .map(|inner| MustUsePath::Boxed(Box::new(inner)))
253 ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
254 ty::Opaque(def, _) => {
255 elaborate_predicates_with_span(
257 cx.tcx.explicit_item_bounds(def).iter().cloned(),
259 .filter_map(|obligation| {
260 // We only look at the `DefId`, so it is safe to skip the binder here.
261 if let ty::PredicateKind::Trait(ref poly_trait_predicate) =
262 obligation.predicate.kind().skip_binder()
264 let def_id = poly_trait_predicate.trait_ref.def_id;
266 is_def_must_use(cx, def_id, span)
271 .map(|inner| MustUsePath::Opaque(Box::new(inner)))
274 ty::Dynamic(binders, _, _) => binders
276 .filter_map(|predicate| {
277 if let ty::ExistentialPredicate::Trait(ref trait_ref) =
278 predicate.skip_binder()
280 let def_id = trait_ref.def_id;
281 is_def_must_use(cx, def_id, span)
285 .map(|inner| MustUsePath::TraitObject(Box::new(inner)))
289 let elem_exprs = if let hir::ExprKind::Tup(elem_exprs) = expr.kind {
290 debug_assert_eq!(elem_exprs.len(), tys.len());
296 // Default to `expr`.
297 let elem_exprs = elem_exprs.iter().chain(iter::repeat(expr));
299 let nested_must_use = tys
303 .filter_map(|(i, (ty, expr))| {
304 is_ty_must_use(cx, ty, expr, expr.span).map(|path| (i, path))
306 .collect::<Vec<_>>();
308 if !nested_must_use.is_empty() {
309 Some(MustUsePath::TupleElement(nested_must_use))
314 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
315 // If the array is empty we don't lint, to avoid false positives
316 Some(0) | None => None,
317 // If the array is definitely non-empty, we can do `#[must_use]` checking.
318 Some(len) => is_ty_must_use(cx, ty, expr, span)
319 .map(|inner| MustUsePath::Array(Box::new(inner), len)),
321 ty::Closure(..) => Some(MustUsePath::Closure(span)),
322 ty::Generator(def_id, ..) => {
323 // async fn should be treated as "implementor of `Future`"
324 let must_use = if matches!(
325 cx.tcx.generator_kind(def_id),
326 Some(hir::GeneratorKind::Async(..))
328 let def_id = cx.tcx.lang_items().future_trait().unwrap();
329 is_def_must_use(cx, def_id, span)
330 .map(|inner| MustUsePath::Opaque(Box::new(inner)))
334 must_use.or(Some(MustUsePath::Generator(span)))
340 fn is_def_must_use(cx: &LateContext<'_>, def_id: DefId, span: Span) -> Option<MustUsePath> {
341 if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
342 // check for #[must_use = "..."]
343 let reason = attr.value_str();
344 Some(MustUsePath::Def(span, def_id, reason))
350 // Returns whether further errors should be suppressed because either a lint has been emitted or the type should be ignored.
351 fn check_must_use_def(
352 cx: &LateContext<'_>,
355 descr_pre_path: &str,
356 descr_post_path: &str,
358 is_def_must_use(cx, def_id, span)
359 .map(|must_use_path| {
360 emit_must_use_untranslated(
371 #[instrument(skip(cx), level = "debug")]
372 fn emit_must_use_untranslated(
373 cx: &LateContext<'_>,
379 let plural_suffix = pluralize!(plural_len);
382 MustUsePath::Suppressed => {}
383 MustUsePath::Boxed(path) => {
384 let descr_pre = &format!("{}boxed ", descr_pre);
385 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
387 MustUsePath::Opaque(path) => {
388 let descr_pre = &format!("{}implementer{} of ", descr_pre, plural_suffix);
389 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
391 MustUsePath::TraitObject(path) => {
392 let descr_post = &format!(" trait object{}{}", plural_suffix, descr_post);
393 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
395 MustUsePath::TupleElement(elems) => {
396 for (index, path) in elems {
397 let descr_post = &format!(" in tuple element {}", index);
398 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
401 MustUsePath::Array(path, len) => {
402 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix);
403 emit_must_use_untranslated(
408 plural_len.saturating_add(usize::try_from(*len).unwrap_or(usize::MAX)),
411 MustUsePath::Closure(span) => {
415 fluent::lint_unused_closure,
417 // FIXME(davidtwco): this isn't properly translatable because of the
419 lint.set_arg("count", plural_len)
420 .set_arg("pre", descr_pre)
421 .set_arg("post", descr_post)
426 MustUsePath::Generator(span) => {
430 fluent::lint_unused_generator,
432 // FIXME(davidtwco): this isn't properly translatable because of the
434 lint.set_arg("count", plural_len)
435 .set_arg("pre", descr_pre)
436 .set_arg("post", descr_post)
441 MustUsePath::Def(span, def_id, reason) => {
442 cx.struct_span_lint(UNUSED_MUST_USE, *span, fluent::lint_unused_def, |lint| {
443 // FIXME(davidtwco): this isn't properly translatable because of the pre/post
445 lint.set_arg("pre", descr_pre);
446 lint.set_arg("post", descr_post);
447 lint.set_arg("def", cx.tcx.def_path_str(*def_id));
448 if let Some(note) = reason {
449 lint.note(note.as_str());
460 /// The `path_statements` lint detects path statements with no effect.
474 /// It is usually a mistake to have a statement that has no effect.
477 "path statements with no effect"
480 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
482 impl<'tcx> LateLintPass<'tcx> for PathStatements {
483 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
484 if let hir::StmtKind::Semi(expr) = s.kind {
485 if let hir::ExprKind::Path(_) = expr.kind {
486 let ty = cx.typeck_results().expr_ty(expr);
487 if ty.needs_drop(cx.tcx, cx.param_env) {
491 fluent::lint_path_statement_drop,
493 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
494 lint.span_suggestion(
497 format!("drop({});", snippet),
498 Applicability::MachineApplicable,
501 lint.span_help(s.span, fluent::suggestion);
510 fluent::lint_path_statement_no_effect,
519 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
520 enum UnusedDelimsCtx {
524 AssignedValueLetElse,
537 impl From<UnusedDelimsCtx> for &'static str {
538 fn from(ctx: UnusedDelimsCtx) -> &'static str {
540 UnusedDelimsCtx::FunctionArg => "function argument",
541 UnusedDelimsCtx::MethodArg => "method argument",
542 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
545 UnusedDelimsCtx::IfCond => "`if` condition",
546 UnusedDelimsCtx::WhileCond => "`while` condition",
547 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
548 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
549 UnusedDelimsCtx::ReturnValue => "`return` value",
550 UnusedDelimsCtx::BlockRetValue => "block return value",
551 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
552 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
553 UnusedDelimsCtx::MatchArmExpr => "match arm expression",
558 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
559 trait UnusedDelimLint {
560 const DELIM_STR: &'static str;
562 /// Due to `ref` pattern, there can be a difference between using
563 /// `{ expr }` and `expr` in pattern-matching contexts. This means
564 /// that we should only lint `unused_parens` and not `unused_braces`
569 /// let ref b = { a }; // We actually borrow a copy of `a` here.
570 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
571 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
573 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
575 // this cannot be a constant is it refers to a static.
576 fn lint(&self) -> &'static Lint;
578 fn check_unused_delims_expr(
580 cx: &EarlyContext<'_>,
582 ctx: UnusedDelimsCtx,
583 followed_by_block: bool,
584 left_pos: Option<BytePos>,
585 right_pos: Option<BytePos>,
588 fn is_expr_delims_necessary(
590 followed_by_block: bool,
591 followed_by_else: bool,
593 if followed_by_else {
595 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
596 _ if classify::expr_trailing_brace(inner).is_some() => return true,
601 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
602 let lhs_needs_parens = {
603 let mut innermost = inner;
605 innermost = match &innermost.kind {
606 ExprKind::Binary(_, lhs, _rhs) => lhs,
607 ExprKind::Call(fn_, _params) => fn_,
608 ExprKind::Cast(expr, _ty) => expr,
609 ExprKind::Type(expr, _ty) => expr,
610 ExprKind::Index(base, _subscript) => base,
613 if !classify::expr_requires_semi_to_be_stmt(innermost) {
620 || (followed_by_block
621 && match &inner.kind {
622 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
623 ExprKind::Range(_lhs, Some(rhs), _limits) => {
624 matches!(rhs.kind, ExprKind::Block(..))
626 _ => parser::contains_exterior_struct_lit(&inner),
630 fn emit_unused_delims_expr(
632 cx: &EarlyContext<'_>,
634 ctx: UnusedDelimsCtx,
635 left_pos: Option<BytePos>,
636 right_pos: Option<BytePos>,
638 let spans = match value.kind {
639 ast::ExprKind::Block(ref block, None) if block.stmts.len() == 1 => {
640 if let StmtKind::Expr(expr) = &block.stmts[0].kind
641 && let ExprKind::Err = expr.kind
645 if let Some(span) = block.stmts[0].span.find_ancestor_inside(value.span) {
646 Some((value.span.with_hi(span.lo()), value.span.with_lo(span.hi())))
651 ast::ExprKind::Paren(ref expr) => {
652 let expr_span = expr.span.find_ancestor_inside(value.span);
653 if let Some(expr_span) = expr_span {
654 Some((value.span.with_hi(expr_span.lo()), value.span.with_lo(expr_span.hi())))
662 left_pos.map_or(false, |s| s >= value.span.lo()),
663 right_pos.map_or(false, |s| s <= value.span.hi()),
665 self.emit_unused_delims(cx, value.span, spans, ctx.into(), keep_space);
668 fn emit_unused_delims(
670 cx: &EarlyContext<'_>,
672 spans: Option<(Span, Span)>,
674 keep_space: (bool, bool),
676 let primary_span = if let Some((lo, hi)) = spans {
677 MultiSpan::from(vec![lo, hi])
679 MultiSpan::from(value_span)
681 cx.struct_span_lint(self.lint(), primary_span, fluent::lint_unused_delim, |lint| {
682 lint.set_arg("delim", Self::DELIM_STR);
683 lint.set_arg("item", msg);
684 if let Some((lo, hi)) = spans {
685 let sm = cx.sess().source_map();
688 let Ok(snip) = sm.span_to_prev_source(lo) && !snip.ends_with(' ') {
696 let Ok(snip) = sm.span_to_next_source(hi) && !snip.starts_with(' ') {
702 let replacement = vec![(lo, lo_replace), (hi, hi_replace)];
703 lint.multipart_suggestion(
706 Applicability::MachineApplicable,
713 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
714 use rustc_ast::ExprKind::*;
715 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
716 // Do not lint `unused_braces` in `if let` expressions.
717 If(ref cond, ref block, _)
718 if !matches!(cond.kind, Let(_, _, _))
719 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
721 let left = e.span.lo() + rustc_span::BytePos(2);
722 let right = block.span.lo();
723 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
726 // Do not lint `unused_braces` in `while let` expressions.
727 While(ref cond, ref block, ..)
728 if !matches!(cond.kind, Let(_, _, _))
729 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
731 let left = e.span.lo() + rustc_span::BytePos(5);
732 let right = block.span.lo();
733 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
736 ForLoop(_, ref cond, ref block, ..) => {
737 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
740 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
741 let left = e.span.lo() + rustc_span::BytePos(5);
742 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
745 Ret(Some(ref value)) => {
746 let left = e.span.lo() + rustc_span::BytePos(3);
747 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
750 Assign(_, ref value, _) | AssignOp(.., ref value) => {
751 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
753 // either function/method call, or something this lint doesn't care about
754 ref call_or_other => {
755 let (args_to_check, ctx) = match *call_or_other {
756 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
757 MethodCall(ref call) => (&call.args[..], UnusedDelimsCtx::MethodArg),
758 // actual catch-all arm
763 // Don't lint if this is a nested macro expansion: otherwise, the lint could
764 // trigger in situations that macro authors shouldn't have to care about, e.g.,
765 // when a parenthesized token tree matched in one macro expansion is matched as
766 // an expression in another and used as a fn/method argument (Issue #47775)
767 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
770 for arg in args_to_check {
771 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
776 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
779 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
781 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
782 if let Some((init, els)) = local.kind.init_else_opt() {
783 let ctx = match els {
784 None => UnusedDelimsCtx::AssignedValue,
785 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
787 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
790 StmtKind::Expr(ref expr) => {
791 self.check_unused_delims_expr(
794 UnusedDelimsCtx::BlockRetValue,
804 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
805 use ast::ItemKind::*;
807 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
808 self.check_unused_delims_expr(
811 UnusedDelimsCtx::AssignedValue,
821 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
822 /// with parentheses; they do not need them.
834 /// The parentheses are not needed, and should be removed. This is the
835 /// preferred style for writing these expressions.
836 pub(super) UNUSED_PARENS,
838 "`if`, `match`, `while` and `return` do not need parentheses"
841 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
843 impl UnusedDelimLint for UnusedParens {
844 const DELIM_STR: &'static str = "parentheses";
846 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
848 fn lint(&self) -> &'static Lint {
852 fn check_unused_delims_expr(
854 cx: &EarlyContext<'_>,
856 ctx: UnusedDelimsCtx,
857 followed_by_block: bool,
858 left_pos: Option<BytePos>,
859 right_pos: Option<BytePos>,
862 ast::ExprKind::Paren(ref inner) => {
863 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
864 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
865 && value.attrs.is_empty()
866 && !value.span.from_expansion()
867 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
868 || !matches!(inner.kind, ast::ExprKind::Binary(
869 rustc_span::source_map::Spanned { node, .. },
874 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
877 ast::ExprKind::Let(_, ref expr, _) => {
878 self.check_unused_delims_expr(
881 UnusedDelimsCtx::LetScrutineeExpr,
893 fn check_unused_parens_pat(
895 cx: &EarlyContext<'_>,
899 keep_space: (bool, bool),
901 use ast::{BindingAnnotation, PatKind};
903 if let PatKind::Paren(inner) = &value.kind {
905 // The lint visitor will visit each subpattern of `p`. We do not want to lint
906 // any range pattern no matter where it occurs in the pattern. For something like
907 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
908 // that if there are unnecessary parens they serve a purpose of readability.
909 PatKind::Range(..) => return,
910 // Avoid `p0 | .. | pn` if we should.
911 PatKind::Or(..) if avoid_or => return,
912 // Avoid `mut x` and `mut x @ p` if we should:
913 PatKind::Ident(BindingAnnotation::MUT, ..) if avoid_mut => {
916 // Otherwise proceed with linting.
919 let spans = if let Some(inner) = inner.span.find_ancestor_inside(value.span) {
920 Some((value.span.with_hi(inner.lo()), value.span.with_lo(inner.hi())))
924 self.emit_unused_delims(cx, value.span, spans, "pattern", keep_space);
929 impl EarlyLintPass for UnusedParens {
930 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
932 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
933 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
935 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
936 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
937 // want to complain about things like `if let 42 = (42)`.
938 ExprKind::If(ref cond, ref block, ref else_)
939 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
941 self.check_unused_delims_expr(
944 UnusedDelimsCtx::LetScrutineeExpr,
949 for stmt in &block.stmts {
950 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
952 if let Some(e) = else_ {
953 <Self as UnusedDelimLint>::check_expr(self, cx, e);
957 ExprKind::Match(ref _expr, ref arm) => {
959 self.check_unused_delims_expr(
962 UnusedDelimsCtx::MatchArmExpr,
972 <Self as UnusedDelimLint>::check_expr(self, cx, e)
975 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
976 use ast::{Mutability, PatKind::*};
977 let keep_space = (false, false);
979 // Do not lint on `(..)` as that will result in the other arms being useless.
981 // The other cases do not contain sub-patterns.
982 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
983 // These are list-like patterns; parens can always be removed.
984 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
985 self.check_unused_parens_pat(cx, p, false, false, keep_space);
987 Struct(_, _, fps, _) => for f in fps {
988 self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
990 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
991 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
992 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
993 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
994 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not, keep_space),
998 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
999 if let StmtKind::Local(ref local) = s.kind {
1000 self.check_unused_parens_pat(cx, &local.pat, true, false, (false, false));
1003 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1006 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
1007 self.check_unused_parens_pat(cx, ¶m.pat, true, false, (false, false));
1010 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
1011 self.check_unused_parens_pat(cx, &arm.pat, false, false, (false, false));
1014 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1015 if let ast::TyKind::Paren(r) = &ty.kind {
1017 ast::TyKind::TraitObject(..) => {}
1018 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
1019 ast::TyKind::Array(_, len) => {
1020 self.check_unused_delims_expr(
1023 UnusedDelimsCtx::ArrayLenExpr,
1030 let spans = if let Some(r) = r.span.find_ancestor_inside(ty.span) {
1031 Some((ty.span.with_hi(r.lo()), ty.span.with_lo(r.hi())))
1035 self.emit_unused_delims(cx, ty.span, spans, "type", (false, false));
1041 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1042 <Self as UnusedDelimLint>::check_item(self, cx, item)
1047 /// The `unused_braces` lint detects unnecessary braces around an
1062 /// The braces are not needed, and should be removed. This is the
1063 /// preferred style for writing these expressions.
1064 pub(super) UNUSED_BRACES,
1066 "unnecessary braces around an expression"
1069 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
1071 impl UnusedDelimLint for UnusedBraces {
1072 const DELIM_STR: &'static str = "braces";
1074 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
1076 fn lint(&self) -> &'static Lint {
1080 fn check_unused_delims_expr(
1082 cx: &EarlyContext<'_>,
1084 ctx: UnusedDelimsCtx,
1085 followed_by_block: bool,
1086 left_pos: Option<BytePos>,
1087 right_pos: Option<BytePos>,
1090 ast::ExprKind::Block(ref inner, None)
1091 if inner.rules == ast::BlockCheckMode::Default =>
1093 // emit a warning under the following conditions:
1095 // - the block does not have a label
1096 // - the block is not `unsafe`
1097 // - the block contains exactly one expression (do not lint `{ expr; }`)
1098 // - `followed_by_block` is true and the internal expr may contain a `{`
1099 // - the block is not multiline (do not lint multiline match arms)
1103 // somewhat_long_expression
1108 // - the block has no attribute and was not created inside a macro
1109 // - if the block is an `anon_const`, the inner expr must be a literal
1110 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
1112 // FIXME(const_generics): handle paths when #67075 is fixed.
1113 if let [stmt] = inner.stmts.as_slice() {
1114 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
1115 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
1116 && (ctx != UnusedDelimsCtx::AnonConst
1117 || matches!(expr.kind, ast::ExprKind::Lit(_)))
1118 && !cx.sess().source_map().is_multiline(value.span)
1119 && value.attrs.is_empty()
1120 && !value.span.from_expansion()
1122 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
1127 ast::ExprKind::Let(_, ref expr, _) => {
1128 self.check_unused_delims_expr(
1131 UnusedDelimsCtx::LetScrutineeExpr,
1142 impl EarlyLintPass for UnusedBraces {
1143 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1144 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1147 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1148 <Self as UnusedDelimLint>::check_expr(self, cx, e);
1150 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1151 self.check_unused_delims_expr(
1154 UnusedDelimsCtx::AnonConst,
1162 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1163 if let ast::GenericArg::Const(ct) = arg {
1164 self.check_unused_delims_expr(
1167 UnusedDelimsCtx::AnonConst,
1175 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1176 if let Some(anon_const) = &v.disr_expr {
1177 self.check_unused_delims_expr(
1180 UnusedDelimsCtx::AnonConst,
1188 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1190 ast::TyKind::Array(_, ref len) => {
1191 self.check_unused_delims_expr(
1194 UnusedDelimsCtx::ArrayLenExpr,
1201 ast::TyKind::Typeof(ref anon_const) => {
1202 self.check_unused_delims_expr(
1205 UnusedDelimsCtx::AnonConst,
1216 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1217 <Self as UnusedDelimLint>::check_item(self, cx, item)
1222 /// The `unused_import_braces` lint catches unnecessary braces around an
1227 /// ```rust,compile_fail
1228 /// #![deny(unused_import_braces)]
1241 /// If there is only a single item, then remove the braces (`use test::A;`
1244 /// This lint is "allow" by default because it is only enforcing a
1245 /// stylistic choice.
1246 UNUSED_IMPORT_BRACES,
1248 "unnecessary braces around an imported item"
1251 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1253 impl UnusedImportBraces {
1254 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1255 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1256 // Recursively check nested UseTrees
1257 for &(ref tree, _) in items {
1258 self.check_use_tree(cx, tree, item);
1261 // Trigger the lint only if there is one nested item
1262 if items.len() != 1 {
1266 // Trigger the lint if the nested item is a non-self single item
1267 let node_name = match items[0].0.kind {
1268 ast::UseTreeKind::Simple(rename, ..) => {
1269 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1270 if orig_ident.name == kw::SelfLower {
1273 rename.unwrap_or(orig_ident).name
1275 ast::UseTreeKind::Glob => Symbol::intern("*"),
1276 ast::UseTreeKind::Nested(_) => return,
1279 cx.struct_span_lint(
1280 UNUSED_IMPORT_BRACES,
1282 fluent::lint_unused_import_braces,
1283 |lint| lint.set_arg("node", node_name),
1289 impl EarlyLintPass for UnusedImportBraces {
1290 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1291 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1292 self.check_use_tree(cx, use_tree, item);
1298 /// The `unused_allocation` lint detects unnecessary allocations that can
1304 /// #![feature(box_syntax)]
1306 /// let a = (box [1, 2, 3]).len();
1314 /// When a `box` expression is immediately coerced to a reference, then
1315 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1316 /// should be used instead to avoid the allocation.
1317 pub(super) UNUSED_ALLOCATION,
1319 "detects unnecessary allocations that can be eliminated"
1322 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1324 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1325 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1327 hir::ExprKind::Box(_) => {}
1331 for adj in cx.typeck_results().expr_adjustments(e) {
1332 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1333 cx.struct_span_lint(
1337 adjustment::AutoBorrowMutability::Not => fluent::lint_unused_allocation,
1338 adjustment::AutoBorrowMutability::Mut { .. } => {
1339 fluent::lint_unused_allocation_mut