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::Alias(ty::Opaque, ty::AliasTy { def_id: 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::Alias(ty::Opaque, ty::AliasTy { def_id: def, .. }) => {
255 elaborate_predicates_with_span(
257 cx.tcx.explicit_item_bounds(def).iter().cloned(),
259 .find_map(|obligation| {
260 // We only look at the `DefId`, so it is safe to skip the binder here.
261 if let ty::PredicateKind::Clause(ty::Clause::Trait(
262 ref poly_trait_predicate,
263 )) = obligation.predicate.kind().skip_binder()
265 let def_id = poly_trait_predicate.trait_ref.def_id;
267 is_def_must_use(cx, def_id, span)
272 .map(|inner| MustUsePath::Opaque(Box::new(inner)))
274 ty::Dynamic(binders, _, _) => binders.iter().find_map(|predicate| {
275 if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate.skip_binder()
277 let def_id = trait_ref.def_id;
278 is_def_must_use(cx, def_id, span)
279 .map(|inner| MustUsePath::TraitObject(Box::new(inner)))
285 let elem_exprs = if let hir::ExprKind::Tup(elem_exprs) = expr.kind {
286 debug_assert_eq!(elem_exprs.len(), tys.len());
292 // Default to `expr`.
293 let elem_exprs = elem_exprs.iter().chain(iter::repeat(expr));
295 let nested_must_use = tys
299 .filter_map(|(i, (ty, expr))| {
300 is_ty_must_use(cx, ty, expr, expr.span).map(|path| (i, path))
302 .collect::<Vec<_>>();
304 if !nested_must_use.is_empty() {
305 Some(MustUsePath::TupleElement(nested_must_use))
310 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
311 // If the array is empty we don't lint, to avoid false positives
312 Some(0) | None => None,
313 // If the array is definitely non-empty, we can do `#[must_use]` checking.
314 Some(len) => is_ty_must_use(cx, ty, expr, span)
315 .map(|inner| MustUsePath::Array(Box::new(inner), len)),
317 ty::Closure(..) => Some(MustUsePath::Closure(span)),
318 ty::Generator(def_id, ..) => {
319 // async fn should be treated as "implementor of `Future`"
320 let must_use = if cx.tcx.generator_is_async(def_id) {
321 let def_id = cx.tcx.lang_items().future_trait().unwrap();
322 is_def_must_use(cx, def_id, span)
323 .map(|inner| MustUsePath::Opaque(Box::new(inner)))
327 must_use.or(Some(MustUsePath::Generator(span)))
333 fn is_def_must_use(cx: &LateContext<'_>, def_id: DefId, span: Span) -> Option<MustUsePath> {
334 if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
335 // check for #[must_use = "..."]
336 let reason = attr.value_str();
337 Some(MustUsePath::Def(span, def_id, reason))
343 // Returns whether further errors should be suppressed because either a lint has been emitted or the type should be ignored.
344 fn check_must_use_def(
345 cx: &LateContext<'_>,
348 descr_pre_path: &str,
349 descr_post_path: &str,
351 is_def_must_use(cx, def_id, span)
352 .map(|must_use_path| {
353 emit_must_use_untranslated(
364 #[instrument(skip(cx), level = "debug")]
365 fn emit_must_use_untranslated(
366 cx: &LateContext<'_>,
372 let plural_suffix = pluralize!(plural_len);
375 MustUsePath::Suppressed => {}
376 MustUsePath::Boxed(path) => {
377 let descr_pre = &format!("{}boxed ", descr_pre);
378 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
380 MustUsePath::Opaque(path) => {
381 let descr_pre = &format!("{}implementer{} of ", descr_pre, plural_suffix);
382 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
384 MustUsePath::TraitObject(path) => {
385 let descr_post = &format!(" trait object{}{}", plural_suffix, descr_post);
386 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
388 MustUsePath::TupleElement(elems) => {
389 for (index, path) in elems {
390 let descr_post = &format!(" in tuple element {}", index);
391 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
394 MustUsePath::Array(path, len) => {
395 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix);
396 emit_must_use_untranslated(
401 plural_len.saturating_add(usize::try_from(*len).unwrap_or(usize::MAX)),
404 MustUsePath::Closure(span) => {
408 fluent::lint_unused_closure,
410 // FIXME(davidtwco): this isn't properly translatable because of the
412 lint.set_arg("count", plural_len)
413 .set_arg("pre", descr_pre)
414 .set_arg("post", descr_post)
419 MustUsePath::Generator(span) => {
423 fluent::lint_unused_generator,
425 // FIXME(davidtwco): this isn't properly translatable because of the
427 lint.set_arg("count", plural_len)
428 .set_arg("pre", descr_pre)
429 .set_arg("post", descr_post)
434 MustUsePath::Def(span, def_id, reason) => {
435 cx.struct_span_lint(UNUSED_MUST_USE, *span, fluent::lint_unused_def, |lint| {
436 // FIXME(davidtwco): this isn't properly translatable because of the pre/post
438 lint.set_arg("pre", descr_pre);
439 lint.set_arg("post", descr_post);
440 lint.set_arg("def", cx.tcx.def_path_str(*def_id));
441 if let Some(note) = reason {
442 lint.note(note.as_str());
453 /// The `path_statements` lint detects path statements with no effect.
467 /// It is usually a mistake to have a statement that has no effect.
470 "path statements with no effect"
473 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
475 impl<'tcx> LateLintPass<'tcx> for PathStatements {
476 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
477 if let hir::StmtKind::Semi(expr) = s.kind {
478 if let hir::ExprKind::Path(_) = expr.kind {
479 let ty = cx.typeck_results().expr_ty(expr);
480 if ty.needs_drop(cx.tcx, cx.param_env) {
484 fluent::lint_path_statement_drop,
486 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
487 lint.span_suggestion(
490 format!("drop({});", snippet),
491 Applicability::MachineApplicable,
494 lint.span_help(s.span, fluent::suggestion);
503 fluent::lint_path_statement_no_effect,
512 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
513 enum UnusedDelimsCtx {
517 AssignedValueLetElse,
530 impl From<UnusedDelimsCtx> for &'static str {
531 fn from(ctx: UnusedDelimsCtx) -> &'static str {
533 UnusedDelimsCtx::FunctionArg => "function argument",
534 UnusedDelimsCtx::MethodArg => "method argument",
535 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
538 UnusedDelimsCtx::IfCond => "`if` condition",
539 UnusedDelimsCtx::WhileCond => "`while` condition",
540 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
541 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
542 UnusedDelimsCtx::ReturnValue => "`return` value",
543 UnusedDelimsCtx::BlockRetValue => "block return value",
544 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
545 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
546 UnusedDelimsCtx::MatchArmExpr => "match arm expression",
551 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
552 trait UnusedDelimLint {
553 const DELIM_STR: &'static str;
555 /// Due to `ref` pattern, there can be a difference between using
556 /// `{ expr }` and `expr` in pattern-matching contexts. This means
557 /// that we should only lint `unused_parens` and not `unused_braces`
562 /// let ref b = { a }; // We actually borrow a copy of `a` here.
563 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
564 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
566 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
568 // this cannot be a constant is it refers to a static.
569 fn lint(&self) -> &'static Lint;
571 fn check_unused_delims_expr(
573 cx: &EarlyContext<'_>,
575 ctx: UnusedDelimsCtx,
576 followed_by_block: bool,
577 left_pos: Option<BytePos>,
578 right_pos: Option<BytePos>,
581 fn is_expr_delims_necessary(
583 followed_by_block: bool,
584 followed_by_else: bool,
586 if followed_by_else {
588 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
589 _ if classify::expr_trailing_brace(inner).is_some() => return true,
594 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
595 let lhs_needs_parens = {
596 let mut innermost = inner;
598 innermost = match &innermost.kind {
599 ExprKind::Binary(_, lhs, _rhs) => lhs,
600 ExprKind::Call(fn_, _params) => fn_,
601 ExprKind::Cast(expr, _ty) => expr,
602 ExprKind::Type(expr, _ty) => expr,
603 ExprKind::Index(base, _subscript) => base,
606 if !classify::expr_requires_semi_to_be_stmt(innermost) {
613 || (followed_by_block
614 && match &inner.kind {
616 | ExprKind::Break(..)
617 | ExprKind::Yield(..)
618 | ExprKind::Yeet(..) => true,
619 ExprKind::Range(_lhs, Some(rhs), _limits) => {
620 matches!(rhs.kind, ExprKind::Block(..))
622 _ => parser::contains_exterior_struct_lit(&inner),
626 fn emit_unused_delims_expr(
628 cx: &EarlyContext<'_>,
630 ctx: UnusedDelimsCtx,
631 left_pos: Option<BytePos>,
632 right_pos: Option<BytePos>,
634 // If `value` has `ExprKind::Err`, unused delim lint can be broken.
635 // For example, the following code caused ICE.
636 // This is because the `ExprKind::Call` in `value` has `ExprKind::Err` as its argument
637 // and this leads to wrong spans. #104897
642 use rustc_ast::visit::{walk_expr, Visitor};
643 struct ErrExprVisitor {
646 impl<'ast> Visitor<'ast> for ErrExprVisitor {
647 fn visit_expr(&mut self, expr: &'ast ast::Expr) {
648 if let ExprKind::Err = expr.kind {
649 self.has_error = true;
652 walk_expr(self, expr)
655 let mut visitor = ErrExprVisitor { has_error: false };
656 visitor.visit_expr(value);
657 if visitor.has_error {
660 let spans = match value.kind {
661 ast::ExprKind::Block(ref block, None) if block.stmts.len() == 1 => {
662 if let Some(span) = block.stmts[0].span.find_ancestor_inside(value.span) {
663 Some((value.span.with_hi(span.lo()), value.span.with_lo(span.hi())))
668 ast::ExprKind::Paren(ref expr) => {
669 let expr_span = expr.span.find_ancestor_inside(value.span);
670 if let Some(expr_span) = expr_span {
671 Some((value.span.with_hi(expr_span.lo()), value.span.with_lo(expr_span.hi())))
679 left_pos.map_or(false, |s| s >= value.span.lo()),
680 right_pos.map_or(false, |s| s <= value.span.hi()),
682 self.emit_unused_delims(cx, value.span, spans, ctx.into(), keep_space);
685 fn emit_unused_delims(
687 cx: &EarlyContext<'_>,
689 spans: Option<(Span, Span)>,
691 keep_space: (bool, bool),
693 let primary_span = if let Some((lo, hi)) = spans {
694 MultiSpan::from(vec![lo, hi])
696 MultiSpan::from(value_span)
698 cx.struct_span_lint(self.lint(), primary_span, fluent::lint_unused_delim, |lint| {
699 lint.set_arg("delim", Self::DELIM_STR);
700 lint.set_arg("item", msg);
701 if let Some((lo, hi)) = spans {
702 let sm = cx.sess().source_map();
705 let Ok(snip) = sm.span_to_prev_source(lo) && !snip.ends_with(' ') {
713 let Ok(snip) = sm.span_to_next_source(hi) && !snip.starts_with(' ') {
719 let replacement = vec![(lo, lo_replace), (hi, hi_replace)];
720 lint.multipart_suggestion(
723 Applicability::MachineApplicable,
730 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
731 use rustc_ast::ExprKind::*;
732 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
733 // Do not lint `unused_braces` in `if let` expressions.
734 If(ref cond, ref block, _)
735 if !matches!(cond.kind, Let(_, _, _))
736 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
738 let left = e.span.lo() + rustc_span::BytePos(2);
739 let right = block.span.lo();
740 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
743 // Do not lint `unused_braces` in `while let` expressions.
744 While(ref cond, ref block, ..)
745 if !matches!(cond.kind, Let(_, _, _))
746 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
748 let left = e.span.lo() + rustc_span::BytePos(5);
749 let right = block.span.lo();
750 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
753 ForLoop(_, ref cond, ref block, ..) => {
754 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
757 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
758 let left = e.span.lo() + rustc_span::BytePos(5);
759 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
762 Ret(Some(ref value)) => {
763 let left = e.span.lo() + rustc_span::BytePos(3);
764 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
767 Assign(_, ref value, _) | AssignOp(.., ref value) => {
768 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
770 // either function/method call, or something this lint doesn't care about
771 ref call_or_other => {
772 let (args_to_check, ctx) = match *call_or_other {
773 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
774 MethodCall(ref call) => (&call.args[..], UnusedDelimsCtx::MethodArg),
775 // actual catch-all arm
780 // Don't lint if this is a nested macro expansion: otherwise, the lint could
781 // trigger in situations that macro authors shouldn't have to care about, e.g.,
782 // when a parenthesized token tree matched in one macro expansion is matched as
783 // an expression in another and used as a fn/method argument (Issue #47775)
784 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
787 for arg in args_to_check {
788 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
793 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
796 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
798 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
799 if let Some((init, els)) = local.kind.init_else_opt() {
800 let ctx = match els {
801 None => UnusedDelimsCtx::AssignedValue,
802 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
804 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
807 StmtKind::Expr(ref expr) => {
808 self.check_unused_delims_expr(
811 UnusedDelimsCtx::BlockRetValue,
821 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
822 use ast::ItemKind::*;
824 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
825 self.check_unused_delims_expr(
828 UnusedDelimsCtx::AssignedValue,
838 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
839 /// with parentheses; they do not need them.
851 /// The parentheses are not needed, and should be removed. This is the
852 /// preferred style for writing these expressions.
853 pub(super) UNUSED_PARENS,
855 "`if`, `match`, `while` and `return` do not need parentheses"
858 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
860 impl UnusedDelimLint for UnusedParens {
861 const DELIM_STR: &'static str = "parentheses";
863 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
865 fn lint(&self) -> &'static Lint {
869 fn check_unused_delims_expr(
871 cx: &EarlyContext<'_>,
873 ctx: UnusedDelimsCtx,
874 followed_by_block: bool,
875 left_pos: Option<BytePos>,
876 right_pos: Option<BytePos>,
879 ast::ExprKind::Paren(ref inner) => {
880 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
881 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
882 && value.attrs.is_empty()
883 && !value.span.from_expansion()
884 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
885 || !matches!(inner.kind, ast::ExprKind::Binary(
886 rustc_span::source_map::Spanned { node, .. },
891 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
894 ast::ExprKind::Let(_, ref expr, _) => {
895 self.check_unused_delims_expr(
898 UnusedDelimsCtx::LetScrutineeExpr,
910 fn check_unused_parens_pat(
912 cx: &EarlyContext<'_>,
916 keep_space: (bool, bool),
918 use ast::{BindingAnnotation, PatKind};
920 if let PatKind::Paren(inner) = &value.kind {
922 // The lint visitor will visit each subpattern of `p`. We do not want to lint
923 // any range pattern no matter where it occurs in the pattern. For something like
924 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
925 // that if there are unnecessary parens they serve a purpose of readability.
926 PatKind::Range(..) => return,
927 // Avoid `p0 | .. | pn` if we should.
928 PatKind::Or(..) if avoid_or => return,
929 // Avoid `mut x` and `mut x @ p` if we should:
930 PatKind::Ident(BindingAnnotation::MUT, ..) if avoid_mut => {
933 // Otherwise proceed with linting.
936 let spans = if let Some(inner) = inner.span.find_ancestor_inside(value.span) {
937 Some((value.span.with_hi(inner.lo()), value.span.with_lo(inner.hi())))
941 self.emit_unused_delims(cx, value.span, spans, "pattern", keep_space);
946 impl EarlyLintPass for UnusedParens {
948 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
950 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
951 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
953 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
954 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
955 // want to complain about things like `if let 42 = (42)`.
956 ExprKind::If(ref cond, ref block, ref else_)
957 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
959 self.check_unused_delims_expr(
962 UnusedDelimsCtx::LetScrutineeExpr,
967 for stmt in &block.stmts {
968 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
970 if let Some(e) = else_ {
971 <Self as UnusedDelimLint>::check_expr(self, cx, e);
975 ExprKind::Match(ref _expr, ref arm) => {
977 self.check_unused_delims_expr(
980 UnusedDelimsCtx::MatchArmExpr,
990 <Self as UnusedDelimLint>::check_expr(self, cx, e)
993 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
994 use ast::{Mutability, PatKind::*};
995 let keep_space = (false, false);
997 // Do not lint on `(..)` as that will result in the other arms being useless.
999 // The other cases do not contain sub-patterns.
1000 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
1001 // These are list-like patterns; parens can always be removed.
1002 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
1003 self.check_unused_parens_pat(cx, p, false, false, keep_space);
1005 Struct(_, _, fps, _) => for f in fps {
1006 self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
1008 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
1009 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
1010 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
1011 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
1012 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not, keep_space),
1016 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1017 if let StmtKind::Local(ref local) = s.kind {
1018 self.check_unused_parens_pat(cx, &local.pat, true, false, (false, false));
1021 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1024 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
1025 self.check_unused_parens_pat(cx, ¶m.pat, true, false, (false, false));
1028 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
1029 self.check_unused_parens_pat(cx, &arm.pat, false, false, (false, false));
1032 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1033 if let ast::TyKind::Paren(r) = &ty.kind {
1035 ast::TyKind::TraitObject(..) => {}
1036 ast::TyKind::BareFn(b) if b.generic_params.len() > 0 => {}
1037 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
1038 ast::TyKind::Array(_, len) => {
1039 self.check_unused_delims_expr(
1042 UnusedDelimsCtx::ArrayLenExpr,
1049 let spans = if let Some(r) = r.span.find_ancestor_inside(ty.span) {
1050 Some((ty.span.with_hi(r.lo()), ty.span.with_lo(r.hi())))
1054 self.emit_unused_delims(cx, ty.span, spans, "type", (false, false));
1060 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1061 <Self as UnusedDelimLint>::check_item(self, cx, item)
1066 /// The `unused_braces` lint detects unnecessary braces around an
1081 /// The braces are not needed, and should be removed. This is the
1082 /// preferred style for writing these expressions.
1083 pub(super) UNUSED_BRACES,
1085 "unnecessary braces around an expression"
1088 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
1090 impl UnusedDelimLint for UnusedBraces {
1091 const DELIM_STR: &'static str = "braces";
1093 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
1095 fn lint(&self) -> &'static Lint {
1099 fn check_unused_delims_expr(
1101 cx: &EarlyContext<'_>,
1103 ctx: UnusedDelimsCtx,
1104 followed_by_block: bool,
1105 left_pos: Option<BytePos>,
1106 right_pos: Option<BytePos>,
1109 ast::ExprKind::Block(ref inner, None)
1110 if inner.rules == ast::BlockCheckMode::Default =>
1112 // emit a warning under the following conditions:
1114 // - the block does not have a label
1115 // - the block is not `unsafe`
1116 // - the block contains exactly one expression (do not lint `{ expr; }`)
1117 // - `followed_by_block` is true and the internal expr may contain a `{`
1118 // - the block is not multiline (do not lint multiline match arms)
1122 // somewhat_long_expression
1127 // - the block has no attribute and was not created inside a macro
1128 // - if the block is an `anon_const`, the inner expr must be a literal
1129 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
1131 // FIXME(const_generics): handle paths when #67075 is fixed.
1132 if let [stmt] = inner.stmts.as_slice() {
1133 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
1134 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
1135 && (ctx != UnusedDelimsCtx::AnonConst
1136 || matches!(expr.kind, ast::ExprKind::Lit(_)))
1137 && !cx.sess().source_map().is_multiline(value.span)
1138 && value.attrs.is_empty()
1139 && !value.span.from_expansion()
1140 && !inner.span.from_expansion()
1142 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
1147 ast::ExprKind::Let(_, ref expr, _) => {
1148 self.check_unused_delims_expr(
1151 UnusedDelimsCtx::LetScrutineeExpr,
1162 impl EarlyLintPass for UnusedBraces {
1163 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1164 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1168 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1169 <Self as UnusedDelimLint>::check_expr(self, cx, e);
1171 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1172 self.check_unused_delims_expr(
1175 UnusedDelimsCtx::AnonConst,
1183 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1184 if let ast::GenericArg::Const(ct) = arg {
1185 self.check_unused_delims_expr(
1188 UnusedDelimsCtx::AnonConst,
1196 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1197 if let Some(anon_const) = &v.disr_expr {
1198 self.check_unused_delims_expr(
1201 UnusedDelimsCtx::AnonConst,
1209 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1211 ast::TyKind::Array(_, ref len) => {
1212 self.check_unused_delims_expr(
1215 UnusedDelimsCtx::ArrayLenExpr,
1222 ast::TyKind::Typeof(ref anon_const) => {
1223 self.check_unused_delims_expr(
1226 UnusedDelimsCtx::AnonConst,
1237 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1238 <Self as UnusedDelimLint>::check_item(self, cx, item)
1243 /// The `unused_import_braces` lint catches unnecessary braces around an
1248 /// ```rust,compile_fail
1249 /// #![deny(unused_import_braces)]
1262 /// If there is only a single item, then remove the braces (`use test::A;`
1265 /// This lint is "allow" by default because it is only enforcing a
1266 /// stylistic choice.
1267 UNUSED_IMPORT_BRACES,
1269 "unnecessary braces around an imported item"
1272 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1274 impl UnusedImportBraces {
1275 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1276 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1277 // Recursively check nested UseTrees
1278 for (tree, _) in items {
1279 self.check_use_tree(cx, tree, item);
1282 // Trigger the lint only if there is one nested item
1283 if items.len() != 1 {
1287 // Trigger the lint if the nested item is a non-self single item
1288 let node_name = match items[0].0.kind {
1289 ast::UseTreeKind::Simple(rename) => {
1290 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1291 if orig_ident.name == kw::SelfLower {
1294 rename.unwrap_or(orig_ident).name
1296 ast::UseTreeKind::Glob => Symbol::intern("*"),
1297 ast::UseTreeKind::Nested(_) => return,
1300 cx.struct_span_lint(
1301 UNUSED_IMPORT_BRACES,
1303 fluent::lint_unused_import_braces,
1304 |lint| lint.set_arg("node", node_name),
1310 impl EarlyLintPass for UnusedImportBraces {
1311 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1312 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1313 self.check_use_tree(cx, use_tree, item);
1319 /// The `unused_allocation` lint detects unnecessary allocations that can
1325 /// #![feature(box_syntax)]
1327 /// let a = (box [1, 2, 3]).len();
1335 /// When a `box` expression is immediately coerced to a reference, then
1336 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1337 /// should be used instead to avoid the allocation.
1338 pub(super) UNUSED_ALLOCATION,
1340 "detects unnecessary allocations that can be eliminated"
1343 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1345 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1346 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1348 hir::ExprKind::Box(_) => {}
1352 for adj in cx.typeck_results().expr_adjustments(e) {
1353 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1354 cx.struct_span_lint(
1358 adjustment::AutoBorrowMutability::Not => fluent::lint_unused_allocation,
1359 adjustment::AutoBorrowMutability::Mut { .. } => {
1360 fluent::lint_unused_allocation_mut