2 PathStatementDrop, PathStatementDropSub, PathStatementNoEffect, UnusedAllocationDiag,
3 UnusedAllocationMutDiag, UnusedClosure, UnusedDef, UnusedDefSuggestion, UnusedDelim,
4 UnusedDelimSuggestion, UnusedGenerator, UnusedImportBracesDiag, UnusedOp, UnusedResult,
7 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
9 use rustc_ast::util::{classify, parser};
10 use rustc_ast::{ExprKind, StmtKind};
11 use rustc_errors::{pluralize, MultiSpan};
13 use rustc_hir::def::{DefKind, Res};
14 use rustc_hir::def_id::DefId;
15 use rustc_infer::traits::util::elaborate_predicates_with_span;
16 use rustc_middle::ty::adjustment;
17 use rustc_middle::ty::{self, DefIdTree, Ty};
18 use rustc_span::symbol::Symbol;
19 use rustc_span::symbol::{kw, sym};
20 use rustc_span::{BytePos, Span};
24 /// The `unused_must_use` lint detects unused result of a type flagged as
30 /// fn returns_result() -> Result<(), ()> {
43 /// The `#[must_use]` attribute is an indicator that it is a mistake to
44 /// ignore the value. See [the reference] for more details.
46 /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
49 "unused result of a type flagged as `#[must_use]`",
50 report_in_external_macro
54 /// The `unused_results` lint checks for the unused result of an
55 /// expression in a statement.
59 /// ```rust,compile_fail
60 /// #![deny(unused_results)]
61 /// fn foo<T>() -> T { panic!() }
72 /// Ignoring the return value of a function may indicate a mistake. In
73 /// cases were it is almost certain that the result should be used, it is
74 /// recommended to annotate the function with the [`must_use` attribute].
75 /// Failure to use such a return value will trigger the [`unused_must_use`
76 /// lint] which is warn-by-default. The `unused_results` lint is
77 /// essentially the same, but triggers for *all* return values.
79 /// This lint is "allow" by default because it can be noisy, and may not be
80 /// an actual problem. For example, calling the `remove` method of a `Vec`
81 /// or `HashMap` returns the previous value, which you may not care about.
82 /// Using this lint would require explicitly ignoring or discarding such
85 /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
86 /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
89 "unused result of an expression in a statement"
92 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
94 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
95 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
96 let hir::StmtKind::Semi(expr) = s.kind else { return; };
98 if let hir::ExprKind::Ret(..) = expr.kind {
102 if let hir::ExprKind::Match(await_expr, _arms, hir::MatchSource::AwaitDesugar) = expr.kind
103 && let ty = cx.typeck_results().expr_ty(&await_expr)
104 && let ty::Alias(ty::Opaque, ty::AliasTy { def_id: future_def_id, .. }) = ty.kind()
105 && cx.tcx.ty_is_opaque_future(ty)
106 // FIXME: This also includes non-async fns that return `impl Future`.
107 && let async_fn_def_id = cx.tcx.parent(*future_def_id)
108 && check_must_use_def(
112 "output of future returned by ",
116 // We have a bare `foo().await;` on an opaque type from an async function that was
117 // annotated with `#[must_use]`.
121 let ty = cx.typeck_results().expr_ty(&expr);
123 let must_use_result = is_ty_must_use(cx, ty, &expr, expr.span);
124 let type_lint_emitted_or_suppressed = match must_use_result {
126 emit_must_use_untranslated(cx, &path, "", "", 1);
132 let fn_warned = check_fn_must_use(cx, expr);
134 if !fn_warned && type_lint_emitted_or_suppressed {
135 // We don't warn about unused unit or uninhabited types.
136 // (See https://github.com/rust-lang/rust/issues/43806 for details.)
140 let must_use_op = match expr.kind {
141 // Hardcoding operators here seemed more expedient than the
142 // refactoring that would be needed to look up the `#[must_use]`
143 // attribute which does exist on the comparison trait methods
144 hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
150 | hir::BinOpKind::Gt => Some("comparison"),
152 | hir::BinOpKind::Sub
153 | hir::BinOpKind::Div
154 | hir::BinOpKind::Mul
155 | hir::BinOpKind::Rem => Some("arithmetic operation"),
156 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
157 hir::BinOpKind::BitXor
158 | hir::BinOpKind::BitAnd
159 | hir::BinOpKind::BitOr
160 | hir::BinOpKind::Shl
161 | hir::BinOpKind::Shr => Some("bitwise operation"),
163 hir::ExprKind::AddrOf(..) => Some("borrow"),
164 hir::ExprKind::Unary(..) => Some("unary operation"),
168 let mut op_warned = false;
170 if let Some(must_use_op) = must_use_op {
171 cx.emit_spanned_lint(
177 suggestion: expr.span.shrink_to_lo(),
183 if !(type_lint_emitted_or_suppressed || fn_warned || op_warned) {
184 cx.emit_spanned_lint(UNUSED_RESULTS, s.span, UnusedResult { ty });
187 fn check_fn_must_use(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
188 let maybe_def_id = match expr.kind {
189 hir::ExprKind::Call(ref callee, _) => {
191 hir::ExprKind::Path(ref qpath) => {
192 match cx.qpath_res(qpath, callee.hir_id) {
193 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
194 // `Res::Local` if it was a closure, for which we
195 // do not currently support must-use linting
202 hir::ExprKind::MethodCall(..) => {
203 cx.typeck_results().type_dependent_def_id(expr.hir_id)
207 if let Some(def_id) = maybe_def_id {
208 check_must_use_def(cx, def_id, expr.span, "return value of ", "")
214 /// A path through a type to a must_use source. Contains useful info for the lint.
217 /// Suppress must_use checking.
219 /// The root of the normal must_use lint with an optional message.
220 Def(Span, DefId, Option<Symbol>),
223 TraitObject(Box<Self>),
224 TupleElement(Vec<(usize, Self)>),
225 Array(Box<Self>, u64),
226 /// The root of the unused_closures lint.
228 /// The root of the unused_generators lint.
232 #[instrument(skip(cx, expr), level = "debug", ret)]
233 fn is_ty_must_use<'tcx>(
234 cx: &LateContext<'tcx>,
236 expr: &hir::Expr<'_>,
238 ) -> Option<MustUsePath> {
240 || !ty.is_inhabited_from(
242 cx.tcx.parent_module(expr.hir_id).to_def_id(),
246 return Some(MustUsePath::Suppressed);
250 ty::Adt(..) if ty.is_box() => {
251 let boxed_ty = ty.boxed_ty();
252 is_ty_must_use(cx, boxed_ty, expr, span)
253 .map(|inner| MustUsePath::Boxed(Box::new(inner)))
255 ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
256 ty::Alias(ty::Opaque, ty::AliasTy { def_id: def, .. }) => {
257 elaborate_predicates_with_span(
259 cx.tcx.explicit_item_bounds(def).iter().cloned(),
261 .find_map(|obligation| {
262 // We only look at the `DefId`, so it is safe to skip the binder here.
263 if let ty::PredicateKind::Clause(ty::Clause::Trait(
264 ref poly_trait_predicate,
265 )) = obligation.predicate.kind().skip_binder()
267 let def_id = poly_trait_predicate.trait_ref.def_id;
269 is_def_must_use(cx, def_id, span)
274 .map(|inner| MustUsePath::Opaque(Box::new(inner)))
276 ty::Dynamic(binders, _, _) => binders.iter().find_map(|predicate| {
277 if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate.skip_binder()
279 let def_id = trait_ref.def_id;
280 is_def_must_use(cx, def_id, span)
281 .map(|inner| MustUsePath::TraitObject(Box::new(inner)))
287 let elem_exprs = if let hir::ExprKind::Tup(elem_exprs) = expr.kind {
288 debug_assert_eq!(elem_exprs.len(), tys.len());
294 // Default to `expr`.
295 let elem_exprs = elem_exprs.iter().chain(iter::repeat(expr));
297 let nested_must_use = tys
301 .filter_map(|(i, (ty, expr))| {
302 is_ty_must_use(cx, ty, expr, expr.span).map(|path| (i, path))
304 .collect::<Vec<_>>();
306 if !nested_must_use.is_empty() {
307 Some(MustUsePath::TupleElement(nested_must_use))
312 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
313 // If the array is empty we don't lint, to avoid false positives
314 Some(0) | None => None,
315 // If the array is definitely non-empty, we can do `#[must_use]` checking.
316 Some(len) => is_ty_must_use(cx, ty, expr, span)
317 .map(|inner| MustUsePath::Array(Box::new(inner), len)),
319 ty::Closure(..) => Some(MustUsePath::Closure(span)),
320 ty::Generator(def_id, ..) => {
321 // async fn should be treated as "implementor of `Future`"
322 let must_use = if cx.tcx.generator_is_async(def_id) {
323 let def_id = cx.tcx.lang_items().future_trait().unwrap();
324 is_def_must_use(cx, def_id, span)
325 .map(|inner| MustUsePath::Opaque(Box::new(inner)))
329 must_use.or(Some(MustUsePath::Generator(span)))
335 fn is_def_must_use(cx: &LateContext<'_>, def_id: DefId, span: Span) -> Option<MustUsePath> {
336 if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
337 // check for #[must_use = "..."]
338 let reason = attr.value_str();
339 Some(MustUsePath::Def(span, def_id, reason))
345 // Returns whether further errors should be suppressed because either a lint has been emitted or the type should be ignored.
346 fn check_must_use_def(
347 cx: &LateContext<'_>,
350 descr_pre_path: &str,
351 descr_post_path: &str,
353 is_def_must_use(cx, def_id, span)
354 .map(|must_use_path| {
355 emit_must_use_untranslated(
366 #[instrument(skip(cx), level = "debug")]
367 fn emit_must_use_untranslated(
368 cx: &LateContext<'_>,
374 let plural_suffix = pluralize!(plural_len);
377 MustUsePath::Suppressed => {}
378 MustUsePath::Boxed(path) => {
379 let descr_pre = &format!("{}boxed ", descr_pre);
380 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
382 MustUsePath::Opaque(path) => {
383 let descr_pre = &format!("{}implementer{} of ", descr_pre, plural_suffix);
384 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
386 MustUsePath::TraitObject(path) => {
387 let descr_post = &format!(" trait object{}{}", plural_suffix, descr_post);
388 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
390 MustUsePath::TupleElement(elems) => {
391 for (index, path) in elems {
392 let descr_post = &format!(" in tuple element {}", index);
393 emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
396 MustUsePath::Array(path, len) => {
397 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix);
398 emit_must_use_untranslated(
403 plural_len.saturating_add(usize::try_from(*len).unwrap_or(usize::MAX)),
406 MustUsePath::Closure(span) => {
407 cx.emit_spanned_lint(
410 UnusedClosure { count: plural_len, pre: descr_pre, post: descr_post },
413 MustUsePath::Generator(span) => {
414 cx.emit_spanned_lint(
417 UnusedGenerator { count: plural_len, pre: descr_pre, post: descr_post },
420 MustUsePath::Def(span, def_id, reason) => {
421 let suggestion = if matches!(
422 cx.tcx.get_diagnostic_name(*def_id),
430 Some(UnusedDefSuggestion::Default { span: span.shrink_to_lo() })
434 cx.emit_spanned_lint(
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) {
481 let sub = if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span)
483 PathStatementDropSub::Suggestion { span: s.span, snippet }
485 PathStatementDropSub::Help { span: s.span }
487 cx.emit_spanned_lint(PATH_STATEMENTS, s.span, PathStatementDrop { sub })
489 cx.emit_spanned_lint(PATH_STATEMENTS, s.span, PathStatementNoEffect);
496 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
497 enum UnusedDelimsCtx {
501 AssignedValueLetElse,
515 impl From<UnusedDelimsCtx> for &'static str {
516 fn from(ctx: UnusedDelimsCtx) -> &'static str {
518 UnusedDelimsCtx::FunctionArg => "function argument",
519 UnusedDelimsCtx::MethodArg => "method argument",
520 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
523 UnusedDelimsCtx::IfCond => "`if` condition",
524 UnusedDelimsCtx::WhileCond => "`while` condition",
525 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
526 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
527 UnusedDelimsCtx::ReturnValue => "`return` value",
528 UnusedDelimsCtx::BlockRetValue => "block return value",
529 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
530 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
531 UnusedDelimsCtx::MatchArmExpr => "match arm expression",
532 UnusedDelimsCtx::IndexExpr => "index expression",
537 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
538 trait UnusedDelimLint {
539 const DELIM_STR: &'static str;
541 /// Due to `ref` pattern, there can be a difference between using
542 /// `{ expr }` and `expr` in pattern-matching contexts. This means
543 /// that we should only lint `unused_parens` and not `unused_braces`
548 /// let ref b = { a }; // We actually borrow a copy of `a` here.
549 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
550 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
552 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
554 // this cannot be a constant is it refers to a static.
555 fn lint(&self) -> &'static Lint;
557 fn check_unused_delims_expr(
559 cx: &EarlyContext<'_>,
561 ctx: UnusedDelimsCtx,
562 followed_by_block: bool,
563 left_pos: Option<BytePos>,
564 right_pos: Option<BytePos>,
567 fn is_expr_delims_necessary(
569 followed_by_block: bool,
570 followed_by_else: bool,
572 if followed_by_else {
574 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
575 _ if classify::expr_trailing_brace(inner).is_some() => return true,
580 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
581 let lhs_needs_parens = {
582 let mut innermost = inner;
584 innermost = match &innermost.kind {
585 ExprKind::Binary(_, lhs, _rhs) => lhs,
586 ExprKind::Call(fn_, _params) => fn_,
587 ExprKind::Cast(expr, _ty) => expr,
588 ExprKind::Type(expr, _ty) => expr,
589 ExprKind::Index(base, _subscript) => base,
592 if !classify::expr_requires_semi_to_be_stmt(innermost) {
599 || (followed_by_block
600 && match &inner.kind {
602 | ExprKind::Break(..)
603 | ExprKind::Yield(..)
604 | ExprKind::Yeet(..) => true,
605 ExprKind::Range(_lhs, Some(rhs), _limits) => {
606 matches!(rhs.kind, ExprKind::Block(..))
608 _ => parser::contains_exterior_struct_lit(&inner),
612 fn emit_unused_delims_expr(
614 cx: &EarlyContext<'_>,
616 ctx: UnusedDelimsCtx,
617 left_pos: Option<BytePos>,
618 right_pos: Option<BytePos>,
620 // If `value` has `ExprKind::Err`, unused delim lint can be broken.
621 // For example, the following code caused ICE.
622 // This is because the `ExprKind::Call` in `value` has `ExprKind::Err` as its argument
623 // and this leads to wrong spans. #104897
628 use rustc_ast::visit::{walk_expr, Visitor};
629 struct ErrExprVisitor {
632 impl<'ast> Visitor<'ast> for ErrExprVisitor {
633 fn visit_expr(&mut self, expr: &'ast ast::Expr) {
634 if let ExprKind::Err = expr.kind {
635 self.has_error = true;
638 walk_expr(self, expr)
641 let mut visitor = ErrExprVisitor { has_error: false };
642 visitor.visit_expr(value);
643 if visitor.has_error {
646 let spans = match value.kind {
647 ast::ExprKind::Block(ref block, None) if block.stmts.len() == 1 => {
648 if let Some(span) = block.stmts[0].span.find_ancestor_inside(value.span) {
649 Some((value.span.with_hi(span.lo()), value.span.with_lo(span.hi())))
654 ast::ExprKind::Paren(ref expr) => {
655 let expr_span = expr.span.find_ancestor_inside(value.span);
656 if let Some(expr_span) = expr_span {
657 Some((value.span.with_hi(expr_span.lo()), value.span.with_lo(expr_span.hi())))
665 left_pos.map_or(false, |s| s >= value.span.lo()),
666 right_pos.map_or(false, |s| s <= value.span.hi()),
668 self.emit_unused_delims(cx, value.span, spans, ctx.into(), keep_space);
671 fn emit_unused_delims(
673 cx: &EarlyContext<'_>,
675 spans: Option<(Span, Span)>,
677 keep_space: (bool, bool),
679 let primary_span = if let Some((lo, hi)) = spans {
681 // do not point at delims that do not exist
684 MultiSpan::from(vec![lo, hi])
686 MultiSpan::from(value_span)
688 let suggestion = spans.map(|(lo, hi)| {
689 let sm = cx.sess().source_map();
692 let Ok(snip) = sm.span_to_prev_source(lo) && !snip.ends_with(' ') {
700 let Ok(snip) = sm.span_to_next_source(hi) && !snip.starts_with(' ') {
705 UnusedDelimSuggestion {
707 start_replace: lo_replace,
709 end_replace: hi_replace,
712 cx.emit_spanned_lint(
715 UnusedDelim { delim: Self::DELIM_STR, item: msg, suggestion },
719 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
720 use rustc_ast::ExprKind::*;
721 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
722 // Do not lint `unused_braces` in `if let` expressions.
723 If(ref cond, ref block, _)
724 if !matches!(cond.kind, Let(_, _, _))
725 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
727 let left = e.span.lo() + rustc_span::BytePos(2);
728 let right = block.span.lo();
729 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
732 // Do not lint `unused_braces` in `while let` expressions.
733 While(ref cond, ref block, ..)
734 if !matches!(cond.kind, Let(_, _, _))
735 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
737 let left = e.span.lo() + rustc_span::BytePos(5);
738 let right = block.span.lo();
739 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
742 ForLoop(_, ref cond, ref block, ..) => {
743 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
746 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
747 let left = e.span.lo() + rustc_span::BytePos(5);
748 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
751 Ret(Some(ref value)) => {
752 let left = e.span.lo() + rustc_span::BytePos(3);
753 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
756 Index(_, ref value) => (value, UnusedDelimsCtx::IndexExpr, false, None, None),
758 Assign(_, ref value, _) | AssignOp(.., ref value) => {
759 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
761 // either function/method call, or something this lint doesn't care about
762 ref call_or_other => {
763 let (args_to_check, ctx) = match *call_or_other {
764 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
765 MethodCall(ref call) => (&call.args[..], UnusedDelimsCtx::MethodArg),
766 // actual catch-all arm
771 // Don't lint if this is a nested macro expansion: otherwise, the lint could
772 // trigger in situations that macro authors shouldn't have to care about, e.g.,
773 // when a parenthesized token tree matched in one macro expansion is matched as
774 // an expression in another and used as a fn/method argument (Issue #47775)
775 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
778 for arg in args_to_check {
779 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
784 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
787 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
789 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
790 if let Some((init, els)) = local.kind.init_else_opt() {
791 let ctx = match els {
792 None => UnusedDelimsCtx::AssignedValue,
793 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
795 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
798 StmtKind::Expr(ref expr) => {
799 self.check_unused_delims_expr(
802 UnusedDelimsCtx::BlockRetValue,
812 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
813 use ast::ItemKind::*;
815 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
816 self.check_unused_delims_expr(
819 UnusedDelimsCtx::AssignedValue,
829 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
830 /// with parentheses; they do not need them.
842 /// The parentheses are not needed, and should be removed. This is the
843 /// preferred style for writing these expressions.
844 pub(super) UNUSED_PARENS,
846 "`if`, `match`, `while` and `return` do not need parentheses"
849 pub struct UnusedParens {
850 with_self_ty_parens: bool,
854 pub fn new() -> Self {
855 Self { with_self_ty_parens: false }
859 impl_lint_pass!(UnusedParens => [UNUSED_PARENS]);
861 impl UnusedDelimLint for UnusedParens {
862 const DELIM_STR: &'static str = "parentheses";
864 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
866 fn lint(&self) -> &'static Lint {
870 fn check_unused_delims_expr(
872 cx: &EarlyContext<'_>,
874 ctx: UnusedDelimsCtx,
875 followed_by_block: bool,
876 left_pos: Option<BytePos>,
877 right_pos: Option<BytePos>,
880 ast::ExprKind::Paren(ref inner) => {
881 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
882 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
883 && value.attrs.is_empty()
884 && !value.span.from_expansion()
885 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
886 || !matches!(inner.kind, ast::ExprKind::Binary(
887 rustc_span::source_map::Spanned { node, .. },
892 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
895 ast::ExprKind::Let(_, ref expr, _) => {
896 self.check_unused_delims_expr(
899 UnusedDelimsCtx::LetScrutineeExpr,
911 fn check_unused_parens_pat(
913 cx: &EarlyContext<'_>,
917 keep_space: (bool, bool),
919 use ast::{BindingAnnotation, PatKind};
921 if let PatKind::Paren(inner) = &value.kind {
923 // The lint visitor will visit each subpattern of `p`. We do not want to lint
924 // any range pattern no matter where it occurs in the pattern. For something like
925 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
926 // that if there are unnecessary parens they serve a purpose of readability.
927 PatKind::Range(..) => return,
928 // Avoid `p0 | .. | pn` if we should.
929 PatKind::Or(..) if avoid_or => return,
930 // Avoid `mut x` and `mut x @ p` if we should:
931 PatKind::Ident(BindingAnnotation::MUT, ..) if avoid_mut => {
934 // Otherwise proceed with linting.
937 let spans = if let Some(inner) = inner.span.find_ancestor_inside(value.span) {
938 Some((value.span.with_hi(inner.lo()), value.span.with_lo(inner.hi())))
942 self.emit_unused_delims(cx, value.span, spans, "pattern", keep_space);
947 impl EarlyLintPass for UnusedParens {
949 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
951 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
952 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
954 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
955 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
956 // want to complain about things like `if let 42 = (42)`.
957 ExprKind::If(ref cond, ref block, ref else_)
958 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
960 self.check_unused_delims_expr(
963 UnusedDelimsCtx::LetScrutineeExpr,
968 for stmt in &block.stmts {
969 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
971 if let Some(e) = else_ {
972 <Self as UnusedDelimLint>::check_expr(self, cx, e);
976 ExprKind::Match(ref _expr, ref arm) => {
978 self.check_unused_delims_expr(
981 UnusedDelimsCtx::MatchArmExpr,
991 <Self as UnusedDelimLint>::check_expr(self, cx, e)
994 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
995 use ast::{Mutability, PatKind::*};
996 let keep_space = (false, false);
998 // Do not lint on `(..)` as that will result in the other arms being useless.
1000 // The other cases do not contain sub-patterns.
1001 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
1002 // These are list-like patterns; parens can always be removed.
1003 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
1004 self.check_unused_parens_pat(cx, p, false, false, keep_space);
1006 Struct(_, _, fps, _) => for f in fps {
1007 self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
1009 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
1010 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
1011 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
1012 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
1013 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not, keep_space),
1017 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1018 if let StmtKind::Local(ref local) = s.kind {
1019 self.check_unused_parens_pat(cx, &local.pat, true, false, (false, false));
1022 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1025 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
1026 self.check_unused_parens_pat(cx, ¶m.pat, true, false, (false, false));
1029 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
1030 self.check_unused_parens_pat(cx, &arm.pat, false, false, (false, false));
1033 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1035 ast::TyKind::Array(_, len) => {
1036 self.check_unused_delims_expr(
1039 UnusedDelimsCtx::ArrayLenExpr,
1045 ast::TyKind::Paren(r) => {
1047 ast::TyKind::TraitObject(..) => {}
1048 ast::TyKind::BareFn(b)
1049 if self.with_self_ty_parens && b.generic_params.len() > 0 => {}
1050 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
1052 let spans = if let Some(r) = r.span.find_ancestor_inside(ty.span) {
1053 Some((ty.span.with_hi(r.lo()), ty.span.with_lo(r.hi())))
1057 self.emit_unused_delims(cx, ty.span, spans, "type", (false, false));
1060 self.with_self_ty_parens = false;
1066 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1067 <Self as UnusedDelimLint>::check_item(self, cx, item)
1070 fn enter_where_predicate(&mut self, _: &EarlyContext<'_>, pred: &ast::WherePredicate) {
1071 use rustc_ast::{WhereBoundPredicate, WherePredicate};
1072 if let WherePredicate::BoundPredicate(WhereBoundPredicate {
1074 bound_generic_params,
1077 let ast::TyKind::Paren(_) = &bounded_ty.kind &&
1078 bound_generic_params.is_empty() {
1079 self.with_self_ty_parens = true;
1083 fn exit_where_predicate(&mut self, _: &EarlyContext<'_>, _: &ast::WherePredicate) {
1084 assert!(!self.with_self_ty_parens);
1089 /// The `unused_braces` lint detects unnecessary braces around an
1104 /// The braces are not needed, and should be removed. This is the
1105 /// preferred style for writing these expressions.
1106 pub(super) UNUSED_BRACES,
1108 "unnecessary braces around an expression"
1111 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
1113 impl UnusedDelimLint for UnusedBraces {
1114 const DELIM_STR: &'static str = "braces";
1116 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
1118 fn lint(&self) -> &'static Lint {
1122 fn check_unused_delims_expr(
1124 cx: &EarlyContext<'_>,
1126 ctx: UnusedDelimsCtx,
1127 followed_by_block: bool,
1128 left_pos: Option<BytePos>,
1129 right_pos: Option<BytePos>,
1132 ast::ExprKind::Block(ref inner, None)
1133 if inner.rules == ast::BlockCheckMode::Default =>
1135 // emit a warning under the following conditions:
1137 // - the block does not have a label
1138 // - the block is not `unsafe`
1139 // - the block contains exactly one expression (do not lint `{ expr; }`)
1140 // - `followed_by_block` is true and the internal expr may contain a `{`
1141 // - the block is not multiline (do not lint multiline match arms)
1145 // somewhat_long_expression
1150 // - the block has no attribute and was not created inside a macro
1151 // - if the block is an `anon_const`, the inner expr must be a literal
1152 // not created by a macro, i.e. do not lint on:
1154 // struct A<const N: usize>;
1155 // let _: A<{ 2 + 3 }>;
1156 // let _: A<{produces_literal!()}>;
1158 // FIXME(const_generics): handle paths when #67075 is fixed.
1159 if let [stmt] = inner.stmts.as_slice() {
1160 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
1161 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
1162 && (ctx != UnusedDelimsCtx::AnonConst
1163 || (matches!(expr.kind, ast::ExprKind::Lit(_))
1164 && !expr.span.from_expansion()))
1165 && !cx.sess().source_map().is_multiline(value.span)
1166 && value.attrs.is_empty()
1167 && !value.span.from_expansion()
1168 && !inner.span.from_expansion()
1170 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
1175 ast::ExprKind::Let(_, ref expr, _) => {
1176 self.check_unused_delims_expr(
1179 UnusedDelimsCtx::LetScrutineeExpr,
1190 impl EarlyLintPass for UnusedBraces {
1191 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1192 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1196 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1197 <Self as UnusedDelimLint>::check_expr(self, cx, e);
1199 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1200 self.check_unused_delims_expr(
1203 UnusedDelimsCtx::AnonConst,
1211 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1212 if let ast::GenericArg::Const(ct) = arg {
1213 self.check_unused_delims_expr(
1216 UnusedDelimsCtx::AnonConst,
1224 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1225 if let Some(anon_const) = &v.disr_expr {
1226 self.check_unused_delims_expr(
1229 UnusedDelimsCtx::AnonConst,
1237 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1239 ast::TyKind::Array(_, ref len) => {
1240 self.check_unused_delims_expr(
1243 UnusedDelimsCtx::ArrayLenExpr,
1250 ast::TyKind::Typeof(ref anon_const) => {
1251 self.check_unused_delims_expr(
1254 UnusedDelimsCtx::AnonConst,
1265 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1266 <Self as UnusedDelimLint>::check_item(self, cx, item)
1271 /// The `unused_import_braces` lint catches unnecessary braces around an
1276 /// ```rust,compile_fail
1277 /// #![deny(unused_import_braces)]
1290 /// If there is only a single item, then remove the braces (`use test::A;`
1293 /// This lint is "allow" by default because it is only enforcing a
1294 /// stylistic choice.
1295 UNUSED_IMPORT_BRACES,
1297 "unnecessary braces around an imported item"
1300 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1302 impl UnusedImportBraces {
1303 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1304 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1305 // Recursively check nested UseTrees
1306 for (tree, _) in items {
1307 self.check_use_tree(cx, tree, item);
1310 // Trigger the lint only if there is one nested item
1311 if items.len() != 1 {
1315 // Trigger the lint if the nested item is a non-self single item
1316 let node_name = match items[0].0.kind {
1317 ast::UseTreeKind::Simple(rename) => {
1318 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1319 if orig_ident.name == kw::SelfLower {
1322 rename.unwrap_or(orig_ident).name
1324 ast::UseTreeKind::Glob => Symbol::intern("*"),
1325 ast::UseTreeKind::Nested(_) => return,
1328 cx.emit_spanned_lint(
1329 UNUSED_IMPORT_BRACES,
1331 UnusedImportBracesDiag { node: node_name },
1337 impl EarlyLintPass for UnusedImportBraces {
1338 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1339 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1340 self.check_use_tree(cx, use_tree, item);
1346 /// The `unused_allocation` lint detects unnecessary allocations that can
1352 /// #![feature(box_syntax)]
1354 /// let a = (box [1, 2, 3]).len();
1362 /// When a `box` expression is immediately coerced to a reference, then
1363 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1364 /// should be used instead to avoid the allocation.
1365 pub(super) UNUSED_ALLOCATION,
1367 "detects unnecessary allocations that can be eliminated"
1370 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1372 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1373 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1375 hir::ExprKind::Box(_) => {}
1379 for adj in cx.typeck_results().expr_adjustments(e) {
1380 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1382 adjustment::AutoBorrowMutability::Not => {
1383 cx.emit_spanned_lint(UNUSED_ALLOCATION, e.span, UnusedAllocationDiag);
1385 adjustment::AutoBorrowMutability::Mut { .. } => {
1386 cx.emit_spanned_lint(UNUSED_ALLOCATION, e.span, UnusedAllocationMutDiag);