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_middle::ty::adjustment;
11 use rustc_middle::ty::{self, Ty};
12 use rustc_span::symbol::Symbol;
13 use rustc_span::symbol::{kw, sym};
14 use rustc_span::{BytePos, Span, DUMMY_SP};
17 /// The `unused_must_use` lint detects unused result of a type flagged as
23 /// fn returns_result() -> Result<(), ()> {
36 /// The `#[must_use]` attribute is an indicator that it is a mistake to
37 /// ignore the value. See [the reference] for more details.
39 /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
42 "unused result of a type flagged as `#[must_use]`",
43 report_in_external_macro
47 /// The `unused_results` lint checks for the unused result of an
48 /// expression in a statement.
52 /// ```rust,compile_fail
53 /// #![deny(unused_results)]
54 /// fn foo<T>() -> T { panic!() }
65 /// Ignoring the return value of a function may indicate a mistake. In
66 /// cases were it is almost certain that the result should be used, it is
67 /// recommended to annotate the function with the [`must_use` attribute].
68 /// Failure to use such a return value will trigger the [`unused_must_use`
69 /// lint] which is warn-by-default. The `unused_results` lint is
70 /// essentially the same, but triggers for *all* return values.
72 /// This lint is "allow" by default because it can be noisy, and may not be
73 /// an actual problem. For example, calling the `remove` method of a `Vec`
74 /// or `HashMap` returns the previous value, which you may not care about.
75 /// Using this lint would require explicitly ignoring or discarding such
78 /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
79 /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
82 "unused result of an expression in a statement"
85 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
87 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
88 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
89 let expr = match s.kind {
90 hir::StmtKind::Semi(ref expr) => &**expr,
94 if let hir::ExprKind::Ret(..) = expr.kind {
98 let ty = cx.typeck_results().expr_ty(&expr);
99 let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);
101 let mut fn_warned = false;
102 let mut op_warned = false;
103 let maybe_def_id = match expr.kind {
104 hir::ExprKind::Call(ref callee, _) => {
106 hir::ExprKind::Path(ref qpath) => {
107 match cx.qpath_res(qpath, callee.hir_id) {
108 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
109 // `Res::Local` if it was a closure, for which we
110 // do not currently support must-use linting
117 hir::ExprKind::MethodCall(..) => cx.typeck_results().type_dependent_def_id(expr.hir_id),
120 if let Some(def_id) = maybe_def_id {
121 fn_warned = check_must_use_def(cx, def_id, s.span, "return value of ", "");
122 } else if type_permits_lack_of_use {
123 // We don't warn about unused unit or uninhabited types.
124 // (See https://github.com/rust-lang/rust/issues/43806 for details.)
128 let must_use_op = match expr.kind {
129 // Hardcoding operators here seemed more expedient than the
130 // refactoring that would be needed to look up the `#[must_use]`
131 // attribute which does exist on the comparison trait methods
132 hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
138 | hir::BinOpKind::Gt => Some("comparison"),
140 | hir::BinOpKind::Sub
141 | hir::BinOpKind::Div
142 | hir::BinOpKind::Mul
143 | hir::BinOpKind::Rem => Some("arithmetic operation"),
144 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
145 hir::BinOpKind::BitXor
146 | hir::BinOpKind::BitAnd
147 | hir::BinOpKind::BitOr
148 | hir::BinOpKind::Shl
149 | hir::BinOpKind::Shr => Some("bitwise operation"),
151 hir::ExprKind::AddrOf(..) => Some("borrow"),
152 hir::ExprKind::Unary(..) => Some("unary operation"),
156 if let Some(must_use_op) = must_use_op {
157 cx.struct_span_lint(UNUSED_MUST_USE, expr.span, |lint| {
158 lint.build(fluent::lint::unused_op)
159 .set_arg("op", must_use_op)
160 .span_label(expr.span, fluent::lint::label)
161 .span_suggestion_verbose(
162 expr.span.shrink_to_lo(),
163 fluent::lint::suggestion,
165 Applicability::MachineApplicable,
172 if !(type_permits_lack_of_use || fn_warned || op_warned) {
173 cx.struct_span_lint(UNUSED_RESULTS, s.span, |lint| {
174 lint.build(fluent::lint::unused_result).set_arg("ty", ty).emit();
178 // Returns whether an error has been emitted (and thus another does not need to be later).
179 fn check_must_use_ty<'tcx>(
180 cx: &LateContext<'tcx>,
182 expr: &hir::Expr<'_>,
189 || cx.tcx.is_ty_uninhabited_from(
190 cx.tcx.parent_module(expr.hir_id).to_def_id(),
198 let plural_suffix = pluralize!(plural_len);
201 ty::Adt(..) if ty.is_box() => {
202 let boxed_ty = ty.boxed_ty();
203 let descr_pre = &format!("{}boxed ", descr_pre);
204 check_must_use_ty(cx, boxed_ty, expr, span, descr_pre, descr_post, plural_len)
206 ty::Adt(def, _) => check_must_use_def(cx, def.did(), span, descr_pre, descr_post),
207 ty::Opaque(def, _) => {
208 let mut has_emitted = false;
209 for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
210 // We only look at the `DefId`, so it is safe to skip the binder here.
211 if let ty::PredicateKind::Trait(ref poly_trait_predicate) =
212 predicate.kind().skip_binder()
214 let def_id = poly_trait_predicate.trait_ref.def_id;
216 &format!("{}implementer{} of ", descr_pre, plural_suffix,);
217 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
225 ty::Dynamic(binder, _) => {
226 let mut has_emitted = false;
227 for predicate in binder.iter() {
228 if let ty::ExistentialPredicate::Trait(ref trait_ref) =
229 predicate.skip_binder()
231 let def_id = trait_ref.def_id;
233 &format!(" trait object{}{}", plural_suffix, descr_post,);
234 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
242 ty::Tuple(ref tys) => {
243 let mut has_emitted = false;
244 let comps = if let hir::ExprKind::Tup(comps) = expr.kind {
245 debug_assert_eq!(comps.len(), tys.len());
250 for (i, ty) in tys.iter().enumerate() {
251 let descr_post = &format!(" in tuple element {}", i);
252 let e = comps.get(i).unwrap_or(expr);
254 if check_must_use_ty(cx, ty, e, span, descr_pre, descr_post, plural_len) {
260 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
261 // If the array is empty we don't lint, to avoid false positives
262 Some(0) | None => false,
263 // If the array is definitely non-empty, we can do `#[must_use]` checking.
265 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix,);
266 check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, n as usize + 1)
270 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
271 // FIXME(davidtwco): this isn't properly translatable becauses of the
273 lint.build(fluent::lint::unused_closure)
274 .set_arg("count", plural_len)
275 .set_arg("pre", descr_pre)
276 .set_arg("post", descr_post)
277 .note(fluent::lint::note)
282 ty::Generator(..) => {
283 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
284 // FIXME(davidtwco): this isn't properly translatable becauses of the
286 lint.build(fluent::lint::unused_generator)
287 .set_arg("count", plural_len)
288 .set_arg("pre", descr_pre)
289 .set_arg("post", descr_post)
290 .note(fluent::lint::note)
299 // Returns whether an error has been emitted (and thus another does not need to be later).
300 // FIXME: Args desc_{pre,post}_path could be made lazy by taking Fn() -> &str, but this
301 // would make calling it a big awkward. Could also take String (so args are moved), but
302 // this would still require a copy into the format string, which would only be executed
304 fn check_must_use_def(
305 cx: &LateContext<'_>,
308 descr_pre_path: &str,
309 descr_post_path: &str,
311 if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
312 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
313 // FIXME(davidtwco): this isn't properly translatable becauses of the pre/post
315 let mut err = lint.build(fluent::lint::unused_def);
316 err.set_arg("pre", descr_pre_path);
317 err.set_arg("post", descr_post_path);
318 err.set_arg("def", cx.tcx.def_path_str(def_id));
319 // check for #[must_use = "..."]
320 if let Some(note) = attr.value_str() {
321 err.note(note.as_str());
334 /// The `path_statements` lint detects path statements with no effect.
348 /// It is usually a mistake to have a statement that has no effect.
351 "path statements with no effect"
354 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
356 impl<'tcx> LateLintPass<'tcx> for PathStatements {
357 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
358 if let hir::StmtKind::Semi(expr) = s.kind {
359 if let hir::ExprKind::Path(_) = expr.kind {
360 cx.struct_span_lint(PATH_STATEMENTS, s.span, |lint| {
361 let ty = cx.typeck_results().expr_ty(expr);
362 if ty.needs_drop(cx.tcx, cx.param_env) {
363 let mut lint = lint.build(fluent::lint::path_statement_drop);
364 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
365 lint.span_suggestion(
367 fluent::lint::suggestion,
368 format!("drop({});", snippet),
369 Applicability::MachineApplicable,
372 lint.span_help(s.span, fluent::lint::suggestion);
376 lint.build(fluent::lint::path_statement_no_effect).emit();
384 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
385 enum UnusedDelimsCtx {
389 AssignedValueLetElse,
402 impl From<UnusedDelimsCtx> for &'static str {
403 fn from(ctx: UnusedDelimsCtx) -> &'static str {
405 UnusedDelimsCtx::FunctionArg => "function argument",
406 UnusedDelimsCtx::MethodArg => "method argument",
407 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
410 UnusedDelimsCtx::IfCond => "`if` condition",
411 UnusedDelimsCtx::WhileCond => "`while` condition",
412 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
413 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
414 UnusedDelimsCtx::ReturnValue => "`return` value",
415 UnusedDelimsCtx::BlockRetValue => "block return value",
416 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
417 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
418 UnusedDelimsCtx::MatchArmExpr => "match arm expression",
423 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
424 trait UnusedDelimLint {
425 const DELIM_STR: &'static str;
427 /// Due to `ref` pattern, there can be a difference between using
428 /// `{ expr }` and `expr` in pattern-matching contexts. This means
429 /// that we should only lint `unused_parens` and not `unused_braces`
434 /// let ref b = { a }; // We actually borrow a copy of `a` here.
435 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
436 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
438 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
440 // this cannot be a constant is it refers to a static.
441 fn lint(&self) -> &'static Lint;
443 fn check_unused_delims_expr(
445 cx: &EarlyContext<'_>,
447 ctx: UnusedDelimsCtx,
448 followed_by_block: bool,
449 left_pos: Option<BytePos>,
450 right_pos: Option<BytePos>,
453 fn is_expr_delims_necessary(
455 followed_by_block: bool,
456 followed_by_else: bool,
458 if followed_by_else {
460 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
461 _ if classify::expr_trailing_brace(inner).is_some() => return true,
466 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
467 let lhs_needs_parens = {
468 let mut innermost = inner;
470 innermost = match &innermost.kind {
471 ExprKind::Binary(_, lhs, _rhs) => lhs,
472 ExprKind::Call(fn_, _params) => fn_,
473 ExprKind::Cast(expr, _ty) => expr,
474 ExprKind::Type(expr, _ty) => expr,
475 ExprKind::Index(base, _subscript) => base,
478 if !classify::expr_requires_semi_to_be_stmt(innermost) {
485 || (followed_by_block
486 && match &inner.kind {
487 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
488 ExprKind::Range(_lhs, Some(rhs), _limits) => {
489 matches!(rhs.kind, ExprKind::Block(..))
491 _ => parser::contains_exterior_struct_lit(&inner),
495 fn emit_unused_delims_expr(
497 cx: &EarlyContext<'_>,
499 ctx: UnusedDelimsCtx,
500 left_pos: Option<BytePos>,
501 right_pos: Option<BytePos>,
503 let spans = match value.kind {
504 ast::ExprKind::Block(ref block, None) if block.stmts.len() > 0 => {
505 let start = block.stmts[0].span;
506 let end = block.stmts[block.stmts.len() - 1].span;
507 if value.span.from_expansion() || start.from_expansion() || end.from_expansion() {
509 value.span.with_hi(value.span.lo() + BytePos(1)),
510 value.span.with_lo(value.span.hi() - BytePos(1)),
513 (value.span.with_hi(start.lo()), value.span.with_lo(end.hi()))
516 ast::ExprKind::Paren(ref expr) => {
517 if value.span.from_expansion() || expr.span.from_expansion() {
519 value.span.with_hi(value.span.lo() + BytePos(1)),
520 value.span.with_lo(value.span.hi() - BytePos(1)),
523 (value.span.with_hi(expr.span.lo()), value.span.with_lo(expr.span.hi()))
529 left_pos.map_or(false, |s| s >= value.span.lo()),
530 right_pos.map_or(false, |s| s <= value.span.hi()),
532 self.emit_unused_delims(cx, spans, ctx.into(), keep_space);
535 fn emit_unused_delims(
537 cx: &EarlyContext<'_>,
540 keep_space: (bool, bool),
542 // FIXME(flip1995): Quick and dirty fix for #70814. This should be fixed in rustdoc
544 if spans.0 == DUMMY_SP || spans.1 == DUMMY_SP {
548 cx.struct_span_lint(self.lint(), MultiSpan::from(vec![spans.0, spans.1]), |lint| {
549 let replacement = vec![
550 (spans.0, if keep_space.0 { " ".into() } else { "".into() }),
551 (spans.1, if keep_space.1 { " ".into() } else { "".into() }),
553 lint.build(fluent::lint::unused_delim)
554 .set_arg("delim", Self::DELIM_STR)
555 .set_arg("item", msg)
556 .multipart_suggestion(
557 fluent::lint::suggestion,
559 Applicability::MachineApplicable,
565 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
566 use rustc_ast::ExprKind::*;
567 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
568 // Do not lint `unused_braces` in `if let` expressions.
569 If(ref cond, ref block, _)
570 if !matches!(cond.kind, Let(_, _, _))
571 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
573 let left = e.span.lo() + rustc_span::BytePos(2);
574 let right = block.span.lo();
575 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
578 // Do not lint `unused_braces` in `while let` expressions.
579 While(ref cond, ref block, ..)
580 if !matches!(cond.kind, Let(_, _, _))
581 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
583 let left = e.span.lo() + rustc_span::BytePos(5);
584 let right = block.span.lo();
585 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
588 ForLoop(_, ref cond, ref block, ..) => {
589 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
592 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
593 let left = e.span.lo() + rustc_span::BytePos(5);
594 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
597 Ret(Some(ref value)) => {
598 let left = e.span.lo() + rustc_span::BytePos(3);
599 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
602 Assign(_, ref value, _) | AssignOp(.., ref value) => {
603 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
605 // either function/method call, or something this lint doesn't care about
606 ref call_or_other => {
607 let (args_to_check, ctx) = match *call_or_other {
608 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
609 // first "argument" is self (which sometimes needs delims)
610 MethodCall(_, ref args, _) => (&args[1..], UnusedDelimsCtx::MethodArg),
611 // actual catch-all arm
616 // Don't lint if this is a nested macro expansion: otherwise, the lint could
617 // trigger in situations that macro authors shouldn't have to care about, e.g.,
618 // when a parenthesized token tree matched in one macro expansion is matched as
619 // an expression in another and used as a fn/method argument (Issue #47775)
620 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
623 for arg in args_to_check {
624 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
629 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
632 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
634 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
635 if let Some((init, els)) = local.kind.init_else_opt() {
636 let ctx = match els {
637 None => UnusedDelimsCtx::AssignedValue,
638 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
640 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
643 StmtKind::Expr(ref expr) => {
644 self.check_unused_delims_expr(
647 UnusedDelimsCtx::BlockRetValue,
657 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
658 use ast::ItemKind::*;
660 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
661 self.check_unused_delims_expr(
664 UnusedDelimsCtx::AssignedValue,
674 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
675 /// with parentheses; they do not need them.
687 /// The parentheses are not needed, and should be removed. This is the
688 /// preferred style for writing these expressions.
689 pub(super) UNUSED_PARENS,
691 "`if`, `match`, `while` and `return` do not need parentheses"
694 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
696 impl UnusedDelimLint for UnusedParens {
697 const DELIM_STR: &'static str = "parentheses";
699 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
701 fn lint(&self) -> &'static Lint {
705 fn check_unused_delims_expr(
707 cx: &EarlyContext<'_>,
709 ctx: UnusedDelimsCtx,
710 followed_by_block: bool,
711 left_pos: Option<BytePos>,
712 right_pos: Option<BytePos>,
715 ast::ExprKind::Paren(ref inner) => {
716 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
717 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
718 && value.attrs.is_empty()
719 && !value.span.from_expansion()
720 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
721 || !matches!(inner.kind, ast::ExprKind::Binary(
722 rustc_span::source_map::Spanned { node, .. },
727 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
730 ast::ExprKind::Let(_, ref expr, _) => {
731 self.check_unused_delims_expr(
734 UnusedDelimsCtx::LetScrutineeExpr,
746 fn check_unused_parens_pat(
748 cx: &EarlyContext<'_>,
753 use ast::{BindingMode, Mutability, PatKind};
755 if let PatKind::Paren(inner) = &value.kind {
757 // The lint visitor will visit each subpattern of `p`. We do not want to lint
758 // any range pattern no matter where it occurs in the pattern. For something like
759 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
760 // that if there are unnecessary parens they serve a purpose of readability.
761 PatKind::Range(..) => return,
762 // Avoid `p0 | .. | pn` if we should.
763 PatKind::Or(..) if avoid_or => return,
764 // Avoid `mut x` and `mut x @ p` if we should:
765 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..) if avoid_mut => return,
766 // Otherwise proceed with linting.
769 let spans = if value.span.from_expansion() || inner.span.from_expansion() {
771 value.span.with_hi(value.span.lo() + BytePos(1)),
772 value.span.with_lo(value.span.hi() - BytePos(1)),
775 (value.span.with_hi(inner.span.lo()), value.span.with_lo(inner.span.hi()))
777 self.emit_unused_delims(cx, spans, "pattern", (false, false));
782 impl EarlyLintPass for UnusedParens {
783 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
785 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
786 self.check_unused_parens_pat(cx, pat, false, false);
788 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
789 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
790 // want to complain about things like `if let 42 = (42)`.
791 ExprKind::If(ref cond, ref block, ref else_)
792 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
794 self.check_unused_delims_expr(
797 UnusedDelimsCtx::LetScrutineeExpr,
802 for stmt in &block.stmts {
803 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
805 if let Some(e) = else_ {
806 <Self as UnusedDelimLint>::check_expr(self, cx, e);
810 ExprKind::Match(ref _expr, ref arm) => {
812 self.check_unused_delims_expr(
815 UnusedDelimsCtx::MatchArmExpr,
825 <Self as UnusedDelimLint>::check_expr(self, cx, e)
828 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
829 use ast::{Mutability, PatKind::*};
831 // Do not lint on `(..)` as that will result in the other arms being useless.
833 // The other cases do not contain sub-patterns.
834 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
835 // These are list-like patterns; parens can always be removed.
836 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
837 self.check_unused_parens_pat(cx, p, false, false);
839 Struct(_, _, fps, _) => for f in fps {
840 self.check_unused_parens_pat(cx, &f.pat, false, false);
842 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
843 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
844 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
845 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
846 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not),
850 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
851 if let StmtKind::Local(ref local) = s.kind {
852 self.check_unused_parens_pat(cx, &local.pat, true, false);
855 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
858 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
859 self.check_unused_parens_pat(cx, ¶m.pat, true, false);
862 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
863 self.check_unused_parens_pat(cx, &arm.pat, false, false);
866 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
867 if let ast::TyKind::Paren(r) = &ty.kind {
869 ast::TyKind::TraitObject(..) => {}
870 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
871 ast::TyKind::Array(_, len) => {
872 self.check_unused_delims_expr(
875 UnusedDelimsCtx::ArrayLenExpr,
882 let spans = if ty.span.from_expansion() || r.span.from_expansion() {
884 ty.span.with_hi(ty.span.lo() + BytePos(1)),
885 ty.span.with_lo(ty.span.hi() - BytePos(1)),
888 (ty.span.with_hi(r.span.lo()), ty.span.with_lo(r.span.hi()))
890 self.emit_unused_delims(cx, spans, "type", (false, false));
896 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
897 <Self as UnusedDelimLint>::check_item(self, cx, item)
902 /// The `unused_braces` lint detects unnecessary braces around an
917 /// The braces are not needed, and should be removed. This is the
918 /// preferred style for writing these expressions.
919 pub(super) UNUSED_BRACES,
921 "unnecessary braces around an expression"
924 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
926 impl UnusedDelimLint for UnusedBraces {
927 const DELIM_STR: &'static str = "braces";
929 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
931 fn lint(&self) -> &'static Lint {
935 fn check_unused_delims_expr(
937 cx: &EarlyContext<'_>,
939 ctx: UnusedDelimsCtx,
940 followed_by_block: bool,
941 left_pos: Option<BytePos>,
942 right_pos: Option<BytePos>,
945 ast::ExprKind::Block(ref inner, None)
946 if inner.rules == ast::BlockCheckMode::Default =>
948 // emit a warning under the following conditions:
950 // - the block does not have a label
951 // - the block is not `unsafe`
952 // - the block contains exactly one expression (do not lint `{ expr; }`)
953 // - `followed_by_block` is true and the internal expr may contain a `{`
954 // - the block is not multiline (do not lint multiline match arms)
958 // somewhat_long_expression
963 // - the block has no attribute and was not created inside a macro
964 // - if the block is an `anon_const`, the inner expr must be a literal
965 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
967 // FIXME(const_generics): handle paths when #67075 is fixed.
968 if let [stmt] = inner.stmts.as_slice() {
969 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
970 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
971 && (ctx != UnusedDelimsCtx::AnonConst
972 || matches!(expr.kind, ast::ExprKind::Lit(_)))
973 && !cx.sess().source_map().is_multiline(value.span)
974 && value.attrs.is_empty()
975 && !value.span.from_expansion()
977 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
982 ast::ExprKind::Let(_, ref expr, _) => {
983 self.check_unused_delims_expr(
986 UnusedDelimsCtx::LetScrutineeExpr,
997 impl EarlyLintPass for UnusedBraces {
998 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
999 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1002 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1003 <Self as UnusedDelimLint>::check_expr(self, cx, e);
1005 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1006 self.check_unused_delims_expr(
1009 UnusedDelimsCtx::AnonConst,
1017 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1018 if let ast::GenericArg::Const(ct) = arg {
1019 self.check_unused_delims_expr(
1022 UnusedDelimsCtx::AnonConst,
1030 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1031 if let Some(anon_const) = &v.disr_expr {
1032 self.check_unused_delims_expr(
1035 UnusedDelimsCtx::AnonConst,
1043 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1045 ast::TyKind::Array(_, ref len) => {
1046 self.check_unused_delims_expr(
1049 UnusedDelimsCtx::ArrayLenExpr,
1056 ast::TyKind::Typeof(ref anon_const) => {
1057 self.check_unused_delims_expr(
1060 UnusedDelimsCtx::AnonConst,
1071 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1072 <Self as UnusedDelimLint>::check_item(self, cx, item)
1077 /// The `unused_import_braces` lint catches unnecessary braces around an
1082 /// ```rust,compile_fail
1083 /// #![deny(unused_import_braces)]
1096 /// If there is only a single item, then remove the braces (`use test::A;`
1099 /// This lint is "allow" by default because it is only enforcing a
1100 /// stylistic choice.
1101 UNUSED_IMPORT_BRACES,
1103 "unnecessary braces around an imported item"
1106 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1108 impl UnusedImportBraces {
1109 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1110 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1111 // Recursively check nested UseTrees
1112 for &(ref tree, _) in items {
1113 self.check_use_tree(cx, tree, item);
1116 // Trigger the lint only if there is one nested item
1117 if items.len() != 1 {
1121 // Trigger the lint if the nested item is a non-self single item
1122 let node_name = match items[0].0.kind {
1123 ast::UseTreeKind::Simple(rename, ..) => {
1124 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1125 if orig_ident.name == kw::SelfLower {
1128 rename.unwrap_or(orig_ident).name
1130 ast::UseTreeKind::Glob => Symbol::intern("*"),
1131 ast::UseTreeKind::Nested(_) => return,
1134 cx.struct_span_lint(UNUSED_IMPORT_BRACES, item.span, |lint| {
1135 lint.build(fluent::lint::unused_import_braces).set_arg("node", node_name).emit();
1141 impl EarlyLintPass for UnusedImportBraces {
1142 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1143 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1144 self.check_use_tree(cx, use_tree, item);
1150 /// The `unused_allocation` lint detects unnecessary allocations that can
1156 /// #![feature(box_syntax)]
1158 /// let a = (box [1, 2, 3]).len();
1166 /// When a `box` expression is immediately coerced to a reference, then
1167 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1168 /// should be used instead to avoid the allocation.
1169 pub(super) UNUSED_ALLOCATION,
1171 "detects unnecessary allocations that can be eliminated"
1174 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1176 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1177 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1179 hir::ExprKind::Box(_) => {}
1183 for adj in cx.typeck_results().expr_adjustments(e) {
1184 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1185 cx.struct_span_lint(UNUSED_ALLOCATION, e.span, |lint| {
1186 lint.build(match m {
1187 adjustment::AutoBorrowMutability::Not => fluent::lint::unused_allocation,
1188 adjustment::AutoBorrowMutability::Mut { .. } => {
1189 fluent::lint::unused_allocation_mut