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,
401 impl From<UnusedDelimsCtx> for &'static str {
402 fn from(ctx: UnusedDelimsCtx) -> &'static str {
404 UnusedDelimsCtx::FunctionArg => "function argument",
405 UnusedDelimsCtx::MethodArg => "method argument",
406 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
409 UnusedDelimsCtx::IfCond => "`if` condition",
410 UnusedDelimsCtx::WhileCond => "`while` condition",
411 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
412 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
413 UnusedDelimsCtx::ReturnValue => "`return` value",
414 UnusedDelimsCtx::BlockRetValue => "block return value",
415 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
416 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
421 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
422 trait UnusedDelimLint {
423 const DELIM_STR: &'static str;
425 /// Due to `ref` pattern, there can be a difference between using
426 /// `{ expr }` and `expr` in pattern-matching contexts. This means
427 /// that we should only lint `unused_parens` and not `unused_braces`
432 /// let ref b = { a }; // We actually borrow a copy of `a` here.
433 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
434 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
436 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
438 // this cannot be a constant is it refers to a static.
439 fn lint(&self) -> &'static Lint;
441 fn check_unused_delims_expr(
443 cx: &EarlyContext<'_>,
445 ctx: UnusedDelimsCtx,
446 followed_by_block: bool,
447 left_pos: Option<BytePos>,
448 right_pos: Option<BytePos>,
451 fn is_expr_delims_necessary(
453 followed_by_block: bool,
454 followed_by_else: bool,
456 if followed_by_else {
458 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
459 _ if classify::expr_trailing_brace(inner).is_some() => return true,
464 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
465 let lhs_needs_parens = {
466 let mut innermost = inner;
468 innermost = match &innermost.kind {
469 ExprKind::Binary(_, lhs, _rhs) => lhs,
470 ExprKind::Call(fn_, _params) => fn_,
471 ExprKind::Cast(expr, _ty) => expr,
472 ExprKind::Type(expr, _ty) => expr,
473 ExprKind::Index(base, _subscript) => base,
476 if !classify::expr_requires_semi_to_be_stmt(innermost) {
483 || (followed_by_block
484 && match &inner.kind {
485 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
486 ExprKind::Range(_lhs, Some(rhs), _limits) => {
487 matches!(rhs.kind, ExprKind::Block(..))
489 _ => parser::contains_exterior_struct_lit(&inner),
493 fn emit_unused_delims_expr(
495 cx: &EarlyContext<'_>,
497 ctx: UnusedDelimsCtx,
498 left_pos: Option<BytePos>,
499 right_pos: Option<BytePos>,
501 let spans = match value.kind {
502 ast::ExprKind::Block(ref block, None) if block.stmts.len() > 0 => {
503 let start = block.stmts[0].span;
504 let end = block.stmts[block.stmts.len() - 1].span;
505 if value.span.from_expansion() || start.from_expansion() || end.from_expansion() {
507 value.span.with_hi(value.span.lo() + BytePos(1)),
508 value.span.with_lo(value.span.hi() - BytePos(1)),
511 (value.span.with_hi(start.lo()), value.span.with_lo(end.hi()))
514 ast::ExprKind::Paren(ref expr) => {
515 if value.span.from_expansion() || expr.span.from_expansion() {
517 value.span.with_hi(value.span.lo() + BytePos(1)),
518 value.span.with_lo(value.span.hi() - BytePos(1)),
521 (value.span.with_hi(expr.span.lo()), value.span.with_lo(expr.span.hi()))
527 left_pos.map_or(false, |s| s >= value.span.lo()),
528 right_pos.map_or(false, |s| s <= value.span.hi()),
530 self.emit_unused_delims(cx, spans, ctx.into(), keep_space);
533 fn emit_unused_delims(
535 cx: &EarlyContext<'_>,
538 keep_space: (bool, bool),
540 // FIXME(flip1995): Quick and dirty fix for #70814. This should be fixed in rustdoc
542 if spans.0 == DUMMY_SP || spans.1 == DUMMY_SP {
546 cx.struct_span_lint(self.lint(), MultiSpan::from(vec![spans.0, spans.1]), |lint| {
547 let span_msg = format!("unnecessary {} around {}", Self::DELIM_STR, msg);
548 let mut err = lint.build(&span_msg);
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 let suggestion = format!("remove these {}", Self::DELIM_STR);
554 err.multipart_suggestion(&suggestion, replacement, Applicability::MachineApplicable);
559 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
560 use rustc_ast::ExprKind::*;
561 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
562 // Do not lint `unused_braces` in `if let` expressions.
563 If(ref cond, ref block, _)
564 if !matches!(cond.kind, Let(_, _, _))
565 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
567 let left = e.span.lo() + rustc_span::BytePos(2);
568 let right = block.span.lo();
569 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
572 // Do not lint `unused_braces` in `while let` expressions.
573 While(ref cond, ref block, ..)
574 if !matches!(cond.kind, Let(_, _, _))
575 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
577 let left = e.span.lo() + rustc_span::BytePos(5);
578 let right = block.span.lo();
579 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
582 ForLoop(_, ref cond, ref block, ..) => {
583 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
586 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
587 let left = e.span.lo() + rustc_span::BytePos(5);
588 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
591 Ret(Some(ref value)) => {
592 let left = e.span.lo() + rustc_span::BytePos(3);
593 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
596 Assign(_, ref value, _) | AssignOp(.., ref value) => {
597 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
599 // either function/method call, or something this lint doesn't care about
600 ref call_or_other => {
601 let (args_to_check, ctx) = match *call_or_other {
602 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
603 // first "argument" is self (which sometimes needs delims)
604 MethodCall(_, ref args, _) => (&args[1..], UnusedDelimsCtx::MethodArg),
605 // actual catch-all arm
610 // Don't lint if this is a nested macro expansion: otherwise, the lint could
611 // trigger in situations that macro authors shouldn't have to care about, e.g.,
612 // when a parenthesized token tree matched in one macro expansion is matched as
613 // an expression in another and used as a fn/method argument (Issue #47775)
614 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
617 for arg in args_to_check {
618 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
623 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
626 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
628 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
629 if let Some((init, els)) = local.kind.init_else_opt() {
630 let ctx = match els {
631 None => UnusedDelimsCtx::AssignedValue,
632 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
634 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
637 StmtKind::Expr(ref expr) => {
638 self.check_unused_delims_expr(
641 UnusedDelimsCtx::BlockRetValue,
651 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
652 use ast::ItemKind::*;
654 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
655 self.check_unused_delims_expr(
658 UnusedDelimsCtx::AssignedValue,
668 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
669 /// with parentheses; they do not need them.
681 /// The parentheses are not needed, and should be removed. This is the
682 /// preferred style for writing these expressions.
683 pub(super) UNUSED_PARENS,
685 "`if`, `match`, `while` and `return` do not need parentheses"
688 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
690 impl UnusedDelimLint for UnusedParens {
691 const DELIM_STR: &'static str = "parentheses";
693 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
695 fn lint(&self) -> &'static Lint {
699 fn check_unused_delims_expr(
701 cx: &EarlyContext<'_>,
703 ctx: UnusedDelimsCtx,
704 followed_by_block: bool,
705 left_pos: Option<BytePos>,
706 right_pos: Option<BytePos>,
709 ast::ExprKind::Paren(ref inner) => {
710 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
711 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
712 && value.attrs.is_empty()
713 && !value.span.from_expansion()
714 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
715 || !matches!(inner.kind, ast::ExprKind::Binary(
716 rustc_span::source_map::Spanned { node, .. },
721 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
724 ast::ExprKind::Let(_, ref expr, _) => {
725 self.check_unused_delims_expr(
728 UnusedDelimsCtx::LetScrutineeExpr,
740 fn check_unused_parens_pat(
742 cx: &EarlyContext<'_>,
747 use ast::{BindingMode, Mutability, PatKind};
749 if let PatKind::Paren(inner) = &value.kind {
751 // The lint visitor will visit each subpattern of `p`. We do not want to lint
752 // any range pattern no matter where it occurs in the pattern. For something like
753 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
754 // that if there are unnecessary parens they serve a purpose of readability.
755 PatKind::Range(..) => return,
756 // Avoid `p0 | .. | pn` if we should.
757 PatKind::Or(..) if avoid_or => return,
758 // Avoid `mut x` and `mut x @ p` if we should:
759 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..) if avoid_mut => return,
760 // Otherwise proceed with linting.
763 let spans = if value.span.from_expansion() || inner.span.from_expansion() {
765 value.span.with_hi(value.span.lo() + BytePos(1)),
766 value.span.with_lo(value.span.hi() - BytePos(1)),
769 (value.span.with_hi(inner.span.lo()), value.span.with_lo(inner.span.hi()))
771 self.emit_unused_delims(cx, spans, "pattern", (false, false));
776 impl EarlyLintPass for UnusedParens {
777 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
779 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
780 self.check_unused_parens_pat(cx, pat, false, false);
782 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
783 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
784 // want to complain about things like `if let 42 = (42)`.
785 ExprKind::If(ref cond, ref block, ref else_)
786 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
788 self.check_unused_delims_expr(
791 UnusedDelimsCtx::LetScrutineeExpr,
796 for stmt in &block.stmts {
797 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
799 if let Some(e) = else_ {
800 <Self as UnusedDelimLint>::check_expr(self, cx, e);
807 <Self as UnusedDelimLint>::check_expr(self, cx, e)
810 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
811 use ast::{Mutability, PatKind::*};
813 // Do not lint on `(..)` as that will result in the other arms being useless.
815 // The other cases do not contain sub-patterns.
816 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
817 // These are list-like patterns; parens can always be removed.
818 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
819 self.check_unused_parens_pat(cx, p, false, false);
821 Struct(_, _, fps, _) => for f in fps {
822 self.check_unused_parens_pat(cx, &f.pat, false, false);
824 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
825 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
826 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
827 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
828 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not),
832 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
833 if let StmtKind::Local(ref local) = s.kind {
834 self.check_unused_parens_pat(cx, &local.pat, true, false);
837 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
840 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
841 self.check_unused_parens_pat(cx, ¶m.pat, true, false);
844 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
845 self.check_unused_parens_pat(cx, &arm.pat, false, false);
848 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
849 if let ast::TyKind::Paren(r) = &ty.kind {
851 ast::TyKind::TraitObject(..) => {}
852 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
853 ast::TyKind::Array(_, len) => {
854 self.check_unused_delims_expr(
857 UnusedDelimsCtx::ArrayLenExpr,
864 let spans = if ty.span.from_expansion() || r.span.from_expansion() {
866 ty.span.with_hi(ty.span.lo() + BytePos(1)),
867 ty.span.with_lo(ty.span.hi() - BytePos(1)),
870 (ty.span.with_hi(r.span.lo()), ty.span.with_lo(r.span.hi()))
872 self.emit_unused_delims(cx, spans, "type", (false, false));
878 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
879 <Self as UnusedDelimLint>::check_item(self, cx, item)
884 /// The `unused_braces` lint detects unnecessary braces around an
899 /// The braces are not needed, and should be removed. This is the
900 /// preferred style for writing these expressions.
901 pub(super) UNUSED_BRACES,
903 "unnecessary braces around an expression"
906 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
908 impl UnusedDelimLint for UnusedBraces {
909 const DELIM_STR: &'static str = "braces";
911 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
913 fn lint(&self) -> &'static Lint {
917 fn check_unused_delims_expr(
919 cx: &EarlyContext<'_>,
921 ctx: UnusedDelimsCtx,
922 followed_by_block: bool,
923 left_pos: Option<BytePos>,
924 right_pos: Option<BytePos>,
927 ast::ExprKind::Block(ref inner, None)
928 if inner.rules == ast::BlockCheckMode::Default =>
930 // emit a warning under the following conditions:
932 // - the block does not have a label
933 // - the block is not `unsafe`
934 // - the block contains exactly one expression (do not lint `{ expr; }`)
935 // - `followed_by_block` is true and the internal expr may contain a `{`
936 // - the block is not multiline (do not lint multiline match arms)
940 // somewhat_long_expression
945 // - the block has no attribute and was not created inside a macro
946 // - if the block is an `anon_const`, the inner expr must be a literal
947 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
949 // FIXME(const_generics): handle paths when #67075 is fixed.
950 if let [stmt] = inner.stmts.as_slice() {
951 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
952 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
953 && (ctx != UnusedDelimsCtx::AnonConst
954 || matches!(expr.kind, ast::ExprKind::Lit(_)))
955 && !cx.sess().source_map().is_multiline(value.span)
956 && value.attrs.is_empty()
957 && !value.span.from_expansion()
959 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
964 ast::ExprKind::Let(_, ref expr, _) => {
965 self.check_unused_delims_expr(
968 UnusedDelimsCtx::LetScrutineeExpr,
979 impl EarlyLintPass for UnusedBraces {
980 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
981 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
984 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
985 <Self as UnusedDelimLint>::check_expr(self, cx, e);
987 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
988 self.check_unused_delims_expr(
991 UnusedDelimsCtx::AnonConst,
999 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1000 if let ast::GenericArg::Const(ct) = arg {
1001 self.check_unused_delims_expr(
1004 UnusedDelimsCtx::AnonConst,
1012 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1013 if let Some(anon_const) = &v.disr_expr {
1014 self.check_unused_delims_expr(
1017 UnusedDelimsCtx::AnonConst,
1025 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1027 ast::TyKind::Array(_, ref len) => {
1028 self.check_unused_delims_expr(
1031 UnusedDelimsCtx::ArrayLenExpr,
1038 ast::TyKind::Typeof(ref anon_const) => {
1039 self.check_unused_delims_expr(
1042 UnusedDelimsCtx::AnonConst,
1053 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1054 <Self as UnusedDelimLint>::check_item(self, cx, item)
1059 /// The `unused_import_braces` lint catches unnecessary braces around an
1064 /// ```rust,compile_fail
1065 /// #![deny(unused_import_braces)]
1078 /// If there is only a single item, then remove the braces (`use test::A;`
1081 /// This lint is "allow" by default because it is only enforcing a
1082 /// stylistic choice.
1083 UNUSED_IMPORT_BRACES,
1085 "unnecessary braces around an imported item"
1088 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1090 impl UnusedImportBraces {
1091 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1092 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1093 // Recursively check nested UseTrees
1094 for &(ref tree, _) in items {
1095 self.check_use_tree(cx, tree, item);
1098 // Trigger the lint only if there is one nested item
1099 if items.len() != 1 {
1103 // Trigger the lint if the nested item is a non-self single item
1104 let node_name = match items[0].0.kind {
1105 ast::UseTreeKind::Simple(rename, ..) => {
1106 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1107 if orig_ident.name == kw::SelfLower {
1110 rename.unwrap_or(orig_ident).name
1112 ast::UseTreeKind::Glob => Symbol::intern("*"),
1113 ast::UseTreeKind::Nested(_) => return,
1116 cx.struct_span_lint(UNUSED_IMPORT_BRACES, item.span, |lint| {
1117 lint.build(&format!("braces around {} is unnecessary", node_name)).emit();
1123 impl EarlyLintPass for UnusedImportBraces {
1124 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1125 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1126 self.check_use_tree(cx, use_tree, item);
1132 /// The `unused_allocation` lint detects unnecessary allocations that can
1138 /// #![feature(box_syntax)]
1140 /// let a = (box [1, 2, 3]).len();
1148 /// When a `box` expression is immediately coerced to a reference, then
1149 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1150 /// should be used instead to avoid the allocation.
1151 pub(super) UNUSED_ALLOCATION,
1153 "detects unnecessary allocations that can be eliminated"
1156 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1158 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1159 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1161 hir::ExprKind::Box(_) => {}
1165 for adj in cx.typeck_results().expr_adjustments(e) {
1166 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1167 cx.struct_span_lint(UNUSED_ALLOCATION, e.span, |lint| {
1169 adjustment::AutoBorrowMutability::Not => {
1170 "unnecessary allocation, use `&` instead"
1172 adjustment::AutoBorrowMutability::Mut { .. } => {
1173 "unnecessary allocation, use `&mut` instead"
1176 lint.build(msg).emit();