2 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
4 use rustc_ast::util::parser;
5 use rustc_ast::{ExprKind, StmtKind};
6 use rustc_ast_pretty::pprust;
7 use rustc_data_structures::fx::FxHashMap;
8 use rustc_errors::{pluralize, Applicability};
9 use rustc_feature::{AttributeType, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
11 use rustc_hir::def::{DefKind, Res};
12 use rustc_hir::def_id::DefId;
13 use rustc_middle::ty::adjustment;
14 use rustc_middle::ty::{self, Ty};
15 use rustc_session::lint::builtin::UNUSED_ATTRIBUTES;
16 use rustc_span::symbol::Symbol;
17 use rustc_span::symbol::{kw, sym};
18 use rustc_span::{BytePos, Span, DUMMY_SP};
23 /// The `unused_must_use` lint detects unused result of a type flagged as
29 /// fn returns_result() -> Result<(), ()> {
42 /// The `#[must_use]` attribute is an indicator that it is a mistake to
43 /// ignore the value. See [the reference] for more details.
45 /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
48 "unused result of a type flagged as `#[must_use]`",
49 report_in_external_macro
53 /// The `unused_results` lint checks for the unused result of an
54 /// expression in a statement.
58 /// ```rust,compile_fail
59 /// #![deny(unused_results)]
60 /// fn foo<T>() -> T { panic!() }
71 /// Ignoring the return value of a function may indicate a mistake. In
72 /// cases were it is almost certain that the result should be used, it is
73 /// recommended to annotate the function with the [`must_use` attribute].
74 /// Failure to use such a return value will trigger the [`unused_must_use`
75 /// lint] which is warn-by-default. The `unused_results` lint is
76 /// essentially the same, but triggers for *all* return values.
78 /// This lint is "allow" by default because it can be noisy, and may not be
79 /// an actual problem. For example, calling the `remove` method of a `Vec`
80 /// or `HashMap` returns the previous value, which you may not care about.
81 /// Using this lint would require explicitly ignoring or discarding such
84 /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
85 /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
88 "unused result of an expression in a statement"
91 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
93 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
94 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
95 let expr = match s.kind {
96 hir::StmtKind::Semi(ref expr) => &**expr,
100 if let hir::ExprKind::Ret(..) = expr.kind {
104 let ty = cx.typeck_results().expr_ty(&expr);
105 let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);
107 let mut fn_warned = false;
108 let mut op_warned = false;
109 let maybe_def_id = match expr.kind {
110 hir::ExprKind::Call(ref callee, _) => {
112 hir::ExprKind::Path(ref qpath) => {
113 match cx.qpath_res(qpath, callee.hir_id) {
114 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
115 // `Res::Local` if it was a closure, for which we
116 // do not currently support must-use linting
123 hir::ExprKind::MethodCall(..) => cx.typeck_results().type_dependent_def_id(expr.hir_id),
126 if let Some(def_id) = maybe_def_id {
127 fn_warned = check_must_use_def(cx, def_id, s.span, "return value of ", "");
128 } else if type_permits_lack_of_use {
129 // We don't warn about unused unit or uninhabited types.
130 // (See https://github.com/rust-lang/rust/issues/43806 for details.)
134 let must_use_op = match expr.kind {
135 // Hardcoding operators here seemed more expedient than the
136 // refactoring that would be needed to look up the `#[must_use]`
137 // attribute which does exist on the comparison trait methods
138 hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
144 | hir::BinOpKind::Gt => Some("comparison"),
146 | hir::BinOpKind::Sub
147 | hir::BinOpKind::Div
148 | hir::BinOpKind::Mul
149 | hir::BinOpKind::Rem => Some("arithmetic operation"),
150 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
151 hir::BinOpKind::BitXor
152 | hir::BinOpKind::BitAnd
153 | hir::BinOpKind::BitOr
154 | hir::BinOpKind::Shl
155 | hir::BinOpKind::Shr => Some("bitwise operation"),
157 hir::ExprKind::Unary(..) => Some("unary operation"),
161 if let Some(must_use_op) = must_use_op {
162 cx.struct_span_lint(UNUSED_MUST_USE, expr.span, |lint| {
163 lint.build(&format!("unused {} that must be used", must_use_op)).emit()
168 if !(type_permits_lack_of_use || fn_warned || op_warned) {
169 cx.struct_span_lint(UNUSED_RESULTS, s.span, |lint| lint.build("unused result").emit());
172 // Returns whether an error has been emitted (and thus another does not need to be later).
173 fn check_must_use_ty<'tcx>(
174 cx: &LateContext<'tcx>,
176 expr: &hir::Expr<'_>,
183 || cx.tcx.is_ty_uninhabited_from(
184 cx.tcx.parent_module(expr.hir_id).to_def_id(),
192 let plural_suffix = pluralize!(plural_len);
195 ty::Adt(..) if ty.is_box() => {
196 let boxed_ty = ty.boxed_ty();
197 let descr_pre = &format!("{}boxed ", descr_pre);
198 check_must_use_ty(cx, boxed_ty, expr, span, descr_pre, descr_post, plural_len)
200 ty::Adt(def, _) => check_must_use_def(cx, def.did, span, descr_pre, descr_post),
201 ty::Opaque(def, _) => {
202 let mut has_emitted = false;
203 for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
204 // We only look at the `DefId`, so it is safe to skip the binder here.
205 if let ty::PredicateAtom::Trait(ref poly_trait_predicate, _) =
206 predicate.skip_binders()
208 let def_id = poly_trait_predicate.trait_ref.def_id;
210 &format!("{}implementer{} of ", descr_pre, plural_suffix,);
211 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
219 ty::Dynamic(binder, _) => {
220 let mut has_emitted = false;
221 for predicate in binder.skip_binder().iter() {
222 if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate {
223 let def_id = trait_ref.def_id;
225 &format!(" trait object{}{}", plural_suffix, descr_post,);
226 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
234 ty::Tuple(ref tys) => {
235 let mut has_emitted = false;
236 let spans = if let hir::ExprKind::Tup(comps) = &expr.kind {
237 debug_assert_eq!(comps.len(), tys.len());
238 comps.iter().map(|e| e.span).collect()
242 for (i, ty) in tys.iter().map(|k| k.expect_ty()).enumerate() {
243 let descr_post = &format!(" in tuple element {}", i);
244 let span = *spans.get(i).unwrap_or(&span);
245 if check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, plural_len)
252 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
253 // If the array is empty we don't lint, to avoid false positives
254 Some(0) | None => false,
255 // If the array is definitely non-empty, we can do `#[must_use]` checking.
257 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix,);
258 check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, n as usize + 1)
262 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
263 let mut err = lint.build(&format!(
264 "unused {}closure{}{} that must be used",
265 descr_pre, plural_suffix, descr_post,
267 err.note("closures are lazy and do nothing unless called");
272 ty::Generator(..) => {
273 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
274 let mut err = lint.build(&format!(
275 "unused {}generator{}{} that must be used",
276 descr_pre, plural_suffix, descr_post,
278 err.note("generators are lazy and do nothing unless resumed");
287 // Returns whether an error has been emitted (and thus another does not need to be later).
288 // FIXME: Args desc_{pre,post}_path could be made lazy by taking Fn() -> &str, but this
289 // would make calling it a big awkward. Could also take String (so args are moved), but
290 // this would still require a copy into the format string, which would only be executed
292 fn check_must_use_def(
293 cx: &LateContext<'_>,
296 descr_pre_path: &str,
297 descr_post_path: &str,
299 for attr in cx.tcx.get_attrs(def_id).iter() {
300 if cx.sess().check_name(attr, sym::must_use) {
301 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
303 "unused {}`{}`{} that must be used",
305 cx.tcx.def_path_str(def_id),
308 let mut err = lint.build(&msg);
309 // check for #[must_use = "..."]
310 if let Some(note) = attr.value_str() {
311 err.note(¬e.as_str());
324 /// The `path_statements` lint detects path statements with no effect.
338 /// It is usually a mistake to have a statement that has no effect.
341 "path statements with no effect"
344 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
346 impl<'tcx> LateLintPass<'tcx> for PathStatements {
347 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
348 if let hir::StmtKind::Semi(expr) = s.kind {
349 if let hir::ExprKind::Path(_) = expr.kind {
350 cx.struct_span_lint(PATH_STATEMENTS, s.span, |lint| {
351 let ty = cx.typeck_results().expr_ty(expr);
352 if ty.needs_drop(cx.tcx, cx.param_env) {
353 let mut lint = lint.build("path statement drops value");
354 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
355 lint.span_suggestion(
357 "use `drop` to clarify the intent",
358 format!("drop({});", snippet),
359 Applicability::MachineApplicable,
362 lint.span_help(s.span, "use `drop` to clarify the intent");
366 lint.build("path statement with no effect").emit()
374 #[derive(Copy, Clone)]
375 pub struct UnusedAttributes {
376 builtin_attributes: &'static FxHashMap<Symbol, &'static BuiltinAttribute>,
379 impl UnusedAttributes {
380 pub fn new() -> Self {
381 UnusedAttributes { builtin_attributes: &*BUILTIN_ATTRIBUTE_MAP }
385 impl_lint_pass!(UnusedAttributes => [UNUSED_ATTRIBUTES]);
387 impl<'tcx> LateLintPass<'tcx> for UnusedAttributes {
388 fn check_attribute(&mut self, cx: &LateContext<'_>, attr: &ast::Attribute) {
389 debug!("checking attribute: {:?}", attr);
391 if attr.is_doc_comment() {
395 let attr_info = attr.ident().and_then(|ident| self.builtin_attributes.get(&ident.name));
397 if let Some(&&(name, ty, ..)) = attr_info {
398 if let AttributeType::AssumedUsed = ty {
399 debug!("{:?} is AssumedUsed", name);
404 if !cx.sess().is_attr_used(attr) {
405 debug!("emitting warning for: {:?}", attr);
406 cx.struct_span_lint(UNUSED_ATTRIBUTES, attr.span, |lint| {
407 lint.build("unused attribute").emit()
409 // Is it a builtin attribute that must be used at the crate level?
410 if attr_info.map_or(false, |(_, ty, ..)| ty == &AttributeType::CrateLevel) {
411 cx.struct_span_lint(UNUSED_ATTRIBUTES, attr.span, |lint| {
412 let msg = match attr.style {
413 ast::AttrStyle::Outer => {
414 "crate-level attribute should be an inner attribute: add an exclamation \
417 ast::AttrStyle::Inner => "crate-level attribute should be in the root module",
419 lint.build(msg).emit()
423 debug!("Attr was used: {:?}", attr);
428 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
429 enum UnusedDelimsCtx {
444 impl From<UnusedDelimsCtx> for &'static str {
445 fn from(ctx: UnusedDelimsCtx) -> &'static str {
447 UnusedDelimsCtx::FunctionArg => "function argument",
448 UnusedDelimsCtx::MethodArg => "method argument",
449 UnusedDelimsCtx::AssignedValue => "assigned value",
450 UnusedDelimsCtx::IfCond => "`if` condition",
451 UnusedDelimsCtx::WhileCond => "`while` condition",
452 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
453 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
454 UnusedDelimsCtx::ReturnValue => "`return` value",
455 UnusedDelimsCtx::BlockRetValue => "block return value",
456 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
457 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
462 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
463 trait UnusedDelimLint {
464 const DELIM_STR: &'static str;
466 /// Due to `ref` pattern, there can be a difference between using
467 /// `{ expr }` and `expr` in pattern-matching contexts. This means
468 /// that we should only lint `unused_parens` and not `unused_braces`
473 /// let ref b = { a }; // We actually borrow a copy of `a` here.
474 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
475 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
477 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
479 // this cannot be a constant is it refers to a static.
480 fn lint(&self) -> &'static Lint;
482 fn check_unused_delims_expr(
484 cx: &EarlyContext<'_>,
486 ctx: UnusedDelimsCtx,
487 followed_by_block: bool,
488 left_pos: Option<BytePos>,
489 right_pos: Option<BytePos>,
492 fn is_expr_delims_necessary(inner: &ast::Expr, followed_by_block: bool) -> bool {
493 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
494 let lhs_needs_parens = {
495 let mut innermost = inner;
497 if let ExprKind::Binary(_, lhs, _rhs) = &innermost.kind {
499 if !rustc_ast::util::classify::expr_requires_semi_to_be_stmt(innermost) {
509 || (followed_by_block
510 && match inner.kind {
511 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
512 _ => parser::contains_exterior_struct_lit(&inner),
516 fn emit_unused_delims_expr(
518 cx: &EarlyContext<'_>,
520 ctx: UnusedDelimsCtx,
521 left_pos: Option<BytePos>,
522 right_pos: Option<BytePos>,
524 let expr_text = if let Ok(snippet) = cx.sess().source_map().span_to_snippet(value.span) {
527 pprust::expr_to_string(value)
530 left_pos.map(|s| s >= value.span.lo()).unwrap_or(false),
531 right_pos.map(|s| s <= value.span.hi()).unwrap_or(false),
533 self.emit_unused_delims(cx, value.span, &expr_text, ctx.into(), keep_space);
536 fn emit_unused_delims(
538 cx: &EarlyContext<'_>,
542 keep_space: (bool, bool),
544 // FIXME(flip1995): Quick and dirty fix for #70814. This should be fixed in rustdoc
546 if span == DUMMY_SP {
550 cx.struct_span_lint(self.lint(), span, |lint| {
551 let span_msg = format!("unnecessary {} around {}", Self::DELIM_STR, msg);
552 let mut err = lint.build(&span_msg);
553 let mut ate_left_paren = false;
554 let mut ate_right_paren = false;
555 let parens_removed = pattern
556 .trim_matches(|c| match c {
561 ate_left_paren = true;
569 ate_right_paren = true;
578 let mut replace = if keep_space.0 {
579 let mut s = String::from(" ");
580 s.push_str(parens_removed);
583 String::from(parens_removed)
592 let suggestion = format!("remove these {}", Self::DELIM_STR);
594 err.span_suggestion_short(span, &suggestion, replace, Applicability::MachineApplicable);
599 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
600 use rustc_ast::ExprKind::*;
601 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
602 // Do not lint `unused_braces` in `if let` expressions.
603 If(ref cond, ref block, ..)
604 if !matches!(cond.kind, Let(_, _)) || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
606 let left = e.span.lo() + rustc_span::BytePos(2);
607 let right = block.span.lo();
608 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
611 // Do not lint `unused_braces` in `while let` expressions.
612 While(ref cond, ref block, ..)
613 if !matches!(cond.kind, Let(_, _)) || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
615 let left = e.span.lo() + rustc_span::BytePos(5);
616 let right = block.span.lo();
617 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
620 ForLoop(_, ref cond, ref block, ..) => {
621 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
624 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
625 let left = e.span.lo() + rustc_span::BytePos(5);
626 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
629 Ret(Some(ref value)) => {
630 let left = e.span.lo() + rustc_span::BytePos(3);
631 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
634 Assign(_, ref value, _) | AssignOp(.., ref value) => {
635 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
637 // either function/method call, or something this lint doesn't care about
638 ref call_or_other => {
639 let (args_to_check, ctx) = match *call_or_other {
640 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
641 // first "argument" is self (which sometimes needs delims)
642 MethodCall(_, ref args, _) => (&args[1..], UnusedDelimsCtx::MethodArg),
643 // actual catch-all arm
648 // Don't lint if this is a nested macro expansion: otherwise, the lint could
649 // trigger in situations that macro authors shouldn't have to care about, e.g.,
650 // when a parenthesized token tree matched in one macro expansion is matched as
651 // an expression in another and used as a fn/method argument (Issue #47775)
652 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
655 for arg in args_to_check {
656 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
661 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
664 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
666 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
667 if let Some(ref value) = local.init {
668 self.check_unused_delims_expr(
671 UnusedDelimsCtx::AssignedValue,
678 StmtKind::Expr(ref expr) => {
679 self.check_unused_delims_expr(
682 UnusedDelimsCtx::BlockRetValue,
692 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
693 use ast::ItemKind::*;
695 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
696 self.check_unused_delims_expr(
699 UnusedDelimsCtx::AssignedValue,
709 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
710 /// with parentheses; they do not need them.
722 /// The parenthesis are not needed, and should be removed. This is the
723 /// preferred style for writing these expressions.
724 pub(super) UNUSED_PARENS,
726 "`if`, `match`, `while` and `return` do not need parentheses"
729 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
731 impl UnusedDelimLint for UnusedParens {
732 const DELIM_STR: &'static str = "parentheses";
734 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
736 fn lint(&self) -> &'static Lint {
740 fn check_unused_delims_expr(
742 cx: &EarlyContext<'_>,
744 ctx: UnusedDelimsCtx,
745 followed_by_block: bool,
746 left_pos: Option<BytePos>,
747 right_pos: Option<BytePos>,
750 ast::ExprKind::Paren(ref inner) => {
751 if !Self::is_expr_delims_necessary(inner, followed_by_block)
752 && value.attrs.is_empty()
753 && !value.span.from_expansion()
754 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
755 || !matches!(inner.kind, ast::ExprKind::Binary(
756 rustc_span::source_map::Spanned { node, .. },
761 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
764 ast::ExprKind::Let(_, ref expr) => {
765 self.check_unused_delims_expr(
768 UnusedDelimsCtx::LetScrutineeExpr,
780 fn check_unused_parens_pat(
782 cx: &EarlyContext<'_>,
787 use ast::{BindingMode, Mutability, PatKind};
789 if let PatKind::Paren(inner) = &value.kind {
791 // The lint visitor will visit each subpattern of `p`. We do not want to lint
792 // any range pattern no matter where it occurs in the pattern. For something like
793 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
794 // that if there are unnecessary parens they serve a purpose of readability.
795 PatKind::Range(..) => return,
796 // Avoid `p0 | .. | pn` if we should.
797 PatKind::Or(..) if avoid_or => return,
798 // Avoid `mut x` and `mut x @ p` if we should:
799 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..) if avoid_mut => return,
800 // Otherwise proceed with linting.
805 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(value.span) {
808 pprust::pat_to_string(value)
810 self.emit_unused_delims(cx, value.span, &pattern_text, "pattern", (false, false));
815 impl EarlyLintPass for UnusedParens {
816 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
817 if let ExprKind::Let(ref pat, ..) | ExprKind::ForLoop(ref pat, ..) = e.kind {
818 self.check_unused_parens_pat(cx, pat, false, false);
821 <Self as UnusedDelimLint>::check_expr(self, cx, e)
824 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
825 use ast::{Mutability, PatKind::*};
827 // Do not lint on `(..)` as that will result in the other arms being useless.
829 // The other cases do not contain sub-patterns.
830 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
831 // These are list-like patterns; parens can always be removed.
832 TupleStruct(_, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
833 self.check_unused_parens_pat(cx, p, false, false);
835 Struct(_, fps, _) => for f in fps {
836 self.check_unused_parens_pat(cx, &f.pat, false, false);
838 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
839 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
840 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
841 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
842 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not),
846 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
847 if let StmtKind::Local(ref local) = s.kind {
848 self.check_unused_parens_pat(cx, &local.pat, false, false);
851 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
854 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
855 self.check_unused_parens_pat(cx, ¶m.pat, true, false);
858 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
859 self.check_unused_parens_pat(cx, &arm.pat, false, false);
862 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
863 if let &ast::TyKind::Paren(ref r) = &ty.kind {
865 &ast::TyKind::TraitObject(..) => {}
866 &ast::TyKind::ImplTrait(_, ref bounds) if bounds.len() > 1 => {}
867 &ast::TyKind::Array(_, ref len) => {
868 self.check_unused_delims_expr(
871 UnusedDelimsCtx::ArrayLenExpr,
879 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(ty.span) {
882 pprust::ty_to_string(ty)
885 self.emit_unused_delims(cx, ty.span, &pattern_text, "type", (false, false));
891 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
892 <Self as UnusedDelimLint>::check_item(self, cx, item)
897 /// The `unused_braces` lint detects unnecessary braces around an
912 /// The braces are not needed, and should be removed. This is the
913 /// preferred style for writing these expressions.
914 pub(super) UNUSED_BRACES,
916 "unnecessary braces around an expression"
919 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
921 impl UnusedDelimLint for UnusedBraces {
922 const DELIM_STR: &'static str = "braces";
924 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
926 fn lint(&self) -> &'static Lint {
930 fn check_unused_delims_expr(
932 cx: &EarlyContext<'_>,
934 ctx: UnusedDelimsCtx,
935 followed_by_block: bool,
936 left_pos: Option<BytePos>,
937 right_pos: Option<BytePos>,
940 ast::ExprKind::Block(ref inner, None)
941 if inner.rules == ast::BlockCheckMode::Default =>
943 // emit a warning under the following conditions:
945 // - the block does not have a label
946 // - the block is not `unsafe`
947 // - the block contains exactly one expression (do not lint `{ expr; }`)
948 // - `followed_by_block` is true and the internal expr may contain a `{`
949 // - the block is not multiline (do not lint multiline match arms)
953 // somewhat_long_expression
958 // - the block has no attribute and was not created inside a macro
959 // - if the block is an `anon_const`, the inner expr must be a literal
960 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
962 // FIXME(const_generics): handle paths when #67075 is fixed.
963 if let [stmt] = inner.stmts.as_slice() {
964 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
965 if !Self::is_expr_delims_necessary(expr, followed_by_block)
966 && (ctx != UnusedDelimsCtx::AnonConst
967 || matches!(expr.kind, ast::ExprKind::Lit(_)))
968 && !cx.sess().source_map().is_multiline(value.span)
969 && value.attrs.is_empty()
970 && !value.span.from_expansion()
972 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
977 ast::ExprKind::Let(_, ref expr) => {
978 // FIXME(#60336): Properly handle `let true = (false && true)`
979 // actually needing the parenthesis.
980 self.check_unused_delims_expr(
983 UnusedDelimsCtx::LetScrutineeExpr,
994 impl EarlyLintPass for UnusedBraces {
995 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
996 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
999 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1000 <Self as UnusedDelimLint>::check_expr(self, cx, e);
1002 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1003 self.check_unused_delims_expr(
1006 UnusedDelimsCtx::AnonConst,
1014 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1015 if let ast::GenericArg::Const(ct) = arg {
1016 self.check_unused_delims_expr(
1019 UnusedDelimsCtx::AnonConst,
1027 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1028 if let Some(anon_const) = &v.disr_expr {
1029 self.check_unused_delims_expr(
1032 UnusedDelimsCtx::AnonConst,
1040 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1042 ast::TyKind::Array(_, ref len) => {
1043 self.check_unused_delims_expr(
1046 UnusedDelimsCtx::ArrayLenExpr,
1053 ast::TyKind::Typeof(ref anon_const) => {
1054 self.check_unused_delims_expr(
1057 UnusedDelimsCtx::AnonConst,
1068 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1069 <Self as UnusedDelimLint>::check_item(self, cx, item)
1074 /// The `unused_import_braces` lint catches unnecessary braces around an
1079 /// ```rust,compile_fail
1080 /// #![deny(unused_import_braces)]
1093 /// If there is only a single item, then remove the braces (`use test::A;`
1096 /// This lint is "allow" by default because it is only enforcing a
1097 /// stylistic choice.
1098 UNUSED_IMPORT_BRACES,
1100 "unnecessary braces around an imported item"
1103 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1105 impl UnusedImportBraces {
1106 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1107 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1108 // Recursively check nested UseTrees
1109 for &(ref tree, _) in items {
1110 self.check_use_tree(cx, tree, item);
1113 // Trigger the lint only if there is one nested item
1114 if items.len() != 1 {
1118 // Trigger the lint if the nested item is a non-self single item
1119 let node_name = match items[0].0.kind {
1120 ast::UseTreeKind::Simple(rename, ..) => {
1121 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1122 if orig_ident.name == kw::SelfLower {
1125 rename.unwrap_or(orig_ident).name
1127 ast::UseTreeKind::Glob => Symbol::intern("*"),
1128 ast::UseTreeKind::Nested(_) => return,
1131 cx.struct_span_lint(UNUSED_IMPORT_BRACES, item.span, |lint| {
1132 lint.build(&format!("braces around {} is unnecessary", node_name)).emit()
1138 impl EarlyLintPass for UnusedImportBraces {
1139 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1140 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1141 self.check_use_tree(cx, use_tree, item);
1147 /// The `unused_allocation` lint detects unnecessary allocations that can
1153 /// #![feature(box_syntax)]
1155 /// let a = (box [1,2,3]).len();
1163 /// When a `box` expression is immediately coerced to a reference, then
1164 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1165 /// should be used instead to avoid the allocation.
1166 pub(super) UNUSED_ALLOCATION,
1168 "detects unnecessary allocations that can be eliminated"
1171 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1173 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1174 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1176 hir::ExprKind::Box(_) => {}
1180 for adj in cx.typeck_results().expr_adjustments(e) {
1181 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1182 cx.struct_span_lint(UNUSED_ALLOCATION, e.span, |lint| {
1184 adjustment::AutoBorrowMutability::Not => {
1185 "unnecessary allocation, use `&` instead"
1187 adjustment::AutoBorrowMutability::Mut { .. } => {
1188 "unnecessary allocation, use `&mut` instead"
1191 lint.build(msg).emit()