2 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
4 use rustc_ast::util::{classify, parser};
5 use rustc_ast::{ExprKind, StmtKind};
6 use rustc_ast_pretty::pprust;
7 use rustc_errors::{pluralize, Applicability};
9 use rustc_hir::def::{DefKind, Res};
10 use rustc_hir::def_id::DefId;
11 use rustc_middle::ty::adjustment;
12 use rustc_middle::ty::{self, Ty};
13 use rustc_span::symbol::Symbol;
14 use rustc_span::symbol::{kw, sym};
15 use rustc_span::{BytePos, Span, DUMMY_SP};
18 /// The `unused_must_use` lint detects unused result of a type flagged as
24 /// fn returns_result() -> Result<(), ()> {
37 /// The `#[must_use]` attribute is an indicator that it is a mistake to
38 /// ignore the value. See [the reference] for more details.
40 /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
43 "unused result of a type flagged as `#[must_use]`",
44 report_in_external_macro
48 /// The `unused_results` lint checks for the unused result of an
49 /// expression in a statement.
53 /// ```rust,compile_fail
54 /// #![deny(unused_results)]
55 /// fn foo<T>() -> T { panic!() }
66 /// Ignoring the return value of a function may indicate a mistake. In
67 /// cases were it is almost certain that the result should be used, it is
68 /// recommended to annotate the function with the [`must_use` attribute].
69 /// Failure to use such a return value will trigger the [`unused_must_use`
70 /// lint] which is warn-by-default. The `unused_results` lint is
71 /// essentially the same, but triggers for *all* return values.
73 /// This lint is "allow" by default because it can be noisy, and may not be
74 /// an actual problem. For example, calling the `remove` method of a `Vec`
75 /// or `HashMap` returns the previous value, which you may not care about.
76 /// Using this lint would require explicitly ignoring or discarding such
79 /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
80 /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
83 "unused result of an expression in a statement"
86 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
88 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
89 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
90 let expr = match s.kind {
91 hir::StmtKind::Semi(ref expr) => &**expr,
95 if let hir::ExprKind::Ret(..) = expr.kind {
99 let ty = cx.typeck_results().expr_ty(&expr);
100 let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);
102 let mut fn_warned = false;
103 let mut op_warned = false;
104 let maybe_def_id = match expr.kind {
105 hir::ExprKind::Call(ref callee, _) => {
107 hir::ExprKind::Path(ref qpath) => {
108 match cx.qpath_res(qpath, callee.hir_id) {
109 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
110 // `Res::Local` if it was a closure, for which we
111 // do not currently support must-use linting
118 hir::ExprKind::MethodCall(..) => cx.typeck_results().type_dependent_def_id(expr.hir_id),
121 if let Some(def_id) = maybe_def_id {
122 fn_warned = check_must_use_def(cx, def_id, s.span, "return value of ", "");
123 } else if type_permits_lack_of_use {
124 // We don't warn about unused unit or uninhabited types.
125 // (See https://github.com/rust-lang/rust/issues/43806 for details.)
129 let must_use_op = match expr.kind {
130 // Hardcoding operators here seemed more expedient than the
131 // refactoring that would be needed to look up the `#[must_use]`
132 // attribute which does exist on the comparison trait methods
133 hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
139 | hir::BinOpKind::Gt => Some("comparison"),
141 | hir::BinOpKind::Sub
142 | hir::BinOpKind::Div
143 | hir::BinOpKind::Mul
144 | hir::BinOpKind::Rem => Some("arithmetic operation"),
145 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
146 hir::BinOpKind::BitXor
147 | hir::BinOpKind::BitAnd
148 | hir::BinOpKind::BitOr
149 | hir::BinOpKind::Shl
150 | hir::BinOpKind::Shr => Some("bitwise operation"),
152 hir::ExprKind::AddrOf(..) => Some("borrow"),
153 hir::ExprKind::Unary(..) => Some("unary operation"),
157 if let Some(must_use_op) = must_use_op {
158 cx.struct_span_lint(UNUSED_MUST_USE, expr.span, |lint| {
159 let mut lint = lint.build(&format!("unused {} that must be used", must_use_op));
160 lint.span_label(expr.span, &format!("the {} produces a value", must_use_op));
161 lint.span_suggestion_verbose(
162 expr.span.shrink_to_lo(),
163 "use `let _ = ...` to ignore the resulting value",
164 "let _ = ".to_string(),
165 Applicability::MachineApplicable,
172 if !(type_permits_lack_of_use || fn_warned || op_warned) {
173 cx.struct_span_lint(UNUSED_RESULTS, s.span, |lint| lint.build("unused result").emit());
176 // Returns whether an error has been emitted (and thus another does not need to be later).
177 fn check_must_use_ty<'tcx>(
178 cx: &LateContext<'tcx>,
180 expr: &hir::Expr<'_>,
187 || cx.tcx.is_ty_uninhabited_from(
188 cx.tcx.parent_module(expr.hir_id).to_def_id(),
196 let plural_suffix = pluralize!(plural_len);
199 ty::Adt(..) if ty.is_box() => {
200 let boxed_ty = ty.boxed_ty();
201 let descr_pre = &format!("{}boxed ", descr_pre);
202 check_must_use_ty(cx, boxed_ty, expr, span, descr_pre, descr_post, plural_len)
204 ty::Adt(def, _) => check_must_use_def(cx, def.did, span, descr_pre, descr_post),
205 ty::Opaque(def, _) => {
206 let mut has_emitted = false;
207 for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
208 // We only look at the `DefId`, so it is safe to skip the binder here.
209 if let ty::PredicateKind::Trait(ref poly_trait_predicate) =
210 predicate.kind().skip_binder()
212 let def_id = poly_trait_predicate.trait_ref.def_id;
214 &format!("{}implementer{} of ", descr_pre, plural_suffix,);
215 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
223 ty::Dynamic(binder, _) => {
224 let mut has_emitted = false;
225 for predicate in binder.iter() {
226 if let ty::ExistentialPredicate::Trait(ref trait_ref) =
227 predicate.skip_binder()
229 let def_id = trait_ref.def_id;
231 &format!(" trait object{}{}", plural_suffix, descr_post,);
232 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
240 ty::Tuple(ref tys) => {
241 let mut has_emitted = false;
242 let spans = if let hir::ExprKind::Tup(comps) = &expr.kind {
243 debug_assert_eq!(comps.len(), tys.len());
244 comps.iter().map(|e| e.span).collect()
248 for (i, ty) in tys.iter().map(|k| k.expect_ty()).enumerate() {
249 let descr_post = &format!(" in tuple element {}", i);
250 let span = *spans.get(i).unwrap_or(&span);
251 if check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, plural_len)
258 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
259 // If the array is empty we don't lint, to avoid false positives
260 Some(0) | None => false,
261 // If the array is definitely non-empty, we can do `#[must_use]` checking.
263 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix,);
264 check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, n as usize + 1)
268 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
269 let mut err = lint.build(&format!(
270 "unused {}closure{}{} that must be used",
271 descr_pre, plural_suffix, descr_post,
273 err.note("closures are lazy and do nothing unless called");
278 ty::Generator(..) => {
279 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
280 let mut err = lint.build(&format!(
281 "unused {}generator{}{} that must be used",
282 descr_pre, plural_suffix, descr_post,
284 err.note("generators are lazy and do nothing unless resumed");
293 // Returns whether an error has been emitted (and thus another does not need to be later).
294 // FIXME: Args desc_{pre,post}_path could be made lazy by taking Fn() -> &str, but this
295 // would make calling it a big awkward. Could also take String (so args are moved), but
296 // this would still require a copy into the format string, which would only be executed
298 fn check_must_use_def(
299 cx: &LateContext<'_>,
302 descr_pre_path: &str,
303 descr_post_path: &str,
305 for attr in cx.tcx.get_attrs(def_id).iter() {
306 if attr.has_name(sym::must_use) {
307 cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {
309 "unused {}`{}`{} that must be used",
311 cx.tcx.def_path_str(def_id),
314 let mut err = lint.build(&msg);
315 // check for #[must_use = "..."]
316 if let Some(note) = attr.value_str() {
317 err.note(¬e.as_str());
330 /// The `path_statements` lint detects path statements with no effect.
344 /// It is usually a mistake to have a statement that has no effect.
347 "path statements with no effect"
350 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
352 impl<'tcx> LateLintPass<'tcx> for PathStatements {
353 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
354 if let hir::StmtKind::Semi(expr) = s.kind {
355 if let hir::ExprKind::Path(_) = expr.kind {
356 cx.struct_span_lint(PATH_STATEMENTS, s.span, |lint| {
357 let ty = cx.typeck_results().expr_ty(expr);
358 if ty.needs_drop(cx.tcx, cx.param_env) {
359 let mut lint = lint.build("path statement drops value");
360 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
361 lint.span_suggestion(
363 "use `drop` to clarify the intent",
364 format!("drop({});", snippet),
365 Applicability::MachineApplicable,
368 lint.span_help(s.span, "use `drop` to clarify the intent");
372 lint.build("path statement with no effect").emit()
380 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
381 enum UnusedDelimsCtx {
385 AssignedValueLetElse,
397 impl From<UnusedDelimsCtx> for &'static str {
398 fn from(ctx: UnusedDelimsCtx) -> &'static str {
400 UnusedDelimsCtx::FunctionArg => "function argument",
401 UnusedDelimsCtx::MethodArg => "method argument",
402 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
405 UnusedDelimsCtx::IfCond => "`if` condition",
406 UnusedDelimsCtx::WhileCond => "`while` condition",
407 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
408 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
409 UnusedDelimsCtx::ReturnValue => "`return` value",
410 UnusedDelimsCtx::BlockRetValue => "block return value",
411 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
412 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
417 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
418 trait UnusedDelimLint {
419 const DELIM_STR: &'static str;
421 /// Due to `ref` pattern, there can be a difference between using
422 /// `{ expr }` and `expr` in pattern-matching contexts. This means
423 /// that we should only lint `unused_parens` and not `unused_braces`
428 /// let ref b = { a }; // We actually borrow a copy of `a` here.
429 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
430 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
432 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
434 // this cannot be a constant is it refers to a static.
435 fn lint(&self) -> &'static Lint;
437 fn check_unused_delims_expr(
439 cx: &EarlyContext<'_>,
441 ctx: UnusedDelimsCtx,
442 followed_by_block: bool,
443 left_pos: Option<BytePos>,
444 right_pos: Option<BytePos>,
447 fn is_expr_delims_necessary(
449 followed_by_block: bool,
450 followed_by_else: bool,
452 if followed_by_else {
454 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
455 _ if classify::expr_trailing_brace(inner).is_some() => return true,
460 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
461 let lhs_needs_parens = {
462 let mut innermost = inner;
464 if let ExprKind::Binary(_, lhs, _rhs) = &innermost.kind {
466 if !classify::expr_requires_semi_to_be_stmt(innermost) {
476 || (followed_by_block
477 && match inner.kind {
478 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
479 _ => parser::contains_exterior_struct_lit(&inner),
483 fn emit_unused_delims_expr(
485 cx: &EarlyContext<'_>,
487 ctx: UnusedDelimsCtx,
488 left_pos: Option<BytePos>,
489 right_pos: Option<BytePos>,
491 let expr_text = if let Ok(snippet) = cx.sess().source_map().span_to_snippet(value.span) {
494 pprust::expr_to_string(value)
497 left_pos.map_or(false, |s| s >= value.span.lo()),
498 right_pos.map_or(false, |s| s <= value.span.hi()),
500 self.emit_unused_delims(cx, value.span, &expr_text, ctx.into(), keep_space);
503 fn emit_unused_delims(
505 cx: &EarlyContext<'_>,
509 keep_space: (bool, bool),
511 // FIXME(flip1995): Quick and dirty fix for #70814. This should be fixed in rustdoc
513 if span == DUMMY_SP {
517 cx.struct_span_lint(self.lint(), span, |lint| {
518 let span_msg = format!("unnecessary {} around {}", Self::DELIM_STR, msg);
519 let mut err = lint.build(&span_msg);
520 let mut ate_left_paren = false;
521 let mut ate_right_paren = false;
522 let parens_removed = pattern
523 .trim_matches(|c| match c {
528 ate_left_paren = true;
536 ate_right_paren = true;
545 let mut replace = if keep_space.0 {
546 let mut s = String::from(" ");
547 s.push_str(parens_removed);
550 String::from(parens_removed)
559 let suggestion = format!("remove these {}", Self::DELIM_STR);
561 err.span_suggestion_short(span, &suggestion, replace, Applicability::MachineApplicable);
566 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
567 use rustc_ast::ExprKind::*;
568 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
569 // Do not lint `unused_braces` in `if let` expressions.
570 If(ref cond, ref block, _)
571 if !matches!(cond.kind, Let(_, _, _))
572 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
574 let left = e.span.lo() + rustc_span::BytePos(2);
575 let right = block.span.lo();
576 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
579 // Do not lint `unused_braces` in `while let` expressions.
580 While(ref cond, ref block, ..)
581 if !matches!(cond.kind, Let(_, _, _))
582 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
584 let left = e.span.lo() + rustc_span::BytePos(5);
585 let right = block.span.lo();
586 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
589 ForLoop(_, ref cond, ref block, ..) => {
590 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
593 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
594 let left = e.span.lo() + rustc_span::BytePos(5);
595 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
598 Ret(Some(ref value)) => {
599 let left = e.span.lo() + rustc_span::BytePos(3);
600 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
603 Assign(_, ref value, _) | AssignOp(.., ref value) => {
604 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
606 // either function/method call, or something this lint doesn't care about
607 ref call_or_other => {
608 let (args_to_check, ctx) = match *call_or_other {
609 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
610 // first "argument" is self (which sometimes needs delims)
611 MethodCall(_, ref args, _) => (&args[1..], UnusedDelimsCtx::MethodArg),
612 // actual catch-all arm
617 // Don't lint if this is a nested macro expansion: otherwise, the lint could
618 // trigger in situations that macro authors shouldn't have to care about, e.g.,
619 // when a parenthesized token tree matched in one macro expansion is matched as
620 // an expression in another and used as a fn/method argument (Issue #47775)
621 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
624 for arg in args_to_check {
625 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
630 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
633 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
635 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
636 if let Some((init, els)) = local.kind.init_else_opt() {
637 let ctx = match els {
638 None => UnusedDelimsCtx::AssignedValue,
639 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
641 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
644 StmtKind::Expr(ref expr) => {
645 self.check_unused_delims_expr(
648 UnusedDelimsCtx::BlockRetValue,
658 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
659 use ast::ItemKind::*;
661 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
662 self.check_unused_delims_expr(
665 UnusedDelimsCtx::AssignedValue,
675 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
676 /// with parentheses; they do not need them.
688 /// The parenthesis are not needed, and should be removed. This is the
689 /// preferred style for writing these expressions.
690 pub(super) UNUSED_PARENS,
692 "`if`, `match`, `while` and `return` do not need parentheses"
695 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
697 impl UnusedDelimLint for UnusedParens {
698 const DELIM_STR: &'static str = "parentheses";
700 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
702 fn lint(&self) -> &'static Lint {
706 fn check_unused_delims_expr(
708 cx: &EarlyContext<'_>,
710 ctx: UnusedDelimsCtx,
711 followed_by_block: bool,
712 left_pos: Option<BytePos>,
713 right_pos: Option<BytePos>,
716 ast::ExprKind::Paren(ref inner) => {
717 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
718 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
719 && value.attrs.is_empty()
720 && !value.span.from_expansion()
721 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
722 || !matches!(inner.kind, ast::ExprKind::Binary(
723 rustc_span::source_map::Spanned { node, .. },
728 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
731 ast::ExprKind::Let(_, ref expr, _) => {
732 self.check_unused_delims_expr(
735 UnusedDelimsCtx::LetScrutineeExpr,
747 fn check_unused_parens_pat(
749 cx: &EarlyContext<'_>,
754 use ast::{BindingMode, Mutability, PatKind};
756 if let PatKind::Paren(inner) = &value.kind {
758 // The lint visitor will visit each subpattern of `p`. We do not want to lint
759 // any range pattern no matter where it occurs in the pattern. For something like
760 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
761 // that if there are unnecessary parens they serve a purpose of readability.
762 PatKind::Range(..) => return,
763 // Avoid `p0 | .. | pn` if we should.
764 PatKind::Or(..) if avoid_or => return,
765 // Avoid `mut x` and `mut x @ p` if we should:
766 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..) if avoid_mut => return,
767 // Otherwise proceed with linting.
772 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(value.span) {
775 pprust::pat_to_string(value)
777 self.emit_unused_delims(cx, value.span, &pattern_text, "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);
813 <Self as UnusedDelimLint>::check_expr(self, cx, e)
816 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
817 use ast::{Mutability, PatKind::*};
819 // Do not lint on `(..)` as that will result in the other arms being useless.
821 // The other cases do not contain sub-patterns.
822 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
823 // These are list-like patterns; parens can always be removed.
824 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
825 self.check_unused_parens_pat(cx, p, false, false);
827 Struct(_, _, fps, _) => for f in fps {
828 self.check_unused_parens_pat(cx, &f.pat, false, false);
830 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
831 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
832 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
833 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
834 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not),
838 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
839 if let StmtKind::Local(ref local) = s.kind {
840 self.check_unused_parens_pat(cx, &local.pat, true, false);
843 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
846 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
847 self.check_unused_parens_pat(cx, ¶m.pat, true, false);
850 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
851 self.check_unused_parens_pat(cx, &arm.pat, false, false);
854 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
855 if let ast::TyKind::Paren(r) = &ty.kind {
857 ast::TyKind::TraitObject(..) => {}
858 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
859 ast::TyKind::Array(_, len) => {
860 self.check_unused_delims_expr(
863 UnusedDelimsCtx::ArrayLenExpr,
871 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(ty.span) {
874 pprust::ty_to_string(ty)
877 self.emit_unused_delims(cx, ty.span, &pattern_text, "type", (false, false));
883 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
884 <Self as UnusedDelimLint>::check_item(self, cx, item)
889 /// The `unused_braces` lint detects unnecessary braces around an
904 /// The braces are not needed, and should be removed. This is the
905 /// preferred style for writing these expressions.
906 pub(super) UNUSED_BRACES,
908 "unnecessary braces around an expression"
911 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
913 impl UnusedDelimLint for UnusedBraces {
914 const DELIM_STR: &'static str = "braces";
916 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
918 fn lint(&self) -> &'static Lint {
922 fn check_unused_delims_expr(
924 cx: &EarlyContext<'_>,
926 ctx: UnusedDelimsCtx,
927 followed_by_block: bool,
928 left_pos: Option<BytePos>,
929 right_pos: Option<BytePos>,
932 ast::ExprKind::Block(ref inner, None)
933 if inner.rules == ast::BlockCheckMode::Default =>
935 // emit a warning under the following conditions:
937 // - the block does not have a label
938 // - the block is not `unsafe`
939 // - the block contains exactly one expression (do not lint `{ expr; }`)
940 // - `followed_by_block` is true and the internal expr may contain a `{`
941 // - the block is not multiline (do not lint multiline match arms)
945 // somewhat_long_expression
950 // - the block has no attribute and was not created inside a macro
951 // - if the block is an `anon_const`, the inner expr must be a literal
952 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
954 // FIXME(const_generics): handle paths when #67075 is fixed.
955 if let [stmt] = inner.stmts.as_slice() {
956 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
957 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
958 && (ctx != UnusedDelimsCtx::AnonConst
959 || matches!(expr.kind, ast::ExprKind::Lit(_)))
960 && !cx.sess().source_map().is_multiline(value.span)
961 && value.attrs.is_empty()
962 && !value.span.from_expansion()
964 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
969 ast::ExprKind::Let(_, ref expr, _) => {
970 self.check_unused_delims_expr(
973 UnusedDelimsCtx::LetScrutineeExpr,
984 impl EarlyLintPass for UnusedBraces {
985 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
986 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
989 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
990 <Self as UnusedDelimLint>::check_expr(self, cx, e);
992 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
993 self.check_unused_delims_expr(
996 UnusedDelimsCtx::AnonConst,
1004 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1005 if let ast::GenericArg::Const(ct) = arg {
1006 self.check_unused_delims_expr(
1009 UnusedDelimsCtx::AnonConst,
1017 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1018 if let Some(anon_const) = &v.disr_expr {
1019 self.check_unused_delims_expr(
1022 UnusedDelimsCtx::AnonConst,
1030 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1032 ast::TyKind::Array(_, ref len) => {
1033 self.check_unused_delims_expr(
1036 UnusedDelimsCtx::ArrayLenExpr,
1043 ast::TyKind::Typeof(ref anon_const) => {
1044 self.check_unused_delims_expr(
1047 UnusedDelimsCtx::AnonConst,
1058 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1059 <Self as UnusedDelimLint>::check_item(self, cx, item)
1064 /// The `unused_import_braces` lint catches unnecessary braces around an
1069 /// ```rust,compile_fail
1070 /// #![deny(unused_import_braces)]
1083 /// If there is only a single item, then remove the braces (`use test::A;`
1086 /// This lint is "allow" by default because it is only enforcing a
1087 /// stylistic choice.
1088 UNUSED_IMPORT_BRACES,
1090 "unnecessary braces around an imported item"
1093 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1095 impl UnusedImportBraces {
1096 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1097 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1098 // Recursively check nested UseTrees
1099 for &(ref tree, _) in items {
1100 self.check_use_tree(cx, tree, item);
1103 // Trigger the lint only if there is one nested item
1104 if items.len() != 1 {
1108 // Trigger the lint if the nested item is a non-self single item
1109 let node_name = match items[0].0.kind {
1110 ast::UseTreeKind::Simple(rename, ..) => {
1111 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1112 if orig_ident.name == kw::SelfLower {
1115 rename.unwrap_or(orig_ident).name
1117 ast::UseTreeKind::Glob => Symbol::intern("*"),
1118 ast::UseTreeKind::Nested(_) => return,
1121 cx.struct_span_lint(UNUSED_IMPORT_BRACES, item.span, |lint| {
1122 lint.build(&format!("braces around {} is unnecessary", node_name)).emit()
1128 impl EarlyLintPass for UnusedImportBraces {
1129 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1130 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1131 self.check_use_tree(cx, use_tree, item);
1137 /// The `unused_allocation` lint detects unnecessary allocations that can
1143 /// #![feature(box_syntax)]
1145 /// let a = (box [1, 2, 3]).len();
1153 /// When a `box` expression is immediately coerced to a reference, then
1154 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1155 /// should be used instead to avoid the allocation.
1156 pub(super) UNUSED_ALLOCATION,
1158 "detects unnecessary allocations that can be eliminated"
1161 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1163 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1164 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1166 hir::ExprKind::Box(_) => {}
1170 for adj in cx.typeck_results().expr_adjustments(e) {
1171 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1172 cx.struct_span_lint(UNUSED_ALLOCATION, e.span, |lint| {
1174 adjustment::AutoBorrowMutability::Not => {
1175 "unnecessary allocation, use `&` instead"
1177 adjustment::AutoBorrowMutability::Mut { .. } => {
1178 "unnecessary allocation, use `&mut` instead"
1181 lint.build(msg).emit()