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_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 {
396 impl From<UnusedDelimsCtx> for &'static str {
397 fn from(ctx: UnusedDelimsCtx) -> &'static str {
399 UnusedDelimsCtx::FunctionArg => "function argument",
400 UnusedDelimsCtx::MethodArg => "method argument",
401 UnusedDelimsCtx::AssignedValue => "assigned value",
402 UnusedDelimsCtx::IfCond => "`if` condition",
403 UnusedDelimsCtx::WhileCond => "`while` condition",
404 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
405 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
406 UnusedDelimsCtx::ReturnValue => "`return` value",
407 UnusedDelimsCtx::BlockRetValue => "block return value",
408 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
409 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
414 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
415 trait UnusedDelimLint {
416 const DELIM_STR: &'static str;
418 /// Due to `ref` pattern, there can be a difference between using
419 /// `{ expr }` and `expr` in pattern-matching contexts. This means
420 /// that we should only lint `unused_parens` and not `unused_braces`
425 /// let ref b = { a }; // We actually borrow a copy of `a` here.
426 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
427 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
429 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
431 // this cannot be a constant is it refers to a static.
432 fn lint(&self) -> &'static Lint;
434 fn check_unused_delims_expr(
436 cx: &EarlyContext<'_>,
438 ctx: UnusedDelimsCtx,
439 followed_by_block: bool,
440 left_pos: Option<BytePos>,
441 right_pos: Option<BytePos>,
444 fn is_expr_delims_necessary(inner: &ast::Expr, followed_by_block: bool) -> bool {
445 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
446 let lhs_needs_parens = {
447 let mut innermost = inner;
449 if let ExprKind::Binary(_, lhs, _rhs) = &innermost.kind {
451 if !rustc_ast::util::classify::expr_requires_semi_to_be_stmt(innermost) {
461 || (followed_by_block
462 && match inner.kind {
463 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
464 _ => parser::contains_exterior_struct_lit(&inner),
468 fn emit_unused_delims_expr(
470 cx: &EarlyContext<'_>,
472 ctx: UnusedDelimsCtx,
473 left_pos: Option<BytePos>,
474 right_pos: Option<BytePos>,
476 let expr_text = if let Ok(snippet) = cx.sess().source_map().span_to_snippet(value.span) {
479 pprust::expr_to_string(value)
482 left_pos.map_or(false, |s| s >= value.span.lo()),
483 right_pos.map_or(false, |s| s <= value.span.hi()),
485 self.emit_unused_delims(cx, value.span, &expr_text, ctx.into(), keep_space);
488 fn emit_unused_delims(
490 cx: &EarlyContext<'_>,
494 keep_space: (bool, bool),
496 // FIXME(flip1995): Quick and dirty fix for #70814. This should be fixed in rustdoc
498 if span == DUMMY_SP {
502 cx.struct_span_lint(self.lint(), span, |lint| {
503 let span_msg = format!("unnecessary {} around {}", Self::DELIM_STR, msg);
504 let mut err = lint.build(&span_msg);
505 let mut ate_left_paren = false;
506 let mut ate_right_paren = false;
507 let parens_removed = pattern
508 .trim_matches(|c| match c {
513 ate_left_paren = true;
521 ate_right_paren = true;
530 let mut replace = if keep_space.0 {
531 let mut s = String::from(" ");
532 s.push_str(parens_removed);
535 String::from(parens_removed)
544 let suggestion = format!("remove these {}", Self::DELIM_STR);
546 err.span_suggestion_short(span, &suggestion, replace, Applicability::MachineApplicable);
551 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
552 use rustc_ast::ExprKind::*;
553 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
554 // Do not lint `unused_braces` in `if let` expressions.
555 If(ref cond, ref block, _)
556 if !matches!(cond.kind, Let(_, _, _))
557 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
559 let left = e.span.lo() + rustc_span::BytePos(2);
560 let right = block.span.lo();
561 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
564 // Do not lint `unused_braces` in `while let` expressions.
565 While(ref cond, ref block, ..)
566 if !matches!(cond.kind, Let(_, _, _))
567 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
569 let left = e.span.lo() + rustc_span::BytePos(5);
570 let right = block.span.lo();
571 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
574 ForLoop(_, ref cond, ref block, ..) => {
575 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
578 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
579 let left = e.span.lo() + rustc_span::BytePos(5);
580 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
583 Ret(Some(ref value)) => {
584 let left = e.span.lo() + rustc_span::BytePos(3);
585 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
588 Assign(_, ref value, _) | AssignOp(.., ref value) => {
589 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
591 // either function/method call, or something this lint doesn't care about
592 ref call_or_other => {
593 let (args_to_check, ctx) = match *call_or_other {
594 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
595 // first "argument" is self (which sometimes needs delims)
596 MethodCall(_, ref args, _) => (&args[1..], UnusedDelimsCtx::MethodArg),
597 // actual catch-all arm
602 // Don't lint if this is a nested macro expansion: otherwise, the lint could
603 // trigger in situations that macro authors shouldn't have to care about, e.g.,
604 // when a parenthesized token tree matched in one macro expansion is matched as
605 // an expression in another and used as a fn/method argument (Issue #47775)
606 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
609 for arg in args_to_check {
610 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
615 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
618 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
620 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
621 if let Some(ref value) = local.init {
622 self.check_unused_delims_expr(
625 UnusedDelimsCtx::AssignedValue,
632 StmtKind::Expr(ref expr) => {
633 self.check_unused_delims_expr(
636 UnusedDelimsCtx::BlockRetValue,
646 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
647 use ast::ItemKind::*;
649 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
650 self.check_unused_delims_expr(
653 UnusedDelimsCtx::AssignedValue,
663 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
664 /// with parentheses; they do not need them.
676 /// The parenthesis are not needed, and should be removed. This is the
677 /// preferred style for writing these expressions.
678 pub(super) UNUSED_PARENS,
680 "`if`, `match`, `while` and `return` do not need parentheses"
683 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
685 impl UnusedDelimLint for UnusedParens {
686 const DELIM_STR: &'static str = "parentheses";
688 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
690 fn lint(&self) -> &'static Lint {
694 fn check_unused_delims_expr(
696 cx: &EarlyContext<'_>,
698 ctx: UnusedDelimsCtx,
699 followed_by_block: bool,
700 left_pos: Option<BytePos>,
701 right_pos: Option<BytePos>,
704 ast::ExprKind::Paren(ref inner) => {
705 if !Self::is_expr_delims_necessary(inner, followed_by_block)
706 && value.attrs.is_empty()
707 && !value.span.from_expansion()
708 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
709 || !matches!(inner.kind, ast::ExprKind::Binary(
710 rustc_span::source_map::Spanned { node, .. },
715 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
718 ast::ExprKind::Let(_, ref expr, _) => {
719 self.check_unused_delims_expr(
722 UnusedDelimsCtx::LetScrutineeExpr,
734 fn check_unused_parens_pat(
736 cx: &EarlyContext<'_>,
741 use ast::{BindingMode, Mutability, PatKind};
743 if let PatKind::Paren(inner) = &value.kind {
745 // The lint visitor will visit each subpattern of `p`. We do not want to lint
746 // any range pattern no matter where it occurs in the pattern. For something like
747 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
748 // that if there are unnecessary parens they serve a purpose of readability.
749 PatKind::Range(..) => return,
750 // Avoid `p0 | .. | pn` if we should.
751 PatKind::Or(..) if avoid_or => return,
752 // Avoid `mut x` and `mut x @ p` if we should:
753 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..) if avoid_mut => return,
754 // Otherwise proceed with linting.
759 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(value.span) {
762 pprust::pat_to_string(value)
764 self.emit_unused_delims(cx, value.span, &pattern_text, "pattern", (false, false));
769 impl EarlyLintPass for UnusedParens {
770 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
772 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
773 self.check_unused_parens_pat(cx, pat, false, false);
775 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
776 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
777 // want to complain about things like `if let 42 = (42)`.
778 ExprKind::If(ref cond, ref block, ref else_)
779 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
781 self.check_unused_delims_expr(
784 UnusedDelimsCtx::LetScrutineeExpr,
789 for stmt in &block.stmts {
790 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
792 if let Some(e) = else_ {
793 <Self as UnusedDelimLint>::check_expr(self, cx, e);
800 <Self as UnusedDelimLint>::check_expr(self, cx, e)
803 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
804 use ast::{Mutability, PatKind::*};
806 // Do not lint on `(..)` as that will result in the other arms being useless.
808 // The other cases do not contain sub-patterns.
809 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
810 // These are list-like patterns; parens can always be removed.
811 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
812 self.check_unused_parens_pat(cx, p, false, false);
814 Struct(_, _, fps, _) => for f in fps {
815 self.check_unused_parens_pat(cx, &f.pat, false, false);
817 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
818 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
819 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
820 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
821 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not),
825 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
826 if let StmtKind::Local(ref local) = s.kind {
827 self.check_unused_parens_pat(cx, &local.pat, true, false);
830 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
833 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
834 self.check_unused_parens_pat(cx, ¶m.pat, true, false);
837 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
838 self.check_unused_parens_pat(cx, &arm.pat, false, false);
841 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
842 if let ast::TyKind::Paren(r) = &ty.kind {
844 ast::TyKind::TraitObject(..) => {}
845 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
846 ast::TyKind::Array(_, len) => {
847 self.check_unused_delims_expr(
850 UnusedDelimsCtx::ArrayLenExpr,
858 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(ty.span) {
861 pprust::ty_to_string(ty)
864 self.emit_unused_delims(cx, ty.span, &pattern_text, "type", (false, false));
870 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
871 <Self as UnusedDelimLint>::check_item(self, cx, item)
876 /// The `unused_braces` lint detects unnecessary braces around an
891 /// The braces are not needed, and should be removed. This is the
892 /// preferred style for writing these expressions.
893 pub(super) UNUSED_BRACES,
895 "unnecessary braces around an expression"
898 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
900 impl UnusedDelimLint for UnusedBraces {
901 const DELIM_STR: &'static str = "braces";
903 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
905 fn lint(&self) -> &'static Lint {
909 fn check_unused_delims_expr(
911 cx: &EarlyContext<'_>,
913 ctx: UnusedDelimsCtx,
914 followed_by_block: bool,
915 left_pos: Option<BytePos>,
916 right_pos: Option<BytePos>,
919 ast::ExprKind::Block(ref inner, None)
920 if inner.rules == ast::BlockCheckMode::Default =>
922 // emit a warning under the following conditions:
924 // - the block does not have a label
925 // - the block is not `unsafe`
926 // - the block contains exactly one expression (do not lint `{ expr; }`)
927 // - `followed_by_block` is true and the internal expr may contain a `{`
928 // - the block is not multiline (do not lint multiline match arms)
932 // somewhat_long_expression
937 // - the block has no attribute and was not created inside a macro
938 // - if the block is an `anon_const`, the inner expr must be a literal
939 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
941 // FIXME(const_generics): handle paths when #67075 is fixed.
942 if let [stmt] = inner.stmts.as_slice() {
943 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
944 if !Self::is_expr_delims_necessary(expr, followed_by_block)
945 && (ctx != UnusedDelimsCtx::AnonConst
946 || matches!(expr.kind, ast::ExprKind::Lit(_)))
947 && !cx.sess().source_map().is_multiline(value.span)
948 && value.attrs.is_empty()
949 && !value.span.from_expansion()
951 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
956 ast::ExprKind::Let(_, ref expr, _) => {
957 self.check_unused_delims_expr(
960 UnusedDelimsCtx::LetScrutineeExpr,
971 impl EarlyLintPass for UnusedBraces {
972 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
973 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
976 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
977 <Self as UnusedDelimLint>::check_expr(self, cx, e);
979 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
980 self.check_unused_delims_expr(
983 UnusedDelimsCtx::AnonConst,
991 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
992 if let ast::GenericArg::Const(ct) = arg {
993 self.check_unused_delims_expr(
996 UnusedDelimsCtx::AnonConst,
1004 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1005 if let Some(anon_const) = &v.disr_expr {
1006 self.check_unused_delims_expr(
1009 UnusedDelimsCtx::AnonConst,
1017 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1019 ast::TyKind::Array(_, ref len) => {
1020 self.check_unused_delims_expr(
1023 UnusedDelimsCtx::ArrayLenExpr,
1030 ast::TyKind::Typeof(ref anon_const) => {
1031 self.check_unused_delims_expr(
1034 UnusedDelimsCtx::AnonConst,
1045 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1046 <Self as UnusedDelimLint>::check_item(self, cx, item)
1051 /// The `unused_import_braces` lint catches unnecessary braces around an
1056 /// ```rust,compile_fail
1057 /// #![deny(unused_import_braces)]
1070 /// If there is only a single item, then remove the braces (`use test::A;`
1073 /// This lint is "allow" by default because it is only enforcing a
1074 /// stylistic choice.
1075 UNUSED_IMPORT_BRACES,
1077 "unnecessary braces around an imported item"
1080 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1082 impl UnusedImportBraces {
1083 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1084 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1085 // Recursively check nested UseTrees
1086 for &(ref tree, _) in items {
1087 self.check_use_tree(cx, tree, item);
1090 // Trigger the lint only if there is one nested item
1091 if items.len() != 1 {
1095 // Trigger the lint if the nested item is a non-self single item
1096 let node_name = match items[0].0.kind {
1097 ast::UseTreeKind::Simple(rename, ..) => {
1098 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1099 if orig_ident.name == kw::SelfLower {
1102 rename.unwrap_or(orig_ident).name
1104 ast::UseTreeKind::Glob => Symbol::intern("*"),
1105 ast::UseTreeKind::Nested(_) => return,
1108 cx.struct_span_lint(UNUSED_IMPORT_BRACES, item.span, |lint| {
1109 lint.build(&format!("braces around {} is unnecessary", node_name)).emit()
1115 impl EarlyLintPass for UnusedImportBraces {
1116 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1117 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1118 self.check_use_tree(cx, use_tree, item);
1124 /// The `unused_allocation` lint detects unnecessary allocations that can
1130 /// #![feature(box_syntax)]
1132 /// let a = (box [1, 2, 3]).len();
1140 /// When a `box` expression is immediately coerced to a reference, then
1141 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1142 /// should be used instead to avoid the allocation.
1143 pub(super) UNUSED_ALLOCATION,
1145 "detects unnecessary allocations that can be eliminated"
1148 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1150 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1151 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1153 hir::ExprKind::Box(_) => {}
1157 for adj in cx.typeck_results().expr_adjustments(e) {
1158 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1159 cx.struct_span_lint(UNUSED_ALLOCATION, e.span, |lint| {
1161 adjustment::AutoBorrowMutability::Not => {
1162 "unnecessary allocation, use `&` instead"
1164 adjustment::AutoBorrowMutability::Mut { .. } => {
1165 "unnecessary allocation, use `&mut` instead"
1168 lint.build(msg).emit()