compiler/rustc_lint/src/unused.rs

   1 use crate::Lint;
   2 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
   3 use rustc_ast as ast;
   4 use rustc_ast::util::{classify, parser};
   5 use rustc_ast::{ExprKind, StmtKind};
   6 use rustc_errors::{fluent, pluralize, Applicability, MultiSpan};
   7 use rustc_hir as hir;
   8 use rustc_hir::def::{DefKind, Res};
   9 use rustc_hir::def_id::DefId;
  10 use rustc_infer::traits::util::elaborate_predicates_with_span;
  11 use rustc_middle::ty::adjustment;
  12 use rustc_middle::ty::{self, DefIdTree, Ty};
  13 use rustc_span::symbol::Symbol;
  14 use rustc_span::symbol::{kw, sym};
  15 use rustc_span::{BytePos, Span};
  16 use std::iter;
  17
  18 declare_lint! {
  19     /// The `unused_must_use` lint detects unused result of a type flagged as
  20     /// `#[must_use]`.
  21     ///
  22     /// ### Example
  23     ///
  24     /// ```rust
  25     /// fn returns_result() -> Result<(), ()> {
  26     ///     Ok(())
  27     /// }
  28     ///
  29     /// fn main() {
  30     ///     returns_result();
  31     /// }
  32     /// ```
  33     ///
  34     /// {{produces}}
  35     ///
  36     /// ### Explanation
  37     ///
  38     /// The `#[must_use]` attribute is an indicator that it is a mistake to
  39     /// ignore the value. See [the reference] for more details.
  40     ///
  41     /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
  42     pub UNUSED_MUST_USE,
  43     Warn,
  44     "unused result of a type flagged as `#[must_use]`",
  45     report_in_external_macro
  46 }
  47
  48 declare_lint! {
  49     /// The `unused_results` lint checks for the unused result of an
  50     /// expression in a statement.
  51     ///
  52     /// ### Example
  53     ///
  54     /// ```rust,compile_fail
  55     /// #![deny(unused_results)]
  56     /// fn foo<T>() -> T { panic!() }
  57     ///
  58     /// fn main() {
  59     ///     foo::<usize>();
  60     /// }
  61     /// ```
  62     ///
  63     /// {{produces}}
  64     ///
  65     /// ### Explanation
  66     ///
  67     /// Ignoring the return value of a function may indicate a mistake. In
  68     /// cases were it is almost certain that the result should be used, it is
  69     /// recommended to annotate the function with the [`must_use` attribute].
  70     /// Failure to use such a return value will trigger the [`unused_must_use`
  71     /// lint] which is warn-by-default. The `unused_results` lint is
  72     /// essentially the same, but triggers for *all* return values.
  73     ///
  74     /// This lint is "allow" by default because it can be noisy, and may not be
  75     /// an actual problem. For example, calling the `remove` method of a `Vec`
  76     /// or `HashMap` returns the previous value, which you may not care about.
  77     /// Using this lint would require explicitly ignoring or discarding such
  78     /// values.
  79     ///
  80     /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
  81     /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
  82     pub UNUSED_RESULTS,
  83     Allow,
  84     "unused result of an expression in a statement"
  85 }
  86
  87 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
  88
  89 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
  90     fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
  91         let hir::StmtKind::Semi(expr) = s.kind else { return; };
  92
  93         if let hir::ExprKind::Ret(..) = expr.kind {
  94             return;
  95         }
  96
  97         if let hir::ExprKind::Match(await_expr, _arms, hir::MatchSource::AwaitDesugar) = expr.kind
  98             && let ty = cx.typeck_results().expr_ty(&await_expr)
  99             && let ty::Opaque(future_def_id, _) = ty.kind()
 100             && cx.tcx.ty_is_opaque_future(ty)
 101             // FIXME: This also includes non-async fns that return `impl Future`.
 102             && let async_fn_def_id = cx.tcx.parent(*future_def_id)
 103             && check_must_use_def(
 104                 cx,
 105                 async_fn_def_id,
 106                 expr.span,
 107                 "output of future returned by ",
 108                 "",
 109             )
 110         {
 111             // We have a bare `foo().await;` on an opaque type from an async function that was
 112             // annotated with `#[must_use]`.
 113             return;
 114         }
 115
 116         let ty = cx.typeck_results().expr_ty(&expr);
 117
 118         let must_use_result = is_ty_must_use(cx, ty, &expr, expr.span);
 119         let type_lint_emitted_or_suppressed = match must_use_result {
 120             Some(path) => {
 121                 emit_must_use_untranslated(cx, &path, "", "", 1);
 122                 true
 123             }
 124             None => false,
 125         };
 126
 127         let fn_warned = check_fn_must_use(cx, expr);
 128
 129         if !fn_warned && type_lint_emitted_or_suppressed {
 130             // We don't warn about unused unit or uninhabited types.
 131             // (See https://github.com/rust-lang/rust/issues/43806 for details.)
 132             return;
 133         }
 134
 135         let must_use_op = match expr.kind {
 136             // Hardcoding operators here seemed more expedient than the
 137             // refactoring that would be needed to look up the `#[must_use]`
 138             // attribute which does exist on the comparison trait methods
 139             hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
 140                 hir::BinOpKind::Eq
 141                 | hir::BinOpKind::Lt
 142                 | hir::BinOpKind::Le
 143                 | hir::BinOpKind::Ne
 144                 | hir::BinOpKind::Ge
 145                 | hir::BinOpKind::Gt => Some("comparison"),
 146                 hir::BinOpKind::Add
 147                 | hir::BinOpKind::Sub
 148                 | hir::BinOpKind::Div
 149                 | hir::BinOpKind::Mul
 150                 | hir::BinOpKind::Rem => Some("arithmetic operation"),
 151                 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
 152                 hir::BinOpKind::BitXor
 153                 | hir::BinOpKind::BitAnd
 154                 | hir::BinOpKind::BitOr
 155                 | hir::BinOpKind::Shl
 156                 | hir::BinOpKind::Shr => Some("bitwise operation"),
 157             },
 158             hir::ExprKind::AddrOf(..) => Some("borrow"),
 159             hir::ExprKind::Unary(..) => Some("unary operation"),
 160             _ => None,
 161         };
 162
 163         let mut op_warned = false;
 164
 165         if let Some(must_use_op) = must_use_op {
 166             cx.struct_span_lint(UNUSED_MUST_USE, expr.span, fluent::lint_unused_op, |lint| {
 167                 lint.set_arg("op", must_use_op)
 168                     .span_label(expr.span, fluent::label)
 169                     .span_suggestion_verbose(
 170                         expr.span.shrink_to_lo(),
 171                         fluent::suggestion,
 172                         "let _ = ",
 173                         Applicability::MachineApplicable,
 174                     )
 175             });
 176             op_warned = true;
 177         }
 178
 179         if !(type_lint_emitted_or_suppressed || fn_warned || op_warned) {
 180             cx.struct_span_lint(UNUSED_RESULTS, s.span, fluent::lint_unused_result, |lint| {
 181                 lint.set_arg("ty", ty)
 182             });
 183         }
 184
 185         fn check_fn_must_use(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
 186             let maybe_def_id = match expr.kind {
 187                 hir::ExprKind::Call(ref callee, _) => {
 188                     match callee.kind {
 189                         hir::ExprKind::Path(ref qpath) => {
 190                             match cx.qpath_res(qpath, callee.hir_id) {
 191                                 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
 192                                 // `Res::Local` if it was a closure, for which we
 193                                 // do not currently support must-use linting
 194                                 _ => None,
 195                             }
 196                         }
 197                         _ => None,
 198                     }
 199                 }
 200                 hir::ExprKind::MethodCall(..) => {
 201                     cx.typeck_results().type_dependent_def_id(expr.hir_id)
 202                 }
 203                 _ => None,
 204             };
 205             if let Some(def_id) = maybe_def_id {
 206                 check_must_use_def(cx, def_id, expr.span, "return value of ", "")
 207             } else {
 208                 false
 209             }
 210         }
 211
 212         /// A path through a type to a must_use source. Contains useful info for the lint.
 213         #[derive(Debug)]
 214         enum MustUsePath {
 215             /// Suppress must_use checking.
 216             Suppressed,
 217             /// The root of the normal must_use lint with an optional message.
 218             Def(Span, DefId, Option<Symbol>),
 219             Boxed(Box<Self>),
 220             Opaque(Box<Self>),
 221             TraitObject(Box<Self>),
 222             TupleElement(Vec<(usize, Self)>),
 223             Array(Box<Self>, u64),
 224             /// The root of the unused_closures lint.
 225             Closure(Span),
 226             /// The root of the unused_generators lint.
 227             Generator(Span),
 228         }
 229
 230         #[instrument(skip(cx, expr), level = "debug", ret)]
 231         fn is_ty_must_use<'tcx>(
 232             cx: &LateContext<'tcx>,
 233             ty: Ty<'tcx>,
 234             expr: &hir::Expr<'_>,
 235             span: Span,
 236         ) -> Option<MustUsePath> {
 237             if ty.is_unit()
 238                 || !ty.is_inhabited_from(
 239                     cx.tcx,
 240                     cx.tcx.parent_module(expr.hir_id).to_def_id(),
 241                     cx.param_env,
 242                 )
 243             {
 244                 return Some(MustUsePath::Suppressed);
 245             }
 246
 247             match *ty.kind() {
 248                 ty::Adt(..) if ty.is_box() => {
 249                     let boxed_ty = ty.boxed_ty();
 250                     is_ty_must_use(cx, boxed_ty, expr, span)
 251                         .map(|inner| MustUsePath::Boxed(Box::new(inner)))
 252                 }
 253                 ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
 254                 ty::Opaque(def, _) => {
 255                     elaborate_predicates_with_span(
 256                         cx.tcx,
 257                         cx.tcx.explicit_item_bounds(def).iter().cloned(),
 258                     )
 259                     .filter_map(|obligation| {
 260                         // We only look at the `DefId`, so it is safe to skip the binder here.
 261                         if let ty::PredicateKind::Clause(ty::Clause::Trait(
 262                             ref poly_trait_predicate,
 263                         )) = obligation.predicate.kind().skip_binder()
 264                         {
 265                             let def_id = poly_trait_predicate.trait_ref.def_id;
 266
 267                             is_def_must_use(cx, def_id, span)
 268                         } else {
 269                             None
 270                         }
 271                     })
 272                     .map(|inner| MustUsePath::Opaque(Box::new(inner)))
 273                     .next()
 274                 }
 275                 ty::Dynamic(binders, _, _) => binders
 276                     .iter()
 277                     .filter_map(|predicate| {
 278                         if let ty::ExistentialPredicate::Trait(ref trait_ref) =
 279                             predicate.skip_binder()
 280                         {
 281                             let def_id = trait_ref.def_id;
 282                             is_def_must_use(cx, def_id, span)
 283                         } else {
 284                             None
 285                         }
 286                         .map(|inner| MustUsePath::TraitObject(Box::new(inner)))
 287                     })
 288                     .next(),
 289                 ty::Tuple(tys) => {
 290                     let elem_exprs = if let hir::ExprKind::Tup(elem_exprs) = expr.kind {
 291                         debug_assert_eq!(elem_exprs.len(), tys.len());
 292                         elem_exprs
 293                     } else {
 294                         &[]
 295                     };
 296
 297                     // Default to `expr`.
 298                     let elem_exprs = elem_exprs.iter().chain(iter::repeat(expr));
 299
 300                     let nested_must_use = tys
 301                         .iter()
 302                         .zip(elem_exprs)
 303                         .enumerate()
 304                         .filter_map(|(i, (ty, expr))| {
 305                             is_ty_must_use(cx, ty, expr, expr.span).map(|path| (i, path))
 306                         })
 307                         .collect::<Vec<_>>();
 308
 309                     if !nested_must_use.is_empty() {
 310                         Some(MustUsePath::TupleElement(nested_must_use))
 311                     } else {
 312                         None
 313                     }
 314                 }
 315                 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
 316                     // If the array is empty we don't lint, to avoid false positives
 317                     Some(0) | None => None,
 318                     // If the array is definitely non-empty, we can do `#[must_use]` checking.
 319                     Some(len) => is_ty_must_use(cx, ty, expr, span)
 320                         .map(|inner| MustUsePath::Array(Box::new(inner), len)),
 321                 },
 322                 ty::Closure(..) => Some(MustUsePath::Closure(span)),
 323                 ty::Generator(def_id, ..) => {
 324                     // async fn should be treated as "implementor of `Future`"
 325                     let must_use = if cx.tcx.generator_is_async(def_id) {
 326                         let def_id = cx.tcx.lang_items().future_trait().unwrap();
 327                         is_def_must_use(cx, def_id, span)
 328                             .map(|inner| MustUsePath::Opaque(Box::new(inner)))
 329                     } else {
 330                         None
 331                     };
 332                     must_use.or(Some(MustUsePath::Generator(span)))
 333                 }
 334                 _ => None,
 335             }
 336         }
 337
 338         fn is_def_must_use(cx: &LateContext<'_>, def_id: DefId, span: Span) -> Option<MustUsePath> {
 339             if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
 340                 // check for #[must_use = "..."]
 341                 let reason = attr.value_str();
 342                 Some(MustUsePath::Def(span, def_id, reason))
 343             } else {
 344                 None
 345             }
 346         }
 347
 348         // Returns whether further errors should be suppressed because either a lint has been emitted or the type should be ignored.
 349         fn check_must_use_def(
 350             cx: &LateContext<'_>,
 351             def_id: DefId,
 352             span: Span,
 353             descr_pre_path: &str,
 354             descr_post_path: &str,
 355         ) -> bool {
 356             is_def_must_use(cx, def_id, span)
 357                 .map(|must_use_path| {
 358                     emit_must_use_untranslated(
 359                         cx,
 360                         &must_use_path,
 361                         descr_pre_path,
 362                         descr_post_path,
 363                         1,
 364                     )
 365                 })
 366                 .is_some()
 367         }
 368
 369         #[instrument(skip(cx), level = "debug")]
 370         fn emit_must_use_untranslated(
 371             cx: &LateContext<'_>,
 372             path: &MustUsePath,
 373             descr_pre: &str,
 374             descr_post: &str,
 375             plural_len: usize,
 376         ) {
 377             let plural_suffix = pluralize!(plural_len);
 378
 379             match path {
 380                 MustUsePath::Suppressed => {}
 381                 MustUsePath::Boxed(path) => {
 382                     let descr_pre = &format!("{}boxed ", descr_pre);
 383                     emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
 384                 }
 385                 MustUsePath::Opaque(path) => {
 386                     let descr_pre = &format!("{}implementer{} of ", descr_pre, plural_suffix);
 387                     emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
 388                 }
 389                 MustUsePath::TraitObject(path) => {
 390                     let descr_post = &format!(" trait object{}{}", plural_suffix, descr_post);
 391                     emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
 392                 }
 393                 MustUsePath::TupleElement(elems) => {
 394                     for (index, path) in elems {
 395                         let descr_post = &format!(" in tuple element {}", index);
 396                         emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
 397                     }
 398                 }
 399                 MustUsePath::Array(path, len) => {
 400                     let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix);
 401                     emit_must_use_untranslated(
 402                         cx,
 403                         path,
 404                         descr_pre,
 405                         descr_post,
 406                         plural_len.saturating_add(usize::try_from(*len).unwrap_or(usize::MAX)),
 407                     );
 408                 }
 409                 MustUsePath::Closure(span) => {
 410                     cx.struct_span_lint(
 411                         UNUSED_MUST_USE,
 412                         *span,
 413                         fluent::lint_unused_closure,
 414                         |lint| {
 415                             // FIXME(davidtwco): this isn't properly translatable because of the
 416                             // pre/post strings
 417                             lint.set_arg("count", plural_len)
 418                                 .set_arg("pre", descr_pre)
 419                                 .set_arg("post", descr_post)
 420                                 .note(fluent::note)
 421                         },
 422                     );
 423                 }
 424                 MustUsePath::Generator(span) => {
 425                     cx.struct_span_lint(
 426                         UNUSED_MUST_USE,
 427                         *span,
 428                         fluent::lint_unused_generator,
 429                         |lint| {
 430                             // FIXME(davidtwco): this isn't properly translatable because of the
 431                             // pre/post strings
 432                             lint.set_arg("count", plural_len)
 433                                 .set_arg("pre", descr_pre)
 434                                 .set_arg("post", descr_post)
 435                                 .note(fluent::note)
 436                         },
 437                     );
 438                 }
 439                 MustUsePath::Def(span, def_id, reason) => {
 440                     cx.struct_span_lint(UNUSED_MUST_USE, *span, fluent::lint_unused_def, |lint| {
 441                         // FIXME(davidtwco): this isn't properly translatable because of the pre/post
 442                         // strings
 443                         lint.set_arg("pre", descr_pre);
 444                         lint.set_arg("post", descr_post);
 445                         lint.set_arg("def", cx.tcx.def_path_str(*def_id));
 446                         if let Some(note) = reason {
 447                             lint.note(note.as_str());
 448                         }
 449                         lint
 450                     });
 451                 }
 452             }
 453         }
 454     }
 455 }
 456
 457 declare_lint! {
 458     /// The `path_statements` lint detects path statements with no effect.
 459     ///
 460     /// ### Example
 461     ///
 462     /// ```rust
 463     /// let x = 42;
 464     ///
 465     /// x;
 466     /// ```
 467     ///
 468     /// {{produces}}
 469     ///
 470     /// ### Explanation
 471     ///
 472     /// It is usually a mistake to have a statement that has no effect.
 473     pub PATH_STATEMENTS,
 474     Warn,
 475     "path statements with no effect"
 476 }
 477
 478 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
 479
 480 impl<'tcx> LateLintPass<'tcx> for PathStatements {
 481     fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
 482         if let hir::StmtKind::Semi(expr) = s.kind {
 483             if let hir::ExprKind::Path(_) = expr.kind {
 484                 let ty = cx.typeck_results().expr_ty(expr);
 485                 if ty.needs_drop(cx.tcx, cx.param_env) {
 486                     cx.struct_span_lint(
 487                         PATH_STATEMENTS,
 488                         s.span,
 489                         fluent::lint_path_statement_drop,
 490                         |lint| {
 491                             if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
 492                                 lint.span_suggestion(
 493                                     s.span,
 494                                     fluent::suggestion,
 495                                     format!("drop({});", snippet),
 496                                     Applicability::MachineApplicable,
 497                                 );
 498                             } else {
 499                                 lint.span_help(s.span, fluent::suggestion);
 500                             }
 501                             lint
 502                         },
 503                     );
 504                 } else {
 505                     cx.struct_span_lint(
 506                         PATH_STATEMENTS,
 507                         s.span,
 508                         fluent::lint_path_statement_no_effect,
 509                         |lint| lint,
 510                     );
 511                 }
 512             }
 513         }
 514     }
 515 }
 516
 517 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 518 enum UnusedDelimsCtx {
 519     FunctionArg,
 520     MethodArg,
 521     AssignedValue,
 522     AssignedValueLetElse,
 523     IfCond,
 524     WhileCond,
 525     ForIterExpr,
 526     MatchScrutineeExpr,
 527     ReturnValue,
 528     BlockRetValue,
 529     LetScrutineeExpr,
 530     ArrayLenExpr,
 531     AnonConst,
 532     MatchArmExpr,
 533 }
 534
 535 impl From<UnusedDelimsCtx> for &'static str {
 536     fn from(ctx: UnusedDelimsCtx) -> &'static str {
 537         match ctx {
 538             UnusedDelimsCtx::FunctionArg => "function argument",
 539             UnusedDelimsCtx::MethodArg => "method argument",
 540             UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
 541                 "assigned value"
 542             }
 543             UnusedDelimsCtx::IfCond => "`if` condition",
 544             UnusedDelimsCtx::WhileCond => "`while` condition",
 545             UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
 546             UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
 547             UnusedDelimsCtx::ReturnValue => "`return` value",
 548             UnusedDelimsCtx::BlockRetValue => "block return value",
 549             UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
 550             UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
 551             UnusedDelimsCtx::MatchArmExpr => "match arm expression",
 552         }
 553     }
 554 }
 555
 556 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
 557 trait UnusedDelimLint {
 558     const DELIM_STR: &'static str;
 559
 560     /// Due to `ref` pattern, there can be a difference between using
 561     /// `{ expr }` and `expr` in pattern-matching contexts. This means
 562     /// that we should only lint `unused_parens` and not `unused_braces`
 563     /// in this case.
 564     ///
 565     /// ```rust
 566     /// let mut a = 7;
 567     /// let ref b = { a }; // We actually borrow a copy of `a` here.
 568     /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
 569     /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
 570     /// ```
 571     const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
 572
 573     // this cannot be a constant is it refers to a static.
 574     fn lint(&self) -> &'static Lint;
 575
 576     fn check_unused_delims_expr(
 577         &self,
 578         cx: &EarlyContext<'_>,
 579         value: &ast::Expr,
 580         ctx: UnusedDelimsCtx,
 581         followed_by_block: bool,
 582         left_pos: Option<BytePos>,
 583         right_pos: Option<BytePos>,
 584     );
 585
 586     fn is_expr_delims_necessary(
 587         inner: &ast::Expr,
 588         followed_by_block: bool,
 589         followed_by_else: bool,
 590     ) -> bool {
 591         if followed_by_else {
 592             match inner.kind {
 593                 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
 594                 _ if classify::expr_trailing_brace(inner).is_some() => return true,
 595                 _ => {}
 596             }
 597         }
 598
 599         // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
 600         let lhs_needs_parens = {
 601             let mut innermost = inner;
 602             loop {
 603                 innermost = match &innermost.kind {
 604                     ExprKind::Binary(_, lhs, _rhs) => lhs,
 605                     ExprKind::Call(fn_, _params) => fn_,
 606                     ExprKind::Cast(expr, _ty) => expr,
 607                     ExprKind::Type(expr, _ty) => expr,
 608                     ExprKind::Index(base, _subscript) => base,
 609                     _ => break false,
 610                 };
 611                 if !classify::expr_requires_semi_to_be_stmt(innermost) {
 612                     break true;
 613                 }
 614             }
 615         };
 616
 617         lhs_needs_parens
 618             || (followed_by_block
 619                 && match &inner.kind {
 620                     ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
 621                     ExprKind::Range(_lhs, Some(rhs), _limits) => {
 622                         matches!(rhs.kind, ExprKind::Block(..))
 623                     }
 624                     _ => parser::contains_exterior_struct_lit(&inner),
 625                 })
 626     }
 627
 628     fn emit_unused_delims_expr(
 629         &self,
 630         cx: &EarlyContext<'_>,
 631         value: &ast::Expr,
 632         ctx: UnusedDelimsCtx,
 633         left_pos: Option<BytePos>,
 634         right_pos: Option<BytePos>,
 635     ) {
 636         // If `value` has `ExprKind::Err`, unused delim lint can be broken.
 637         // For example, the following code caused ICE.
 638         // This is because the `ExprKind::Call` in `value` has `ExprKind::Err` as its argument
 639         // and this leads to wrong spans. #104897
 640         //
 641         // ```
 642         // fn f(){(print!(á
 643         // ```
 644         use rustc_ast::visit::{walk_expr, Visitor};
 645         struct ErrExprVisitor {
 646             has_error: bool,
 647         }
 648         impl<'ast> Visitor<'ast> for ErrExprVisitor {
 649             fn visit_expr(&mut self, expr: &'ast ast::Expr) {
 650                 if let ExprKind::Err = expr.kind {
 651                     self.has_error = true;
 652                     return;
 653                 }
 654                 walk_expr(self, expr)
 655             }
 656         }
 657         let mut visitor = ErrExprVisitor { has_error: false };
 658         visitor.visit_expr(value);
 659         if visitor.has_error {
 660             return;
 661         }
 662         let spans = match value.kind {
 663             ast::ExprKind::Block(ref block, None) if block.stmts.len() == 1 => {
 664                 if let Some(span) = block.stmts[0].span.find_ancestor_inside(value.span) {
 665                     Some((value.span.with_hi(span.lo()), value.span.with_lo(span.hi())))
 666                 } else {
 667                     None
 668                 }
 669             }
 670             ast::ExprKind::Paren(ref expr) => {
 671                 let expr_span = expr.span.find_ancestor_inside(value.span);
 672                 if let Some(expr_span) = expr_span {
 673                     Some((value.span.with_hi(expr_span.lo()), value.span.with_lo(expr_span.hi())))
 674                 } else {
 675                     None
 676                 }
 677             }
 678             _ => return,
 679         };
 680         let keep_space = (
 681             left_pos.map_or(false, |s| s >= value.span.lo()),
 682             right_pos.map_or(false, |s| s <= value.span.hi()),
 683         );
 684         self.emit_unused_delims(cx, value.span, spans, ctx.into(), keep_space);
 685     }
 686
 687     fn emit_unused_delims(
 688         &self,
 689         cx: &EarlyContext<'_>,
 690         value_span: Span,
 691         spans: Option<(Span, Span)>,
 692         msg: &str,
 693         keep_space: (bool, bool),
 694     ) {
 695         let primary_span = if let Some((lo, hi)) = spans {
 696             MultiSpan::from(vec![lo, hi])
 697         } else {
 698             MultiSpan::from(value_span)
 699         };
 700         cx.struct_span_lint(self.lint(), primary_span, fluent::lint_unused_delim, |lint| {
 701             lint.set_arg("delim", Self::DELIM_STR);
 702             lint.set_arg("item", msg);
 703             if let Some((lo, hi)) = spans {
 704                 let sm = cx.sess().source_map();
 705                 let lo_replace =
 706                     if keep_space.0 &&
 707                         let Ok(snip) = sm.span_to_prev_source(lo) && !snip.ends_with(' ') {
 708                         " ".to_string()
 709                         } else {
 710                             "".to_string()
 711                         };
 712
 713                 let hi_replace =
 714                     if keep_space.1 &&
 715                         let Ok(snip) = sm.span_to_next_source(hi) && !snip.starts_with(' ') {
 716                         " ".to_string()
 717                         } else {
 718                             "".to_string()
 719                         };
 720
 721                 let replacement = vec![(lo, lo_replace), (hi, hi_replace)];
 722                 lint.multipart_suggestion(
 723                     fluent::suggestion,
 724                     replacement,
 725                     Applicability::MachineApplicable,
 726                 );
 727             }
 728             lint
 729         });
 730     }
 731
 732     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 733         use rustc_ast::ExprKind::*;
 734         let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
 735             // Do not lint `unused_braces` in `if let` expressions.
 736             If(ref cond, ref block, _)
 737                 if !matches!(cond.kind, Let(_, _, _))
 738                     || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
 739             {
 740                 let left = e.span.lo() + rustc_span::BytePos(2);
 741                 let right = block.span.lo();
 742                 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
 743             }
 744
 745             // Do not lint `unused_braces` in `while let` expressions.
 746             While(ref cond, ref block, ..)
 747                 if !matches!(cond.kind, Let(_, _, _))
 748                     || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
 749             {
 750                 let left = e.span.lo() + rustc_span::BytePos(5);
 751                 let right = block.span.lo();
 752                 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
 753             }
 754
 755             ForLoop(_, ref cond, ref block, ..) => {
 756                 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
 757             }
 758
 759             Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
 760                 let left = e.span.lo() + rustc_span::BytePos(5);
 761                 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
 762             }
 763
 764             Ret(Some(ref value)) => {
 765                 let left = e.span.lo() + rustc_span::BytePos(3);
 766                 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
 767             }
 768
 769             Assign(_, ref value, _) | AssignOp(.., ref value) => {
 770                 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
 771             }
 772             // either function/method call, or something this lint doesn't care about
 773             ref call_or_other => {
 774                 let (args_to_check, ctx) = match *call_or_other {
 775                     Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
 776                     MethodCall(ref call) => (&call.args[..], UnusedDelimsCtx::MethodArg),
 777                     // actual catch-all arm
 778                     _ => {
 779                         return;
 780                     }
 781                 };
 782                 // Don't lint if this is a nested macro expansion: otherwise, the lint could
 783                 // trigger in situations that macro authors shouldn't have to care about, e.g.,
 784                 // when a parenthesized token tree matched in one macro expansion is matched as
 785                 // an expression in another and used as a fn/method argument (Issue #47775)
 786                 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
 787                     return;
 788                 }
 789                 for arg in args_to_check {
 790                     self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
 791                 }
 792                 return;
 793             }
 794         };
 795         self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
 796     }
 797
 798     fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
 799         match s.kind {
 800             StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
 801                 if let Some((init, els)) = local.kind.init_else_opt() {
 802                     let ctx = match els {
 803                         None => UnusedDelimsCtx::AssignedValue,
 804                         Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
 805                     };
 806                     self.check_unused_delims_expr(cx, init, ctx, false, None, None);
 807                 }
 808             }
 809             StmtKind::Expr(ref expr) => {
 810                 self.check_unused_delims_expr(
 811                     cx,
 812                     &expr,
 813                     UnusedDelimsCtx::BlockRetValue,
 814                     false,
 815                     None,
 816                     None,
 817                 );
 818             }
 819             _ => {}
 820         }
 821     }
 822
 823     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
 824         use ast::ItemKind::*;
 825
 826         if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
 827             self.check_unused_delims_expr(
 828                 cx,
 829                 expr,
 830                 UnusedDelimsCtx::AssignedValue,
 831                 false,
 832                 None,
 833                 None,
 834             );
 835         }
 836     }
 837 }
 838
 839 declare_lint! {
 840     /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
 841     /// with parentheses; they do not need them.
 842     ///
 843     /// ### Examples
 844     ///
 845     /// ```rust
 846     /// if(true) {}
 847     /// ```
 848     ///
 849     /// {{produces}}
 850     ///
 851     /// ### Explanation
 852     ///
 853     /// The parentheses are not needed, and should be removed. This is the
 854     /// preferred style for writing these expressions.
 855     pub(super) UNUSED_PARENS,
 856     Warn,
 857     "`if`, `match`, `while` and `return` do not need parentheses"
 858 }
 859
 860 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
 861
 862 impl UnusedDelimLint for UnusedParens {
 863     const DELIM_STR: &'static str = "parentheses";
 864
 865     const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
 866
 867     fn lint(&self) -> &'static Lint {
 868         UNUSED_PARENS
 869     }
 870
 871     fn check_unused_delims_expr(
 872         &self,
 873         cx: &EarlyContext<'_>,
 874         value: &ast::Expr,
 875         ctx: UnusedDelimsCtx,
 876         followed_by_block: bool,
 877         left_pos: Option<BytePos>,
 878         right_pos: Option<BytePos>,
 879     ) {
 880         match value.kind {
 881             ast::ExprKind::Paren(ref inner) => {
 882                 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
 883                 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
 884                     && value.attrs.is_empty()
 885                     && !value.span.from_expansion()
 886                     && (ctx != UnusedDelimsCtx::LetScrutineeExpr
 887                         || !matches!(inner.kind, ast::ExprKind::Binary(
 888                                 rustc_span::source_map::Spanned { node, .. },
 889                                 _,
 890                                 _,
 891                             ) if node.lazy()))
 892                 {
 893                     self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
 894                 }
 895             }
 896             ast::ExprKind::Let(_, ref expr, _) => {
 897                 self.check_unused_delims_expr(
 898                     cx,
 899                     expr,
 900                     UnusedDelimsCtx::LetScrutineeExpr,
 901                     followed_by_block,
 902                     None,
 903                     None,
 904                 );
 905             }
 906             _ => {}
 907         }
 908     }
 909 }
 910
 911 impl UnusedParens {
 912     fn check_unused_parens_pat(
 913         &self,
 914         cx: &EarlyContext<'_>,
 915         value: &ast::Pat,
 916         avoid_or: bool,
 917         avoid_mut: bool,
 918         keep_space: (bool, bool),
 919     ) {
 920         use ast::{BindingAnnotation, PatKind};
 921
 922         if let PatKind::Paren(inner) = &value.kind {
 923             match inner.kind {
 924                 // The lint visitor will visit each subpattern of `p`. We do not want to lint
 925                 // any range pattern no matter where it occurs in the pattern. For something like
 926                 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
 927                 // that if there are unnecessary parens they serve a purpose of readability.
 928                 PatKind::Range(..) => return,
 929                 // Avoid `p0 | .. | pn` if we should.
 930                 PatKind::Or(..) if avoid_or => return,
 931                 // Avoid `mut x` and `mut x @ p` if we should:
 932                 PatKind::Ident(BindingAnnotation::MUT, ..) if avoid_mut => {
 933                     return;
 934                 }
 935                 // Otherwise proceed with linting.
 936                 _ => {}
 937             }
 938             let spans = if let Some(inner) = inner.span.find_ancestor_inside(value.span) {
 939                 Some((value.span.with_hi(inner.lo()), value.span.with_lo(inner.hi())))
 940             } else {
 941                 None
 942             };
 943             self.emit_unused_delims(cx, value.span, spans, "pattern", keep_space);
 944         }
 945     }
 946 }
 947
 948 impl EarlyLintPass for UnusedParens {
 949     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 950         match e.kind {
 951             ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
 952                 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
 953             }
 954             // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
 955             // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
 956             // want to complain about things like `if let 42 = (42)`.
 957             ExprKind::If(ref cond, ref block, ref else_)
 958                 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
 959             {
 960                 self.check_unused_delims_expr(
 961                     cx,
 962                     cond.peel_parens(),
 963                     UnusedDelimsCtx::LetScrutineeExpr,
 964                     true,
 965                     None,
 966                     None,
 967                 );
 968                 for stmt in &block.stmts {
 969                     <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
 970                 }
 971                 if let Some(e) = else_ {
 972                     <Self as UnusedDelimLint>::check_expr(self, cx, e);
 973                 }
 974                 return;
 975             }
 976             ExprKind::Match(ref _expr, ref arm) => {
 977                 for a in arm {
 978                     self.check_unused_delims_expr(
 979                         cx,
 980                         &a.body,
 981                         UnusedDelimsCtx::MatchArmExpr,
 982                         false,
 983                         None,
 984                         None,
 985                     );
 986                 }
 987             }
 988             _ => {}
 989         }
 990
 991         <Self as UnusedDelimLint>::check_expr(self, cx, e)
 992     }
 993
 994     fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
 995         use ast::{Mutability, PatKind::*};
 996         let keep_space = (false, false);
 997         match &p.kind {
 998             // Do not lint on `(..)` as that will result in the other arms being useless.
 999             Paren(_)
1000             // The other cases do not contain sub-patterns.
1001             | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
1002             // These are list-like patterns; parens can always be removed.
1003             TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
1004                 self.check_unused_parens_pat(cx, p, false, false, keep_space);
1005             },
1006             Struct(_, _, fps, _) => for f in fps {
1007                 self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
1008             },
1009             // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
1010             Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
1011             // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
1012             // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
1013             Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not, keep_space),
1014         }
1015     }
1016
1017     fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1018         if let StmtKind::Local(ref local) = s.kind {
1019             self.check_unused_parens_pat(cx, &local.pat, true, false, (false, false));
1020         }
1021
1022         <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1023     }
1024
1025     fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
1026         self.check_unused_parens_pat(cx, &param.pat, true, false, (false, false));
1027     }
1028
1029     fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
1030         self.check_unused_parens_pat(cx, &arm.pat, false, false, (false, false));
1031     }
1032
1033     fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1034         if let ast::TyKind::Paren(r) = &ty.kind {
1035             match &r.kind {
1036                 ast::TyKind::TraitObject(..) => {}
1037                 ast::TyKind::BareFn(b) if b.generic_params.len() > 0 => {}
1038                 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
1039                 ast::TyKind::Array(_, len) => {
1040                     self.check_unused_delims_expr(
1041                         cx,
1042                         &len.value,
1043                         UnusedDelimsCtx::ArrayLenExpr,
1044                         false,
1045                         None,
1046                         None,
1047                     );
1048                 }
1049                 _ => {
1050                     let spans = if let Some(r) = r.span.find_ancestor_inside(ty.span) {
1051                         Some((ty.span.with_hi(r.lo()), ty.span.with_lo(r.hi())))
1052                     } else {
1053                         None
1054                     };
1055                     self.emit_unused_delims(cx, ty.span, spans, "type", (false, false));
1056                 }
1057             }
1058         }
1059     }
1060
1061     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1062         <Self as UnusedDelimLint>::check_item(self, cx, item)
1063     }
1064 }
1065
1066 declare_lint! {
1067     /// The `unused_braces` lint detects unnecessary braces around an
1068     /// expression.
1069     ///
1070     /// ### Example
1071     ///
1072     /// ```rust
1073     /// if { true } {
1074     ///     // ...
1075     /// }
1076     /// ```
1077     ///
1078     /// {{produces}}
1079     ///
1080     /// ### Explanation
1081     ///
1082     /// The braces are not needed, and should be removed. This is the
1083     /// preferred style for writing these expressions.
1084     pub(super) UNUSED_BRACES,
1085     Warn,
1086     "unnecessary braces around an expression"
1087 }
1088
1089 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
1090
1091 impl UnusedDelimLint for UnusedBraces {
1092     const DELIM_STR: &'static str = "braces";
1093
1094     const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
1095
1096     fn lint(&self) -> &'static Lint {
1097         UNUSED_BRACES
1098     }
1099
1100     fn check_unused_delims_expr(
1101         &self,
1102         cx: &EarlyContext<'_>,
1103         value: &ast::Expr,
1104         ctx: UnusedDelimsCtx,
1105         followed_by_block: bool,
1106         left_pos: Option<BytePos>,
1107         right_pos: Option<BytePos>,
1108     ) {
1109         match value.kind {
1110             ast::ExprKind::Block(ref inner, None)
1111                 if inner.rules == ast::BlockCheckMode::Default =>
1112             {
1113                 // emit a warning under the following conditions:
1114                 //
1115                 // - the block does not have a label
1116                 // - the block is not `unsafe`
1117                 // - the block contains exactly one expression (do not lint `{ expr; }`)
1118                 // - `followed_by_block` is true and the internal expr may contain a `{`
1119                 // - the block is not multiline (do not lint multiline match arms)
1120                 //      ```
1121                 //      match expr {
1122                 //          Pattern => {
1123                 //              somewhat_long_expression
1124                 //          }
1125                 //          // ...
1126                 //      }
1127                 //      ```
1128                 // - the block has no attribute and was not created inside a macro
1129                 // - if the block is an `anon_const`, the inner expr must be a literal
1130                 //      (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
1131                 //
1132                 // FIXME(const_generics): handle paths when #67075 is fixed.
1133                 if let [stmt] = inner.stmts.as_slice() {
1134                     if let ast::StmtKind::Expr(ref expr) = stmt.kind {
1135                         if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
1136                             && (ctx != UnusedDelimsCtx::AnonConst
1137                                 || matches!(expr.kind, ast::ExprKind::Lit(_)))
1138                             && !cx.sess().source_map().is_multiline(value.span)
1139                             && value.attrs.is_empty()
1140                             && !value.span.from_expansion()
1141                         {
1142                             self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
1143                         }
1144                     }
1145                 }
1146             }
1147             ast::ExprKind::Let(_, ref expr, _) => {
1148                 self.check_unused_delims_expr(
1149                     cx,
1150                     expr,
1151                     UnusedDelimsCtx::LetScrutineeExpr,
1152                     followed_by_block,
1153                     None,
1154                     None,
1155                 );
1156             }
1157             _ => {}
1158         }
1159     }
1160 }
1161
1162 impl EarlyLintPass for UnusedBraces {
1163     fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1164         <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1165     }
1166
1167     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1168         <Self as UnusedDelimLint>::check_expr(self, cx, e);
1169
1170         if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1171             self.check_unused_delims_expr(
1172                 cx,
1173                 &anon_const.value,
1174                 UnusedDelimsCtx::AnonConst,
1175                 false,
1176                 None,
1177                 None,
1178             );
1179         }
1180     }
1181
1182     fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1183         if let ast::GenericArg::Const(ct) = arg {
1184             self.check_unused_delims_expr(
1185                 cx,
1186                 &ct.value,
1187                 UnusedDelimsCtx::AnonConst,
1188                 false,
1189                 None,
1190                 None,
1191             );
1192         }
1193     }
1194
1195     fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1196         if let Some(anon_const) = &v.disr_expr {
1197             self.check_unused_delims_expr(
1198                 cx,
1199                 &anon_const.value,
1200                 UnusedDelimsCtx::AnonConst,
1201                 false,
1202                 None,
1203                 None,
1204             );
1205         }
1206     }
1207
1208     fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1209         match ty.kind {
1210             ast::TyKind::Array(_, ref len) => {
1211                 self.check_unused_delims_expr(
1212                     cx,
1213                     &len.value,
1214                     UnusedDelimsCtx::ArrayLenExpr,
1215                     false,
1216                     None,
1217                     None,
1218                 );
1219             }
1220
1221             ast::TyKind::Typeof(ref anon_const) => {
1222                 self.check_unused_delims_expr(
1223                     cx,
1224                     &anon_const.value,
1225                     UnusedDelimsCtx::AnonConst,
1226                     false,
1227                     None,
1228                     None,
1229                 );
1230             }
1231
1232             _ => {}
1233         }
1234     }
1235
1236     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1237         <Self as UnusedDelimLint>::check_item(self, cx, item)
1238     }
1239 }
1240
1241 declare_lint! {
1242     /// The `unused_import_braces` lint catches unnecessary braces around an
1243     /// imported item.
1244     ///
1245     /// ### Example
1246     ///
1247     /// ```rust,compile_fail
1248     /// #![deny(unused_import_braces)]
1249     /// use test::{A};
1250     ///
1251     /// pub mod test {
1252     ///     pub struct A;
1253     /// }
1254     /// # fn main() {}
1255     /// ```
1256     ///
1257     /// {{produces}}
1258     ///
1259     /// ### Explanation
1260     ///
1261     /// If there is only a single item, then remove the braces (`use test::A;`
1262     /// for example).
1263     ///
1264     /// This lint is "allow" by default because it is only enforcing a
1265     /// stylistic choice.
1266     UNUSED_IMPORT_BRACES,
1267     Allow,
1268     "unnecessary braces around an imported item"
1269 }
1270
1271 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1272
1273 impl UnusedImportBraces {
1274     fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1275         if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1276             // Recursively check nested UseTrees
1277             for &(ref tree, _) in items {
1278                 self.check_use_tree(cx, tree, item);
1279             }
1280
1281             // Trigger the lint only if there is one nested item
1282             if items.len() != 1 {
1283                 return;
1284             }
1285
1286             // Trigger the lint if the nested item is a non-self single item
1287             let node_name = match items[0].0.kind {
1288                 ast::UseTreeKind::Simple(rename) => {
1289                     let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1290                     if orig_ident.name == kw::SelfLower {
1291                         return;
1292                     }
1293                     rename.unwrap_or(orig_ident).name
1294                 }
1295                 ast::UseTreeKind::Glob => Symbol::intern("*"),
1296                 ast::UseTreeKind::Nested(_) => return,
1297             };
1298
1299             cx.struct_span_lint(
1300                 UNUSED_IMPORT_BRACES,
1301                 item.span,
1302                 fluent::lint_unused_import_braces,
1303                 |lint| lint.set_arg("node", node_name),
1304             );
1305         }
1306     }
1307 }
1308
1309 impl EarlyLintPass for UnusedImportBraces {
1310     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1311         if let ast::ItemKind::Use(ref use_tree) = item.kind {
1312             self.check_use_tree(cx, use_tree, item);
1313         }
1314     }
1315 }
1316
1317 declare_lint! {
1318     /// The `unused_allocation` lint detects unnecessary allocations that can
1319     /// be eliminated.
1320     ///
1321     /// ### Example
1322     ///
1323     /// ```rust
1324     /// #![feature(box_syntax)]
1325     /// fn main() {
1326     ///     let a = (box [1, 2, 3]).len();
1327     /// }
1328     /// ```
1329     ///
1330     /// {{produces}}
1331     ///
1332     /// ### Explanation
1333     ///
1334     /// When a `box` expression is immediately coerced to a reference, then
1335     /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1336     /// should be used instead to avoid the allocation.
1337     pub(super) UNUSED_ALLOCATION,
1338     Warn,
1339     "detects unnecessary allocations that can be eliminated"
1340 }
1341
1342 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1343
1344 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1345     fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1346         match e.kind {
1347             hir::ExprKind::Box(_) => {}
1348             _ => return,
1349         }
1350
1351         for adj in cx.typeck_results().expr_adjustments(e) {
1352             if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1353                 cx.struct_span_lint(
1354                     UNUSED_ALLOCATION,
1355                     e.span,
1356                     match m {
1357                         adjustment::AutoBorrowMutability::Not => fluent::lint_unused_allocation,
1358                         adjustment::AutoBorrowMutability::Mut { .. } => {
1359                             fluent::lint_unused_allocation_mut
1360                         }
1361                     },
1362                     |lint| lint,
1363                 );
1364             }
1365         }
1366     }
1367 }