compiler/rustc_typeck/src/check/_match.rs

   1 use crate::check::coercion::{AsCoercionSite, CoerceMany};
   2 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
   3 use rustc_errors::{Applicability, DiagnosticBuilder};
   4 use rustc_hir::{self as hir, ExprKind};
   5 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
   6 use rustc_infer::traits::Obligation;
   7 use rustc_middle::ty::{self, ToPredicate, Ty};
   8 use rustc_span::{MultiSpan, Span};
   9 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  10 use rustc_trait_selection::traits::{
  11     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
  12     StatementAsExpression,
  13 };
  14
  15 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  16     #[instrument(skip(self), level = "debug")]
  17     pub fn check_match(
  18         &self,
  19         expr: &'tcx hir::Expr<'tcx>,
  20         scrut: &'tcx hir::Expr<'tcx>,
  21         arms: &'tcx [hir::Arm<'tcx>],
  22         orig_expected: Expectation<'tcx>,
  23         match_src: hir::MatchSource,
  24     ) -> Ty<'tcx> {
  25         let tcx = self.tcx;
  26
  27         let acrb = arms_contain_ref_bindings(arms);
  28         let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
  29         debug!(?scrutinee_ty);
  30
  31         // If there are no arms, that is a diverging match; a special case.
  32         if arms.is_empty() {
  33             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
  34             return tcx.types.never;
  35         }
  36
  37         self.warn_arms_when_scrutinee_diverges(arms);
  38
  39         // Otherwise, we have to union together the types that the arms produce and so forth.
  40         let scrut_diverges = self.diverges.replace(Diverges::Maybe);
  41
  42         // #55810: Type check patterns first so we get types for all bindings.
  43         for arm in arms {
  44             self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut.span), true);
  45         }
  46
  47         // Now typecheck the blocks.
  48         //
  49         // The result of the match is the common supertype of all the
  50         // arms. Start out the value as bottom, since it's the, well,
  51         // bottom the type lattice, and we'll be moving up the lattice as
  52         // we process each arm. (Note that any match with 0 arms is matching
  53         // on any empty type and is therefore unreachable; should the flow
  54         // of execution reach it, we will panic, so bottom is an appropriate
  55         // type in that case)
  56         let mut all_arms_diverge = Diverges::WarnedAlways;
  57
  58         let expected = orig_expected.adjust_for_branches(self);
  59
  60         let mut coercion = {
  61             let coerce_first = match expected {
  62                 // We don't coerce to `()` so that if the match expression is a
  63                 // statement it's branches can have any consistent type. That allows
  64                 // us to give better error messages (pointing to a usually better
  65                 // arm for inconsistent arms or to the whole match when a `()` type
  66                 // is required).
  67                 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
  68                 _ => self.next_ty_var(TypeVariableOrigin {
  69                     kind: TypeVariableOriginKind::MiscVariable,
  70                     span: expr.span,
  71                 }),
  72             };
  73             CoerceMany::with_coercion_sites(coerce_first, arms)
  74         };
  75
  76         let mut other_arms = vec![]; // Used only for diagnostics.
  77         let mut prior_arm_ty = None;
  78         for (i, arm) in arms.iter().enumerate() {
  79             if let Some(g) = &arm.guard {
  80                 self.diverges.set(Diverges::Maybe);
  81                 match g {
  82                     hir::Guard::If(e) => {
  83                         self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
  84                     }
  85                     hir::Guard::IfLet(pat, e) => {
  86                         let scrutinee_ty = self.demand_scrutinee_type(
  87                             e,
  88                             pat.contains_explicit_ref_binding(),
  89                             false,
  90                         );
  91                         self.check_pat_top(&pat, scrutinee_ty, None, true);
  92                     }
  93                 };
  94             }
  95
  96             self.diverges.set(Diverges::Maybe);
  97
  98             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
  99             all_arms_diverge &= self.diverges.get();
 100
 101             let opt_suggest_box_span =
 102                 self.opt_suggest_box_span(arm.body.span, arm_ty, orig_expected);
 103
 104             let (arm_span, semi_span) =
 105                 self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
 106             let (span, code) = match i {
 107                 // The reason for the first arm to fail is not that the match arms diverge,
 108                 // but rather that there's a prior obligation that doesn't hold.
 109                 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
 110                 _ => (
 111                     expr.span,
 112                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
 113                         arm_span,
 114                         scrut_span: scrut.span,
 115                         semi_span,
 116                         source: match_src,
 117                         prior_arms: other_arms.clone(),
 118                         last_ty: prior_arm_ty.unwrap(),
 119                         scrut_hir_id: scrut.hir_id,
 120                         opt_suggest_box_span,
 121                     })),
 122                 ),
 123             };
 124             let cause = self.cause(span, code);
 125
 126             // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
 127             // We use it this way to be able to expand on the potential error and detect when a
 128             // `match` tail statement could be a tail expression instead. If so, we suggest
 129             // removing the stray semicolon.
 130             coercion.coerce_inner(
 131                 self,
 132                 &cause,
 133                 Some(&arm.body),
 134                 arm_ty,
 135                 Some(&mut |err: &mut DiagnosticBuilder<'_>| {
 136                     let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 137                         Some(ret_coercion) if self.in_tail_expr => {
 138                             let ret_ty = ret_coercion.borrow().expected_ty();
 139                             let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 140                             self.can_coerce(arm_ty, ret_ty)
 141                                 && prior_arm_ty.map_or(true, |t| self.can_coerce(t, ret_ty))
 142                                 // The match arms need to unify for the case of `impl Trait`.
 143                                 && !matches!(ret_ty.kind(), ty::Opaque(..))
 144                         }
 145                         _ => false,
 146                     };
 147                     if let (Expectation::IsLast(stmt), Some(ret), true) =
 148                         (orig_expected, self.ret_type_span, can_coerce_to_return_ty)
 149                     {
 150                         let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 151                         let mut ret_span: MultiSpan = semi_span.into();
 152                         ret_span.push_span_label(
 153                             expr.span,
 154                             "this could be implicitly returned but it is a statement, not a \
 155                                 tail expression"
 156                                 .to_owned(),
 157                         );
 158                         ret_span.push_span_label(
 159                             ret,
 160                             "the `match` arms can conform to this return type".to_owned(),
 161                         );
 162                         ret_span.push_span_label(
 163                             semi_span,
 164                             "the `match` is a statement because of this semicolon, consider \
 165                                 removing it"
 166                                 .to_owned(),
 167                         );
 168                         err.span_note(
 169                             ret_span,
 170                             "you might have meant to return the `match` expression",
 171                         );
 172                         err.tool_only_span_suggestion(
 173                             semi_span,
 174                             "remove this semicolon",
 175                             String::new(),
 176                             Applicability::MaybeIncorrect,
 177                         );
 178                     }
 179                 }),
 180                 false,
 181             );
 182
 183             other_arms.push(arm_span);
 184             if other_arms.len() > 5 {
 185                 other_arms.remove(0);
 186             }
 187             prior_arm_ty = Some(arm_ty);
 188         }
 189
 190         // If all of the arms in the `match` diverge,
 191         // and we're dealing with an actual `match` block
 192         // (as opposed to a `match` desugared from something else'),
 193         // we can emit a better note. Rather than pointing
 194         // at a diverging expression in an arbitrary arm,
 195         // we can point at the entire `match` expression
 196         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 197             all_arms_diverge = Diverges::Always {
 198                 span: expr.span,
 199                 custom_note: Some(
 200                     "any code following this `match` expression is unreachable, as all arms diverge",
 201                 ),
 202             };
 203         }
 204
 205         // We won't diverge unless the scrutinee or all arms diverge.
 206         self.diverges.set(scrut_diverges | all_arms_diverge);
 207
 208         coercion.complete(self)
 209     }
 210
 211     fn get_appropriate_arm_semicolon_removal_span(
 212         &self,
 213         arms: &'tcx [hir::Arm<'tcx>],
 214         i: usize,
 215         prior_arm_ty: Option<Ty<'tcx>>,
 216         arm_ty: Ty<'tcx>,
 217     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 218         let arm = &arms[i];
 219         let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
 220             self.find_block_span(blk, prior_arm_ty)
 221         } else {
 222             (arm.body.span, None)
 223         };
 224         if semi_span.is_none() && i > 0 {
 225             if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
 226                 let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
 227                 semi_span = semi_span_prev;
 228             }
 229         }
 230         (arm_span, semi_span)
 231     }
 232
 233     /// When the previously checked expression (the scrutinee) diverges,
 234     /// warn the user about the match arms being unreachable.
 235     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 236         for arm in arms {
 237             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 238         }
 239     }
 240
 241     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 242     ///
 243     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 244     pub(super) fn if_fallback_coercion<T>(
 245         &self,
 246         span: Span,
 247         then_expr: &'tcx hir::Expr<'tcx>,
 248         coercion: &mut CoerceMany<'tcx, '_, T>,
 249     ) -> bool
 250     where
 251         T: AsCoercionSite,
 252     {
 253         // If this `if` expr is the parent's function return expr,
 254         // the cause of the type coercion is the return type, point at it. (#25228)
 255         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
 256         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 257         let mut error = false;
 258         coercion.coerce_forced_unit(
 259             self,
 260             &cause,
 261             &mut |err| {
 262                 if let Some((span, msg)) = &ret_reason {
 263                     err.span_label(*span, msg.as_str());
 264                 } else if let ExprKind::Block(block, _) = &then_expr.kind {
 265                     if let Some(expr) = &block.expr {
 266                         err.span_label(expr.span, "found here".to_string());
 267                     }
 268                 }
 269                 err.note("`if` expressions without `else` evaluate to `()`");
 270                 err.help("consider adding an `else` block that evaluates to the expected type");
 271                 error = true;
 272             },
 273             ret_reason.is_none(),
 274         );
 275         error
 276     }
 277
 278     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
 279         let node = {
 280             let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id));
 281             self.tcx.hir().get(rslt)
 282         };
 283         if let hir::Node::Block(block) = node {
 284             // check that the body's parent is an fn
 285             let parent = self
 286                 .tcx
 287                 .hir()
 288                 .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)));
 289             if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
 290                 (&block.expr, parent)
 291             {
 292                 // check that the `if` expr without `else` is the fn body's expr
 293                 if expr.span == sp {
 294                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
 295                         let span = fn_decl.output.span();
 296                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
 297                         Some((span, format!("expected `{}` because of this return type", snippet)))
 298                     });
 299                 }
 300             }
 301         }
 302         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 303             return Some((pat.span, "expected because of this assignment".to_string()));
 304         }
 305         None
 306     }
 307
 308     pub(crate) fn if_cause(
 309         &self,
 310         span: Span,
 311         then_expr: &'tcx hir::Expr<'tcx>,
 312         else_expr: &'tcx hir::Expr<'tcx>,
 313         then_ty: Ty<'tcx>,
 314         else_ty: Ty<'tcx>,
 315         opt_suggest_box_span: Option<Span>,
 316     ) -> ObligationCause<'tcx> {
 317         let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
 318             // The `if`/`else` isn't in one line in the output, include some context to make it
 319             // clear it is an if/else expression:
 320             // ```
 321             // LL |      let x = if true {
 322             //    | _____________-
 323             // LL ||         10i32
 324             //    ||         ----- expected because of this
 325             // LL ||     } else {
 326             // LL ||         10u32
 327             //    ||         ^^^^^ expected `i32`, found `u32`
 328             // LL ||     };
 329             //    ||_____- `if` and `else` have incompatible types
 330             // ```
 331             Some(span)
 332         } else {
 333             // The entire expression is in one line, only point at the arms
 334             // ```
 335             // LL |     let x = if true { 10i32 } else { 10u32 };
 336             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 337             //    |                       |
 338             //    |                       expected because of this
 339             // ```
 340             None
 341         };
 342
 343         let mut remove_semicolon = None;
 344         let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
 345             let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
 346             remove_semicolon = semi_sp;
 347             if block.expr.is_none() && block.stmts.is_empty() {
 348                 // Avoid overlapping spans that aren't as readable:
 349                 // ```
 350                 // 2 |        let x = if true {
 351                 //   |   _____________-
 352                 // 3 |  |         3
 353                 //   |  |         - expected because of this
 354                 // 4 |  |     } else {
 355                 //   |  |____________^
 356                 // 5 | ||
 357                 // 6 | ||     };
 358                 //   | ||     ^
 359                 //   | ||_____|
 360                 //   | |______if and else have incompatible types
 361                 //   |        expected integer, found `()`
 362                 // ```
 363                 // by not pointing at the entire expression:
 364                 // ```
 365                 // 2 |       let x = if true {
 366                 //   |               ------- `if` and `else` have incompatible types
 367                 // 3 |           3
 368                 //   |           - expected because of this
 369                 // 4 |       } else {
 370                 //   |  ____________^
 371                 // 5 | |
 372                 // 6 | |     };
 373                 //   | |_____^ expected integer, found `()`
 374                 // ```
 375                 if outer_sp.is_some() {
 376                     outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
 377                 }
 378             }
 379             error_sp
 380         } else {
 381             // shouldn't happen unless the parser has done something weird
 382             else_expr.span
 383         };
 384
 385         // Compute `Span` of `then` part of `if`-expression.
 386         let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
 387             let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
 388             remove_semicolon = remove_semicolon.or(semi_sp);
 389             if block.expr.is_none() && block.stmts.is_empty() {
 390                 outer_sp = None; // same as in `error_sp`; cleanup output
 391             }
 392             then_sp
 393         } else {
 394             // shouldn't happen unless the parser has done something weird
 395             then_expr.span
 396         };
 397
 398         // Finally construct the cause:
 399         self.cause(
 400             error_sp,
 401             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 402                 then: then_sp,
 403                 else_sp: error_sp,
 404                 outer: outer_sp,
 405                 semicolon: remove_semicolon,
 406                 opt_suggest_box_span,
 407             })),
 408         )
 409     }
 410
 411     pub(super) fn demand_scrutinee_type(
 412         &self,
 413         scrut: &'tcx hir::Expr<'tcx>,
 414         contains_ref_bindings: Option<hir::Mutability>,
 415         no_arms: bool,
 416     ) -> Ty<'tcx> {
 417         // Not entirely obvious: if matches may create ref bindings, we want to
 418         // use the *precise* type of the scrutinee, *not* some supertype, as
 419         // the "scrutinee type" (issue #23116).
 420         //
 421         // arielb1 [writes here in this comment thread][c] that there
 422         // is certainly *some* potential danger, e.g., for an example
 423         // like:
 424         //
 425         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 426         //
 427         // ```
 428         // let Foo(x) = f()[0];
 429         // ```
 430         //
 431         // Then if the pattern matches by reference, we want to match
 432         // `f()[0]` as a lexpr, so we can't allow it to be
 433         // coerced. But if the pattern matches by value, `f()[0]` is
 434         // still syntactically a lexpr, but we *do* want to allow
 435         // coercions.
 436         //
 437         // However, *likely* we are ok with allowing coercions to
 438         // happen if there are no explicit ref mut patterns - all
 439         // implicit ref mut patterns must occur behind a reference, so
 440         // they will have the "correct" variance and lifetime.
 441         //
 442         // This does mean that the following pattern would be legal:
 443         //
 444         // ```
 445         // struct Foo(Bar);
 446         // struct Bar(u32);
 447         // impl Deref for Foo {
 448         //     type Target = Bar;
 449         //     fn deref(&self) -> &Bar { &self.0 }
 450         // }
 451         // impl DerefMut for Foo {
 452         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 453         // }
 454         // fn foo(x: &mut Foo) {
 455         //     {
 456         //         let Bar(z): &mut Bar = x;
 457         //         *z = 42;
 458         //     }
 459         //     assert_eq!(foo.0.0, 42);
 460         // }
 461         // ```
 462         //
 463         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 464         // is problematic as the HIR is being scraped, but ref bindings may be
 465         // implicit after #42640. We need to make sure that pat_adjustments
 466         // (once introduced) is populated by the time we get here.
 467         //
 468         // See #44848.
 469         if let Some(m) = contains_ref_bindings {
 470             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 471         } else if no_arms {
 472             self.check_expr(scrut)
 473         } else {
 474             // ...but otherwise we want to use any supertype of the
 475             // scrutinee. This is sort of a workaround, see note (*) in
 476             // `check_pat` for some details.
 477             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 478                 kind: TypeVariableOriginKind::TypeInference,
 479                 span: scrut.span,
 480             });
 481             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 482             scrut_ty
 483         }
 484     }
 485
 486     fn find_block_span(
 487         &self,
 488         block: &'tcx hir::Block<'tcx>,
 489         expected_ty: Option<Ty<'tcx>>,
 490     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 491         if let Some(expr) = &block.expr {
 492             (expr.span, None)
 493         } else if let Some(stmt) = block.stmts.last() {
 494             // possibly incorrect trailing `;` in the else arm
 495             (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
 496         } else {
 497             // empty block; point at its entirety
 498             (block.span, None)
 499         }
 500     }
 501
 502     // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 503     // we check if the different arms would work with boxed trait objects instead and
 504     // provide a structured suggestion in that case.
 505     pub(crate) fn opt_suggest_box_span(
 506         &self,
 507         span: Span,
 508         outer_ty: Ty<'tcx>,
 509         orig_expected: Expectation<'tcx>,
 510     ) -> Option<Span> {
 511         match (orig_expected, self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty))) {
 512             (Expectation::ExpectHasType(expected), Some((_id, ty)))
 513                 if self.in_tail_expr && self.can_coerce(outer_ty, expected) =>
 514             {
 515                 let impl_trait_ret_ty =
 516                     self.infcx.instantiate_opaque_types(self.body_id, self.param_env, ty, span);
 517                 assert!(
 518                     impl_trait_ret_ty.obligations.is_empty(),
 519                     "we should never get new obligations here"
 520                 );
 521                 let obligations = self.fulfillment_cx.borrow().pending_obligations();
 522                 let mut suggest_box = !obligations.is_empty();
 523                 for o in obligations {
 524                     match o.predicate.kind().skip_binder() {
 525                         ty::PredicateKind::Trait(t) => {
 526                             let pred =
 527                                 ty::Binder::dummy(ty::PredicateKind::Trait(ty::TraitPredicate {
 528                                     trait_ref: ty::TraitRef {
 529                                         def_id: t.def_id(),
 530                                         substs: self.infcx.tcx.mk_substs_trait(outer_ty, &[]),
 531                                     },
 532                                     constness: t.constness,
 533                                     polarity: t.polarity,
 534                                 }));
 535                             let obl = Obligation::new(
 536                                 o.cause.clone(),
 537                                 self.param_env,
 538                                 pred.to_predicate(self.infcx.tcx),
 539                             );
 540                             suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
 541                             if !suggest_box {
 542                                 // We've encountered some obligation that didn't hold, so the
 543                                 // return expression can't just be boxed. We don't need to
 544                                 // evaluate the rest of the obligations.
 545                                 break;
 546                             }
 547                         }
 548                         _ => {}
 549                     }
 550                 }
 551                 // If all the obligations hold (or there are no obligations) the tail expression
 552                 // we can suggest to return a boxed trait object instead of an opaque type.
 553                 if suggest_box { self.ret_type_span } else { None }
 554             }
 555             _ => None,
 556         }
 557     }
 558 }
 559
 560 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 561     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
 562         hir::Mutability::Mut => 1,
 563         hir::Mutability::Not => 0,
 564     })
 565 }