compiler/rustc_hir_typeck/src/_match.rs

   1 use crate::coercion::{AsCoercionSite, CoerceMany};
   2 use crate::{Diverges, Expectation, FnCtxt, Needs};
   3 use rustc_errors::Diagnostic;
   4 use rustc_hir::{self as hir, ExprKind};
   5 use rustc_hir_pretty::ty_to_string;
   6 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
   7 use rustc_infer::traits::Obligation;
   8 use rustc_middle::ty::{self, Ty};
   9 use rustc_span::Span;
  10 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  11 use rustc_trait_selection::traits::{
  12     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
  13 };
  14
  15 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  16     #[instrument(skip(self), level = "debug", ret)]
  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         let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
  44         for arm in arms {
  45             self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut_span), Some(scrut), None);
  46         }
  47
  48         // Now typecheck the blocks.
  49         //
  50         // The result of the match is the common supertype of all the
  51         // arms. Start out the value as bottom, since it's the, well,
  52         // bottom the type lattice, and we'll be moving up the lattice as
  53         // we process each arm. (Note that any match with 0 arms is matching
  54         // on any empty type and is therefore unreachable; should the flow
  55         // of execution reach it, we will panic, so bottom is an appropriate
  56         // type in that case)
  57         let mut all_arms_diverge = Diverges::WarnedAlways;
  58
  59         let expected = orig_expected.adjust_for_branches(self);
  60         debug!(?expected);
  61
  62         let mut coercion = {
  63             let coerce_first = match expected {
  64                 // We don't coerce to `()` so that if the match expression is a
  65                 // statement it's branches can have any consistent type. That allows
  66                 // us to give better error messages (pointing to a usually better
  67                 // arm for inconsistent arms or to the whole match when a `()` type
  68                 // is required).
  69                 Expectation::ExpectHasType(ety) if ety != Ty::new_unit(self.tcx) => ety,
  70                 _ => self.next_ty_var(TypeVariableOrigin {
  71                     kind: TypeVariableOriginKind::MiscVariable,
  72                     span: expr.span,
  73                 }),
  74             };
  75             CoerceMany::with_coercion_sites(coerce_first, arms)
  76         };
  77
  78         let mut other_arms = vec![]; // Used only for diagnostics.
  79         let mut prior_arm = None;
  80         for arm in arms {
  81             if let Some(g) = &arm.guard {
  82                 self.diverges.set(Diverges::Maybe);
  83                 match g {
  84                     hir::Guard::If(e) => {
  85                         self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
  86                     }
  87                     hir::Guard::IfLet(l) => {
  88                         self.check_expr_let(l);
  89                     }
  90                 };
  91             }
  92
  93             self.diverges.set(Diverges::Maybe);
  94
  95             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
  96             all_arms_diverge &= self.diverges.get();
  97
  98             let opt_suggest_box_span = prior_arm.and_then(|(_, prior_arm_ty, _)| {
  99                 self.opt_suggest_box_span(prior_arm_ty, arm_ty, orig_expected)
 100             });
 101
 102             let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
 103                 (Some(blk.hir_id), self.find_block_span(blk))
 104             } else {
 105                 (None, arm.body.span)
 106             };
 107
 108             let (span, code) = match prior_arm {
 109                 // The reason for the first arm to fail is not that the match arms diverge,
 110                 // but rather that there's a prior obligation that doesn't hold.
 111                 None => {
 112                     (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id, match_src))
 113                 }
 114                 Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
 115                     expr.span,
 116                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
 117                         arm_block_id,
 118                         arm_span,
 119                         arm_ty,
 120                         prior_arm_block_id,
 121                         prior_arm_ty,
 122                         prior_arm_span,
 123                         scrut_span: scrut.span,
 124                         source: match_src,
 125                         prior_arms: other_arms.clone(),
 126                         opt_suggest_box_span,
 127                     })),
 128                 ),
 129             };
 130             let cause = self.cause(span, code);
 131
 132             // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
 133             // We use it this way to be able to expand on the potential error and detect when a
 134             // `match` tail statement could be a tail expression instead. If so, we suggest
 135             // removing the stray semicolon.
 136             coercion.coerce_inner(
 137                 self,
 138                 &cause,
 139                 Some(&arm.body),
 140                 arm_ty,
 141                 |err| self.suggest_removing_semicolon_for_coerce(err, expr, arm_ty, prior_arm),
 142                 false,
 143             );
 144
 145             other_arms.push(arm_span);
 146             if other_arms.len() > 5 {
 147                 other_arms.remove(0);
 148             }
 149
 150             if !arm_ty.is_never() {
 151                 // When a match arm has type `!`, then it doesn't influence the expected type for
 152                 // the following arm. If all of the prior arms are `!`, then the influence comes
 153                 // from elsewhere and we shouldn't point to any previous arm.
 154                 prior_arm = Some((arm_block_id, arm_ty, arm_span));
 155             }
 156         }
 157
 158         // If all of the arms in the `match` diverge,
 159         // and we're dealing with an actual `match` block
 160         // (as opposed to a `match` desugared from something else'),
 161         // we can emit a better note. Rather than pointing
 162         // at a diverging expression in an arbitrary arm,
 163         // we can point at the entire `match` expression
 164         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 165             all_arms_diverge = Diverges::Always {
 166                 span: expr.span,
 167                 custom_note: Some(
 168                     "any code following this `match` expression is unreachable, as all arms diverge",
 169                 ),
 170             };
 171         }
 172
 173         // We won't diverge unless the scrutinee or all arms diverge.
 174         self.diverges.set(scrut_diverges | all_arms_diverge);
 175
 176         coercion.complete(self)
 177     }
 178
 179     fn suggest_removing_semicolon_for_coerce(
 180         &self,
 181         diag: &mut Diagnostic,
 182         expr: &hir::Expr<'tcx>,
 183         arm_ty: Ty<'tcx>,
 184         prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
 185     ) {
 186         let hir = self.tcx.hir();
 187
 188         // First, check that we're actually in the tail of a function.
 189         let Some(body_id) = hir.maybe_body_owned_by(self.body_id) else {
 190             return;
 191         };
 192         let body = hir.body(body_id);
 193         let hir::ExprKind::Block(block, _) = body.value.kind else {
 194             return;
 195         };
 196         let Some(hir::Stmt { kind: hir::StmtKind::Semi(last_expr), span: semi_span, .. }) =
 197             block.innermost_block().stmts.last()
 198         else {
 199             return;
 200         };
 201         if last_expr.hir_id != expr.hir_id {
 202             return;
 203         }
 204
 205         // Next, make sure that we have no type expectation.
 206         let Some(ret) = hir
 207             .find_by_def_id(self.body_id)
 208             .and_then(|owner| owner.fn_decl())
 209             .map(|decl| decl.output.span())
 210         else {
 211             return;
 212         };
 213
 214         let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 215             Some(ret_coercion) => {
 216                 let ret_ty = ret_coercion.borrow().expected_ty();
 217                 let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 218                 self.can_coerce(arm_ty, ret_ty)
 219                     && prior_arm.map_or(true, |(_, ty, _)| self.can_coerce(ty, ret_ty))
 220                     // The match arms need to unify for the case of `impl Trait`.
 221                     && !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
 222             }
 223             _ => false,
 224         };
 225         if !can_coerce_to_return_ty {
 226             return;
 227         }
 228
 229         let semi = expr.span.shrink_to_hi().with_hi(semi_span.hi());
 230         let sugg = crate::errors::RemoveSemiForCoerce { expr: expr.span, ret, semi };
 231         diag.subdiagnostic(sugg);
 232     }
 233
 234     /// When the previously checked expression (the scrutinee) diverges,
 235     /// warn the user about the match arms being unreachable.
 236     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 237         for arm in arms {
 238             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 239         }
 240     }
 241
 242     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 243     ///
 244     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 245     pub(super) fn if_fallback_coercion<T>(
 246         &self,
 247         span: Span,
 248         then_expr: &'tcx hir::Expr<'tcx>,
 249         coercion: &mut CoerceMany<'tcx, '_, T>,
 250     ) -> bool
 251     where
 252         T: AsCoercionSite,
 253     {
 254         // If this `if` expr is the parent's function return expr,
 255         // the cause of the type coercion is the return type, point at it. (#25228)
 256         let hir_id = self.tcx.hir().parent_id(self.tcx.hir().parent_id(then_expr.hir_id));
 257         let ret_reason = self.maybe_get_coercion_reason(hir_id, span);
 258         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 259         let mut error = false;
 260         coercion.coerce_forced_unit(
 261             self,
 262             &cause,
 263             |err| {
 264                 if let Some((span, msg)) = &ret_reason {
 265                     err.span_label(*span, msg.clone());
 266                 } else if let ExprKind::Block(block, _) = &then_expr.kind
 267                     && let Some(expr) = &block.expr
 268                 {
 269                     err.span_label(expr.span, "found here");
 270                 }
 271                 err.note("`if` expressions without `else` evaluate to `()`");
 272                 err.help("consider adding an `else` block that evaluates to the expected type");
 273                 error = true;
 274             },
 275             false,
 276         );
 277         error
 278     }
 279
 280     pub fn maybe_get_coercion_reason(
 281         &self,
 282         hir_id: hir::HirId,
 283         sp: Span,
 284     ) -> Option<(Span, String)> {
 285         let node = self.tcx.hir().get(hir_id);
 286         if let hir::Node::Block(block) = node {
 287             // check that the body's parent is an fn
 288             let parent = self.tcx.hir().get_parent(self.tcx.hir().parent_id(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 (ty, span) = match fn_decl.output {
 296                             hir::FnRetTy::DefaultReturn(span) => ("()".to_string(), span),
 297                             hir::FnRetTy::Return(ty) => (ty_to_string(ty), ty.span),
 298                         };
 299                         Some((span, format!("expected `{ty}` because of this return type")))
 300                     });
 301                 }
 302             }
 303         }
 304         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 305             return Some((pat.span, "expected because of this assignment".to_string()));
 306         }
 307         None
 308     }
 309
 310     pub(crate) fn if_cause(
 311         &self,
 312         span: Span,
 313         cond_span: Span,
 314         then_expr: &'tcx hir::Expr<'tcx>,
 315         else_expr: &'tcx hir::Expr<'tcx>,
 316         then_ty: Ty<'tcx>,
 317         else_ty: Ty<'tcx>,
 318         opt_suggest_box_span: Option<Span>,
 319     ) -> ObligationCause<'tcx> {
 320         let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
 321             // The `if`/`else` isn't in one line in the output, include some context to make it
 322             // clear it is an if/else expression:
 323             // ```
 324             // LL |      let x = if true {
 325             //    | _____________-
 326             // LL ||         10i32
 327             //    ||         ----- expected because of this
 328             // LL ||     } else {
 329             // LL ||         10u32
 330             //    ||         ^^^^^ expected `i32`, found `u32`
 331             // LL ||     };
 332             //    ||_____- `if` and `else` have incompatible types
 333             // ```
 334             Some(span)
 335         } else {
 336             // The entire expression is in one line, only point at the arms
 337             // ```
 338             // LL |     let x = if true { 10i32 } else { 10u32 };
 339             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 340             //    |                       |
 341             //    |                       expected because of this
 342             // ```
 343             None
 344         };
 345
 346         let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
 347             let block = block.innermost_block();
 348
 349             // Avoid overlapping spans that aren't as readable:
 350             // ```
 351             // 2 |        let x = if true {
 352             //   |   _____________-
 353             // 3 |  |         3
 354             //   |  |         - expected because of this
 355             // 4 |  |     } else {
 356             //   |  |____________^
 357             // 5 | ||
 358             // 6 | ||     };
 359             //   | ||     ^
 360             //   | ||_____|
 361             //   | |______if and else have incompatible types
 362             //   |        expected integer, found `()`
 363             // ```
 364             // by not pointing at the entire expression:
 365             // ```
 366             // 2 |       let x = if true {
 367             //   |               ------- `if` and `else` have incompatible types
 368             // 3 |           3
 369             //   |           - expected because of this
 370             // 4 |       } else {
 371             //   |  ____________^
 372             // 5 | |
 373             // 6 | |     };
 374             //   | |_____^ expected integer, found `()`
 375             // ```
 376             if block.expr.is_none() && block.stmts.is_empty()
 377                 && let Some(outer_span) = &mut outer_span
 378                 && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
 379             {
 380                 *outer_span = outer_span.with_hi(cond_span.hi())
 381             }
 382
 383             (self.find_block_span(block), block.hir_id)
 384         } else {
 385             (else_expr.span, else_expr.hir_id)
 386         };
 387
 388         let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
 389             let block = block.innermost_block();
 390             // Exclude overlapping spans
 391             if block.expr.is_none() && block.stmts.is_empty() {
 392                 outer_span = None;
 393             }
 394             block.hir_id
 395         } else {
 396             then_expr.hir_id
 397         };
 398
 399         // Finally construct the cause:
 400         self.cause(
 401             error_sp,
 402             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 403                 else_id,
 404                 then_id,
 405                 then_ty,
 406                 else_ty,
 407                 outer_span,
 408                 opt_suggest_box_span,
 409             })),
 410         )
 411     }
 412
 413     pub(super) fn demand_scrutinee_type(
 414         &self,
 415         scrut: &'tcx hir::Expr<'tcx>,
 416         contains_ref_bindings: Option<hir::Mutability>,
 417         no_arms: bool,
 418     ) -> Ty<'tcx> {
 419         // Not entirely obvious: if matches may create ref bindings, we want to
 420         // use the *precise* type of the scrutinee, *not* some supertype, as
 421         // the "scrutinee type" (issue #23116).
 422         //
 423         // arielb1 [writes here in this comment thread][c] that there
 424         // is certainly *some* potential danger, e.g., for an example
 425         // like:
 426         //
 427         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 428         //
 429         // ```
 430         // let Foo(x) = f()[0];
 431         // ```
 432         //
 433         // Then if the pattern matches by reference, we want to match
 434         // `f()[0]` as a lexpr, so we can't allow it to be
 435         // coerced. But if the pattern matches by value, `f()[0]` is
 436         // still syntactically a lexpr, but we *do* want to allow
 437         // coercions.
 438         //
 439         // However, *likely* we are ok with allowing coercions to
 440         // happen if there are no explicit ref mut patterns - all
 441         // implicit ref mut patterns must occur behind a reference, so
 442         // they will have the "correct" variance and lifetime.
 443         //
 444         // This does mean that the following pattern would be legal:
 445         //
 446         // ```
 447         // struct Foo(Bar);
 448         // struct Bar(u32);
 449         // impl Deref for Foo {
 450         //     type Target = Bar;
 451         //     fn deref(&self) -> &Bar { &self.0 }
 452         // }
 453         // impl DerefMut for Foo {
 454         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 455         // }
 456         // fn foo(x: &mut Foo) {
 457         //     {
 458         //         let Bar(z): &mut Bar = x;
 459         //         *z = 42;
 460         //     }
 461         //     assert_eq!(foo.0.0, 42);
 462         // }
 463         // ```
 464         //
 465         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 466         // is problematic as the HIR is being scraped, but ref bindings may be
 467         // implicit after #42640. We need to make sure that pat_adjustments
 468         // (once introduced) is populated by the time we get here.
 469         //
 470         // See #44848.
 471         if let Some(m) = contains_ref_bindings {
 472             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 473         } else if no_arms {
 474             self.check_expr(scrut)
 475         } else {
 476             // ...but otherwise we want to use any supertype of the
 477             // scrutinee. This is sort of a workaround, see note (*) in
 478             // `check_pat` for some details.
 479             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 480                 kind: TypeVariableOriginKind::TypeInference,
 481                 span: scrut.span,
 482             });
 483             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 484             scrut_ty
 485         }
 486     }
 487
 488     /// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 489     /// we check if the different arms would work with boxed trait objects instead and
 490     /// provide a structured suggestion in that case.
 491     pub(crate) fn opt_suggest_box_span(
 492         &self,
 493         first_ty: Ty<'tcx>,
 494         second_ty: Ty<'tcx>,
 495         orig_expected: Expectation<'tcx>,
 496     ) -> Option<Span> {
 497         // FIXME(compiler-errors): This really shouldn't need to be done during the
 498         // "good" path of typeck, but here we are.
 499         match orig_expected {
 500             Expectation::ExpectHasType(expected) => {
 501                 let TypeVariableOrigin {
 502                     span,
 503                     kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id),
 504                     ..
 505                 } = self.type_var_origin(expected)?
 506                 else {
 507                     return None;
 508                 };
 509
 510                 let Some(rpit_local_def_id) = rpit_def_id.as_local() else {
 511                     return None;
 512                 };
 513                 if !matches!(
 514                     self.tcx.hir().expect_item(rpit_local_def_id).expect_opaque_ty().origin,
 515                     hir::OpaqueTyOrigin::FnReturn(..)
 516                 ) {
 517                     return None;
 518                 }
 519
 520                 let sig = self.body_fn_sig()?;
 521
 522                 let args = sig.output().walk().find_map(|arg| {
 523                     if let ty::GenericArgKind::Type(ty) = arg.unpack()
 524                         && let ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) = *ty.kind()
 525                         && def_id == rpit_def_id
 526                     {
 527                         Some(args)
 528                     } else {
 529                         None
 530                     }
 531                 })?;
 532
 533                 if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) {
 534                     return None;
 535                 }
 536
 537                 for ty in [first_ty, second_ty] {
 538                     for (clause, _) in self
 539                         .tcx
 540                         .explicit_item_bounds(rpit_def_id)
 541                         .iter_instantiated_copied(self.tcx, args)
 542                     {
 543                         let pred = clause.kind().rebind(match clause.kind().skip_binder() {
 544                             ty::ClauseKind::Trait(trait_pred) => {
 545                                 assert!(matches!(
 546                                     *trait_pred.trait_ref.self_ty().kind(),
 547                                     ty::Alias(ty::Opaque, ty::AliasTy { def_id, args: alias_args, .. })
 548                                     if def_id == rpit_def_id && args == alias_args
 549                                 ));
 550                                 ty::ClauseKind::Trait(trait_pred.with_self_ty(self.tcx, ty))
 551                             }
 552                             ty::ClauseKind::Projection(mut proj_pred) => {
 553                                 assert!(matches!(
 554                                     *proj_pred.projection_ty.self_ty().kind(),
 555                                     ty::Alias(ty::Opaque, ty::AliasTy { def_id, args: alias_args, .. })
 556                                     if def_id == rpit_def_id && args == alias_args
 557                                 ));
 558                                 proj_pred = proj_pred.with_self_ty(self.tcx, ty);
 559                                 ty::ClauseKind::Projection(proj_pred)
 560                             }
 561                             _ => continue,
 562                         });
 563                         if !self.predicate_must_hold_modulo_regions(&Obligation::new(
 564                             self.tcx,
 565                             ObligationCause::misc(span, self.body_id),
 566                             self.param_env,
 567                             pred,
 568                         )) {
 569                             return None;
 570                         }
 571                     }
 572                 }
 573
 574                 Some(span)
 575             }
 576             _ => None,
 577         }
 578     }
 579 }
 580
 581 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 582     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max()
 583 }