compiler/rustc_hir_typeck/src/_match.rs

   1 use crate::coercion::{AsCoercionSite, CoerceMany};
   2 use crate::{Diverges, Expectation, FnCtxt, Needs};
   3 use rustc_errors::{Applicability, Diagnostic, MultiSpan};
   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, Ty};
   8 use rustc_span::Span;
   9 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  10 use rustc_trait_selection::traits::{
  11     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
  12 };
  13
  14 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  15     #[instrument(skip(self), level = "debug", ret)]
  16     pub fn check_match(
  17         &self,
  18         expr: &'tcx hir::Expr<'tcx>,
  19         scrut: &'tcx hir::Expr<'tcx>,
  20         arms: &'tcx [hir::Arm<'tcx>],
  21         orig_expected: Expectation<'tcx>,
  22         match_src: hir::MatchSource,
  23     ) -> Ty<'tcx> {
  24         let tcx = self.tcx;
  25
  26         let acrb = arms_contain_ref_bindings(arms);
  27         let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
  28         debug!(?scrutinee_ty);
  29
  30         // If there are no arms, that is a diverging match; a special case.
  31         if arms.is_empty() {
  32             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
  33             return tcx.types.never;
  34         }
  35
  36         self.warn_arms_when_scrutinee_diverges(arms);
  37
  38         // Otherwise, we have to union together the types that the arms produce and so forth.
  39         let scrut_diverges = self.diverges.replace(Diverges::Maybe);
  40
  41         // #55810: Type check patterns first so we get types for all bindings.
  42         let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
  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         debug!(?expected);
  60
  61         let mut coercion = {
  62             let coerce_first = match expected {
  63                 // We don't coerce to `()` so that if the match expression is a
  64                 // statement it's branches can have any consistent type. That allows
  65                 // us to give better error messages (pointing to a usually better
  66                 // arm for inconsistent arms or to the whole match when a `()` type
  67                 // is required).
  68                 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
  69                 _ => self.next_ty_var(TypeVariableOrigin {
  70                     kind: TypeVariableOriginKind::MiscVariable,
  71                     span: expr.span,
  72                 }),
  73             };
  74             CoerceMany::with_coercion_sites(coerce_first, arms)
  75         };
  76
  77         let mut other_arms = vec![]; // Used only for diagnostics.
  78         let mut prior_arm = None;
  79         for arm in arms {
  80             if let Some(g) = &arm.guard {
  81                 self.diverges.set(Diverges::Maybe);
  82                 match g {
  83                     hir::Guard::If(e) => {
  84                         self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
  85                     }
  86                     hir::Guard::IfLet(l) => {
  87                         self.check_expr_let(l);
  88                     }
  89                 };
  90             }
  91
  92             self.diverges.set(Diverges::Maybe);
  93
  94             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
  95             all_arms_diverge &= self.diverges.get();
  96
  97             let opt_suggest_box_span = prior_arm.and_then(|(_, prior_arm_ty, _)| {
  98                 self.opt_suggest_box_span(prior_arm_ty, arm_ty, orig_expected)
  99             });
 100
 101             let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
 102                 (Some(blk.hir_id), self.find_block_span(blk))
 103             } else {
 104                 (None, arm.body.span)
 105             };
 106
 107             let (span, code) = match prior_arm {
 108                 // The reason for the first arm to fail is not that the match arms diverge,
 109                 // but rather that there's a prior obligation that doesn't hold.
 110                 None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
 111                 Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
 112                     expr.span,
 113                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
 114                         arm_block_id,
 115                         arm_span,
 116                         arm_ty,
 117                         prior_arm_block_id,
 118                         prior_arm_ty,
 119                         prior_arm_span,
 120                         scrut_span: scrut.span,
 121                         source: match_src,
 122                         prior_arms: other_arms.clone(),
 123                         scrut_hir_id: scrut.hir_id,
 124                         opt_suggest_box_span,
 125                     })),
 126                 ),
 127             };
 128             let cause = self.cause(span, code);
 129
 130             // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
 131             // We use it this way to be able to expand on the potential error and detect when a
 132             // `match` tail statement could be a tail expression instead. If so, we suggest
 133             // removing the stray semicolon.
 134             coercion.coerce_inner(
 135                 self,
 136                 &cause,
 137                 Some(&arm.body),
 138                 arm_ty,
 139                 Some(&mut |err| {
 140                     self.suggest_removing_semicolon_for_coerce(
 141                         err,
 142                         expr,
 143                         orig_expected,
 144                         arm_ty,
 145                         prior_arm,
 146                     )
 147                 }),
 148                 false,
 149             );
 150
 151             other_arms.push(arm_span);
 152             if other_arms.len() > 5 {
 153                 other_arms.remove(0);
 154             }
 155
 156             prior_arm = Some((arm_block_id, arm_ty, arm_span));
 157         }
 158
 159         // If all of the arms in the `match` diverge,
 160         // and we're dealing with an actual `match` block
 161         // (as opposed to a `match` desugared from something else'),
 162         // we can emit a better note. Rather than pointing
 163         // at a diverging expression in an arbitrary arm,
 164         // we can point at the entire `match` expression
 165         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 166             all_arms_diverge = Diverges::Always {
 167                 span: expr.span,
 168                 custom_note: Some(
 169                     "any code following this `match` expression is unreachable, as all arms diverge",
 170                 ),
 171             };
 172         }
 173
 174         // We won't diverge unless the scrutinee or all arms diverge.
 175         self.diverges.set(scrut_diverges | all_arms_diverge);
 176
 177         coercion.complete(self)
 178     }
 179
 180     fn suggest_removing_semicolon_for_coerce(
 181         &self,
 182         diag: &mut Diagnostic,
 183         expr: &hir::Expr<'tcx>,
 184         expectation: Expectation<'tcx>,
 185         arm_ty: Ty<'tcx>,
 186         prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
 187     ) {
 188         let hir = self.tcx.hir();
 189
 190         // First, check that we're actually in the tail of a function.
 191         let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Block(block, _), .. }) =
 192             hir.get(self.body_id) else { return; };
 193         let Some(hir::Stmt { kind: hir::StmtKind::Semi(last_expr), .. })
 194             = block.innermost_block().stmts.last() else {  return; };
 195         if last_expr.hir_id != expr.hir_id {
 196             return;
 197         }
 198
 199         // Next, make sure that we have no type expectation.
 200         let Some(ret) = hir
 201             .find_by_def_id(self.body_id.owner.def_id)
 202             .and_then(|owner| owner.fn_decl())
 203             .map(|decl| decl.output.span()) else { return; };
 204         let Expectation::IsLast(stmt) = expectation else {
 205             return;
 206         };
 207
 208         let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 209             Some(ret_coercion) => {
 210                 let ret_ty = ret_coercion.borrow().expected_ty();
 211                 let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 212                 self.can_coerce(arm_ty, ret_ty)
 213                     && prior_arm.map_or(true, |(_, ty, _)| self.can_coerce(ty, ret_ty))
 214                     // The match arms need to unify for the case of `impl Trait`.
 215                     && !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
 216             }
 217             _ => false,
 218         };
 219         if !can_coerce_to_return_ty {
 220             return;
 221         }
 222
 223         let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 224         let mut ret_span: MultiSpan = semi_span.into();
 225         ret_span.push_span_label(
 226             expr.span,
 227             "this could be implicitly returned but it is a statement, not a \
 228                             tail expression",
 229         );
 230         ret_span.push_span_label(ret, "the `match` arms can conform to this return type");
 231         ret_span.push_span_label(
 232             semi_span,
 233             "the `match` is a statement because of this semicolon, consider \
 234                             removing it",
 235         );
 236         diag.span_note(ret_span, "you might have meant to return the `match` expression");
 237         diag.tool_only_span_suggestion(
 238             semi_span,
 239             "remove this semicolon",
 240             "",
 241             Applicability::MaybeIncorrect,
 242         );
 243     }
 244
 245     /// When the previously checked expression (the scrutinee) diverges,
 246     /// warn the user about the match arms being unreachable.
 247     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 248         for arm in arms {
 249             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 250         }
 251     }
 252
 253     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 254     ///
 255     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 256     pub(super) fn if_fallback_coercion<T>(
 257         &self,
 258         span: Span,
 259         then_expr: &'tcx hir::Expr<'tcx>,
 260         coercion: &mut CoerceMany<'tcx, '_, T>,
 261     ) -> bool
 262     where
 263         T: AsCoercionSite,
 264     {
 265         // If this `if` expr is the parent's function return expr,
 266         // the cause of the type coercion is the return type, point at it. (#25228)
 267         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
 268         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 269         let mut error = false;
 270         coercion.coerce_forced_unit(
 271             self,
 272             &cause,
 273             &mut |err| {
 274                 if let Some((span, msg)) = &ret_reason {
 275                     err.span_label(*span, msg);
 276                 } else if let ExprKind::Block(block, _) = &then_expr.kind
 277                     && let Some(expr) = &block.expr
 278                 {
 279                     err.span_label(expr.span, "found here");
 280                 }
 281                 err.note("`if` expressions without `else` evaluate to `()`");
 282                 err.help("consider adding an `else` block that evaluates to the expected type");
 283                 error = true;
 284             },
 285             false,
 286         );
 287         error
 288     }
 289
 290     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
 291         let node = {
 292             let rslt = self.tcx.hir().parent_id(self.tcx.hir().parent_id(hir_id));
 293             self.tcx.hir().get(rslt)
 294         };
 295         if let hir::Node::Block(block) = node {
 296             // check that the body's parent is an fn
 297             let parent = self.tcx.hir().get_parent(self.tcx.hir().parent_id(block.hir_id));
 298             if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
 299                 (&block.expr, parent)
 300             {
 301                 // check that the `if` expr without `else` is the fn body's expr
 302                 if expr.span == sp {
 303                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
 304                         let span = fn_decl.output.span();
 305                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
 306                         Some((span, format!("expected `{snippet}` because of this return type")))
 307                     });
 308                 }
 309             }
 310         }
 311         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 312             return Some((pat.span, "expected because of this assignment".to_string()));
 313         }
 314         None
 315     }
 316
 317     pub(crate) fn if_cause(
 318         &self,
 319         span: Span,
 320         cond_span: Span,
 321         then_expr: &'tcx hir::Expr<'tcx>,
 322         else_expr: &'tcx hir::Expr<'tcx>,
 323         then_ty: Ty<'tcx>,
 324         else_ty: Ty<'tcx>,
 325         opt_suggest_box_span: Option<Span>,
 326     ) -> ObligationCause<'tcx> {
 327         let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
 328             // The `if`/`else` isn't in one line in the output, include some context to make it
 329             // clear it is an if/else expression:
 330             // ```
 331             // LL |      let x = if true {
 332             //    | _____________-
 333             // LL ||         10i32
 334             //    ||         ----- expected because of this
 335             // LL ||     } else {
 336             // LL ||         10u32
 337             //    ||         ^^^^^ expected `i32`, found `u32`
 338             // LL ||     };
 339             //    ||_____- `if` and `else` have incompatible types
 340             // ```
 341             Some(span)
 342         } else {
 343             // The entire expression is in one line, only point at the arms
 344             // ```
 345             // LL |     let x = if true { 10i32 } else { 10u32 };
 346             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 347             //    |                       |
 348             //    |                       expected because of this
 349             // ```
 350             None
 351         };
 352
 353         let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
 354             let block = block.innermost_block();
 355
 356             // Avoid overlapping spans that aren't as readable:
 357             // ```
 358             // 2 |        let x = if true {
 359             //   |   _____________-
 360             // 3 |  |         3
 361             //   |  |         - expected because of this
 362             // 4 |  |     } else {
 363             //   |  |____________^
 364             // 5 | ||
 365             // 6 | ||     };
 366             //   | ||     ^
 367             //   | ||_____|
 368             //   | |______if and else have incompatible types
 369             //   |        expected integer, found `()`
 370             // ```
 371             // by not pointing at the entire expression:
 372             // ```
 373             // 2 |       let x = if true {
 374             //   |               ------- `if` and `else` have incompatible types
 375             // 3 |           3
 376             //   |           - expected because of this
 377             // 4 |       } else {
 378             //   |  ____________^
 379             // 5 | |
 380             // 6 | |     };
 381             //   | |_____^ expected integer, found `()`
 382             // ```
 383             if block.expr.is_none() && block.stmts.is_empty()
 384                 && let Some(outer_span) = &mut outer_span
 385                 && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
 386             {
 387                 *outer_span = outer_span.with_hi(cond_span.hi())
 388             }
 389
 390             (self.find_block_span(block), block.hir_id)
 391         } else {
 392             (else_expr.span, else_expr.hir_id)
 393         };
 394
 395         let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
 396             let block = block.innermost_block();
 397             // Exclude overlapping spans
 398             if block.expr.is_none() && block.stmts.is_empty() {
 399                 outer_span = None;
 400             }
 401             block.hir_id
 402         } else {
 403             then_expr.hir_id
 404         };
 405
 406         // Finally construct the cause:
 407         self.cause(
 408             error_sp,
 409             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 410                 else_id,
 411                 then_id,
 412                 then_ty,
 413                 else_ty,
 414                 outer_span,
 415                 opt_suggest_box_span,
 416             })),
 417         )
 418     }
 419
 420     pub(super) fn demand_scrutinee_type(
 421         &self,
 422         scrut: &'tcx hir::Expr<'tcx>,
 423         contains_ref_bindings: Option<hir::Mutability>,
 424         no_arms: bool,
 425     ) -> Ty<'tcx> {
 426         // Not entirely obvious: if matches may create ref bindings, we want to
 427         // use the *precise* type of the scrutinee, *not* some supertype, as
 428         // the "scrutinee type" (issue #23116).
 429         //
 430         // arielb1 [writes here in this comment thread][c] that there
 431         // is certainly *some* potential danger, e.g., for an example
 432         // like:
 433         //
 434         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 435         //
 436         // ```
 437         // let Foo(x) = f()[0];
 438         // ```
 439         //
 440         // Then if the pattern matches by reference, we want to match
 441         // `f()[0]` as a lexpr, so we can't allow it to be
 442         // coerced. But if the pattern matches by value, `f()[0]` is
 443         // still syntactically a lexpr, but we *do* want to allow
 444         // coercions.
 445         //
 446         // However, *likely* we are ok with allowing coercions to
 447         // happen if there are no explicit ref mut patterns - all
 448         // implicit ref mut patterns must occur behind a reference, so
 449         // they will have the "correct" variance and lifetime.
 450         //
 451         // This does mean that the following pattern would be legal:
 452         //
 453         // ```
 454         // struct Foo(Bar);
 455         // struct Bar(u32);
 456         // impl Deref for Foo {
 457         //     type Target = Bar;
 458         //     fn deref(&self) -> &Bar { &self.0 }
 459         // }
 460         // impl DerefMut for Foo {
 461         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 462         // }
 463         // fn foo(x: &mut Foo) {
 464         //     {
 465         //         let Bar(z): &mut Bar = x;
 466         //         *z = 42;
 467         //     }
 468         //     assert_eq!(foo.0.0, 42);
 469         // }
 470         // ```
 471         //
 472         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 473         // is problematic as the HIR is being scraped, but ref bindings may be
 474         // implicit after #42640. We need to make sure that pat_adjustments
 475         // (once introduced) is populated by the time we get here.
 476         //
 477         // See #44848.
 478         if let Some(m) = contains_ref_bindings {
 479             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 480         } else if no_arms {
 481             self.check_expr(scrut)
 482         } else {
 483             // ...but otherwise we want to use any supertype of the
 484             // scrutinee. This is sort of a workaround, see note (*) in
 485             // `check_pat` for some details.
 486             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 487                 kind: TypeVariableOriginKind::TypeInference,
 488                 span: scrut.span,
 489             });
 490             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 491             scrut_ty
 492         }
 493     }
 494
 495     /// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 496     /// we check if the different arms would work with boxed trait objects instead and
 497     /// provide a structured suggestion in that case.
 498     pub(crate) fn opt_suggest_box_span(
 499         &self,
 500         first_ty: Ty<'tcx>,
 501         second_ty: Ty<'tcx>,
 502         orig_expected: Expectation<'tcx>,
 503     ) -> Option<Span> {
 504         // FIXME(compiler-errors): This really shouldn't need to be done during the
 505         // "good" path of typeck, but here we are.
 506         match orig_expected {
 507             Expectation::ExpectHasType(expected) => {
 508                 let TypeVariableOrigin {
 509                     span,
 510                     kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id),
 511                     ..
 512                 } = self.type_var_origin(expected)? else { return None; };
 513
 514                 let sig = self.body_fn_sig()?;
 515
 516                 let substs = sig.output().walk().find_map(|arg| {
 517                     if let ty::GenericArgKind::Type(ty) = arg.unpack()
 518                         && let ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) = *ty.kind()
 519                         && def_id == rpit_def_id
 520                     {
 521                         Some(substs)
 522                     } else {
 523                         None
 524                     }
 525                 })?;
 526
 527                 if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) {
 528                     return None;
 529                 }
 530
 531                 for ty in [first_ty, second_ty] {
 532                     for (pred, _) in self
 533                         .tcx
 534                         .bound_explicit_item_bounds(rpit_def_id)
 535                         .subst_iter_copied(self.tcx, substs)
 536                     {
 537                         let pred = pred.kind().rebind(match pred.kind().skip_binder() {
 538                             ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) => {
 539                                 // FIXME(rpitit): This will need to be fixed when we move to associated types
 540                                 assert!(matches!(
 541                                     *trait_pred.trait_ref.self_ty().kind(),
 542                                     ty::Alias(_, ty::AliasTy { def_id, substs, .. })
 543                                     if def_id == rpit_def_id && substs == substs
 544                                 ));
 545                                 ty::PredicateKind::Clause(ty::Clause::Trait(
 546                                     trait_pred.with_self_ty(self.tcx, ty),
 547                                 ))
 548                             }
 549                             ty::PredicateKind::Clause(ty::Clause::Projection(mut proj_pred)) => {
 550                                 assert!(matches!(
 551                                     *proj_pred.projection_ty.self_ty().kind(),
 552                                     ty::Alias(_, ty::AliasTy { def_id, substs, .. })
 553                                     if def_id == rpit_def_id && substs == substs
 554                                 ));
 555                                 proj_pred = proj_pred.with_self_ty(self.tcx, ty);
 556                                 ty::PredicateKind::Clause(ty::Clause::Projection(proj_pred))
 557                             }
 558                             _ => continue,
 559                         });
 560                         if !self.predicate_must_hold_modulo_regions(&Obligation::new(
 561                             self.tcx,
 562                             ObligationCause::misc(span, self.body_id),
 563                             self.param_env,
 564                             pred,
 565                         )) {
 566                             return None;
 567                         }
 568                     }
 569                 }
 570
 571                 Some(span)
 572             }
 573             _ => None,
 574         }
 575     }
 576 }
 577
 578 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 579     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max()
 580 }