compiler/rustc_hir_analysis/src/check/_match.rs

   1 use crate::check::coercion::{AsCoercionSite, CoerceMany};
   2 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
   3 use rustc_errors::{Applicability, 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, ToPredicate, 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                     let Some(ret) = self
 141                         .tcx
 142                         .hir()
 143                         .find_by_def_id(self.body_id.owner.def_id)
 144                         .and_then(|owner| owner.fn_decl())
 145                         .map(|decl| decl.output.span())
 146                     else { return; };
 147                     let Expectation::IsLast(stmt) = orig_expected else {
 148                         return
 149                     };
 150                     let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 151                         Some(ret_coercion) if self.in_tail_expr => {
 152                             let ret_ty = ret_coercion.borrow().expected_ty();
 153                             let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 154                             self.can_coerce(arm_ty, ret_ty)
 155                                 && prior_arm.map_or(true, |(_, t, _)| self.can_coerce(t, ret_ty))
 156                                 // The match arms need to unify for the case of `impl Trait`.
 157                                 && !matches!(ret_ty.kind(), ty::Opaque(..))
 158                         }
 159                         _ => false,
 160                     };
 161                     if !can_coerce_to_return_ty {
 162                         return;
 163                     }
 164
 165                     let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 166                     let mut ret_span: MultiSpan = semi_span.into();
 167                     ret_span.push_span_label(
 168                         expr.span,
 169                         "this could be implicitly returned but it is a statement, not a \
 170                             tail expression",
 171                     );
 172                     ret_span
 173                         .push_span_label(ret, "the `match` arms can conform to this return type");
 174                     ret_span.push_span_label(
 175                         semi_span,
 176                         "the `match` is a statement because of this semicolon, consider \
 177                             removing it",
 178                     );
 179                     err.span_note(
 180                         ret_span,
 181                         "you might have meant to return the `match` expression",
 182                     );
 183                     err.tool_only_span_suggestion(
 184                         semi_span,
 185                         "remove this semicolon",
 186                         "",
 187                         Applicability::MaybeIncorrect,
 188                     );
 189                 }),
 190                 false,
 191             );
 192
 193             other_arms.push(arm_span);
 194             if other_arms.len() > 5 {
 195                 other_arms.remove(0);
 196             }
 197
 198             prior_arm = Some((arm_block_id, arm_ty, arm_span));
 199         }
 200
 201         // If all of the arms in the `match` diverge,
 202         // and we're dealing with an actual `match` block
 203         // (as opposed to a `match` desugared from something else'),
 204         // we can emit a better note. Rather than pointing
 205         // at a diverging expression in an arbitrary arm,
 206         // we can point at the entire `match` expression
 207         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 208             all_arms_diverge = Diverges::Always {
 209                 span: expr.span,
 210                 custom_note: Some(
 211                     "any code following this `match` expression is unreachable, as all arms diverge",
 212                 ),
 213             };
 214         }
 215
 216         // We won't diverge unless the scrutinee or all arms diverge.
 217         self.diverges.set(scrut_diverges | all_arms_diverge);
 218
 219         coercion.complete(self)
 220     }
 221
 222     /// When the previously checked expression (the scrutinee) diverges,
 223     /// warn the user about the match arms being unreachable.
 224     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 225         for arm in arms {
 226             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 227         }
 228     }
 229
 230     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 231     ///
 232     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 233     pub(super) fn if_fallback_coercion<T>(
 234         &self,
 235         span: Span,
 236         then_expr: &'tcx hir::Expr<'tcx>,
 237         coercion: &mut CoerceMany<'tcx, '_, T>,
 238     ) -> bool
 239     where
 240         T: AsCoercionSite,
 241     {
 242         // If this `if` expr is the parent's function return expr,
 243         // the cause of the type coercion is the return type, point at it. (#25228)
 244         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
 245         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 246         let mut error = false;
 247         coercion.coerce_forced_unit(
 248             self,
 249             &cause,
 250             &mut |err| {
 251                 if let Some((span, msg)) = &ret_reason {
 252                     err.span_label(*span, msg);
 253                 } else if let ExprKind::Block(block, _) = &then_expr.kind
 254                     && let Some(expr) = &block.expr
 255                 {
 256                     err.span_label(expr.span, "found here");
 257                 }
 258                 err.note("`if` expressions without `else` evaluate to `()`");
 259                 err.help("consider adding an `else` block that evaluates to the expected type");
 260                 error = true;
 261             },
 262             ret_reason.is_none(),
 263         );
 264         error
 265     }
 266
 267     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
 268         let node = {
 269             let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id));
 270             self.tcx.hir().get(rslt)
 271         };
 272         if let hir::Node::Block(block) = node {
 273             // check that the body's parent is an fn
 274             let parent = self
 275                 .tcx
 276                 .hir()
 277                 .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)));
 278             if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
 279                 (&block.expr, parent)
 280             {
 281                 // check that the `if` expr without `else` is the fn body's expr
 282                 if expr.span == sp {
 283                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
 284                         let span = fn_decl.output.span();
 285                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
 286                         Some((span, format!("expected `{snippet}` because of this return type")))
 287                     });
 288                 }
 289             }
 290         }
 291         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 292             return Some((pat.span, "expected because of this assignment".to_string()));
 293         }
 294         None
 295     }
 296
 297     pub(crate) fn if_cause(
 298         &self,
 299         span: Span,
 300         cond_span: Span,
 301         then_expr: &'tcx hir::Expr<'tcx>,
 302         else_expr: &'tcx hir::Expr<'tcx>,
 303         then_ty: Ty<'tcx>,
 304         else_ty: Ty<'tcx>,
 305         opt_suggest_box_span: Option<Span>,
 306     ) -> ObligationCause<'tcx> {
 307         let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
 308             // The `if`/`else` isn't in one line in the output, include some context to make it
 309             // clear it is an if/else expression:
 310             // ```
 311             // LL |      let x = if true {
 312             //    | _____________-
 313             // LL ||         10i32
 314             //    ||         ----- expected because of this
 315             // LL ||     } else {
 316             // LL ||         10u32
 317             //    ||         ^^^^^ expected `i32`, found `u32`
 318             // LL ||     };
 319             //    ||_____- `if` and `else` have incompatible types
 320             // ```
 321             Some(span)
 322         } else {
 323             // The entire expression is in one line, only point at the arms
 324             // ```
 325             // LL |     let x = if true { 10i32 } else { 10u32 };
 326             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 327             //    |                       |
 328             //    |                       expected because of this
 329             // ```
 330             None
 331         };
 332
 333         let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
 334             let block = block.innermost_block();
 335
 336             // Avoid overlapping spans that aren't as readable:
 337             // ```
 338             // 2 |        let x = if true {
 339             //   |   _____________-
 340             // 3 |  |         3
 341             //   |  |         - expected because of this
 342             // 4 |  |     } else {
 343             //   |  |____________^
 344             // 5 | ||
 345             // 6 | ||     };
 346             //   | ||     ^
 347             //   | ||_____|
 348             //   | |______if and else have incompatible types
 349             //   |        expected integer, found `()`
 350             // ```
 351             // by not pointing at the entire expression:
 352             // ```
 353             // 2 |       let x = if true {
 354             //   |               ------- `if` and `else` have incompatible types
 355             // 3 |           3
 356             //   |           - expected because of this
 357             // 4 |       } else {
 358             //   |  ____________^
 359             // 5 | |
 360             // 6 | |     };
 361             //   | |_____^ expected integer, found `()`
 362             // ```
 363             if block.expr.is_none() && block.stmts.is_empty()
 364                 && let Some(outer_span) = &mut outer_span
 365                 && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
 366             {
 367                 *outer_span = outer_span.with_hi(cond_span.hi())
 368             }
 369
 370             (self.find_block_span(block), block.hir_id)
 371         } else {
 372             (else_expr.span, else_expr.hir_id)
 373         };
 374
 375         let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
 376             let block = block.innermost_block();
 377             // Exclude overlapping spans
 378             if block.expr.is_none() && block.stmts.is_empty() {
 379                 outer_span = None;
 380             }
 381             block.hir_id
 382         } else {
 383             then_expr.hir_id
 384         };
 385
 386         // Finally construct the cause:
 387         self.cause(
 388             error_sp,
 389             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 390                 else_id,
 391                 then_id,
 392                 then_ty,
 393                 else_ty,
 394                 outer_span,
 395                 opt_suggest_box_span,
 396             })),
 397         )
 398     }
 399
 400     pub(super) fn demand_scrutinee_type(
 401         &self,
 402         scrut: &'tcx hir::Expr<'tcx>,
 403         contains_ref_bindings: Option<hir::Mutability>,
 404         no_arms: bool,
 405     ) -> Ty<'tcx> {
 406         // Not entirely obvious: if matches may create ref bindings, we want to
 407         // use the *precise* type of the scrutinee, *not* some supertype, as
 408         // the "scrutinee type" (issue #23116).
 409         //
 410         // arielb1 [writes here in this comment thread][c] that there
 411         // is certainly *some* potential danger, e.g., for an example
 412         // like:
 413         //
 414         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 415         //
 416         // ```
 417         // let Foo(x) = f()[0];
 418         // ```
 419         //
 420         // Then if the pattern matches by reference, we want to match
 421         // `f()[0]` as a lexpr, so we can't allow it to be
 422         // coerced. But if the pattern matches by value, `f()[0]` is
 423         // still syntactically a lexpr, but we *do* want to allow
 424         // coercions.
 425         //
 426         // However, *likely* we are ok with allowing coercions to
 427         // happen if there are no explicit ref mut patterns - all
 428         // implicit ref mut patterns must occur behind a reference, so
 429         // they will have the "correct" variance and lifetime.
 430         //
 431         // This does mean that the following pattern would be legal:
 432         //
 433         // ```
 434         // struct Foo(Bar);
 435         // struct Bar(u32);
 436         // impl Deref for Foo {
 437         //     type Target = Bar;
 438         //     fn deref(&self) -> &Bar { &self.0 }
 439         // }
 440         // impl DerefMut for Foo {
 441         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 442         // }
 443         // fn foo(x: &mut Foo) {
 444         //     {
 445         //         let Bar(z): &mut Bar = x;
 446         //         *z = 42;
 447         //     }
 448         //     assert_eq!(foo.0.0, 42);
 449         // }
 450         // ```
 451         //
 452         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 453         // is problematic as the HIR is being scraped, but ref bindings may be
 454         // implicit after #42640. We need to make sure that pat_adjustments
 455         // (once introduced) is populated by the time we get here.
 456         //
 457         // See #44848.
 458         if let Some(m) = contains_ref_bindings {
 459             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 460         } else if no_arms {
 461             self.check_expr(scrut)
 462         } else {
 463             // ...but otherwise we want to use any supertype of the
 464             // scrutinee. This is sort of a workaround, see note (*) in
 465             // `check_pat` for some details.
 466             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 467                 kind: TypeVariableOriginKind::TypeInference,
 468                 span: scrut.span,
 469             });
 470             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 471             scrut_ty
 472         }
 473     }
 474
 475     /// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 476     /// we check if the different arms would work with boxed trait objects instead and
 477     /// provide a structured suggestion in that case.
 478     pub(crate) fn opt_suggest_box_span(
 479         &self,
 480         first_ty: Ty<'tcx>,
 481         second_ty: Ty<'tcx>,
 482         orig_expected: Expectation<'tcx>,
 483     ) -> Option<Span> {
 484         // FIXME(compiler-errors): This really shouldn't need to be done during the
 485         // "good" path of typeck, but here we are.
 486         match orig_expected {
 487             Expectation::ExpectHasType(expected) => {
 488                 let TypeVariableOrigin {
 489                     span,
 490                     kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id),
 491                     ..
 492                 } = self.type_var_origin(expected)? else { return None; };
 493
 494                 let sig = *self
 495                     .typeck_results
 496                     .borrow()
 497                     .liberated_fn_sigs()
 498                     .get(hir::HirId::make_owner(self.body_id.owner.def_id))?;
 499
 500                 let substs = sig.output().walk().find_map(|arg| {
 501                     if let ty::GenericArgKind::Type(ty) = arg.unpack()
 502                         && let ty::Opaque(def_id, substs) = *ty.kind()
 503                         && def_id == rpit_def_id
 504                     {
 505                         Some(substs)
 506                     } else {
 507                         None
 508                     }
 509                 })?;
 510                 let opaque_ty = self.tcx.mk_opaque(rpit_def_id, substs);
 511
 512                 if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) {
 513                     return None;
 514                 }
 515
 516                 for ty in [first_ty, second_ty] {
 517                     for pred in self.tcx.bound_explicit_item_bounds(rpit_def_id).transpose_iter() {
 518                         let pred = pred.map_bound(|(pred, _)| *pred).subst(self.tcx, substs);
 519                         let pred = match pred.kind().skip_binder() {
 520                             ty::PredicateKind::Trait(mut trait_pred) => {
 521                                 assert_eq!(trait_pred.trait_ref.self_ty(), opaque_ty);
 522                                 trait_pred.trait_ref.substs =
 523                                     self.tcx.mk_substs_trait(ty, &trait_pred.trait_ref.substs[1..]);
 524                                 pred.kind().rebind(trait_pred).to_predicate(self.tcx)
 525                             }
 526                             ty::PredicateKind::Projection(mut proj_pred) => {
 527                                 assert_eq!(proj_pred.projection_ty.self_ty(), opaque_ty);
 528                                 proj_pred.projection_ty.substs = self
 529                                     .tcx
 530                                     .mk_substs_trait(ty, &proj_pred.projection_ty.substs[1..]);
 531                                 pred.kind().rebind(proj_pred).to_predicate(self.tcx)
 532                             }
 533                             _ => continue,
 534                         };
 535                         if !self.predicate_must_hold_modulo_regions(&Obligation::new(
 536                             ObligationCause::misc(span, self.body_id),
 537                             self.param_env,
 538                             pred,
 539                         )) {
 540                             return None;
 541                         }
 542                     }
 543                 }
 544
 545                 Some(span)
 546             }
 547             _ => None,
 548         }
 549     }
 550 }
 551
 552 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 553     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
 554         hir::Mutability::Mut => 1,
 555         hir::Mutability::Not => 0,
 556     })
 557 }