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