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