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, DiagnosticBuilder};
   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, TyS};
   8 use rustc_span::{MultiSpan, Span};
   9 use rustc_trait_selection::opaque_types::InferCtxtExt as _;
  10 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  11 use rustc_trait_selection::traits::{
  12     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
  13     StatementAsExpression,
  14 };
  15
  16 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  17     #[instrument(skip(self), level = "debug")]
  18     pub fn check_match(
  19         &self,
  20         expr: &'tcx hir::Expr<'tcx>,
  21         scrut: &'tcx hir::Expr<'tcx>,
  22         arms: &'tcx [hir::Arm<'tcx>],
  23         orig_expected: Expectation<'tcx>,
  24         match_src: hir::MatchSource,
  25     ) -> Ty<'tcx> {
  26         let tcx = self.tcx;
  27
  28         let acrb = arms_contain_ref_bindings(arms);
  29         let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
  30         debug!(?scrutinee_ty);
  31
  32         // If there are no arms, that is a diverging match; a special case.
  33         if arms.is_empty() {
  34             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
  35             return tcx.types.never;
  36         }
  37
  38         self.warn_arms_when_scrutinee_diverges(arms);
  39
  40         // Otherwise, we have to union together the types that the arms produce and so forth.
  41         let scrut_diverges = self.diverges.replace(Diverges::Maybe);
  42
  43         // #55810: Type check patterns first so we get types for all bindings.
  44         for arm in arms {
  45             self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut.span), true);
  46         }
  47
  48         // Now typecheck the blocks.
  49         //
  50         // The result of the match is the common supertype of all the
  51         // arms. Start out the value as bottom, since it's the, well,
  52         // bottom the type lattice, and we'll be moving up the lattice as
  53         // we process each arm. (Note that any match with 0 arms is matching
  54         // on any empty type and is therefore unreachable; should the flow
  55         // of execution reach it, we will panic, so bottom is an appropriate
  56         // type in that case)
  57         let mut all_arms_diverge = Diverges::WarnedAlways;
  58
  59         let expected = orig_expected.adjust_for_branches(self);
  60
  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(pat, e) => {
  87                         let scrutinee_ty = self.demand_scrutinee_type(
  88                             e,
  89                             pat.contains_explicit_ref_binding(),
  90                             false,
  91                         );
  92                         self.check_pat_top(&pat, scrutinee_ty, None, true);
  93                     }
  94                 };
  95             }
  96
  97             self.diverges.set(Diverges::Maybe);
  98
  99             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
 100             all_arms_diverge &= self.diverges.get();
 101
 102             let opt_suggest_box_span =
 103                 self.opt_suggest_box_span(arm.body.span, arm_ty, orig_expected);
 104
 105             let (arm_span, semi_span) =
 106                 self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
 107             let (span, code) = match i {
 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                 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
 111                 _ => (
 112                     expr.span,
 113                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
 114                         arm_span,
 115                         scrut_span: scrut.span,
 116                         semi_span,
 117                         source: match_src,
 118                         prior_arms: other_arms.clone(),
 119                         last_ty: prior_arm_ty.unwrap(),
 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: &mut DiagnosticBuilder<'_>| {
 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 let (Expectation::IsLast(stmt), Some(ret), true) =
 149                         (orig_expected, self.ret_type_span, can_coerce_to_return_ty)
 150                     {
 151                         let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 152                         let mut ret_span: MultiSpan = semi_span.into();
 153                         ret_span.push_span_label(
 154                             expr.span,
 155                             "this could be implicitly returned but it is a statement, not a \
 156                                 tail expression"
 157                                 .to_owned(),
 158                         );
 159                         ret_span.push_span_label(
 160                             ret,
 161                             "the `match` arms can conform to this return type".to_owned(),
 162                         );
 163                         ret_span.push_span_label(
 164                             semi_span,
 165                             "the `match` is a statement because of this semicolon, consider \
 166                                 removing it"
 167                                 .to_owned(),
 168                         );
 169                         err.span_note(
 170                             ret_span,
 171                             "you might have meant to return the `match` expression",
 172                         );
 173                         err.tool_only_span_suggestion(
 174                             semi_span,
 175                             "remove this semicolon",
 176                             String::new(),
 177                             Applicability::MaybeIncorrect,
 178                         );
 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         coercion.complete(self)
 210     }
 211
 212     fn get_appropriate_arm_semicolon_removal_span(
 213         &self,
 214         arms: &'tcx [hir::Arm<'tcx>],
 215         i: usize,
 216         prior_arm_ty: Option<Ty<'tcx>>,
 217         arm_ty: Ty<'tcx>,
 218     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 219         let arm = &arms[i];
 220         let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
 221             self.find_block_span(blk, prior_arm_ty)
 222         } else {
 223             (arm.body.span, None)
 224         };
 225         if semi_span.is_none() && i > 0 {
 226             if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
 227                 let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
 228                 semi_span = semi_span_prev;
 229             }
 230         }
 231         (arm_span, semi_span)
 232     }
 233
 234     /// When the previously checked expression (the scrutinee) diverges,
 235     /// warn the user about the match arms being unreachable.
 236     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 237         for arm in arms {
 238             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 239         }
 240     }
 241
 242     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 243     ///
 244     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 245     pub(super) fn if_fallback_coercion<T>(
 246         &self,
 247         span: Span,
 248         then_expr: &'tcx hir::Expr<'tcx>,
 249         coercion: &mut CoerceMany<'tcx, '_, T>,
 250     ) -> bool
 251     where
 252         T: AsCoercionSite,
 253     {
 254         // If this `if` expr is the parent's function return expr,
 255         // the cause of the type coercion is the return type, point at it. (#25228)
 256         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
 257         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 258         let mut error = false;
 259         coercion.coerce_forced_unit(
 260             self,
 261             &cause,
 262             &mut |err| {
 263                 if let Some((span, msg)) = &ret_reason {
 264                     err.span_label(*span, msg.as_str());
 265                 } else if let ExprKind::Block(block, _) = &then_expr.kind {
 266                     if let Some(expr) = &block.expr {
 267                         err.span_label(expr.span, "found here".to_string());
 268                     }
 269                 }
 270                 err.note("`if` expressions without `else` evaluate to `()`");
 271                 err.help("consider adding an `else` block that evaluates to the expected type");
 272                 error = true;
 273             },
 274             ret_reason.is_none(),
 275         );
 276         error
 277     }
 278
 279     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
 280         let node = {
 281             let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id));
 282             self.tcx.hir().get(rslt)
 283         };
 284         if let hir::Node::Block(block) = node {
 285             // check that the body's parent is an fn
 286             let parent = self
 287                 .tcx
 288                 .hir()
 289                 .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)));
 290             if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
 291                 (&block.expr, parent)
 292             {
 293                 // check that the `if` expr without `else` is the fn body's expr
 294                 if expr.span == sp {
 295                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
 296                         let span = fn_decl.output.span();
 297                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
 298                         Some((span, format!("expected `{}` because of this return type", snippet)))
 299                     });
 300                 }
 301             }
 302         }
 303         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 304             return Some((pat.span, "expected because of this assignment".to_string()));
 305         }
 306         None
 307     }
 308
 309     pub(crate) fn if_cause(
 310         &self,
 311         span: Span,
 312         then_expr: &'tcx hir::Expr<'tcx>,
 313         else_expr: &'tcx hir::Expr<'tcx>,
 314         then_ty: Ty<'tcx>,
 315         else_ty: Ty<'tcx>,
 316         opt_suggest_box_span: Option<Span>,
 317     ) -> ObligationCause<'tcx> {
 318         let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
 319             // The `if`/`else` isn't in one line in the output, include some context to make it
 320             // clear it is an if/else expression:
 321             // ```
 322             // LL |      let x = if true {
 323             //    | _____________-
 324             // LL ||         10i32
 325             //    ||         ----- expected because of this
 326             // LL ||     } else {
 327             // LL ||         10u32
 328             //    ||         ^^^^^ expected `i32`, found `u32`
 329             // LL ||     };
 330             //    ||_____- `if` and `else` have incompatible types
 331             // ```
 332             Some(span)
 333         } else {
 334             // The entire expression is in one line, only point at the arms
 335             // ```
 336             // LL |     let x = if true { 10i32 } else { 10u32 };
 337             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 338             //    |                       |
 339             //    |                       expected because of this
 340             // ```
 341             None
 342         };
 343
 344         let mut remove_semicolon = None;
 345         let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
 346             let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
 347             remove_semicolon = semi_sp;
 348             if block.expr.is_none() && block.stmts.is_empty() {
 349                 // Avoid overlapping spans that aren't as readable:
 350                 // ```
 351                 // 2 |        let x = if true {
 352                 //   |   _____________-
 353                 // 3 |  |         3
 354                 //   |  |         - expected because of this
 355                 // 4 |  |     } else {
 356                 //   |  |____________^
 357                 // 5 | ||
 358                 // 6 | ||     };
 359                 //   | ||     ^
 360                 //   | ||_____|
 361                 //   | |______if and else have incompatible types
 362                 //   |        expected integer, found `()`
 363                 // ```
 364                 // by not pointing at the entire expression:
 365                 // ```
 366                 // 2 |       let x = if true {
 367                 //   |               ------- `if` and `else` have incompatible types
 368                 // 3 |           3
 369                 //   |           - expected because of this
 370                 // 4 |       } else {
 371                 //   |  ____________^
 372                 // 5 | |
 373                 // 6 | |     };
 374                 //   | |_____^ expected integer, found `()`
 375                 // ```
 376                 if outer_sp.is_some() {
 377                     outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
 378                 }
 379             }
 380             error_sp
 381         } else {
 382             // shouldn't happen unless the parser has done something weird
 383             else_expr.span
 384         };
 385
 386         // Compute `Span` of `then` part of `if`-expression.
 387         let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
 388             let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
 389             remove_semicolon = remove_semicolon.or(semi_sp);
 390             if block.expr.is_none() && block.stmts.is_empty() {
 391                 outer_sp = None; // same as in `error_sp`; cleanup output
 392             }
 393             then_sp
 394         } else {
 395             // shouldn't happen unless the parser has done something weird
 396             then_expr.span
 397         };
 398
 399         // Finally construct the cause:
 400         self.cause(
 401             error_sp,
 402             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 403                 then: then_sp,
 404                 else_sp: error_sp,
 405                 outer: outer_sp,
 406                 semicolon: remove_semicolon,
 407                 opt_suggest_box_span,
 408             })),
 409         )
 410     }
 411
 412     pub(super) fn demand_scrutinee_type(
 413         &self,
 414         scrut: &'tcx hir::Expr<'tcx>,
 415         contains_ref_bindings: Option<hir::Mutability>,
 416         no_arms: bool,
 417     ) -> Ty<'tcx> {
 418         // Not entirely obvious: if matches may create ref bindings, we want to
 419         // use the *precise* type of the scrutinee, *not* some supertype, as
 420         // the "scrutinee type" (issue #23116).
 421         //
 422         // arielb1 [writes here in this comment thread][c] that there
 423         // is certainly *some* potential danger, e.g., for an example
 424         // like:
 425         //
 426         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 427         //
 428         // ```
 429         // let Foo(x) = f()[0];
 430         // ```
 431         //
 432         // Then if the pattern matches by reference, we want to match
 433         // `f()[0]` as a lexpr, so we can't allow it to be
 434         // coerced. But if the pattern matches by value, `f()[0]` is
 435         // still syntactically a lexpr, but we *do* want to allow
 436         // coercions.
 437         //
 438         // However, *likely* we are ok with allowing coercions to
 439         // happen if there are no explicit ref mut patterns - all
 440         // implicit ref mut patterns must occur behind a reference, so
 441         // they will have the "correct" variance and lifetime.
 442         //
 443         // This does mean that the following pattern would be legal:
 444         //
 445         // ```
 446         // struct Foo(Bar);
 447         // struct Bar(u32);
 448         // impl Deref for Foo {
 449         //     type Target = Bar;
 450         //     fn deref(&self) -> &Bar { &self.0 }
 451         // }
 452         // impl DerefMut for Foo {
 453         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 454         // }
 455         // fn foo(x: &mut Foo) {
 456         //     {
 457         //         let Bar(z): &mut Bar = x;
 458         //         *z = 42;
 459         //     }
 460         //     assert_eq!(foo.0.0, 42);
 461         // }
 462         // ```
 463         //
 464         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 465         // is problematic as the HIR is being scraped, but ref bindings may be
 466         // implicit after #42640. We need to make sure that pat_adjustments
 467         // (once introduced) is populated by the time we get here.
 468         //
 469         // See #44848.
 470         if let Some(m) = contains_ref_bindings {
 471             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 472         } else if no_arms {
 473             self.check_expr(scrut)
 474         } else {
 475             // ...but otherwise we want to use any supertype of the
 476             // scrutinee. This is sort of a workaround, see note (*) in
 477             // `check_pat` for some details.
 478             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 479                 kind: TypeVariableOriginKind::TypeInference,
 480                 span: scrut.span,
 481             });
 482             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 483             scrut_ty
 484         }
 485     }
 486
 487     fn find_block_span(
 488         &self,
 489         block: &'tcx hir::Block<'tcx>,
 490         expected_ty: Option<Ty<'tcx>>,
 491     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 492         if let Some(expr) = &block.expr {
 493             (expr.span, None)
 494         } else if let Some(stmt) = block.stmts.last() {
 495             // possibly incorrect trailing `;` in the else arm
 496             (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
 497         } else {
 498             // empty block; point at its entirety
 499             (block.span, None)
 500         }
 501     }
 502
 503     // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 504     // we check if the different arms would work with boxed trait objects instead and
 505     // provide a structured suggestion in that case.
 506     pub(crate) fn opt_suggest_box_span(
 507         &self,
 508         span: Span,
 509         outer_ty: &'tcx TyS<'tcx>,
 510         orig_expected: Expectation<'tcx>,
 511     ) -> Option<Span> {
 512         match (orig_expected, self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty))) {
 513             (Expectation::ExpectHasType(expected), Some((_id, ty)))
 514                 if self.in_tail_expr && self.can_coerce(outer_ty, expected) =>
 515             {
 516                 let impl_trait_ret_ty =
 517                     self.infcx.instantiate_opaque_types(self.body_id, self.param_env, ty, span);
 518                 assert!(
 519                     impl_trait_ret_ty.obligations.is_empty(),
 520                     "we should never get new obligations here"
 521                 );
 522                 let obligations = self.fulfillment_cx.borrow().pending_obligations();
 523                 let mut suggest_box = !obligations.is_empty();
 524                 for o in obligations {
 525                     match o.predicate.kind().skip_binder() {
 526                         ty::PredicateKind::Trait(t) => {
 527                             let pred =
 528                                 ty::Binder::dummy(ty::PredicateKind::Trait(ty::TraitPredicate {
 529                                     trait_ref: ty::TraitRef {
 530                                         def_id: t.def_id(),
 531                                         substs: self.infcx.tcx.mk_substs_trait(outer_ty, &[]),
 532                                     },
 533                                     constness: t.constness,
 534                                 }));
 535                             let obl = Obligation::new(
 536                                 o.cause.clone(),
 537                                 self.param_env,
 538                                 pred.to_predicate(self.infcx.tcx),
 539                             );
 540                             suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
 541                             if !suggest_box {
 542                                 // We've encountered some obligation that didn't hold, so the
 543                                 // return expression can't just be boxed. We don't need to
 544                                 // evaluate the rest of the obligations.
 545                                 break;
 546                             }
 547                         }
 548                         _ => {}
 549                     }
 550                 }
 551                 // If all the obligations hold (or there are no obligations) the tail expression
 552                 // we can suggest to return a boxed trait object instead of an opaque type.
 553                 if suggest_box { self.ret_type_span } else { None }
 554             }
 555             _ => None,
 556         }
 557     }
 558 }
 559
 560 fn arms_contain_ref_bindings(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 561     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
 562         hir::Mutability::Mut => 1,
 563         hir::Mutability::Not => 0,
 564     })
 565 }