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
  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_ty = None;
  78         for (i, arm) in arms.iter().enumerate() {
  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(pat, e) => {
  86                         let scrutinee_ty = self.demand_scrutinee_type(
  87                             e,
  88                             pat.contains_explicit_ref_binding(),
  89                             false,
  90                         );
  91                         self.check_pat_top(&pat, scrutinee_ty, None, true);
  92                     }
  93                 };
  94             }
  95
  96             self.diverges.set(Diverges::Maybe);
  97
  98             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
  99             all_arms_diverge &= self.diverges.get();
 100
 101             let opt_suggest_box_span = self.opt_suggest_box_span(arm_ty, orig_expected);
 102
 103             let (arm_span, semi_span) =
 104                 self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
 105             let (span, code) = match i {
 106                 // The reason for the first arm to fail is not that the match arms diverge,
 107                 // but rather that there's a prior obligation that doesn't hold.
 108                 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
 109                 _ => (
 110                     expr.span,
 111                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
 112                         arm_span,
 113                         scrut_span: scrut.span,
 114                         semi_span,
 115                         source: match_src,
 116                         prior_arms: other_arms.clone(),
 117                         last_ty: prior_arm_ty.unwrap(),
 118                         scrut_hir_id: scrut.hir_id,
 119                         opt_suggest_box_span,
 120                     })),
 121                 ),
 122             };
 123             let cause = self.cause(span, code);
 124
 125             // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
 126             // We use it this way to be able to expand on the potential error and detect when a
 127             // `match` tail statement could be a tail expression instead. If so, we suggest
 128             // removing the stray semicolon.
 129             coercion.coerce_inner(
 130                 self,
 131                 &cause,
 132                 Some(&arm.body),
 133                 arm_ty,
 134                 Some(&mut |err: &mut Diagnostic| {
 135                     let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 136                         Some(ret_coercion) if self.in_tail_expr => {
 137                             let ret_ty = ret_coercion.borrow().expected_ty();
 138                             let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 139                             self.can_coerce(arm_ty, ret_ty)
 140                                 && prior_arm_ty.map_or(true, |t| self.can_coerce(t, ret_ty))
 141                                 // The match arms need to unify for the case of `impl Trait`.
 142                                 && !matches!(ret_ty.kind(), ty::Opaque(..))
 143                         }
 144                         _ => false,
 145                     };
 146                     if let (Expectation::IsLast(stmt), Some(ret), true) =
 147                         (orig_expected, self.ret_type_span, can_coerce_to_return_ty)
 148                     {
 149                         let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 150                         let mut ret_span: MultiSpan = semi_span.into();
 151                         ret_span.push_span_label(
 152                             expr.span,
 153                             "this could be implicitly returned but it is a statement, not a \
 154                                 tail expression"
 155                                 .to_owned(),
 156                         );
 157                         ret_span.push_span_label(
 158                             ret,
 159                             "the `match` arms can conform to this return type".to_owned(),
 160                         );
 161                         ret_span.push_span_label(
 162                             semi_span,
 163                             "the `match` is a statement because of this semicolon, consider \
 164                                 removing it"
 165                                 .to_owned(),
 166                         );
 167                         err.span_note(
 168                             ret_span,
 169                             "you might have meant to return the `match` expression",
 170                         );
 171                         err.tool_only_span_suggestion(
 172                             semi_span,
 173                             "remove this semicolon",
 174                             String::new(),
 175                             Applicability::MaybeIncorrect,
 176                         );
 177                     }
 178                 }),
 179                 false,
 180             );
 181
 182             other_arms.push(arm_span);
 183             if other_arms.len() > 5 {
 184                 other_arms.remove(0);
 185             }
 186             prior_arm_ty = Some(arm_ty);
 187         }
 188
 189         // If all of the arms in the `match` diverge,
 190         // and we're dealing with an actual `match` block
 191         // (as opposed to a `match` desugared from something else'),
 192         // we can emit a better note. Rather than pointing
 193         // at a diverging expression in an arbitrary arm,
 194         // we can point at the entire `match` expression
 195         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 196             all_arms_diverge = Diverges::Always {
 197                 span: expr.span,
 198                 custom_note: Some(
 199                     "any code following this `match` expression is unreachable, as all arms diverge",
 200                 ),
 201             };
 202         }
 203
 204         // We won't diverge unless the scrutinee or all arms diverge.
 205         self.diverges.set(scrut_diverges | all_arms_diverge);
 206
 207         coercion.complete(self)
 208     }
 209
 210     fn get_appropriate_arm_semicolon_removal_span(
 211         &self,
 212         arms: &'tcx [hir::Arm<'tcx>],
 213         i: usize,
 214         prior_arm_ty: Option<Ty<'tcx>>,
 215         arm_ty: Ty<'tcx>,
 216     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 217         let arm = &arms[i];
 218         let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
 219             self.find_block_span(blk, prior_arm_ty)
 220         } else {
 221             (arm.body.span, None)
 222         };
 223         if semi_span.is_none() && i > 0 {
 224             if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
 225                 let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
 226                 semi_span = semi_span_prev;
 227             }
 228         }
 229         (arm_span, semi_span)
 230     }
 231
 232     /// When the previously checked expression (the scrutinee) diverges,
 233     /// warn the user about the match arms being unreachable.
 234     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 235         for arm in arms {
 236             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 237         }
 238     }
 239
 240     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 241     ///
 242     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 243     pub(super) fn if_fallback_coercion<T>(
 244         &self,
 245         span: Span,
 246         then_expr: &'tcx hir::Expr<'tcx>,
 247         coercion: &mut CoerceMany<'tcx, '_, T>,
 248     ) -> bool
 249     where
 250         T: AsCoercionSite,
 251     {
 252         // If this `if` expr is the parent's function return expr,
 253         // the cause of the type coercion is the return type, point at it. (#25228)
 254         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
 255         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 256         let mut error = false;
 257         coercion.coerce_forced_unit(
 258             self,
 259             &cause,
 260             &mut |err| {
 261                 if let Some((span, msg)) = &ret_reason {
 262                     err.span_label(*span, msg.as_str());
 263                 } else if let ExprKind::Block(block, _) = &then_expr.kind
 264                     && let Some(expr) = &block.expr
 265                 {
 266                     err.span_label(expr.span, "found here".to_string());
 267                 }
 268                 err.note("`if` expressions without `else` evaluate to `()`");
 269                 err.help("consider adding an `else` block that evaluates to the expected type");
 270                 error = true;
 271             },
 272             ret_reason.is_none(),
 273         );
 274         error
 275     }
 276
 277     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
 278         let node = {
 279             let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id));
 280             self.tcx.hir().get(rslt)
 281         };
 282         if let hir::Node::Block(block) = node {
 283             // check that the body's parent is an fn
 284             let parent = self
 285                 .tcx
 286                 .hir()
 287                 .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)));
 288             if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
 289                 (&block.expr, parent)
 290             {
 291                 // check that the `if` expr without `else` is the fn body's expr
 292                 if expr.span == sp {
 293                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
 294                         let span = fn_decl.output.span();
 295                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
 296                         Some((span, format!("expected `{snippet}` because of this return type")))
 297                     });
 298                 }
 299             }
 300         }
 301         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 302             return Some((pat.span, "expected because of this assignment".to_string()));
 303         }
 304         None
 305     }
 306
 307     pub(crate) fn if_cause(
 308         &self,
 309         span: Span,
 310         then_expr: &'tcx hir::Expr<'tcx>,
 311         else_expr: &'tcx hir::Expr<'tcx>,
 312         then_ty: Ty<'tcx>,
 313         else_ty: Ty<'tcx>,
 314         opt_suggest_box_span: Option<Span>,
 315     ) -> ObligationCause<'tcx> {
 316         let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
 317             // The `if`/`else` isn't in one line in the output, include some context to make it
 318             // clear it is an if/else expression:
 319             // ```
 320             // LL |      let x = if true {
 321             //    | _____________-
 322             // LL ||         10i32
 323             //    ||         ----- expected because of this
 324             // LL ||     } else {
 325             // LL ||         10u32
 326             //    ||         ^^^^^ expected `i32`, found `u32`
 327             // LL ||     };
 328             //    ||_____- `if` and `else` have incompatible types
 329             // ```
 330             Some(span)
 331         } else {
 332             // The entire expression is in one line, only point at the arms
 333             // ```
 334             // LL |     let x = if true { 10i32 } else { 10u32 };
 335             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 336             //    |                       |
 337             //    |                       expected because of this
 338             // ```
 339             None
 340         };
 341
 342         let mut remove_semicolon = None;
 343         let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
 344             let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
 345             remove_semicolon = semi_sp;
 346             if block.expr.is_none() && block.stmts.is_empty() {
 347                 // Avoid overlapping spans that aren't as readable:
 348                 // ```
 349                 // 2 |        let x = if true {
 350                 //   |   _____________-
 351                 // 3 |  |         3
 352                 //   |  |         - expected because of this
 353                 // 4 |  |     } else {
 354                 //   |  |____________^
 355                 // 5 | ||
 356                 // 6 | ||     };
 357                 //   | ||     ^
 358                 //   | ||_____|
 359                 //   | |______if and else have incompatible types
 360                 //   |        expected integer, found `()`
 361                 // ```
 362                 // by not pointing at the entire expression:
 363                 // ```
 364                 // 2 |       let x = if true {
 365                 //   |               ------- `if` and `else` have incompatible types
 366                 // 3 |           3
 367                 //   |           - expected because of this
 368                 // 4 |       } else {
 369                 //   |  ____________^
 370                 // 5 | |
 371                 // 6 | |     };
 372                 //   | |_____^ expected integer, found `()`
 373                 // ```
 374                 if outer_sp.is_some() {
 375                     outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
 376                 }
 377             }
 378             error_sp
 379         } else {
 380             // shouldn't happen unless the parser has done something weird
 381             else_expr.span
 382         };
 383
 384         // Compute `Span` of `then` part of `if`-expression.
 385         let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
 386             let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
 387             remove_semicolon = remove_semicolon.or(semi_sp);
 388             if block.expr.is_none() && block.stmts.is_empty() {
 389                 outer_sp = None; // same as in `error_sp`; cleanup output
 390             }
 391             then_sp
 392         } else {
 393             // shouldn't happen unless the parser has done something weird
 394             then_expr.span
 395         };
 396
 397         // Finally construct the cause:
 398         self.cause(
 399             error_sp,
 400             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 401                 then: then_sp,
 402                 else_sp: error_sp,
 403                 outer: outer_sp,
 404                 semicolon: remove_semicolon,
 405                 opt_suggest_box_span,
 406             })),
 407         )
 408     }
 409
 410     pub(super) fn demand_scrutinee_type(
 411         &self,
 412         scrut: &'tcx hir::Expr<'tcx>,
 413         contains_ref_bindings: Option<hir::Mutability>,
 414         no_arms: bool,
 415     ) -> Ty<'tcx> {
 416         // Not entirely obvious: if matches may create ref bindings, we want to
 417         // use the *precise* type of the scrutinee, *not* some supertype, as
 418         // the "scrutinee type" (issue #23116).
 419         //
 420         // arielb1 [writes here in this comment thread][c] that there
 421         // is certainly *some* potential danger, e.g., for an example
 422         // like:
 423         //
 424         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 425         //
 426         // ```
 427         // let Foo(x) = f()[0];
 428         // ```
 429         //
 430         // Then if the pattern matches by reference, we want to match
 431         // `f()[0]` as a lexpr, so we can't allow it to be
 432         // coerced. But if the pattern matches by value, `f()[0]` is
 433         // still syntactically a lexpr, but we *do* want to allow
 434         // coercions.
 435         //
 436         // However, *likely* we are ok with allowing coercions to
 437         // happen if there are no explicit ref mut patterns - all
 438         // implicit ref mut patterns must occur behind a reference, so
 439         // they will have the "correct" variance and lifetime.
 440         //
 441         // This does mean that the following pattern would be legal:
 442         //
 443         // ```
 444         // struct Foo(Bar);
 445         // struct Bar(u32);
 446         // impl Deref for Foo {
 447         //     type Target = Bar;
 448         //     fn deref(&self) -> &Bar { &self.0 }
 449         // }
 450         // impl DerefMut for Foo {
 451         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 452         // }
 453         // fn foo(x: &mut Foo) {
 454         //     {
 455         //         let Bar(z): &mut Bar = x;
 456         //         *z = 42;
 457         //     }
 458         //     assert_eq!(foo.0.0, 42);
 459         // }
 460         // ```
 461         //
 462         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 463         // is problematic as the HIR is being scraped, but ref bindings may be
 464         // implicit after #42640. We need to make sure that pat_adjustments
 465         // (once introduced) is populated by the time we get here.
 466         //
 467         // See #44848.
 468         if let Some(m) = contains_ref_bindings {
 469             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 470         } else if no_arms {
 471             self.check_expr(scrut)
 472         } else {
 473             // ...but otherwise we want to use any supertype of the
 474             // scrutinee. This is sort of a workaround, see note (*) in
 475             // `check_pat` for some details.
 476             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 477                 kind: TypeVariableOriginKind::TypeInference,
 478                 span: scrut.span,
 479             });
 480             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 481             scrut_ty
 482         }
 483     }
 484
 485     fn find_block_span(
 486         &self,
 487         block: &'tcx hir::Block<'tcx>,
 488         expected_ty: Option<Ty<'tcx>>,
 489     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 490         if let Some(expr) = &block.expr {
 491             (expr.span, None)
 492         } else if let Some(stmt) = block.stmts.last() {
 493             // possibly incorrect trailing `;` in the else arm
 494             (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
 495         } else {
 496             // empty block; point at its entirety
 497             (block.span, None)
 498         }
 499     }
 500
 501     // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 502     // we check if the different arms would work with boxed trait objects instead and
 503     // provide a structured suggestion in that case.
 504     pub(crate) fn opt_suggest_box_span(
 505         &self,
 506         outer_ty: Ty<'tcx>,
 507         orig_expected: Expectation<'tcx>,
 508     ) -> Option<Span> {
 509         match orig_expected {
 510             Expectation::ExpectHasType(expected)
 511                 if self.in_tail_expr
 512                     && self.ret_coercion.as_ref()?.borrow().merged_ty().has_opaque_types()
 513                     && self.can_coerce(outer_ty, expected) =>
 514             {
 515                 let obligations = self.fulfillment_cx.borrow().pending_obligations();
 516                 let mut suggest_box = !obligations.is_empty();
 517                 for o in obligations {
 518                     match o.predicate.kind().skip_binder() {
 519                         ty::PredicateKind::Trait(t) => {
 520                             let pred =
 521                                 ty::Binder::dummy(ty::PredicateKind::Trait(ty::TraitPredicate {
 522                                     trait_ref: ty::TraitRef {
 523                                         def_id: t.def_id(),
 524                                         substs: self.infcx.tcx.mk_substs_trait(outer_ty, &[]),
 525                                     },
 526                                     constness: t.constness,
 527                                     polarity: t.polarity,
 528                                 }));
 529                             let obl = Obligation::new(
 530                                 o.cause.clone(),
 531                                 self.param_env,
 532                                 pred.to_predicate(self.infcx.tcx),
 533                             );
 534                             suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
 535                             if !suggest_box {
 536                                 // We've encountered some obligation that didn't hold, so the
 537                                 // return expression can't just be boxed. We don't need to
 538                                 // evaluate the rest of the obligations.
 539                                 break;
 540                             }
 541                         }
 542                         _ => {}
 543                     }
 544                 }
 545                 // If all the obligations hold (or there are no obligations) the tail expression
 546                 // we can suggest to return a boxed trait object instead of an opaque type.
 547                 if suggest_box { self.ret_type_span } else { None }
 548             }
 549             _ => None,
 550         }
 551     }
 552 }
 553
 554 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 555     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
 556         hir::Mutability::Mut => 1,
 557         hir::Mutability::Not => 0,
 558     })
 559 }