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};
   8 use rustc_span::Span;
   9 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  10 use rustc_trait_selection::traits::{
  11     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
  12 };
  13
  14 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  15     #[instrument(skip(self), level = "debug", ret)]
  16     pub fn check_match(
  17         &self,
  18         expr: &'tcx hir::Expr<'tcx>,
  19         scrut: &'tcx hir::Expr<'tcx>,
  20         arms: &'tcx [hir::Arm<'tcx>],
  21         orig_expected: Expectation<'tcx>,
  22         match_src: hir::MatchSource,
  23     ) -> Ty<'tcx> {
  24         let tcx = self.tcx;
  25
  26         let acrb = arms_contain_ref_bindings(arms);
  27         let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
  28         debug!(?scrutinee_ty);
  29
  30         // If there are no arms, that is a diverging match; a special case.
  31         if arms.is_empty() {
  32             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
  33             return tcx.types.never;
  34         }
  35
  36         self.warn_arms_when_scrutinee_diverges(arms);
  37
  38         // Otherwise, we have to union together the types that the arms produce and so forth.
  39         let scrut_diverges = self.diverges.replace(Diverges::Maybe);
  40
  41         // #55810: Type check patterns first so we get types for all bindings.
  42         let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
  43         for arm in arms {
  44             self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut_span), true);
  45         }
  46
  47         // Now typecheck the blocks.
  48         //
  49         // The result of the match is the common supertype of all the
  50         // arms. Start out the value as bottom, since it's the, well,
  51         // bottom the type lattice, and we'll be moving up the lattice as
  52         // we process each arm. (Note that any match with 0 arms is matching
  53         // on any empty type and is therefore unreachable; should the flow
  54         // of execution reach it, we will panic, so bottom is an appropriate
  55         // type in that case)
  56         let mut all_arms_diverge = Diverges::WarnedAlways;
  57
  58         let expected = orig_expected.adjust_for_branches(self);
  59         debug!(?expected);
  60
  61         let mut coercion = {
  62             let coerce_first = match expected {
  63                 // We don't coerce to `()` so that if the match expression is a
  64                 // statement it's branches can have any consistent type. That allows
  65                 // us to give better error messages (pointing to a usually better
  66                 // arm for inconsistent arms or to the whole match when a `()` type
  67                 // is required).
  68                 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
  69                 _ => self.next_ty_var(TypeVariableOrigin {
  70                     kind: TypeVariableOriginKind::MiscVariable,
  71                     span: expr.span,
  72                 }),
  73             };
  74             CoerceMany::with_coercion_sites(coerce_first, arms)
  75         };
  76
  77         let mut other_arms = vec![]; // Used only for diagnostics.
  78         let mut prior_arm = None;
  79         for arm in arms {
  80             if let Some(g) = &arm.guard {
  81                 self.diverges.set(Diverges::Maybe);
  82                 match g {
  83                     hir::Guard::If(e) => {
  84                         self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
  85                     }
  86                     hir::Guard::IfLet(l) => {
  87                         self.check_expr_let(l);
  88                     }
  89                 };
  90             }
  91
  92             self.diverges.set(Diverges::Maybe);
  93
  94             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
  95             all_arms_diverge &= self.diverges.get();
  96
  97             let opt_suggest_box_span = prior_arm.and_then(|(_, prior_arm_ty, _)| {
  98                 self.opt_suggest_box_span(prior_arm_ty, arm_ty, orig_expected)
  99             });
 100
 101             let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
 102                 (Some(blk.hir_id), self.find_block_span(blk))
 103             } else {
 104                 (None, arm.body.span)
 105             };
 106
 107             let (span, code) = match prior_arm {
 108                 // The reason for the first arm to fail is not that the match arms diverge,
 109                 // but rather that there's a prior obligation that doesn't hold.
 110                 None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
 111                 Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
 112                     expr.span,
 113                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
 114                         arm_block_id,
 115                         arm_span,
 116                         arm_ty,
 117                         prior_arm_block_id,
 118                         prior_arm_ty,
 119                         prior_arm_span,
 120                         scrut_span: scrut.span,
 121                         source: match_src,
 122                         prior_arms: other_arms.clone(),
 123                         scrut_hir_id: scrut.hir_id,
 124                         opt_suggest_box_span,
 125                     })),
 126                 ),
 127             };
 128             let cause = self.cause(span, code);
 129
 130             // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
 131             // We use it this way to be able to expand on the potential error and detect when a
 132             // `match` tail statement could be a tail expression instead. If so, we suggest
 133             // removing the stray semicolon.
 134             coercion.coerce_inner(
 135                 self,
 136                 &cause,
 137                 Some(&arm.body),
 138                 arm_ty,
 139                 Some(&mut |err| {
 140                     let Some(ret) = self.ret_type_span else {
 141                         return;
 142                     };
 143                     let Expectation::IsLast(stmt) = orig_expected else {
 144                         return
 145                     };
 146                     let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 147                         Some(ret_coercion) if self.in_tail_expr => {
 148                             let ret_ty = ret_coercion.borrow().expected_ty();
 149                             let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 150                             self.can_coerce(arm_ty, ret_ty)
 151                                 && prior_arm.map_or(true, |(_, t, _)| self.can_coerce(t, ret_ty))
 152                                 // The match arms need to unify for the case of `impl Trait`.
 153                                 && !matches!(ret_ty.kind(), ty::Opaque(..))
 154                         }
 155                         _ => false,
 156                     };
 157                     if !can_coerce_to_return_ty {
 158                         return;
 159                     }
 160
 161                     let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 162                     let mut ret_span: MultiSpan = semi_span.into();
 163                     ret_span.push_span_label(
 164                         expr.span,
 165                         "this could be implicitly returned but it is a statement, not a \
 166                             tail expression",
 167                     );
 168                     ret_span
 169                         .push_span_label(ret, "the `match` arms can conform to this return type");
 170                     ret_span.push_span_label(
 171                         semi_span,
 172                         "the `match` is a statement because of this semicolon, consider \
 173                             removing it",
 174                     );
 175                     err.span_note(
 176                         ret_span,
 177                         "you might have meant to return the `match` expression",
 178                     );
 179                     err.tool_only_span_suggestion(
 180                         semi_span,
 181                         "remove this semicolon",
 182                         "",
 183                         Applicability::MaybeIncorrect,
 184                     );
 185                 }),
 186                 false,
 187             );
 188
 189             other_arms.push(arm_span);
 190             if other_arms.len() > 5 {
 191                 other_arms.remove(0);
 192             }
 193
 194             prior_arm = Some((arm_block_id, arm_ty, arm_span));
 195         }
 196
 197         // If all of the arms in the `match` diverge,
 198         // and we're dealing with an actual `match` block
 199         // (as opposed to a `match` desugared from something else'),
 200         // we can emit a better note. Rather than pointing
 201         // at a diverging expression in an arbitrary arm,
 202         // we can point at the entire `match` expression
 203         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 204             all_arms_diverge = Diverges::Always {
 205                 span: expr.span,
 206                 custom_note: Some(
 207                     "any code following this `match` expression is unreachable, as all arms diverge",
 208                 ),
 209             };
 210         }
 211
 212         // We won't diverge unless the scrutinee or all arms diverge.
 213         self.diverges.set(scrut_diverges | all_arms_diverge);
 214
 215         coercion.complete(self)
 216     }
 217
 218     /// When the previously checked expression (the scrutinee) diverges,
 219     /// warn the user about the match arms being unreachable.
 220     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
 221         for arm in arms {
 222             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
 223         }
 224     }
 225
 226     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 227     ///
 228     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 229     pub(super) fn if_fallback_coercion<T>(
 230         &self,
 231         span: Span,
 232         then_expr: &'tcx hir::Expr<'tcx>,
 233         coercion: &mut CoerceMany<'tcx, '_, T>,
 234     ) -> bool
 235     where
 236         T: AsCoercionSite,
 237     {
 238         // If this `if` expr is the parent's function return expr,
 239         // the cause of the type coercion is the return type, point at it. (#25228)
 240         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
 241         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 242         let mut error = false;
 243         coercion.coerce_forced_unit(
 244             self,
 245             &cause,
 246             &mut |err| {
 247                 if let Some((span, msg)) = &ret_reason {
 248                     err.span_label(*span, msg);
 249                 } else if let ExprKind::Block(block, _) = &then_expr.kind
 250                     && let Some(expr) = &block.expr
 251                 {
 252                     err.span_label(expr.span, "found here");
 253                 }
 254                 err.note("`if` expressions without `else` evaluate to `()`");
 255                 err.help("consider adding an `else` block that evaluates to the expected type");
 256                 error = true;
 257             },
 258             ret_reason.is_none(),
 259         );
 260         error
 261     }
 262
 263     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
 264         let node = {
 265             let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id));
 266             self.tcx.hir().get(rslt)
 267         };
 268         if let hir::Node::Block(block) = node {
 269             // check that the body's parent is an fn
 270             let parent = self
 271                 .tcx
 272                 .hir()
 273                 .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)));
 274             if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
 275                 (&block.expr, parent)
 276             {
 277                 // check that the `if` expr without `else` is the fn body's expr
 278                 if expr.span == sp {
 279                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
 280                         let span = fn_decl.output.span();
 281                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
 282                         Some((span, format!("expected `{snippet}` because of this return type")))
 283                     });
 284                 }
 285             }
 286         }
 287         if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
 288             return Some((pat.span, "expected because of this assignment".to_string()));
 289         }
 290         None
 291     }
 292
 293     pub(crate) fn if_cause(
 294         &self,
 295         span: Span,
 296         cond_span: Span,
 297         then_expr: &'tcx hir::Expr<'tcx>,
 298         else_expr: &'tcx hir::Expr<'tcx>,
 299         then_ty: Ty<'tcx>,
 300         else_ty: Ty<'tcx>,
 301         opt_suggest_box_span: Option<Span>,
 302     ) -> ObligationCause<'tcx> {
 303         let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
 304             // The `if`/`else` isn't in one line in the output, include some context to make it
 305             // clear it is an if/else expression:
 306             // ```
 307             // LL |      let x = if true {
 308             //    | _____________-
 309             // LL ||         10i32
 310             //    ||         ----- expected because of this
 311             // LL ||     } else {
 312             // LL ||         10u32
 313             //    ||         ^^^^^ expected `i32`, found `u32`
 314             // LL ||     };
 315             //    ||_____- `if` and `else` have incompatible types
 316             // ```
 317             Some(span)
 318         } else {
 319             // The entire expression is in one line, only point at the arms
 320             // ```
 321             // LL |     let x = if true { 10i32 } else { 10u32 };
 322             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 323             //    |                       |
 324             //    |                       expected because of this
 325             // ```
 326             None
 327         };
 328
 329         let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
 330             let block = block.innermost_block();
 331
 332             // Avoid overlapping spans that aren't as readable:
 333             // ```
 334             // 2 |        let x = if true {
 335             //   |   _____________-
 336             // 3 |  |         3
 337             //   |  |         - expected because of this
 338             // 4 |  |     } else {
 339             //   |  |____________^
 340             // 5 | ||
 341             // 6 | ||     };
 342             //   | ||     ^
 343             //   | ||_____|
 344             //   | |______if and else have incompatible types
 345             //   |        expected integer, found `()`
 346             // ```
 347             // by not pointing at the entire expression:
 348             // ```
 349             // 2 |       let x = if true {
 350             //   |               ------- `if` and `else` have incompatible types
 351             // 3 |           3
 352             //   |           - expected because of this
 353             // 4 |       } else {
 354             //   |  ____________^
 355             // 5 | |
 356             // 6 | |     };
 357             //   | |_____^ expected integer, found `()`
 358             // ```
 359             if block.expr.is_none() && block.stmts.is_empty()
 360                 && let Some(outer_span) = &mut outer_span
 361                 && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
 362             {
 363                 *outer_span = outer_span.with_hi(cond_span.hi())
 364             }
 365
 366             (self.find_block_span(block), block.hir_id)
 367         } else {
 368             (else_expr.span, else_expr.hir_id)
 369         };
 370
 371         let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
 372             let block = block.innermost_block();
 373             // Exclude overlapping spans
 374             if block.expr.is_none() && block.stmts.is_empty() {
 375                 outer_span = None;
 376             }
 377             block.hir_id
 378         } else {
 379             then_expr.hir_id
 380         };
 381
 382         // Finally construct the cause:
 383         self.cause(
 384             error_sp,
 385             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
 386                 else_id,
 387                 then_id,
 388                 then_ty,
 389                 else_ty,
 390                 outer_span,
 391                 opt_suggest_box_span,
 392             })),
 393         )
 394     }
 395
 396     pub(super) fn demand_scrutinee_type(
 397         &self,
 398         scrut: &'tcx hir::Expr<'tcx>,
 399         contains_ref_bindings: Option<hir::Mutability>,
 400         no_arms: bool,
 401     ) -> Ty<'tcx> {
 402         // Not entirely obvious: if matches may create ref bindings, we want to
 403         // use the *precise* type of the scrutinee, *not* some supertype, as
 404         // the "scrutinee type" (issue #23116).
 405         //
 406         // arielb1 [writes here in this comment thread][c] that there
 407         // is certainly *some* potential danger, e.g., for an example
 408         // like:
 409         //
 410         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 411         //
 412         // ```
 413         // let Foo(x) = f()[0];
 414         // ```
 415         //
 416         // Then if the pattern matches by reference, we want to match
 417         // `f()[0]` as a lexpr, so we can't allow it to be
 418         // coerced. But if the pattern matches by value, `f()[0]` is
 419         // still syntactically a lexpr, but we *do* want to allow
 420         // coercions.
 421         //
 422         // However, *likely* we are ok with allowing coercions to
 423         // happen if there are no explicit ref mut patterns - all
 424         // implicit ref mut patterns must occur behind a reference, so
 425         // they will have the "correct" variance and lifetime.
 426         //
 427         // This does mean that the following pattern would be legal:
 428         //
 429         // ```
 430         // struct Foo(Bar);
 431         // struct Bar(u32);
 432         // impl Deref for Foo {
 433         //     type Target = Bar;
 434         //     fn deref(&self) -> &Bar { &self.0 }
 435         // }
 436         // impl DerefMut for Foo {
 437         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 438         // }
 439         // fn foo(x: &mut Foo) {
 440         //     {
 441         //         let Bar(z): &mut Bar = x;
 442         //         *z = 42;
 443         //     }
 444         //     assert_eq!(foo.0.0, 42);
 445         // }
 446         // ```
 447         //
 448         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 449         // is problematic as the HIR is being scraped, but ref bindings may be
 450         // implicit after #42640. We need to make sure that pat_adjustments
 451         // (once introduced) is populated by the time we get here.
 452         //
 453         // See #44848.
 454         if let Some(m) = contains_ref_bindings {
 455             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 456         } else if no_arms {
 457             self.check_expr(scrut)
 458         } else {
 459             // ...but otherwise we want to use any supertype of the
 460             // scrutinee. This is sort of a workaround, see note (*) in
 461             // `check_pat` for some details.
 462             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 463                 kind: TypeVariableOriginKind::TypeInference,
 464                 span: scrut.span,
 465             });
 466             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 467             scrut_ty
 468         }
 469     }
 470
 471     // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 472     // we check if the different arms would work with boxed trait objects instead and
 473     // provide a structured suggestion in that case.
 474     pub(crate) fn opt_suggest_box_span(
 475         &self,
 476         first_ty: Ty<'tcx>,
 477         second_ty: Ty<'tcx>,
 478         orig_expected: Expectation<'tcx>,
 479     ) -> Option<Span> {
 480         match orig_expected {
 481             Expectation::ExpectHasType(expected)
 482                 if self.in_tail_expr
 483                     && self.return_type_has_opaque
 484                     && self.can_coerce(first_ty, expected)
 485                     && self.can_coerce(second_ty, expected) =>
 486             {
 487                 let obligations = self.fulfillment_cx.borrow().pending_obligations();
 488                 let mut suggest_box = !obligations.is_empty();
 489                 'outer: for o in obligations {
 490                     for outer_ty in &[first_ty, second_ty] {
 491                         match o.predicate.kind().skip_binder() {
 492                             ty::PredicateKind::Trait(t) => {
 493                                 let pred = ty::Binder::dummy(ty::PredicateKind::Trait(
 494                                     ty::TraitPredicate {
 495                                         trait_ref: ty::TraitRef {
 496                                             def_id: t.def_id(),
 497                                             substs: self.tcx.mk_substs_trait(*outer_ty, &[]),
 498                                         },
 499                                         constness: t.constness,
 500                                         polarity: t.polarity,
 501                                     },
 502                                 ));
 503                                 let obl = Obligation::new(
 504                                     o.cause.clone(),
 505                                     self.param_env,
 506                                     pred.to_predicate(self.tcx),
 507                                 );
 508                                 suggest_box &= self.predicate_must_hold_modulo_regions(&obl);
 509                                 if !suggest_box {
 510                                     // We've encountered some obligation that didn't hold, so the
 511                                     // return expression can't just be boxed. We don't need to
 512                                     // evaluate the rest of the obligations.
 513                                     break 'outer;
 514                                 }
 515                             }
 516                             _ => {}
 517                         }
 518                     }
 519                 }
 520                 // If all the obligations hold (or there are no obligations) the tail expression
 521                 // we can suggest to return a boxed trait object instead of an opaque type.
 522                 if suggest_box { self.ret_type_span } else { None }
 523             }
 524             _ => None,
 525         }
 526     }
 527 }
 528
 529 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 530     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
 531         hir::Mutability::Mut => 1,
 532         hir::Mutability::Not => 0,
 533     })
 534 }