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 macro_rules! create_maybe_get_coercion_reason {
  17     ($fn_name:ident, $node:expr) => {
  18         pub(crate) fn $fn_name(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
  19             let node = $node(self.tcx.hir(), hir_id);
  20             if let hir::Node::Block(block) = node {
  21                 // check that the body's parent is an fn
  22                 let parent = self.tcx.hir().get(
  23                     self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)),
  24                 );
  25                 if let (
  26                     Some(expr),
  27                     hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. }),
  28                 ) = (&block.expr, parent)
  29                 {
  30                     // check that the `if` expr without `else` is the fn body's expr
  31                     if expr.span == sp {
  32                         return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
  33                             let span = fn_decl.output.span();
  34                             let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
  35                             Some((
  36                                 span,
  37                                 format!("expected `{}` because of this return type", snippet),
  38                             ))
  39                         });
  40                     }
  41                 }
  42             }
  43             if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
  44                 return Some((pat.span, "expected because of this assignment".to_string()));
  45             }
  46             None
  47         }
  48     };
  49 }
  50
  51 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  52     pub fn check_match(
  53         &self,
  54         expr: &'tcx hir::Expr<'tcx>,
  55         scrut: &'tcx hir::Expr<'tcx>,
  56         arms: &'tcx [hir::Arm<'tcx>],
  57         orig_expected: Expectation<'tcx>,
  58         match_src: hir::MatchSource,
  59     ) -> Ty<'tcx> {
  60         let tcx = self.tcx;
  61
  62         use hir::MatchSource::*;
  63         let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
  64             IfLetDesugar { contains_else_clause, .. } => (true, !contains_else_clause, false),
  65             WhileDesugar => (false, false, true),
  66             _ => (false, false, false),
  67         };
  68
  69         // Type check the discriminant and get its type.
  70         let scrutinee_ty = if force_scrutinee_bool {
  71             // Here we want to ensure:
  72             //
  73             // 1. That default match bindings are *not* accepted in the condition of an
  74             //    `if` expression. E.g. given `fn foo() -> &bool;` we reject `if foo() { .. }`.
  75             //
  76             // 2. By expecting `bool` for `expr` we get nice diagnostics for e.g. `if x = y { .. }`.
  77             //
  78             // FIXME(60707): Consider removing hack with principled solution.
  79             self.check_expr_has_type_or_error(scrut, self.tcx.types.bool, |_| {})
  80         } else {
  81             self.demand_scrutinee_type(scrut, arms_contain_ref_bindings(arms), arms.is_empty())
  82         };
  83
  84         // If there are no arms, that is a diverging match; a special case.
  85         if arms.is_empty() {
  86             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
  87             return tcx.types.never;
  88         }
  89
  90         self.warn_arms_when_scrutinee_diverges(arms, match_src);
  91
  92         // Otherwise, we have to union together the types that the arms produce and so forth.
  93         let scrut_diverges = self.diverges.replace(Diverges::Maybe);
  94
  95         // #55810: Type check patterns first so we get types for all bindings.
  96         for arm in arms {
  97             self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut.span), true);
  98         }
  99
 100         // Now typecheck the blocks.
 101         //
 102         // The result of the match is the common supertype of all the
 103         // arms. Start out the value as bottom, since it's the, well,
 104         // bottom the type lattice, and we'll be moving up the lattice as
 105         // we process each arm. (Note that any match with 0 arms is matching
 106         // on any empty type and is therefore unreachable; should the flow
 107         // of execution reach it, we will panic, so bottom is an appropriate
 108         // type in that case)
 109         let mut all_arms_diverge = Diverges::WarnedAlways;
 110
 111         let expected = orig_expected.adjust_for_branches(self);
 112
 113         let mut coercion = {
 114             let coerce_first = match expected {
 115                 // We don't coerce to `()` so that if the match expression is a
 116                 // statement it's branches can have any consistent type. That allows
 117                 // us to give better error messages (pointing to a usually better
 118                 // arm for inconsistent arms or to the whole match when a `()` type
 119                 // is required).
 120                 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
 121                 _ => self.next_ty_var(TypeVariableOrigin {
 122                     kind: TypeVariableOriginKind::MiscVariable,
 123                     span: expr.span,
 124                 }),
 125             };
 126             CoerceMany::with_coercion_sites(coerce_first, arms)
 127         };
 128
 129         let mut other_arms = vec![]; // Used only for diagnostics.
 130         let mut prior_arm_ty = None;
 131         for (i, arm) in arms.iter().enumerate() {
 132             if let Some(g) = &arm.guard {
 133                 self.diverges.set(Diverges::Maybe);
 134                 match g {
 135                     hir::Guard::If(e) => {
 136                         self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
 137                     }
 138                     hir::Guard::IfLet(pat, e) => {
 139                         let scrutinee_ty = self.demand_scrutinee_type(
 140                             e,
 141                             pat.contains_explicit_ref_binding(),
 142                             false,
 143                         );
 144                         self.check_pat_top(&pat, scrutinee_ty, None, true);
 145                     }
 146                 };
 147             }
 148
 149             self.diverges.set(Diverges::Maybe);
 150             let arm_ty = if source_if
 151                 && if_no_else
 152                 && i != 0
 153                 && self.if_fallback_coercion(
 154                     expr.span,
 155                     &arms[0].body,
 156                     &mut coercion,
 157                     |hir_id, span| self.maybe_get_coercion_reason(hir_id, span),
 158                 ) {
 159                 tcx.ty_error()
 160             } else {
 161                 // Only call this if this is not an `if` expr with an expected type and no `else`
 162                 // clause to avoid duplicated type errors. (#60254)
 163                 self.check_expr_with_expectation(&arm.body, expected)
 164             };
 165             all_arms_diverge &= self.diverges.get();
 166
 167             let opt_suggest_box_span =
 168                 self.opt_suggest_box_span(arm.body.span, arm_ty, orig_expected);
 169
 170             if source_if {
 171                 let then_expr = &arms[0].body;
 172                 match (i, if_no_else) {
 173                     (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
 174                     (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
 175                     (_, _) => {
 176                         let then_ty = prior_arm_ty.unwrap();
 177                         let cause = self.if_cause(
 178                             expr.span,
 179                             then_expr,
 180                             &arm.body,
 181                             then_ty,
 182                             arm_ty,
 183                             opt_suggest_box_span,
 184                         );
 185                         coercion.coerce(self, &cause, &arm.body, arm_ty);
 186                     }
 187                 }
 188             } else {
 189                 let (arm_span, semi_span) =
 190                     self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
 191                 let (span, code) = match i {
 192                     // The reason for the first arm to fail is not that the match arms diverge,
 193                     // but rather that there's a prior obligation that doesn't hold.
 194                     0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
 195                     _ => (
 196                         expr.span,
 197                         ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
 198                             arm_span,
 199                             scrut_span: scrut.span,
 200                             semi_span,
 201                             source: match_src,
 202                             prior_arms: other_arms.clone(),
 203                             last_ty: prior_arm_ty.unwrap(),
 204                             scrut_hir_id: scrut.hir_id,
 205                             opt_suggest_box_span,
 206                         }),
 207                     ),
 208                 };
 209                 let cause = self.cause(span, code);
 210
 211                 // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
 212                 // We use it this way to be able to expand on the potential error and detect when a
 213                 // `match` tail statement could be a tail expression instead. If so, we suggest
 214                 // removing the stray semicolon.
 215                 coercion.coerce_inner(
 216                     self,
 217                     &cause,
 218                     Some(&arm.body),
 219                     arm_ty,
 220                     Some(&mut |err: &mut DiagnosticBuilder<'_>| {
 221                         let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
 222                             Some(ret_coercion) if self.in_tail_expr => {
 223                                 let ret_ty = ret_coercion.borrow().expected_ty();
 224                                 let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 225                                 self.can_coerce(arm_ty, ret_ty)
 226                                     && prior_arm_ty.map_or(true, |t| self.can_coerce(t, ret_ty))
 227                                     // The match arms need to unify for the case of `impl Trait`.
 228                                     && !matches!(ret_ty.kind(), ty::Opaque(..))
 229                             }
 230                             _ => false,
 231                         };
 232                         if let (Expectation::IsLast(stmt), Some(ret), true) =
 233                             (orig_expected, self.ret_type_span, can_coerce_to_return_ty)
 234                         {
 235                             let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
 236                             let mut ret_span: MultiSpan = semi_span.into();
 237                             ret_span.push_span_label(
 238                                 expr.span,
 239                                 "this could be implicitly returned but it is a statement, not a \
 240                                  tail expression"
 241                                     .to_owned(),
 242                             );
 243                             ret_span.push_span_label(
 244                                 ret,
 245                                 "the `match` arms can conform to this return type".to_owned(),
 246                             );
 247                             ret_span.push_span_label(
 248                                 semi_span,
 249                                 "the `match` is a statement because of this semicolon, consider \
 250                                  removing it"
 251                                     .to_owned(),
 252                             );
 253                             err.span_note(
 254                                 ret_span,
 255                                 "you might have meant to return the `match` expression",
 256                             );
 257                             err.tool_only_span_suggestion(
 258                                 semi_span,
 259                                 "remove this semicolon",
 260                                 String::new(),
 261                                 Applicability::MaybeIncorrect,
 262                             );
 263                         }
 264                     }),
 265                     false,
 266                 );
 267
 268                 other_arms.push(arm_span);
 269                 if other_arms.len() > 5 {
 270                     other_arms.remove(0);
 271                 }
 272             }
 273             prior_arm_ty = Some(arm_ty);
 274         }
 275
 276         // If all of the arms in the `match` diverge,
 277         // and we're dealing with an actual `match` block
 278         // (as opposed to a `match` desugared from something else'),
 279         // we can emit a better note. Rather than pointing
 280         // at a diverging expression in an arbitrary arm,
 281         // we can point at the entire `match` expression
 282         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
 283             all_arms_diverge = Diverges::Always {
 284                 span: expr.span,
 285                 custom_note: Some(
 286                     "any code following this `match` expression is unreachable, as all arms diverge",
 287                 ),
 288             };
 289         }
 290
 291         // We won't diverge unless the scrutinee or all arms diverge.
 292         self.diverges.set(scrut_diverges | all_arms_diverge);
 293
 294         coercion.complete(self)
 295     }
 296
 297     fn get_appropriate_arm_semicolon_removal_span(
 298         &self,
 299         arms: &'tcx [hir::Arm<'tcx>],
 300         i: usize,
 301         prior_arm_ty: Option<Ty<'tcx>>,
 302         arm_ty: Ty<'tcx>,
 303     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 304         let arm = &arms[i];
 305         let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
 306             self.find_block_span(blk, prior_arm_ty)
 307         } else {
 308             (arm.body.span, None)
 309         };
 310         if semi_span.is_none() && i > 0 {
 311             if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
 312                 let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
 313                 semi_span = semi_span_prev;
 314             }
 315         }
 316         (arm_span, semi_span)
 317     }
 318
 319     /// When the previously checked expression (the scrutinee) diverges,
 320     /// warn the user about the match arms being unreachable.
 321     fn warn_arms_when_scrutinee_diverges(
 322         &self,
 323         arms: &'tcx [hir::Arm<'tcx>],
 324         source: hir::MatchSource,
 325     ) {
 326         use hir::MatchSource::*;
 327         let msg = match source {
 328             IfLetDesugar { .. } => "block in `if` expression",
 329             WhileDesugar { .. } | WhileLetDesugar { .. } => "block in `while` expression",
 330             _ => "arm",
 331         };
 332         for arm in arms {
 333             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, msg);
 334         }
 335     }
 336
 337     /// Handle the fallback arm of a desugared if(-let) like a missing else.
 338     ///
 339     /// Returns `true` if there was an error forcing the coercion to the `()` type.
 340     pub(crate) fn if_fallback_coercion<F, T>(
 341         &self,
 342         span: Span,
 343         then_expr: &'tcx hir::Expr<'tcx>,
 344         coercion: &mut CoerceMany<'tcx, '_, T>,
 345         ret_reason: F,
 346     ) -> bool
 347     where
 348         F: Fn(hir::HirId, Span) -> Option<(Span, String)>,
 349         T: AsCoercionSite,
 350     {
 351         // If this `if` expr is the parent's function return expr,
 352         // the cause of the type coercion is the return type, point at it. (#25228)
 353         let ret_reason = ret_reason(then_expr.hir_id, span);
 354         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
 355         let mut error = false;
 356         coercion.coerce_forced_unit(
 357             self,
 358             &cause,
 359             &mut |err| {
 360                 if let Some((span, msg)) = &ret_reason {
 361                     err.span_label(*span, msg.as_str());
 362                 } else if let ExprKind::Block(block, _) = &then_expr.kind {
 363                     if let Some(expr) = &block.expr {
 364                         err.span_label(expr.span, "found here".to_string());
 365                     }
 366                 }
 367                 err.note("`if` expressions without `else` evaluate to `()`");
 368                 err.help("consider adding an `else` block that evaluates to the expected type");
 369                 error = true;
 370             },
 371             ret_reason.is_none(),
 372         );
 373         error
 374     }
 375
 376     create_maybe_get_coercion_reason!(
 377         maybe_get_coercion_reason,
 378         |hir: rustc_middle::hir::map::Map<'a>, id| {
 379             let arm_id = hir.get_parent_node(id);
 380             let match_id = hir.get_parent_node(arm_id);
 381             let containing_id = hir.get_parent_node(match_id);
 382             hir.get(containing_id)
 383         }
 384     );
 385
 386     create_maybe_get_coercion_reason!(
 387         maybe_get_coercion_reason_if,
 388         |hir: rustc_middle::hir::map::Map<'a>, id| {
 389             let rslt = hir.get_parent_node(hir.get_parent_node(id));
 390             hir.get(rslt)
 391         }
 392     );
 393
 394     pub(crate) fn if_cause(
 395         &self,
 396         span: Span,
 397         then_expr: &'tcx hir::Expr<'tcx>,
 398         else_expr: &'tcx hir::Expr<'tcx>,
 399         then_ty: Ty<'tcx>,
 400         else_ty: Ty<'tcx>,
 401         opt_suggest_box_span: Option<Span>,
 402     ) -> ObligationCause<'tcx> {
 403         let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
 404             // The `if`/`else` isn't in one line in the output, include some context to make it
 405             // clear it is an if/else expression:
 406             // ```
 407             // LL |      let x = if true {
 408             //    | _____________-
 409             // LL ||         10i32
 410             //    ||         ----- expected because of this
 411             // LL ||     } else {
 412             // LL ||         10u32
 413             //    ||         ^^^^^ expected `i32`, found `u32`
 414             // LL ||     };
 415             //    ||_____- `if` and `else` have incompatible types
 416             // ```
 417             Some(span)
 418         } else {
 419             // The entire expression is in one line, only point at the arms
 420             // ```
 421             // LL |     let x = if true { 10i32 } else { 10u32 };
 422             //    |                       -----          ^^^^^ expected `i32`, found `u32`
 423             //    |                       |
 424             //    |                       expected because of this
 425             // ```
 426             None
 427         };
 428
 429         let mut remove_semicolon = None;
 430         let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
 431             let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
 432             remove_semicolon = semi_sp;
 433             if block.expr.is_none() && block.stmts.is_empty() {
 434                 // Avoid overlapping spans that aren't as readable:
 435                 // ```
 436                 // 2 |        let x = if true {
 437                 //   |   _____________-
 438                 // 3 |  |         3
 439                 //   |  |         - expected because of this
 440                 // 4 |  |     } else {
 441                 //   |  |____________^
 442                 // 5 | ||
 443                 // 6 | ||     };
 444                 //   | ||     ^
 445                 //   | ||_____|
 446                 //   | |______if and else have incompatible types
 447                 //   |        expected integer, found `()`
 448                 // ```
 449                 // by not pointing at the entire expression:
 450                 // ```
 451                 // 2 |       let x = if true {
 452                 //   |               ------- `if` and `else` have incompatible types
 453                 // 3 |           3
 454                 //   |           - expected because of this
 455                 // 4 |       } else {
 456                 //   |  ____________^
 457                 // 5 | |
 458                 // 6 | |     };
 459                 //   | |_____^ expected integer, found `()`
 460                 // ```
 461                 if outer_sp.is_some() {
 462                     outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
 463                 }
 464             }
 465             error_sp
 466         } else {
 467             // shouldn't happen unless the parser has done something weird
 468             else_expr.span
 469         };
 470
 471         // Compute `Span` of `then` part of `if`-expression.
 472         let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
 473             let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
 474             remove_semicolon = remove_semicolon.or(semi_sp);
 475             if block.expr.is_none() && block.stmts.is_empty() {
 476                 outer_sp = None; // same as in `error_sp`; cleanup output
 477             }
 478             then_sp
 479         } else {
 480             // shouldn't happen unless the parser has done something weird
 481             then_expr.span
 482         };
 483
 484         // Finally construct the cause:
 485         self.cause(
 486             error_sp,
 487             ObligationCauseCode::IfExpression(box IfExpressionCause {
 488                 then: then_sp,
 489                 else_sp: error_sp,
 490                 outer: outer_sp,
 491                 semicolon: remove_semicolon,
 492                 opt_suggest_box_span,
 493             }),
 494         )
 495     }
 496
 497     fn demand_scrutinee_type(
 498         &self,
 499         scrut: &'tcx hir::Expr<'tcx>,
 500         contains_ref_bindings: Option<hir::Mutability>,
 501         no_arms: bool,
 502     ) -> Ty<'tcx> {
 503         // Not entirely obvious: if matches may create ref bindings, we want to
 504         // use the *precise* type of the scrutinee, *not* some supertype, as
 505         // the "scrutinee type" (issue #23116).
 506         //
 507         // arielb1 [writes here in this comment thread][c] that there
 508         // is certainly *some* potential danger, e.g., for an example
 509         // like:
 510         //
 511         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
 512         //
 513         // ```
 514         // let Foo(x) = f()[0];
 515         // ```
 516         //
 517         // Then if the pattern matches by reference, we want to match
 518         // `f()[0]` as a lexpr, so we can't allow it to be
 519         // coerced. But if the pattern matches by value, `f()[0]` is
 520         // still syntactically a lexpr, but we *do* want to allow
 521         // coercions.
 522         //
 523         // However, *likely* we are ok with allowing coercions to
 524         // happen if there are no explicit ref mut patterns - all
 525         // implicit ref mut patterns must occur behind a reference, so
 526         // they will have the "correct" variance and lifetime.
 527         //
 528         // This does mean that the following pattern would be legal:
 529         //
 530         // ```
 531         // struct Foo(Bar);
 532         // struct Bar(u32);
 533         // impl Deref for Foo {
 534         //     type Target = Bar;
 535         //     fn deref(&self) -> &Bar { &self.0 }
 536         // }
 537         // impl DerefMut for Foo {
 538         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
 539         // }
 540         // fn foo(x: &mut Foo) {
 541         //     {
 542         //         let Bar(z): &mut Bar = x;
 543         //         *z = 42;
 544         //     }
 545         //     assert_eq!(foo.0.0, 42);
 546         // }
 547         // ```
 548         //
 549         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
 550         // is problematic as the HIR is being scraped, but ref bindings may be
 551         // implicit after #42640. We need to make sure that pat_adjustments
 552         // (once introduced) is populated by the time we get here.
 553         //
 554         // See #44848.
 555         if let Some(m) = contains_ref_bindings {
 556             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
 557         } else if no_arms {
 558             self.check_expr(scrut)
 559         } else {
 560             // ...but otherwise we want to use any supertype of the
 561             // scrutinee. This is sort of a workaround, see note (*) in
 562             // `check_pat` for some details.
 563             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
 564                 kind: TypeVariableOriginKind::TypeInference,
 565                 span: scrut.span,
 566             });
 567             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
 568             scrut_ty
 569         }
 570     }
 571
 572     fn find_block_span(
 573         &self,
 574         block: &'tcx hir::Block<'tcx>,
 575         expected_ty: Option<Ty<'tcx>>,
 576     ) -> (Span, Option<(Span, StatementAsExpression)>) {
 577         if let Some(expr) = &block.expr {
 578             (expr.span, None)
 579         } else if let Some(stmt) = block.stmts.last() {
 580             // possibly incorrect trailing `;` in the else arm
 581             (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
 582         } else {
 583             // empty block; point at its entirety
 584             (block.span, None)
 585         }
 586     }
 587
 588     // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
 589     // we check if the different arms would work with boxed trait objects instead and
 590     // provide a structured suggestion in that case.
 591     pub(crate) fn opt_suggest_box_span(
 592         &self,
 593         span: Span,
 594         outer_ty: &'tcx TyS<'tcx>,
 595         orig_expected: Expectation<'tcx>,
 596     ) -> Option<Span> {
 597         match (orig_expected, self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty))) {
 598             (Expectation::ExpectHasType(expected), Some((id, ty)))
 599                 if self.in_tail_expr && self.can_coerce(outer_ty, expected) =>
 600             {
 601                 let impl_trait_ret_ty =
 602                     self.infcx.instantiate_opaque_types(id, self.body_id, self.param_env, ty, span);
 603                 let mut suggest_box = !impl_trait_ret_ty.obligations.is_empty();
 604                 for o in impl_trait_ret_ty.obligations {
 605                     match o.predicate.kind().skip_binder() {
 606                         ty::PredicateKind::Trait(t, constness) => {
 607                             let pred = ty::PredicateKind::Trait(
 608                                 ty::TraitPredicate {
 609                                     trait_ref: ty::TraitRef {
 610                                         def_id: t.def_id(),
 611                                         substs: self.infcx.tcx.mk_substs_trait(outer_ty, &[]),
 612                                     },
 613                                 },
 614                                 constness,
 615                             );
 616                             let obl = Obligation::new(
 617                                 o.cause.clone(),
 618                                 self.param_env,
 619                                 pred.to_predicate(self.infcx.tcx),
 620                             );
 621                             suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
 622                             if !suggest_box {
 623                                 // We've encountered some obligation that didn't hold, so the
 624                                 // return expression can't just be boxed. We don't need to
 625                                 // evaluate the rest of the obligations.
 626                                 break;
 627                             }
 628                         }
 629                         _ => {}
 630                     }
 631                 }
 632                 // If all the obligations hold (or there are no obligations) the tail expression
 633                 // we can suggest to return a boxed trait object instead of an opaque type.
 634                 if suggest_box { self.ret_type_span } else { None }
 635             }
 636             _ => None,
 637         }
 638     }
 639 }
 640
 641 fn arms_contain_ref_bindings(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
 642     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
 643         hir::Mutability::Mut => 1,
 644         hir::Mutability::Not => 0,
 645     })
 646 }