compiler/rustc_typeck/src/check/_match.rs - third_party/rust - Git at Google

 use crate::check::coercion::CoerceMany;
 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
 use rustc_hir::{self as hir, ExprKind};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::traits::Obligation;
 use rustc_middle::ty::{self, ToPredicate, Ty};
 use rustc_span::Span;
 use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
 };

 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn check_match(
         &self,
         expr: &'tcx hir::Expr<'tcx>,
         scrut: &'tcx hir::Expr<'tcx>,
         arms: &'tcx [hir::Arm<'tcx>],
         orig_expected: Expectation<'tcx>,
         match_src: hir::MatchSource,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;

         use hir::MatchSource::*;
         let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
             IfDesugar { contains_else_clause } => (true, !contains_else_clause, true),
             IfLetDesugar { contains_else_clause, .. } => (true, !contains_else_clause, false),
             WhileDesugar => (false, false, true),
             _ => (false, false, false),
         };

         // Type check the descriminant and get its type.
         let scrutinee_ty = if force_scrutinee_bool {
             // Here we want to ensure:
             //
             // 1. That default match bindings are *not* accepted in the condition of an
             //    `if` expression. E.g. given `fn foo() -> &bool;` we reject `if foo() { .. }`.
             //
             // 2. By expecting `bool` for `expr` we get nice diagnostics for e.g. `if x = y { .. }`.
             //
             // FIXME(60707): Consider removing hack with principled solution.
             self.check_expr_has_type_or_error(scrut, self.tcx.types.bool, |_| {})
         } else {
             self.demand_scrutinee_type(arms, scrut)
         };

         // If there are no arms, that is a diverging match; a special case.
         if arms.is_empty() {
             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
             return tcx.types.never;
         }

         self.warn_arms_when_scrutinee_diverges(arms, match_src);

         // Otherwise, we have to union together the types that the arms produce and so forth.
         let scrut_diverges = self.diverges.replace(Diverges::Maybe);

         // #55810: Type check patterns first so we get types for all bindings.
         for arm in arms {
             self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut.span), true);
         }

         // Now typecheck the blocks.
         //
         // The result of the match is the common supertype of all the
         // arms. Start out the value as bottom, since it's the, well,
         // bottom the type lattice, and we'll be moving up the lattice as
         // we process each arm. (Note that any match with 0 arms is matching
         // on any empty type and is therefore unreachable; should the flow
         // of execution reach it, we will panic, so bottom is an appropriate
         // type in that case)
         let mut all_arms_diverge = Diverges::WarnedAlways;

         let expected = orig_expected.adjust_for_branches(self);

         let mut coercion = {
             let coerce_first = match expected {
                 // We don't coerce to `()` so that if the match expression is a
                 // statement it's branches can have any consistent type. That allows
                 // us to give better error messages (pointing to a usually better
                 // arm for inconsistent arms or to the whole match when a `()` type
                 // is required).
                 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
                 _ => self.next_ty_var(TypeVariableOrigin {
                     kind: TypeVariableOriginKind::MiscVariable,
                     span: expr.span,
                 }),
             };
             CoerceMany::with_coercion_sites(coerce_first, arms)
         };

         let mut other_arms = vec![]; // Used only for diagnostics.
         let mut prior_arm_ty = None;
         for (i, arm) in arms.iter().enumerate() {
             if let Some(g) = &arm.guard {
                 self.diverges.set(Diverges::Maybe);
                 match g {
                     hir::Guard::If(e) => {
                         self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {})
                     }
                 };
             }

             self.diverges.set(Diverges::Maybe);
             let arm_ty = if source_if
                 && if_no_else
                 && i != 0
                 && self.if_fallback_coercion(expr.span, &arms[0].body, &mut coercion)
             {
                 tcx.ty_error()
             } else {
                 // Only call this if this is not an `if` expr with an expected type and no `else`
                 // clause to avoid duplicated type errors. (#60254)
                 self.check_expr_with_expectation(&arm.body, expected)
             };
             all_arms_diverge &= self.diverges.get();

             // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
             // we check if the different arms would work with boxed trait objects instead and
             // provide a structured suggestion in that case.
             let opt_suggest_box_span = match (
                 orig_expected,
                 self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty)),
             ) {
                 (Expectation::ExpectHasType(expected), Some((id, ty)))
                     if self.in_tail_expr && self.can_coerce(arm_ty, expected) =>
                 {
                     let impl_trait_ret_ty = self.infcx.instantiate_opaque_types(
                         id,
                         self.body_id,
                         self.param_env,
                         &ty,
                         arm.body.span,
                     );
                     let mut suggest_box = !impl_trait_ret_ty.obligations.is_empty();
                     for o in impl_trait_ret_ty.obligations {
                         match o.predicate.skip_binders_unchecked() {
                             ty::PredicateAtom::Trait(t, constness) => {
                                 let pred = ty::PredicateAtom::Trait(
                                     ty::TraitPredicate {
                                         trait_ref: ty::TraitRef {
                                             def_id: t.def_id(),
                                             substs: self.infcx.tcx.mk_substs_trait(arm_ty, &[]),
                                         },
                                     },
                                     constness,
                                 );
                                 let obl = Obligation::new(
                                     o.cause.clone(),
                                     self.param_env,
                                     pred.to_predicate(self.infcx.tcx),
                                 );
                                 suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
                                 if !suggest_box {
                                     // We've encountered some obligation that didn't hold, so the
                                     // return expression can't just be boxed. We don't need to
                                     // evaluate the rest of the obligations.
                                     break;
                                 }
                             }
                             _ => {}
                         }
                     }
                     // If all the obligations hold (or there are no obligations) the tail expression
                     // we can suggest to return a boxed trait object instead of an opaque type.
                     if suggest_box { self.ret_type_span } else { None }
                 }
                 _ => None,
             };

             if source_if {
                 let then_expr = &arms[0].body;
                 match (i, if_no_else) {
                     (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
                     (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
                     (_, _) => {
                         let then_ty = prior_arm_ty.unwrap();
                         let cause = self.if_cause(
                             expr.span,
                             then_expr,
                             &arm.body,
                             then_ty,
                             arm_ty,
                             opt_suggest_box_span,
                         );
                         coercion.coerce(self, &cause, &arm.body, arm_ty);
                     }
                 }
             } else {
                 let (arm_span, semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
                     self.find_block_span(blk, prior_arm_ty)
                 } else {
                     (arm.body.span, None)
                 };
                 let (span, code) = match i {
                     // The reason for the first arm to fail is not that the match arms diverge,
                     // but rather that there's a prior obligation that doesn't hold.
                     0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
                     _ => (
                         expr.span,
                         ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                             arm_span,
                             semi_span,
                             source: match_src,
                             prior_arms: other_arms.clone(),
                             last_ty: prior_arm_ty.unwrap(),
                             scrut_hir_id: scrut.hir_id,
                             opt_suggest_box_span,
                         }),
                     ),
                 };
                 let cause = self.cause(span, code);
                 coercion.coerce(self, &cause, &arm.body, arm_ty);
                 other_arms.push(arm_span);
                 if other_arms.len() > 5 {
                     other_arms.remove(0);
                 }
             }
             prior_arm_ty = Some(arm_ty);
         }

         // If all of the arms in the `match` diverge,
         // and we're dealing with an actual `match` block
         // (as opposed to a `match` desugared from something else'),
         // we can emit a better note. Rather than pointing
         // at a diverging expression in an arbitrary arm,
         // we can point at the entire `match` expression
         if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
             all_arms_diverge = Diverges::Always {
                 span: expr.span,
                 custom_note: Some(
                     "any code following this `match` expression is unreachable, as all arms diverge",
                 ),
             };
         }

         // We won't diverge unless the scrutinee or all arms diverge.
         self.diverges.set(scrut_diverges | all_arms_diverge);

         coercion.complete(self)
     }

     /// When the previously checked expression (the scrutinee) diverges,
     /// warn the user about the match arms being unreachable.
     fn warn_arms_when_scrutinee_diverges(
         &self,
         arms: &'tcx [hir::Arm<'tcx>],
         source: hir::MatchSource,
     ) {
         use hir::MatchSource::*;
         let msg = match source {
             IfDesugar { .. } | IfLetDesugar { .. } => "block in `if` expression",
             WhileDesugar { .. } | WhileLetDesugar { .. } => "block in `while` expression",
             _ => "arm",
         };
         for arm in arms {
             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, msg);
         }
     }

     /// Handle the fallback arm of a desugared if(-let) like a missing else.
     ///
     /// Returns `true` if there was an error forcing the coercion to the `()` type.
     fn if_fallback_coercion(
         &self,
         span: Span,
         then_expr: &'tcx hir::Expr<'tcx>,
         coercion: &mut CoerceMany<'tcx, '_, rustc_hir::Arm<'tcx>>,
     ) -> bool {
         // If this `if` expr is the parent's function return expr,
         // the cause of the type coercion is the return type, point at it. (#25228)
         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
         let mut error = false;
         coercion.coerce_forced_unit(
             self,
             &cause,
             &mut |err| {
                 if let Some((span, msg)) = &ret_reason {
                     err.span_label(*span, msg.as_str());
                 } else if let ExprKind::Block(block, _) = &then_expr.kind {
                     if let Some(expr) = &block.expr {
                         err.span_label(expr.span, "found here".to_string());
                     }
                 }
                 err.note("`if` expressions without `else` evaluate to `()`");
                 err.help("consider adding an `else` block that evaluates to the expected type");
                 error = true;
             },
             ret_reason.is_none(),
         );
         error
     }

     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, span: Span) -> Option<(Span, String)> {
         use hir::Node::{Block, Item, Local};

         let hir = self.tcx.hir();
         let arm_id = hir.get_parent_node(hir_id);
         let match_id = hir.get_parent_node(arm_id);
         let containing_id = hir.get_parent_node(match_id);

         let node = hir.get(containing_id);
         if let Block(block) = node {
             // check that the body's parent is an fn
             let parent = hir.get(hir.get_parent_node(hir.get_parent_node(block.hir_id)));
             if let (Some(expr), Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
                 (&block.expr, parent)
             {
                 // check that the `if` expr without `else` is the fn body's expr
                 if expr.span == span {
                     return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
                         let span = fn_decl.output.span();
                         let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
                         Some((span, format!("expected `{}` because of this return type", snippet)))
                     });
                 }
             }
         }
         if let Local(hir::Local { ty: Some(_), pat, .. }) = node {
             return Some((pat.span, "expected because of this assignment".to_string()));
         }
         None
     }

     fn if_cause(
         &self,
         span: Span,
         then_expr: &'tcx hir::Expr<'tcx>,
         else_expr: &'tcx hir::Expr<'tcx>,
         then_ty: Ty<'tcx>,
         else_ty: Ty<'tcx>,
         opt_suggest_box_span: Option<Span>,
     ) -> ObligationCause<'tcx> {
         let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
             // The `if`/`else` isn't in one line in the output, include some context to make it
             // clear it is an if/else expression:
             // ```
             // LL |      let x = if true {
             //    | _____________-
             // LL ||         10i32
             //    ||         ----- expected because of this
             // LL ||     } else {
             // LL ||         10u32
             //    ||         ^^^^^ expected `i32`, found `u32`
             // LL ||     };
             //    ||_____- `if` and `else` have incompatible types
             // ```
             Some(span)
         } else {
             // The entire expression is in one line, only point at the arms
             // ```
             // LL |     let x = if true { 10i32 } else { 10u32 };
             //    |                       -----          ^^^^^ expected `i32`, found `u32`
             //    |                       |
             //    |                       expected because of this
             // ```
             None
         };

         let mut remove_semicolon = None;
         let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
             let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
             remove_semicolon = semi_sp;
             if block.expr.is_none() && block.stmts.is_empty() {
                 // Avoid overlapping spans that aren't as readable:
                 // ```
                 // 2 |        let x = if true {
                 //   |   _____________-
                 // 3 |  |         3
                 //   |  |         - expected because of this
                 // 4 |  |     } else {
                 //   |  |____________^
                 // 5 | ||
                 // 6 | ||     };
                 //   | ||     ^
                 //   | ||_____|
                 //   | |______if and else have incompatible types
                 //   |        expected integer, found `()`
                 // ```
                 // by not pointing at the entire expression:
                 // ```
                 // 2 |       let x = if true {
                 //   |               ------- `if` and `else` have incompatible types
                 // 3 |           3
                 //   |           - expected because of this
                 // 4 |       } else {
                 //   |  ____________^
                 // 5 | |
                 // 6 | |     };
                 //   | |_____^ expected integer, found `()`
                 // ```
                 if outer_sp.is_some() {
                     outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
                 }
             }
             error_sp
         } else {
             // shouldn't happen unless the parser has done something weird
             else_expr.span
         };

         // Compute `Span` of `then` part of `if`-expression.
         let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
             let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
             remove_semicolon = remove_semicolon.or(semi_sp);
             if block.expr.is_none() && block.stmts.is_empty() {
                 outer_sp = None; // same as in `error_sp`; cleanup output
             }
             then_sp
         } else {
             // shouldn't happen unless the parser has done something weird
             then_expr.span
         };

         // Finally construct the cause:
         self.cause(
             error_sp,
             ObligationCauseCode::IfExpression(box IfExpressionCause {
                 then: then_sp,
                 else_sp: error_sp,
                 outer: outer_sp,
                 semicolon: remove_semicolon,
                 opt_suggest_box_span,
             }),
         )
     }

     fn demand_scrutinee_type(
         &self,
         arms: &'tcx [hir::Arm<'tcx>],
         scrut: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         // Not entirely obvious: if matches may create ref bindings, we want to
         // use the *precise* type of the scrutinee, *not* some supertype, as
         // the "scrutinee type" (issue #23116).
         //
         // arielb1 [writes here in this comment thread][c] that there
         // is certainly *some* potential danger, e.g., for an example
         // like:
         //
         // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
         //
         // ```
         // let Foo(x) = f()[0];
         // ```
         //
         // Then if the pattern matches by reference, we want to match
         // `f()[0]` as a lexpr, so we can't allow it to be
         // coerced. But if the pattern matches by value, `f()[0]` is
         // still syntactically a lexpr, but we *do* want to allow
         // coercions.
         //
         // However, *likely* we are ok with allowing coercions to
         // happen if there are no explicit ref mut patterns - all
         // implicit ref mut patterns must occur behind a reference, so
         // they will have the "correct" variance and lifetime.
         //
         // This does mean that the following pattern would be legal:
         //
         // ```
         // struct Foo(Bar);
         // struct Bar(u32);
         // impl Deref for Foo {
         //     type Target = Bar;
         //     fn deref(&self) -> &Bar { &self.0 }
         // }
         // impl DerefMut for Foo {
         //     fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
         // }
         // fn foo(x: &mut Foo) {
         //     {
         //         let Bar(z): &mut Bar = x;
         //         *z = 42;
         //     }
         //     assert_eq!(foo.0.0, 42);
         // }
         // ```
         //
         // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
         // is problematic as the HIR is being scraped, but ref bindings may be
         // implicit after #42640. We need to make sure that pat_adjustments
         // (once introduced) is populated by the time we get here.
         //
         // See #44848.
         let contains_ref_bindings = arms
             .iter()
             .filter_map(|a| a.pat.contains_explicit_ref_binding())
             .max_by_key(|m| match *m {
                 hir::Mutability::Mut => 1,
                 hir::Mutability::Not => 0,
             });

         if let Some(m) = contains_ref_bindings {
             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
         } else if arms.is_empty() {
             self.check_expr(scrut)
         } else {
             // ...but otherwise we want to use any supertype of the
             // scrutinee. This is sort of a workaround, see note (*) in
             // `check_pat` for some details.
             let scrut_ty = self.next_ty_var(TypeVariableOrigin {
                 kind: TypeVariableOriginKind::TypeInference,
                 span: scrut.span,
             });
             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
             scrut_ty
         }
     }

     fn find_block_span(
         &self,
         block: &'tcx hir::Block<'tcx>,
         expected_ty: Option<Ty<'tcx>>,
     ) -> (Span, Option<Span>) {
         if let Some(expr) = &block.expr {
             (expr.span, None)
         } else if let Some(stmt) = block.stmts.last() {
             // possibly incorrect trailing `;` in the else arm
             (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
         } else {
             // empty block; point at its entirety
             (block.span, None)
         }
     }
 }
	use crate::check::coercion::CoerceMany;
	use crate::check::{Diverges, Expectation, FnCtxt, Needs};
	use rustc_hir::{self as hir, ExprKind};
	use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
	use rustc_infer::traits::Obligation;
	use rustc_middle::ty::{self, ToPredicate, Ty};
	use rustc_span::Span;
	use rustc_trait_selection::opaque_types::InferCtxtExt as _;
	use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
	use rustc_trait_selection::traits::{
	IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
	};

	impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
	pub fn check_match(
	&self,
	expr: &'tcx hir::Expr<'tcx>,
	scrut: &'tcx hir::Expr<'tcx>,
	arms: &'tcx [hir::Arm<'tcx>],
	orig_expected: Expectation<'tcx>,
	match_src: hir::MatchSource,
	) -> Ty<'tcx> {
	let tcx = self.tcx;

	use hir::MatchSource::*;
	let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
	IfDesugar { contains_else_clause } => (true, !contains_else_clause, true),
	IfLetDesugar { contains_else_clause, .. } => (true, !contains_else_clause, false),
	WhileDesugar => (false, false, true),
	_ => (false, false, false),
	};

	// Type check the descriminant and get its type.
	let scrutinee_ty = if force_scrutinee_bool {
	// Here we want to ensure:
	//
	// 1. That default match bindings are not accepted in the condition of an
	// `if` expression. E.g. given `fn foo() -> &bool;` we reject `if foo() { .. }`.
	//
	// 2. By expecting `bool` for `expr` we get nice diagnostics for e.g. `if x = y { .. }`.
	//
	// FIXME(60707): Consider removing hack with principled solution.
	self.check_expr_has_type_or_error(scrut, self.tcx.types.bool, \|_\| {})
	} else {
	self.demand_scrutinee_type(arms, scrut)
	};

	// If there are no arms, that is a diverging match; a special case.
	if arms.is_empty() {
	self.diverges.set(self.diverges.get() \| Diverges::always(expr.span));
	return tcx.types.never;
	}

	self.warn_arms_when_scrutinee_diverges(arms, match_src);

	// Otherwise, we have to union together the types that the arms produce and so forth.
	let scrut_diverges = self.diverges.replace(Diverges::Maybe);

	// #55810: Type check patterns first so we get types for all bindings.
	for arm in arms {
	self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut.span), true);
	}

	// Now typecheck the blocks.
	//
	// The result of the match is the common supertype of all the
	// arms. Start out the value as bottom, since it's the, well,
	// bottom the type lattice, and we'll be moving up the lattice as
	// we process each arm. (Note that any match with 0 arms is matching
	// on any empty type and is therefore unreachable; should the flow
	// of execution reach it, we will panic, so bottom is an appropriate
	// type in that case)
	let mut all_arms_diverge = Diverges::WarnedAlways;

	let expected = orig_expected.adjust_for_branches(self);

	let mut coercion = {
	let coerce_first = match expected {
	// We don't coerce to `()` so that if the match expression is a
	// statement it's branches can have any consistent type. That allows
	// us to give better error messages (pointing to a usually better
	// arm for inconsistent arms or to the whole match when a `()` type
	// is required).
	Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
	_ => self.next_ty_var(TypeVariableOrigin {
	kind: TypeVariableOriginKind::MiscVariable,
	span: expr.span,
	}),
	};
	CoerceMany::with_coercion_sites(coerce_first, arms)
	};

	let mut other_arms = vec![]; // Used only for diagnostics.
	let mut prior_arm_ty = None;
	for (i, arm) in arms.iter().enumerate() {
	if let Some(g) = &arm.guard {
	self.diverges.set(Diverges::Maybe);
	match g {
	hir::Guard::If(e) => {
	self.check_expr_has_type_or_error(e, tcx.types.bool, \|_\| {})
	}
	};
	}

	self.diverges.set(Diverges::Maybe);
	let arm_ty = if source_if
	&& if_no_else
	&& i != 0
	&& self.if_fallback_coercion(expr.span, &arms[0].body, &mut coercion)
	{
	tcx.ty_error()
	} else {
	// Only call this if this is not an `if` expr with an expected type and no `else`
	// clause to avoid duplicated type errors. (#60254)
	self.check_expr_with_expectation(&arm.body, expected)
	};
	all_arms_diverge &= self.diverges.get();

	// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
	// we check if the different arms would work with boxed trait objects instead and
	// provide a structured suggestion in that case.
	let opt_suggest_box_span = match (
	orig_expected,
	self.ret_coercion_impl_trait.map(\|ty\| (self.body_id.owner, ty)),
	) {
	(Expectation::ExpectHasType(expected), Some((id, ty)))
	if self.in_tail_expr && self.can_coerce(arm_ty, expected) =>
	{
	let impl_trait_ret_ty = self.infcx.instantiate_opaque_types(
	id,
	self.body_id,
	self.param_env,
	&ty,
	arm.body.span,
	);
	let mut suggest_box = !impl_trait_ret_ty.obligations.is_empty();
	for o in impl_trait_ret_ty.obligations {
	match o.predicate.skip_binders_unchecked() {
	ty::PredicateAtom::Trait(t, constness) => {
	let pred = ty::PredicateAtom::Trait(
	ty::TraitPredicate {
	trait_ref: ty::TraitRef {
	def_id: t.def_id(),
	substs: self.infcx.tcx.mk_substs_trait(arm_ty, &[]),
	},
	},
	constness,
	);
	let obl = Obligation::new(
	o.cause.clone(),
	self.param_env,
	pred.to_predicate(self.infcx.tcx),
	);
	suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
	if !suggest_box {
	// We've encountered some obligation that didn't hold, so the
	// return expression can't just be boxed. We don't need to
	// evaluate the rest of the obligations.
	break;
	}
	}
	_ => {}
	}
	}
	// If all the obligations hold (or there are no obligations) the tail expression
	// we can suggest to return a boxed trait object instead of an opaque type.
	if suggest_box { self.ret_type_span } else { None }
	}
	_ => None,
	};

	if source_if {
	let then_expr = &arms[0].body;
	match (i, if_no_else) {
	(0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
	(_, true) => {} // Handled above to avoid duplicated type errors (#60254).
	(_, _) => {
	let then_ty = prior_arm_ty.unwrap();
	let cause = self.if_cause(
	expr.span,
	then_expr,
	&arm.body,
	then_ty,
	arm_ty,
	opt_suggest_box_span,
	);
	coercion.coerce(self, &cause, &arm.body, arm_ty);
	}
	}
	} else {
	let (arm_span, semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
	self.find_block_span(blk, prior_arm_ty)
	} else {
	(arm.body.span, None)
	};
	let (span, code) = match i {
	// The reason for the first arm to fail is not that the match arms diverge,
	// but rather that there's a prior obligation that doesn't hold.
	0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
	_ => (
	expr.span,
	ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
	arm_span,
	semi_span,
	source: match_src,
	prior_arms: other_arms.clone(),
	last_ty: prior_arm_ty.unwrap(),
	scrut_hir_id: scrut.hir_id,
	opt_suggest_box_span,
	}),
	),
	};
	let cause = self.cause(span, code);
	coercion.coerce(self, &cause, &arm.body, arm_ty);
	other_arms.push(arm_span);
	if other_arms.len() > 5 {
	other_arms.remove(0);
	}
	}
	prior_arm_ty = Some(arm_ty);
	}

	// If all of the arms in the `match` diverge,
	// and we're dealing with an actual `match` block
	// (as opposed to a `match` desugared from something else'),
	// we can emit a better note. Rather than pointing
	// at a diverging expression in an arbitrary arm,
	// we can point at the entire `match` expression
	if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
	all_arms_diverge = Diverges::Always {
	span: expr.span,
	custom_note: Some(
	"any code following this `match` expression is unreachable, as all arms diverge",
	),
	};
	}

	// We won't diverge unless the scrutinee or all arms diverge.
	self.diverges.set(scrut_diverges \| all_arms_diverge);

	coercion.complete(self)
	}

	/// When the previously checked expression (the scrutinee) diverges,
	/// warn the user about the match arms being unreachable.
	fn warn_arms_when_scrutinee_diverges(
	&self,
	arms: &'tcx [hir::Arm<'tcx>],
	source: hir::MatchSource,
	) {
	use hir::MatchSource::*;
	let msg = match source {
	IfDesugar { .. } \| IfLetDesugar { .. } => "block in `if` expression",
	WhileDesugar { .. } \| WhileLetDesugar { .. } => "block in `while` expression",
	_ => "arm",
	};
	for arm in arms {
	self.warn_if_unreachable(arm.body.hir_id, arm.body.span, msg);
	}
	}

	/// Handle the fallback arm of a desugared if(-let) like a missing else.
	///
	/// Returns `true` if there was an error forcing the coercion to the `()` type.
	fn if_fallback_coercion(
	&self,
	span: Span,
	then_expr: &'tcx hir::Expr<'tcx>,
	coercion: &mut CoerceMany<'tcx, '_, rustc_hir::Arm<'tcx>>,
	) -> bool {
	// If this `if` expr is the parent's function return expr,
	// the cause of the type coercion is the return type, point at it. (#25228)
	let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
	let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
	let mut error = false;
	coercion.coerce_forced_unit(
	self,
	&cause,
	&mut \|err\| {
	if let Some((span, msg)) = &ret_reason {
	err.span_label(*span, msg.as_str());
	} else if let ExprKind::Block(block, _) = &then_expr.kind {
	if let Some(expr) = &block.expr {
	err.span_label(expr.span, "found here".to_string());
	}
	}
	err.note("`if` expressions without `else` evaluate to `()`");
	err.help("consider adding an `else` block that evaluates to the expected type");
	error = true;
	},
	ret_reason.is_none(),
	);
	error
	}

	fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, span: Span) -> Option<(Span, String)> {
	use hir::Node::{Block, Item, Local};

	let hir = self.tcx.hir();
	let arm_id = hir.get_parent_node(hir_id);
	let match_id = hir.get_parent_node(arm_id);
	let containing_id = hir.get_parent_node(match_id);

	let node = hir.get(containing_id);
	if let Block(block) = node {
	// check that the body's parent is an fn
	let parent = hir.get(hir.get_parent_node(hir.get_parent_node(block.hir_id)));
	if let (Some(expr), Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
	(&block.expr, parent)
	{
	// check that the `if` expr without `else` is the fn body's expr
	if expr.span == span {
	return self.get_fn_decl(hir_id).and_then(\|(fn_decl, _)\| {
	let span = fn_decl.output.span();
	let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
	Some((span, format!("expected `{}` because of this return type", snippet)))
	});
	}
	}
	}
	if let Local(hir::Local { ty: Some(_), pat, .. }) = node {
	return Some((pat.span, "expected because of this assignment".to_string()));
	}
	None
	}

	fn if_cause(
	&self,
	span: Span,
	then_expr: &'tcx hir::Expr<'tcx>,
	else_expr: &'tcx hir::Expr<'tcx>,
	then_ty: Ty<'tcx>,
	else_ty: Ty<'tcx>,
	opt_suggest_box_span: Option<Span>,
	) -> ObligationCause<'tcx> {
	let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
	// The `if`/`else` isn't in one line in the output, include some context to make it
	// clear it is an if/else expression:
	// ```
	// LL \| let x = if true {
	// \| _____________-
	// LL \|\| 10i32
	// \|\| ----- expected because of this
	// LL \|\| } else {
	// LL \|\| 10u32
	// \|\| ^^^^^ expected `i32`, found `u32`
	// LL \|\| };
	// \|\|_____- `if` and `else` have incompatible types
	// ```
	Some(span)
	} else {
	// The entire expression is in one line, only point at the arms
	// ```
	// LL \| let x = if true { 10i32 } else { 10u32 };
	// \| ----- ^^^^^ expected `i32`, found `u32`
	// \| \|
	// \| expected because of this
	// ```
	None
	};

	let mut remove_semicolon = None;
	let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
	let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
	remove_semicolon = semi_sp;
	if block.expr.is_none() && block.stmts.is_empty() {
	// Avoid overlapping spans that aren't as readable:
	// ```
	// 2 \| let x = if true {
	// \| _____________-
	// 3 \| \| 3
	// \| \| - expected because of this
	// 4 \| \| } else {
	// \| \|____________^
	// 5 \| \|\|
	// 6 \| \|\| };
	// \| \|\| ^
	// \| \|\|_____\|
	// \| \|______if and else have incompatible types
	// \| expected integer, found `()`
	// ```
	// by not pointing at the entire expression:
	// ```
	// 2 \| let x = if true {
	// \| ------- `if` and `else` have incompatible types
	// 3 \| 3
	// \| - expected because of this
	// 4 \| } else {
	// \| ____________^
	// 5 \| \|
	// 6 \| \| };
	// \| \|_____^ expected integer, found `()`
	// ```
	if outer_sp.is_some() {
	outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
	}
	}
	error_sp
	} else {
	// shouldn't happen unless the parser has done something weird
	else_expr.span
	};

	// Compute `Span` of `then` part of `if`-expression.
	let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
	let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
	remove_semicolon = remove_semicolon.or(semi_sp);
	if block.expr.is_none() && block.stmts.is_empty() {
	outer_sp = None; // same as in `error_sp`; cleanup output
	}
	then_sp
	} else {
	// shouldn't happen unless the parser has done something weird
	then_expr.span
	};

	// Finally construct the cause:
	self.cause(
	error_sp,
	ObligationCauseCode::IfExpression(box IfExpressionCause {
	then: then_sp,
	else_sp: error_sp,
	outer: outer_sp,
	semicolon: remove_semicolon,
	opt_suggest_box_span,
	}),
	)
	}

	fn demand_scrutinee_type(
	&self,
	arms: &'tcx [hir::Arm<'tcx>],
	scrut: &'tcx hir::Expr<'tcx>,
	) -> Ty<'tcx> {
	// Not entirely obvious: if matches may create ref bindings, we want to
	// use the precise type of the scrutinee, not some supertype, as
	// the "scrutinee type" (issue #23116).
	//
	// arielb1 [writes here in this comment thread][c] that there
	// is certainly some potential danger, e.g., for an example
	// like:
	//
	// [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
	//
	// ```
	// let Foo(x) = f()[0];
	// ```
	//
	// Then if the pattern matches by reference, we want to match
	// `f()[0]` as a lexpr, so we can't allow it to be
	// coerced. But if the pattern matches by value, `f()[0]` is
	// still syntactically a lexpr, but we do want to allow
	// coercions.
	//
	// However, likely we are ok with allowing coercions to
	// happen if there are no explicit ref mut patterns - all
	// implicit ref mut patterns must occur behind a reference, so
	// they will have the "correct" variance and lifetime.
	//
	// This does mean that the following pattern would be legal:
	//
	// ```
	// struct Foo(Bar);
	// struct Bar(u32);
	// impl Deref for Foo {
	// type Target = Bar;
	// fn deref(&self) -> &Bar { &self.0 }
	// }
	// impl DerefMut for Foo {
	// fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
	// }
	// fn foo(x: &mut Foo) {
	// {
	// let Bar(z): &mut Bar = x;
	// *z = 42;
	// }
	// assert_eq!(foo.0.0, 42);
	// }
	// ```
	//
	// FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
	// is problematic as the HIR is being scraped, but ref bindings may be
	// implicit after #42640. We need to make sure that pat_adjustments
	// (once introduced) is populated by the time we get here.
	//
	// See #44848.
	let contains_ref_bindings = arms
	.iter()
	.filter_map(\|a\| a.pat.contains_explicit_ref_binding())
	.max_by_key(\|m\| match *m {
	hir::Mutability::Mut => 1,
	hir::Mutability::Not => 0,
	});

	if let Some(m) = contains_ref_bindings {
	self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
	} else if arms.is_empty() {
	self.check_expr(scrut)
	} else {
	// ...but otherwise we want to use any supertype of the
	// scrutinee. This is sort of a workaround, see note (*) in
	// `check_pat` for some details.
	let scrut_ty = self.next_ty_var(TypeVariableOrigin {
	kind: TypeVariableOriginKind::TypeInference,
	span: scrut.span,
	});
	self.check_expr_has_type_or_error(scrut, scrut_ty, \|_\| {});
	scrut_ty
	}
	}

	fn find_block_span(
	&self,
	block: &'tcx hir::Block<'tcx>,
	expected_ty: Option<Ty<'tcx>>,
	) -> (Span, Option<Span>) {
	if let Some(expr) = &block.expr {
	(expr.span, None)
	} else if let Some(stmt) = block.stmts.last() {
	// possibly incorrect trailing `;` in the else arm
	(stmt.span, expected_ty.and_then(\|ty\| self.could_remove_semicolon(block, ty)))
	} else {
	// empty block; point at its entirety
	(block.span, None)
	}
	}
	}