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

 use crate::check::{FnCtxt, Expectation, Diverges, Needs};
 use crate::check::coercion::CoerceMany;
 use rustc::hir::{self, ExprKind};
 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc::traits::{IfExpressionCause, MatchExpressionArmCause, ObligationCause};
 use rustc::traits::{ObligationCauseCode};
 use rustc::ty::Ty;
 use syntax_pos::Span;

 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn check_match(
         &self,
         expr: &'tcx hir::Expr,
         discrim: &'tcx hir::Expr,
         arms: &'tcx [hir::Arm],
         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 discrim_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(discrim, self.tcx.types.bool)
         } else {
             self.demand_discriminant_type(arms, discrim)
         };

         // 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);
             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 discrim_diverges = self.diverges.get();
         self.diverges.set(Diverges::Maybe);

         // rust-lang/rust#55810: Typecheck patterns first (via eager
         // collection into `Vec`), so we get types for all bindings.
         let all_arm_pats_diverge: Vec<_> = arms.iter().map(|arm| {
             let mut all_pats_diverge = Diverges::WarnedAlways;
             for p in &arm.pats {
                 self.diverges.set(Diverges::Maybe);
                 self.check_pat_top(&p, discrim_ty, Some(discrim.span));
                 all_pats_diverge &= self.diverges.get();
             }

             // As discussed with @eddyb, this is for disabling unreachable_code
             // warnings on patterns (they're now subsumed by unreachable_patterns
             // warnings).
             match all_pats_diverge {
                 Diverges::Maybe => Diverges::Maybe,
                 Diverges::Always | Diverges::WarnedAlways => Diverges::WarnedAlways,
             }
         }).collect();

         // 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 = 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, pats_diverge)) in arms.iter().zip(all_arm_pats_diverge).enumerate() {
             if let Some(g) = &arm.guard {
                 self.diverges.set(pats_diverge);
                 match g {
                     hir::Guard::If(e) => self.check_expr_has_type_or_error(e, tcx.types.bool),
                 };
             }

             self.diverges.set(pats_diverge);
             let arm_ty = if source_if && if_no_else && i != 0 && self.if_fallback_coercion(
                 expr.span,
                 &arms[0].body,
                 &mut coercion,
             ) {
                 tcx.types.err
             } 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();
             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);
                         coercion.coerce(self, &cause, &arm.body, arm_ty);
                     }
                 }
             } else {
                 let arm_span = if let hir::ExprKind::Block(blk, _) = &arm.body.node {
                     // Point at the block expr instead of the entire block
                     blk.expr.as_ref().map(|e| e.span).unwrap_or(arm.body.span)
                 } else {
                     arm.body.span
                 };
                 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,
                             source: match_src,
                             prior_arms: other_arms.clone(),
                             last_ty: prior_arm_ty.unwrap(),
                             discrim_hir_id: discrim.hir_id,
                           })
                          ),
                 };
                 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);
         }

         // We won't diverge unless the discriminant or all arms diverge.
         self.diverges.set(discrim_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], source: hir::MatchSource) {
         if self.diverges.get().always() {
             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,
         coercion: &mut CoerceMany<'tcx, '_, rustc::hir::Arm>,
     ) -> 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.node {
                 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 {
                 node: 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).map(|(fn_decl, _)| (
                         fn_decl.output.span(),
                         format!("expected `{}` because of this return type", fn_decl.output),
                     ));
                 }
             }
         }
         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,
         else_expr: &'tcx hir::Expr,
         then_ty: Ty<'tcx>,
         else_ty: Ty<'tcx>,
     ) -> 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.node {
             if let Some(expr) = &block.expr {
                 expr.span
             } else if let Some(stmt) = block.stmts.last() {
                 // possibly incorrect trailing `;` in the else arm
                 remove_semicolon = self.could_remove_semicolon(block, then_ty);
                 stmt.span
             } else { // empty block; point at its entirety
                 // 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().def_span(span));
                 }
                 else_expr.span
             }
         } 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.node {
             if let Some(expr) = &block.expr {
                 expr.span
             } else if let Some(stmt) = block.stmts.last() {
                 // possibly incorrect trailing `;` in the else arm
                 remove_semicolon = remove_semicolon.or(self.could_remove_semicolon(block, else_ty));
                 stmt.span
             } else { // empty block; point at its entirety
                 outer_sp = None;  // same as in `error_sp`; cleanup output
                 then_expr.span
             }
         } 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,
             outer: outer_sp,
             semicolon: remove_semicolon,
         }))
     }

     fn demand_discriminant_type(
         &self,
         arms: &'tcx [hir::Arm],
         discrim: &'tcx hir::Expr,
     ) -> Ty<'tcx> {
         // Not entirely obvious: if matches may create ref bindings, we want to
         // use the *precise* type of the discriminant, *not* some supertype, as
         // the "discriminant 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.contains_explicit_ref_binding())
                                         .max_by_key(|m| match *m {
                                             hir::MutMutable => 1,
                                             hir::MutImmutable => 0,
                                         });

         if let Some(m) = contains_ref_bindings {
             self.check_expr_with_needs(discrim, Needs::maybe_mut_place(m))
         } else {
             // ...but otherwise we want to use any supertype of the
             // discriminant. This is sort of a workaround, see note (*) in
             // `check_pat` for some details.
             let discrim_ty = self.next_ty_var(TypeVariableOrigin {
                 kind: TypeVariableOriginKind::TypeInference,
                 span: discrim.span,
             });
             self.check_expr_has_type_or_error(discrim, discrim_ty);
             discrim_ty
         }
     }
 }
	use crate::check::{FnCtxt, Expectation, Diverges, Needs};
	use crate::check::coercion::CoerceMany;
	use rustc::hir::{self, ExprKind};
	use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
	use rustc::traits::{IfExpressionCause, MatchExpressionArmCause, ObligationCause};
	use rustc::traits::{ObligationCauseCode};
	use rustc::ty::Ty;
	use syntax_pos::Span;

	impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
	pub fn check_match(
	&self,
	expr: &'tcx hir::Expr,
	discrim: &'tcx hir::Expr,
	arms: &'tcx [hir::Arm],
	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 discrim_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(discrim, self.tcx.types.bool)
	} else {
	self.demand_discriminant_type(arms, discrim)
	};

	// 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);
	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 discrim_diverges = self.diverges.get();
	self.diverges.set(Diverges::Maybe);

	// rust-lang/rust#55810: Typecheck patterns first (via eager
	// collection into `Vec`), so we get types for all bindings.
	let all_arm_pats_diverge: Vec<_> = arms.iter().map(\|arm\| {
	let mut all_pats_diverge = Diverges::WarnedAlways;
	for p in &arm.pats {
	self.diverges.set(Diverges::Maybe);
	self.check_pat_top(&p, discrim_ty, Some(discrim.span));
	all_pats_diverge &= self.diverges.get();
	}

	// As discussed with @eddyb, this is for disabling unreachable_code
	// warnings on patterns (they're now subsumed by unreachable_patterns
	// warnings).
	match all_pats_diverge {
	Diverges::Maybe => Diverges::Maybe,
	Diverges::Always \| Diverges::WarnedAlways => Diverges::WarnedAlways,
	}
	}).collect();

	// 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 = 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, pats_diverge)) in arms.iter().zip(all_arm_pats_diverge).enumerate() {
	if let Some(g) = &arm.guard {
	self.diverges.set(pats_diverge);
	match g {
	hir::Guard::If(e) => self.check_expr_has_type_or_error(e, tcx.types.bool),
	};
	}

	self.diverges.set(pats_diverge);
	let arm_ty = if source_if && if_no_else && i != 0 && self.if_fallback_coercion(
	expr.span,
	&arms[0].body,
	&mut coercion,
	) {
	tcx.types.err
	} 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();
	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);
	coercion.coerce(self, &cause, &arm.body, arm_ty);
	}
	}
	} else {
	let arm_span = if let hir::ExprKind::Block(blk, _) = &arm.body.node {
	// Point at the block expr instead of the entire block
	blk.expr.as_ref().map(\|e\| e.span).unwrap_or(arm.body.span)
	} else {
	arm.body.span
	};
	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,
	source: match_src,
	prior_arms: other_arms.clone(),
	last_ty: prior_arm_ty.unwrap(),
	discrim_hir_id: discrim.hir_id,
	})
	),
	};
	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);
	}

	// We won't diverge unless the discriminant or all arms diverge.
	self.diverges.set(discrim_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], source: hir::MatchSource) {
	if self.diverges.get().always() {
	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,
	coercion: &mut CoerceMany<'tcx, '_, rustc::hir::Arm>,
	) -> 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.node {
	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 {
	node: 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).map(\|(fn_decl, _)\| (
	fn_decl.output.span(),
	format!("expected `{}` because of this return type", fn_decl.output),
	));
	}
	}
	}
	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,
	else_expr: &'tcx hir::Expr,
	then_ty: Ty<'tcx>,
	else_ty: Ty<'tcx>,
	) -> 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.node {
	if let Some(expr) = &block.expr {
	expr.span
	} else if let Some(stmt) = block.stmts.last() {
	// possibly incorrect trailing `;` in the else arm
	remove_semicolon = self.could_remove_semicolon(block, then_ty);
	stmt.span
	} else { // empty block; point at its entirety
	// 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().def_span(span));
	}
	else_expr.span
	}
	} 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.node {
	if let Some(expr) = &block.expr {
	expr.span
	} else if let Some(stmt) = block.stmts.last() {
	// possibly incorrect trailing `;` in the else arm
	remove_semicolon = remove_semicolon.or(self.could_remove_semicolon(block, else_ty));
	stmt.span
	} else { // empty block; point at its entirety
	outer_sp = None; // same as in `error_sp`; cleanup output
	then_expr.span
	}
	} 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,
	outer: outer_sp,
	semicolon: remove_semicolon,
	}))
	}

	fn demand_discriminant_type(
	&self,
	arms: &'tcx [hir::Arm],
	discrim: &'tcx hir::Expr,
	) -> Ty<'tcx> {
	// Not entirely obvious: if matches may create ref bindings, we want to
	// use the precise type of the discriminant, not some supertype, as
	// the "discriminant 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.contains_explicit_ref_binding())
	.max_by_key(\|m\| match *m {
	hir::MutMutable => 1,
	hir::MutImmutable => 0,
	});

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