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

 use crate::coercion::{AsCoercionSite, CoerceMany};
 use crate::{Diverges, Expectation, FnCtxt, Needs};
 use rustc_errors::{Applicability, Diag};
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::{self as hir, ExprKind, PatKind};
 use rustc_hir_pretty::ty_to_string;
 use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
 use rustc_trait_selection::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
 };

 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(skip(self), level = "debug", ret)]
     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;

         let acrb = arms_contain_ref_bindings(arms);
         let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
         debug!(?scrutinee_ty);

         // 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);

         // 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.
         let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
         for arm in arms {
             self.check_pat_top(arm.pat, scrutinee_ty, Some(scrut_span), Some(scrut), None);
         }

         // 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);
         debug!(?expected);

         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 != tcx.types.unit => ety,
                 _ => self.next_ty_var(expr.span),
             };
             CoerceMany::with_coercion_sites(coerce_first, arms)
         };

         let mut prior_non_diverging_arms = vec![]; // Used only for diagnostics.
         let mut prior_arm = None;
         for arm in arms {
             if let Some(e) = &arm.guard {
                 self.diverges.set(Diverges::Maybe);
                 self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
             }

             self.diverges.set(Diverges::Maybe);

             let arm_ty = self.check_expr_with_expectation(arm.body, expected);
             all_arms_diverge &= self.diverges.get();
             let tail_defines_return_position_impl_trait =
                 self.return_position_impl_trait_from_match_expectation(orig_expected);

             let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
                 (Some(blk.hir_id), self.find_block_span(blk))
             } else {
                 (None, arm.body.span)
             };

             let (span, code) = match prior_arm {
                 // 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.
                 None => {
                     (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id, match_src))
                 }
                 Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
                     expr.span,
                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
                         arm_block_id,
                         arm_span,
                         arm_ty,
                         prior_arm_block_id,
                         prior_arm_ty,
                         prior_arm_span,
                         scrut_span: scrut.span,
                         source: match_src,
                         prior_non_diverging_arms: prior_non_diverging_arms.clone(),
                         tail_defines_return_position_impl_trait,
                     })),
                 ),
             };
             let cause = self.cause(span, code);

             // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
             // We use it this way to be able to expand on the potential error and detect when a
             // `match` tail statement could be a tail expression instead. If so, we suggest
             // removing the stray semicolon.
             coercion.coerce_inner(
                 self,
                 &cause,
                 Some(arm.body),
                 arm_ty,
                 |err| {
                     self.explain_never_type_coerced_to_unit(err, arm, arm_ty, prior_arm, expr);
                 },
                 false,
             );

             if !arm_ty.is_never() {
                 // When a match arm has type `!`, then it doesn't influence the expected type for
                 // the following arm. If all of the prior arms are `!`, then the influence comes
                 // from elsewhere and we shouldn't point to any previous arm.
                 prior_arm = Some((arm_block_id, arm_ty, arm_span));

                 prior_non_diverging_arms.push(arm_span);
                 if prior_non_diverging_arms.len() > 5 {
                     prior_non_diverging_arms.remove(0);
                 }
             }
         }

         // 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)
     }

     fn explain_never_type_coerced_to_unit(
         &self,
         err: &mut Diag<'_>,
         arm: &hir::Arm<'tcx>,
         arm_ty: Ty<'tcx>,
         prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
         expr: &hir::Expr<'tcx>,
     ) {
         if let hir::ExprKind::Block(block, _) = arm.body.kind
             && let Some(expr) = block.expr
             && let arm_tail_ty = self.node_ty(expr.hir_id)
             && arm_tail_ty.is_never()
             && !arm_ty.is_never()
         {
             err.span_label(
                 expr.span,
                 format!(
                     "this expression is of type `!`, but it is coerced to `{arm_ty}` due to its \
                      surrounding expression",
                 ),
             );
             self.suggest_mismatched_types_on_tail(
                 err,
                 expr,
                 arm_ty,
                 prior_arm.map_or(arm_tail_ty, |(_, ty, _)| ty),
                 expr.hir_id,
             );
         }
         self.suggest_removing_semicolon_for_coerce(err, expr, arm_ty, prior_arm)
     }

     fn suggest_removing_semicolon_for_coerce(
         &self,
         diag: &mut Diag<'_>,
         expr: &hir::Expr<'tcx>,
         arm_ty: Ty<'tcx>,
         prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
     ) {
         let hir = self.tcx.hir();

         // First, check that we're actually in the tail of a function.
         let Some(body_id) = hir.maybe_body_owned_by(self.body_id) else {
             return;
         };
         let body = hir.body(body_id);
         let hir::ExprKind::Block(block, _) = body.value.kind else {
             return;
         };
         let Some(hir::Stmt { kind: hir::StmtKind::Semi(last_expr), span: semi_span, .. }) =
             block.innermost_block().stmts.last()
         else {
             return;
         };
         if last_expr.hir_id != expr.hir_id {
             return;
         }

         // Next, make sure that we have no type expectation.
         let Some(ret) =
             self.tcx.hir_node_by_def_id(self.body_id).fn_decl().map(|decl| decl.output.span())
         else {
             return;
         };

         let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
             Some(ret_coercion) => {
                 let ret_ty = ret_coercion.borrow().expected_ty();
                 let ret_ty = self.infcx.shallow_resolve(ret_ty);
                 self.can_coerce(arm_ty, ret_ty)
                     && prior_arm.map_or(true, |(_, ty, _)| self.can_coerce(ty, ret_ty))
                     // The match arms need to unify for the case of `impl Trait`.
                     && !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
             }
             _ => false,
         };
         if !can_coerce_to_return_ty {
             return;
         }

         let semi = expr.span.shrink_to_hi().with_hi(semi_span.hi());
         let sugg = crate::errors::RemoveSemiForCoerce { expr: expr.span, ret, semi };
         diag.subdiagnostic(self.dcx(), sugg);
     }

     /// 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>]) {
         for arm in arms {
             self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
         }
     }

     /// 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.
     pub(super) fn if_fallback_coercion<T>(
         &self,
         if_span: Span,
         cond_expr: &'tcx hir::Expr<'tcx>,
         then_expr: &'tcx hir::Expr<'tcx>,
         coercion: &mut CoerceMany<'tcx, '_, T>,
     ) -> bool
     where
         T: AsCoercionSite,
     {
         // 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 hir_id = self.tcx.parent_hir_id(self.tcx.parent_hir_id(then_expr.hir_id));
         let ret_reason = self.maybe_get_coercion_reason(hir_id, if_span);
         let cause = self.cause(if_span, ObligationCauseCode::IfExpressionWithNoElse);
         let mut error = false;
         coercion.coerce_forced_unit(
             self,
             &cause,
             |err| self.explain_if_expr(err, ret_reason, if_span, cond_expr, then_expr, &mut error),
             false,
         );
         error
     }

     /// Explain why `if` expressions without `else` evaluate to `()` and detect likely irrefutable
     /// `if let PAT = EXPR {}` expressions that could be turned into `let PAT = EXPR;`.
     fn explain_if_expr(
         &self,
         err: &mut Diag<'_>,
         ret_reason: Option<(Span, String)>,
         if_span: Span,
         cond_expr: &'tcx hir::Expr<'tcx>,
         then_expr: &'tcx hir::Expr<'tcx>,
         error: &mut bool,
     ) {
         if let Some((if_span, msg)) = ret_reason {
             err.span_label(if_span, msg);
         } else if let ExprKind::Block(block, _) = then_expr.kind
             && let Some(expr) = block.expr
         {
             err.span_label(expr.span, "found here");
         }
         err.note("`if` expressions without `else` evaluate to `()`");
         err.help("consider adding an `else` block that evaluates to the expected type");
         *error = true;
         if let ExprKind::Let(hir::LetExpr { span, pat, init, .. }) = cond_expr.kind
             && let ExprKind::Block(block, _) = then_expr.kind
             // Refutability checks occur on the MIR, so we approximate it here by checking
             // if we have an enum with a single variant or a struct in the pattern.
             && let PatKind::TupleStruct(qpath, ..) | PatKind::Struct(qpath, ..) = pat.kind
             && let hir::QPath::Resolved(_, path) = qpath
         {
             match path.res {
                 Res::Def(DefKind::Ctor(CtorOf::Struct, _), _) => {
                     // Structs are always irrefutable. Their fields might not be, but we
                     // don't check for that here, it's only an approximation.
                 }
                 Res::Def(DefKind::Ctor(CtorOf::Variant, _), def_id)
                     if self
                         .tcx
                         .adt_def(self.tcx.parent(self.tcx.parent(def_id)))
                         .variants()
                         .len()
                         == 1 =>
                 {
                     // There's only a single variant in the `enum`, so we can suggest the
                     // irrefutable `let` instead of `if let`.
                 }
                 _ => return,
             }

             let mut sugg = vec![
                 // Remove the `if`
                 (if_span.until(*span), String::new()),
             ];
             match (block.stmts, block.expr) {
                 ([first, ..], Some(expr)) => {
                     let padding = self
                         .tcx
                         .sess
                         .source_map()
                         .indentation_before(first.span)
                         .unwrap_or_else(|| String::new());
                     sugg.extend([
                         (init.span.between(first.span), format!(";\n{padding}")),
                         (expr.span.shrink_to_hi().with_hi(block.span.hi()), String::new()),
                     ]);
                 }
                 ([], Some(expr)) => {
                     let padding = self
                         .tcx
                         .sess
                         .source_map()
                         .indentation_before(expr.span)
                         .unwrap_or_else(|| String::new());
                     sugg.extend([
                         (init.span.between(expr.span), format!(";\n{padding}")),
                         (expr.span.shrink_to_hi().with_hi(block.span.hi()), String::new()),
                     ]);
                 }
                 // If there's no value in the body, then the `if` expression would already
                 // be of type `()`, so checking for those cases is unnecessary.
                 (_, None) => return,
             }
             err.multipart_suggestion(
                 "consider using an irrefutable `let` binding instead",
                 sugg,
                 Applicability::MaybeIncorrect,
             );
         }
     }

     pub fn maybe_get_coercion_reason(
         &self,
         hir_id: hir::HirId,
         sp: Span,
     ) -> Option<(Span, String)> {
         let node = self.tcx.hir_node(hir_id);
         if let hir::Node::Block(block) = node {
             // check that the body's parent is an fn
             let parent = self.tcx.parent_hir_node(self.tcx.parent_hir_id(block.hir_id));
             if let (Some(expr), hir::Node::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 == sp {
                     return self.get_fn_decl(hir_id).map(|(_, fn_decl, _)| {
                         let (ty, span) = match fn_decl.output {
                             hir::FnRetTy::DefaultReturn(span) => ("()".to_string(), span),
                             hir::FnRetTy::Return(ty) => (ty_to_string(&self.tcx, ty), ty.span),
                         };
                         (span, format!("expected `{ty}` because of this return type"))
                     });
                 }
             }
         }
         if let hir::Node::LetStmt(hir::LetStmt { ty: Some(_), pat, .. }) = node {
             return Some((pat.span, "expected because of this assignment".to_string()));
         }
         None
     }

     pub(crate) fn if_cause(
         &self,
         span: Span,
         cond_span: Span,
         then_expr: &'tcx hir::Expr<'tcx>,
         else_expr: &'tcx hir::Expr<'tcx>,
         then_ty: Ty<'tcx>,
         else_ty: Ty<'tcx>,
         tail_defines_return_position_impl_trait: Option<LocalDefId>,
     ) -> ObligationCause<'tcx> {
         let mut outer_span = 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 (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
             let block = block.innermost_block();

             // 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 block.expr.is_none()
                 && block.stmts.is_empty()
                 && let Some(outer_span) = &mut outer_span
                 && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
             {
                 *outer_span = outer_span.with_hi(cond_span.hi())
             }

             (self.find_block_span(block), block.hir_id)
         } else {
             (else_expr.span, else_expr.hir_id)
         };

         let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
             let block = block.innermost_block();
             // Exclude overlapping spans
             if block.expr.is_none() && block.stmts.is_empty() {
                 outer_span = None;
             }
             block.hir_id
         } else {
             then_expr.hir_id
         };

         // Finally construct the cause:
         self.cause(
             error_sp,
             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
                 else_id,
                 then_id,
                 then_ty,
                 else_ty,
                 outer_span,
                 tail_defines_return_position_impl_trait,
             })),
         )
     }

     pub(super) fn demand_scrutinee_type(
         &self,
         scrut: &'tcx hir::Expr<'tcx>,
         contains_ref_bindings: Option<hir::Mutability>,
         no_arms: bool,
     ) -> 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.
         if let Some(m) = contains_ref_bindings {
             self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
         } else if no_arms {
             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(scrut.span);
             self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
             scrut_ty
         }
     }

     // Does the expectation of the match define an RPIT?
     // (e.g. we're in the tail of a function body)
     //
     // Returns the `LocalDefId` of the RPIT, which is always identity-substituted.
     pub fn return_position_impl_trait_from_match_expectation(
         &self,
         expectation: Expectation<'tcx>,
     ) -> Option<LocalDefId> {
         let expected_ty = expectation.to_option(self)?;
         let (def_id, args) = match *expected_ty.kind() {
             // FIXME: Could also check that the RPIT is not defined
             ty::Alias(ty::Opaque, alias_ty) => (alias_ty.def_id.as_local()?, alias_ty.args),
             // FIXME(-Znext-solver): Remove this branch once `replace_opaque_types_with_infer` is gone.
             ty::Infer(ty::TyVar(_)) => self
                 .inner
                 .borrow()
                 .iter_opaque_types()
                 .find(|(_, v)| v.ty == expected_ty)
                 .map(|(k, _)| (k.def_id, k.args))?,
             _ => return None,
         };
         let hir::OpaqueTyOrigin::FnReturn(parent_def_id) = self.tcx.opaque_type_origin(def_id)
         else {
             return None;
         };
         if &args[0..self.tcx.generics_of(parent_def_id).count()]
             != ty::GenericArgs::identity_for_item(self.tcx, parent_def_id).as_slice()
         {
             return None;
         }
         Some(def_id)
     }
 }

 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
     arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max()
 }
	use crate::coercion::{AsCoercionSite, CoerceMany};
	use crate::{Diverges, Expectation, FnCtxt, Needs};
	use rustc_errors::{Applicability, Diag};
	use rustc_hir::def::{CtorOf, DefKind, Res};
	use rustc_hir::def_id::LocalDefId;
	use rustc_hir::{self as hir, ExprKind, PatKind};
	use rustc_hir_pretty::ty_to_string;
	use rustc_middle::ty::{self, Ty};
	use rustc_span::Span;
	use rustc_trait_selection::traits::{
	IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
	};

	impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
	#[instrument(skip(self), level = "debug", ret)]
	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;

	let acrb = arms_contain_ref_bindings(arms);
	let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
	debug!(?scrutinee_ty);

	// 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);

	// 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.
	let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
	for arm in arms {
	self.check_pat_top(arm.pat, scrutinee_ty, Some(scrut_span), Some(scrut), None);
	}

	// 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);
	debug!(?expected);

	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 != tcx.types.unit => ety,
	_ => self.next_ty_var(expr.span),
	};
	CoerceMany::with_coercion_sites(coerce_first, arms)
	};

	let mut prior_non_diverging_arms = vec![]; // Used only for diagnostics.
	let mut prior_arm = None;
	for arm in arms {
	if let Some(e) = &arm.guard {
	self.diverges.set(Diverges::Maybe);
	self.check_expr_has_type_or_error(e, tcx.types.bool, \|_\| {});
	}

	self.diverges.set(Diverges::Maybe);

	let arm_ty = self.check_expr_with_expectation(arm.body, expected);
	all_arms_diverge &= self.diverges.get();
	let tail_defines_return_position_impl_trait =
	self.return_position_impl_trait_from_match_expectation(orig_expected);

	let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
	(Some(blk.hir_id), self.find_block_span(blk))
	} else {
	(None, arm.body.span)
	};

	let (span, code) = match prior_arm {
	// 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.
	None => {
	(arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id, match_src))
	}
	Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
	expr.span,
	ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
	arm_block_id,
	arm_span,
	arm_ty,
	prior_arm_block_id,
	prior_arm_ty,
	prior_arm_span,
	scrut_span: scrut.span,
	source: match_src,
	prior_non_diverging_arms: prior_non_diverging_arms.clone(),
	tail_defines_return_position_impl_trait,
	})),
	),
	};
	let cause = self.cause(span, code);

	// This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
	// We use it this way to be able to expand on the potential error and detect when a
	// `match` tail statement could be a tail expression instead. If so, we suggest
	// removing the stray semicolon.
	coercion.coerce_inner(
	self,
	&cause,
	Some(arm.body),
	arm_ty,
	\|err\| {
	self.explain_never_type_coerced_to_unit(err, arm, arm_ty, prior_arm, expr);
	},
	false,
	);

	if !arm_ty.is_never() {
	// When a match arm has type `!`, then it doesn't influence the expected type for
	// the following arm. If all of the prior arms are `!`, then the influence comes
	// from elsewhere and we shouldn't point to any previous arm.
	prior_arm = Some((arm_block_id, arm_ty, arm_span));

	prior_non_diverging_arms.push(arm_span);
	if prior_non_diverging_arms.len() > 5 {
	prior_non_diverging_arms.remove(0);
	}
	}
	}

	// 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)
	}

	fn explain_never_type_coerced_to_unit(
	&self,
	err: &mut Diag<'_>,
	arm: &hir::Arm<'tcx>,
	arm_ty: Ty<'tcx>,
	prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
	expr: &hir::Expr<'tcx>,
	) {
	if let hir::ExprKind::Block(block, _) = arm.body.kind
	&& let Some(expr) = block.expr
	&& let arm_tail_ty = self.node_ty(expr.hir_id)
	&& arm_tail_ty.is_never()
	&& !arm_ty.is_never()
	{
	err.span_label(
	expr.span,
	format!(
	"this expression is of type `!`, but it is coerced to `{arm_ty}` due to its \
	surrounding expression",
	),
	);
	self.suggest_mismatched_types_on_tail(
	err,
	expr,
	arm_ty,
	prior_arm.map_or(arm_tail_ty, \|(_, ty, _)\| ty),
	expr.hir_id,
	);
	}
	self.suggest_removing_semicolon_for_coerce(err, expr, arm_ty, prior_arm)
	}

	fn suggest_removing_semicolon_for_coerce(
	&self,
	diag: &mut Diag<'_>,
	expr: &hir::Expr<'tcx>,
	arm_ty: Ty<'tcx>,
	prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
	) {
	let hir = self.tcx.hir();

	// First, check that we're actually in the tail of a function.
	let Some(body_id) = hir.maybe_body_owned_by(self.body_id) else {
	return;
	};
	let body = hir.body(body_id);
	let hir::ExprKind::Block(block, _) = body.value.kind else {
	return;
	};
	let Some(hir::Stmt { kind: hir::StmtKind::Semi(last_expr), span: semi_span, .. }) =
	block.innermost_block().stmts.last()
	else {
	return;
	};
	if last_expr.hir_id != expr.hir_id {
	return;
	}

	// Next, make sure that we have no type expectation.
	let Some(ret) =
	self.tcx.hir_node_by_def_id(self.body_id).fn_decl().map(\|decl\| decl.output.span())
	else {
	return;
	};

	let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
	Some(ret_coercion) => {
	let ret_ty = ret_coercion.borrow().expected_ty();
	let ret_ty = self.infcx.shallow_resolve(ret_ty);
	self.can_coerce(arm_ty, ret_ty)
	&& prior_arm.map_or(true, \|(_, ty, _)\| self.can_coerce(ty, ret_ty))
	// The match arms need to unify for the case of `impl Trait`.
	&& !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
	}
	_ => false,
	};
	if !can_coerce_to_return_ty {
	return;
	}

	let semi = expr.span.shrink_to_hi().with_hi(semi_span.hi());
	let sugg = crate::errors::RemoveSemiForCoerce { expr: expr.span, ret, semi };
	diag.subdiagnostic(self.dcx(), sugg);
	}

	/// 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>]) {
	for arm in arms {
	self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
	}
	}

	/// 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.
	pub(super) fn if_fallback_coercion<T>(
	&self,
	if_span: Span,
	cond_expr: &'tcx hir::Expr<'tcx>,
	then_expr: &'tcx hir::Expr<'tcx>,
	coercion: &mut CoerceMany<'tcx, '_, T>,
	) -> bool
	where
	T: AsCoercionSite,
	{
	// 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 hir_id = self.tcx.parent_hir_id(self.tcx.parent_hir_id(then_expr.hir_id));
	let ret_reason = self.maybe_get_coercion_reason(hir_id, if_span);
	let cause = self.cause(if_span, ObligationCauseCode::IfExpressionWithNoElse);
	let mut error = false;
	coercion.coerce_forced_unit(
	self,
	&cause,
	\|err\| self.explain_if_expr(err, ret_reason, if_span, cond_expr, then_expr, &mut error),
	false,
	);
	error
	}

	/// Explain why `if` expressions without `else` evaluate to `()` and detect likely irrefutable
	/// `if let PAT = EXPR {}` expressions that could be turned into `let PAT = EXPR;`.
	fn explain_if_expr(
	&self,
	err: &mut Diag<'_>,
	ret_reason: Option<(Span, String)>,
	if_span: Span,
	cond_expr: &'tcx hir::Expr<'tcx>,
	then_expr: &'tcx hir::Expr<'tcx>,
	error: &mut bool,
	) {
	if let Some((if_span, msg)) = ret_reason {
	err.span_label(if_span, msg);
	} else if let ExprKind::Block(block, _) = then_expr.kind
	&& let Some(expr) = block.expr
	{
	err.span_label(expr.span, "found here");
	}
	err.note("`if` expressions without `else` evaluate to `()`");
	err.help("consider adding an `else` block that evaluates to the expected type");
	*error = true;
	if let ExprKind::Let(hir::LetExpr { span, pat, init, .. }) = cond_expr.kind
	&& let ExprKind::Block(block, _) = then_expr.kind
	// Refutability checks occur on the MIR, so we approximate it here by checking
	// if we have an enum with a single variant or a struct in the pattern.
	&& let PatKind::TupleStruct(qpath, ..) \| PatKind::Struct(qpath, ..) = pat.kind
	&& let hir::QPath::Resolved(_, path) = qpath
	{
	match path.res {
	Res::Def(DefKind::Ctor(CtorOf::Struct, _), _) => {
	// Structs are always irrefutable. Their fields might not be, but we
	// don't check for that here, it's only an approximation.
	}
	Res::Def(DefKind::Ctor(CtorOf::Variant, _), def_id)
	if self
	.tcx
	.adt_def(self.tcx.parent(self.tcx.parent(def_id)))
	.variants()
	.len()
	== 1 =>
	{
	// There's only a single variant in the `enum`, so we can suggest the
	// irrefutable `let` instead of `if let`.
	}
	_ => return,
	}

	let mut sugg = vec![
	// Remove the `if`
	(if_span.until(*span), String::new()),
	];
	match (block.stmts, block.expr) {
	([first, ..], Some(expr)) => {
	let padding = self
	.tcx
	.sess
	.source_map()
	.indentation_before(first.span)
	.unwrap_or_else(\|\| String::new());
	sugg.extend([
	(init.span.between(first.span), format!(";\n{padding}")),
	(expr.span.shrink_to_hi().with_hi(block.span.hi()), String::new()),
	]);
	}
	([], Some(expr)) => {
	let padding = self
	.tcx
	.sess
	.source_map()
	.indentation_before(expr.span)
	.unwrap_or_else(\|\| String::new());
	sugg.extend([
	(init.span.between(expr.span), format!(";\n{padding}")),
	(expr.span.shrink_to_hi().with_hi(block.span.hi()), String::new()),
	]);
	}
	// If there's no value in the body, then the `if` expression would already
	// be of type `()`, so checking for those cases is unnecessary.
	(_, None) => return,
	}
	err.multipart_suggestion(
	"consider using an irrefutable `let` binding instead",
	sugg,
	Applicability::MaybeIncorrect,
	);
	}
	}

	pub fn maybe_get_coercion_reason(
	&self,
	hir_id: hir::HirId,
	sp: Span,
	) -> Option<(Span, String)> {
	let node = self.tcx.hir_node(hir_id);
	if let hir::Node::Block(block) = node {
	// check that the body's parent is an fn
	let parent = self.tcx.parent_hir_node(self.tcx.parent_hir_id(block.hir_id));
	if let (Some(expr), hir::Node::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 == sp {
	return self.get_fn_decl(hir_id).map(\|(_, fn_decl, _)\| {
	let (ty, span) = match fn_decl.output {
	hir::FnRetTy::DefaultReturn(span) => ("()".to_string(), span),
	hir::FnRetTy::Return(ty) => (ty_to_string(&self.tcx, ty), ty.span),
	};
	(span, format!("expected `{ty}` because of this return type"))
	});
	}
	}
	}
	if let hir::Node::LetStmt(hir::LetStmt { ty: Some(_), pat, .. }) = node {
	return Some((pat.span, "expected because of this assignment".to_string()));
	}
	None
	}

	pub(crate) fn if_cause(
	&self,
	span: Span,
	cond_span: Span,
	then_expr: &'tcx hir::Expr<'tcx>,
	else_expr: &'tcx hir::Expr<'tcx>,
	then_ty: Ty<'tcx>,
	else_ty: Ty<'tcx>,
	tail_defines_return_position_impl_trait: Option<LocalDefId>,
	) -> ObligationCause<'tcx> {
	let mut outer_span = 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 (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
	let block = block.innermost_block();

	// 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 block.expr.is_none()
	&& block.stmts.is_empty()
	&& let Some(outer_span) = &mut outer_span
	&& let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
	{
	*outer_span = outer_span.with_hi(cond_span.hi())
	}

	(self.find_block_span(block), block.hir_id)
	} else {
	(else_expr.span, else_expr.hir_id)
	};

	let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
	let block = block.innermost_block();
	// Exclude overlapping spans
	if block.expr.is_none() && block.stmts.is_empty() {
	outer_span = None;
	}
	block.hir_id
	} else {
	then_expr.hir_id
	};

	// Finally construct the cause:
	self.cause(
	error_sp,
	ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
	else_id,
	then_id,
	then_ty,
	else_ty,
	outer_span,
	tail_defines_return_position_impl_trait,
	})),
	)
	}

	pub(super) fn demand_scrutinee_type(
	&self,
	scrut: &'tcx hir::Expr<'tcx>,
	contains_ref_bindings: Option<hir::Mutability>,
	no_arms: bool,
	) -> 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.
	if let Some(m) = contains_ref_bindings {
	self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
	} else if no_arms {
	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(scrut.span);
	self.check_expr_has_type_or_error(scrut, scrut_ty, \|_\| {});
	scrut_ty
	}
	}

	// Does the expectation of the match define an RPIT?
	// (e.g. we're in the tail of a function body)
	//
	// Returns the `LocalDefId` of the RPIT, which is always identity-substituted.
	pub fn return_position_impl_trait_from_match_expectation(
	&self,
	expectation: Expectation<'tcx>,
	) -> Option<LocalDefId> {
	let expected_ty = expectation.to_option(self)?;
	let (def_id, args) = match *expected_ty.kind() {
	// FIXME: Could also check that the RPIT is not defined
	ty::Alias(ty::Opaque, alias_ty) => (alias_ty.def_id.as_local()?, alias_ty.args),
	// FIXME(-Znext-solver): Remove this branch once `replace_opaque_types_with_infer` is gone.
	ty::Infer(ty::TyVar(_)) => self
	.inner
	.borrow()
	.iter_opaque_types()
	.find(\|(_, v)\| v.ty == expected_ty)
	.map(\|(k, _)\| (k.def_id, k.args))?,
	_ => return None,
	};
	let hir::OpaqueTyOrigin::FnReturn(parent_def_id) = self.tcx.opaque_type_origin(def_id)
	else {
	return None;
	};
	if &args[0..self.tcx.generics_of(parent_def_id).count()]
	!= ty::GenericArgs::identity_for_item(self.tcx, parent_def_id).as_slice()
	{
	return None;
	}
	Some(def_id)
	}
	}

	fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
	arms.iter().filter_map(\|a\| a.pat.contains_explicit_ref_binding()).max()
	}