[rust.git] / src / librustc_typeck / check / _match.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use rustc::hir::{self, PatKind};
use rustc::hir::def::{Def, CtorKind};
use rustc::hir::pat_util::EnumerateAndAdjustIterator;
use rustc::infer;
use rustc::infer::type_variable::TypeVariableOrigin;
use rustc::traits::ObligationCauseCode;
use rustc::ty::{self, Ty, TypeFoldable};
use check::{FnCtxt, Expectation, Diverges, Needs};
use check::coercion::CoerceMany;
use util::nodemap::FxHashMap;

use std::collections::hash_map::Entry::{Occupied, Vacant};
use std::cmp;
use syntax::ast;
use syntax::codemap::Spanned;
use syntax::ptr::P;
use syntax_pos::Span;

impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
    /// The `is_arg` argument indicates whether this pattern is the
    /// *outermost* pattern in an argument (e.g., in `fn foo(&x:
    /// &u32)`, it is true for the `&x` pattern but not `x`). This is
    /// used to tailor error reporting.
    pub fn check_pat_walk(
        &self,
        pat: &'gcx hir::Pat,
        mut expected: Ty<'tcx>,
        mut def_bm: ty::BindingMode,
        is_arg: bool)
    {
        let tcx = self.tcx;

        debug!("check_pat_walk(pat={:?},expected={:?},def_bm={:?},is_arg={})",
            pat, expected, def_bm, is_arg);

        let is_non_ref_pat = match pat.node {
            PatKind::Struct(..) |
            PatKind::TupleStruct(..) |
            PatKind::Tuple(..) |
            PatKind::Box(_) |
            PatKind::Range(..) |
            PatKind::Slice(..) => true,
            PatKind::Lit(ref lt) => {
                let ty = self.check_expr(lt);
                match ty.sty {
                    ty::TypeVariants::TyRef(..) => false,
                    _ => true,
                }
            }
            PatKind::Path(ref qpath) => {
                let (def, _, _) = self.resolve_ty_and_def_ufcs(qpath, pat.id, pat.span);
                match def {
                    Def::Const(..) | Def::AssociatedConst(..) => false,
                    _ => true,
                }
            }
            PatKind::Wild |
            PatKind::Binding(..) |
            PatKind::Ref(..) => false,
        };
        if is_non_ref_pat {
            debug!("pattern is non reference pattern");
            let mut exp_ty = self.resolve_type_vars_with_obligations(&expected);

            // Peel off as many `&` or `&mut` from the discriminant as possible. For example,
            // for `match &&&mut Some(5)` the loop runs three times, aborting when it reaches
            // the `Some(5)` which is not of type TyRef.
            //
            // For each ampersand peeled off, update the binding mode and push the original
            // type into the adjustments vector.
            //
            // See the examples in `run-pass/match-defbm*.rs`.
            let mut pat_adjustments = vec![];
            expected = loop {
                debug!("inspecting {:?} with type {:?}", exp_ty, exp_ty.sty);
                match exp_ty.sty {
                    ty::TypeVariants::TyRef(_, ty::TypeAndMut{
                        ty: inner_ty, mutbl: inner_mutability,
                    }) => {
                        debug!("current discriminant is TyRef, inserting implicit deref");
                        // Preserve the reference type. We'll need it later during HAIR lowering.
                        pat_adjustments.push(exp_ty);

                        exp_ty = inner_ty;
                        def_bm = match def_bm {
                            // If default binding mode is by value, make it `ref` or `ref mut`
                            // (depending on whether we observe `&` or `&mut`).
                            ty::BindByValue(_) =>
                                ty::BindByReference(inner_mutability),

                            // Once a `ref`, always a `ref`. This is because a `& &mut` can't mutate
                            // the underlying value.
                            ty::BindByReference(hir::Mutability::MutImmutable) =>
                                ty::BindByReference(hir::Mutability::MutImmutable),

                            // When `ref mut`, stay a `ref mut` (on `&mut`) or downgrade to `ref`
                            // (on `&`).
                            ty::BindByReference(hir::Mutability::MutMutable) =>
                                ty::BindByReference(inner_mutability),
                        };
                    },
                    _ => break exp_ty,
                }
            };
            if pat_adjustments.len() > 0 {
                debug!("default binding mode is now {:?}", def_bm);
                self.inh.tables.borrow_mut()
                    .pat_adjustments_mut()
                    .insert(pat.hir_id, pat_adjustments);
            }
        } else if let PatKind::Ref(..) = pat.node {
            // When you encounter a `&pat` pattern, reset to "by
            // value". This is so that `x` and `y` here are by value,
            // as they appear to be:
            //
            // ```
            // match &(&22, &44) {
            //   (&x, &y) => ...
            // }
            // ```
            //
            // cc #46688
            def_bm = ty::BindByValue(hir::MutImmutable);
        }

        // Lose mutability now that we know binding mode and discriminant type.
        let def_bm = def_bm;
        let expected = expected;

        let ty = match pat.node {
            PatKind::Wild => {
                expected
            }
            PatKind::Lit(ref lt) => {
                // We've already computed the type above (when checking for a non-ref pat), so
                // avoid computing it again.
                let ty = self.node_ty(lt.hir_id);

                // Byte string patterns behave the same way as array patterns
                // They can denote both statically and dynamically sized byte arrays
                let mut pat_ty = ty;
                if let hir::ExprLit(ref lt) = lt.node {
                    if let ast::LitKind::ByteStr(_) = lt.node {
                        let expected_ty = self.structurally_resolved_type(pat.span, expected);
                        if let ty::TyRef(_, mt) = expected_ty.sty {
                            if let ty::TySlice(_) = mt.ty.sty {
                                pat_ty = tcx.mk_imm_ref(tcx.types.re_static,
                                                         tcx.mk_slice(tcx.types.u8))
                            }
                        }
                    }
                }

                // somewhat surprising: in this case, the subtyping
                // relation goes the opposite way as the other
                // cases. Actually what we really want is not a subtyping
                // relation at all but rather that there exists a LUB (so
                // that they can be compared). However, in practice,
                // constants are always scalars or strings.  For scalars
                // subtyping is irrelevant, and for strings `ty` is
                // type is `&'static str`, so if we say that
                //
                //     &'static str <: expected
                //
                // that's equivalent to there existing a LUB.
                self.demand_suptype(pat.span, expected, pat_ty);
                pat_ty
            }
            PatKind::Range(ref begin, ref end, _) => {
                let lhs_ty = self.check_expr(begin);
                let rhs_ty = self.check_expr(end);

                // Check that both end-points are of numeric or char type.
                let numeric_or_char = |ty: Ty| ty.is_numeric() || ty.is_char();
                let lhs_compat = numeric_or_char(lhs_ty);
                let rhs_compat = numeric_or_char(rhs_ty);

                if !lhs_compat || !rhs_compat {
                    let span = if !lhs_compat && !rhs_compat {
                        pat.span
                    } else if !lhs_compat {
                        begin.span
                    } else {
                        end.span
                    };

                    let mut err = struct_span_err!(
                        tcx.sess,
                        span,
                        E0029,
                        "only char and numeric types are allowed in range patterns"
                    );
                    err.span_label(span, "ranges require char or numeric types");
                    err.note(&format!("start type: {}", self.ty_to_string(lhs_ty)));
                    err.note(&format!("end type: {}", self.ty_to_string(rhs_ty)));
                    if tcx.sess.teach(&err.get_code().unwrap()) {
                        err.note(
                            "In a match expression, only numbers and characters can be matched \
                             against a range. This is because the compiler checks that the range \
                             is non-empty at compile-time, and is unable to evaluate arbitrary \
                             comparison functions. If you want to capture values of an orderable \
                             type between two end-points, you can use a guard."
                         );
                    }
                    err.emit();
                    return;
                }

                // Now that we know the types can be unified we find the unified type and use
                // it to type the entire expression.
                let common_type = self.resolve_type_vars_if_possible(&lhs_ty);

                // subtyping doesn't matter here, as the value is some kind of scalar
                self.demand_eqtype(pat.span, expected, lhs_ty);
                self.demand_eqtype(pat.span, expected, rhs_ty);
                common_type
            }
            PatKind::Binding(ba, var_id, _, ref sub) => {
                let bm = if ba == hir::BindingAnnotation::Unannotated {
                    def_bm
                } else {
                    ty::BindingMode::convert(ba)
                };
                self.inh
                    .tables
                    .borrow_mut()
                    .pat_binding_modes_mut()
                    .insert(pat.hir_id, bm);
                let typ = self.local_ty(pat.span, pat.id);
                match bm {
                    ty::BindByReference(mutbl) => {
                        // if the binding is like
                        //    ref x | ref const x | ref mut x
                        // then `x` is assigned a value of type `&M T` where M is the mutability
                        // and T is the expected type.
                        let region_var = self.next_region_var(infer::PatternRegion(pat.span));
                        let mt = ty::TypeAndMut { ty: expected, mutbl: mutbl };
                        let region_ty = tcx.mk_ref(region_var, mt);

                        // `x` is assigned a value of type `&M T`, hence `&M T <: typeof(x)` is
                        // required. However, we use equality, which is stronger. See (*) for
                        // an explanation.
                        self.demand_eqtype(pat.span, region_ty, typ);
                    }
                    // otherwise the type of x is the expected type T
                    ty::BindByValue(_) => {
                        // As above, `T <: typeof(x)` is required but we
                        // use equality, see (*) below.
                        self.demand_eqtype(pat.span, expected, typ);
                    }
                }

                // if there are multiple arms, make sure they all agree on
                // what the type of the binding `x` ought to be
                if var_id != pat.id {
                    let vt = self.local_ty(pat.span, var_id);
                    self.demand_eqtype(pat.span, vt, typ);
                }

                if let Some(ref p) = *sub {
                    self.check_pat_walk(&p, expected, def_bm, true);
                }

                typ
            }
            PatKind::TupleStruct(ref qpath, ref subpats, ddpos) => {
                self.check_pat_tuple_struct(pat, qpath, &subpats, ddpos, expected, def_bm)
            }
            PatKind::Path(ref qpath) => {
                self.check_pat_path(pat, qpath, expected)
            }
            PatKind::Struct(ref qpath, ref fields, etc) => {
                self.check_pat_struct(pat, qpath, fields, etc, expected, def_bm)
            }
            PatKind::Tuple(ref elements, ddpos) => {
                let mut expected_len = elements.len();
                if ddpos.is_some() {
                    // Require known type only when `..` is present
                    if let ty::TyTuple(ref tys) =
                            self.structurally_resolved_type(pat.span, expected).sty {
                        expected_len = tys.len();
                    }
                }
                let max_len = cmp::max(expected_len, elements.len());

                let element_tys_iter = (0..max_len).map(|_| self.next_ty_var(
                    // FIXME: MiscVariable for now, obtaining the span and name information
                    //       from all tuple elements isn't trivial.
                    ty::UniverseIndex::ROOT,
                    TypeVariableOrigin::TypeInference(pat.span)));
                let element_tys = tcx.mk_type_list(element_tys_iter);
                let pat_ty = tcx.mk_ty(ty::TyTuple(element_tys));
                self.demand_eqtype(pat.span, expected, pat_ty);
                for (i, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
                    self.check_pat_walk(elem, &element_tys[i], def_bm, true);
                }
                pat_ty
            }
            PatKind::Box(ref inner) => {
                let inner_ty = self.next_ty_var(ty::UniverseIndex::ROOT,
                                                TypeVariableOrigin::TypeInference(inner.span));
                let uniq_ty = tcx.mk_box(inner_ty);

                if self.check_dereferencable(pat.span, expected, &inner) {
                    // Here, `demand::subtype` is good enough, but I don't
                    // think any errors can be introduced by using
                    // `demand::eqtype`.
                    self.demand_eqtype(pat.span, expected, uniq_ty);
                    self.check_pat_walk(&inner, inner_ty, def_bm, true);
                    uniq_ty
                } else {
                    self.check_pat_walk(&inner, tcx.types.err, def_bm, true);
                    tcx.types.err
                }
            }
            PatKind::Ref(ref inner, mutbl) => {
                let expected = self.shallow_resolve(expected);
                if self.check_dereferencable(pat.span, expected, &inner) {
                    // `demand::subtype` would be good enough, but using
                    // `eqtype` turns out to be equally general. See (*)
                    // below for details.

                    // Take region, inner-type from expected type if we
                    // can, to avoid creating needless variables.  This
                    // also helps with the bad interactions of the given
                    // hack detailed in (*) below.
                    debug!("check_pat_walk: expected={:?}", expected);
                    let (rptr_ty, inner_ty) = match expected.sty {
                        ty::TyRef(_, mt) if mt.mutbl == mutbl => {
                            (expected, mt.ty)
                        }
                        _ => {
                            let inner_ty = self.next_ty_var(
                                ty::UniverseIndex::ROOT,
                                TypeVariableOrigin::TypeInference(inner.span));
                            let mt = ty::TypeAndMut { ty: inner_ty, mutbl: mutbl };
                            let region = self.next_region_var(infer::PatternRegion(pat.span));
                            let rptr_ty = tcx.mk_ref(region, mt);
                            debug!("check_pat_walk: demanding {:?} = {:?}", expected, rptr_ty);
                            let err = self.demand_eqtype_diag(pat.span, expected, rptr_ty);

                            // Look for a case like `fn foo(&foo: u32)` and suggest
                            // `fn foo(foo: &u32)`
                            if let Some(mut err) = err {
                                if is_arg {
                                    if let PatKind::Binding(..) = inner.node {
                                        if let Ok(snippet) = tcx.sess.codemap()
                                                                     .span_to_snippet(pat.span)
                                        {
                                            err.help(&format!("did you mean `{}: &{}`?",
                                                              &snippet[1..],
                                                              expected));
                                        }
                                    }
                                }
                                err.emit();
                            }
                            (rptr_ty, inner_ty)
                        }
                    };

                    self.check_pat_walk(&inner, inner_ty, def_bm, true);
                    rptr_ty
                } else {
                    self.check_pat_walk(&inner, tcx.types.err, def_bm, true);
                    tcx.types.err
                }
            }
            PatKind::Slice(ref before, ref slice, ref after) => {
                let expected_ty = self.structurally_resolved_type(pat.span, expected);
                let (inner_ty, slice_ty) = match expected_ty.sty {
                    ty::TyArray(inner_ty, size) => {
                        let size = size.val.unwrap_u64();
                        let min_len = before.len() as u64 + after.len() as u64;
                        if slice.is_none() {
                            if min_len != size {
                                struct_span_err!(
                                    tcx.sess, pat.span, E0527,
                                    "pattern requires {} elements but array has {}",
                                    min_len, size)
                                    .span_label(pat.span, format!("expected {} elements",size))
                                    .emit();
                            }
                            (inner_ty, tcx.types.err)
                        } else if let Some(rest) = size.checked_sub(min_len) {
                            (inner_ty, tcx.mk_array(inner_ty, rest))
                        } else {
                            struct_span_err!(tcx.sess, pat.span, E0528,
                                    "pattern requires at least {} elements but array has {}",
                                    min_len, size)
                                .span_label(pat.span,
                                    format!("pattern cannot match array of {} elements", size))
                                .emit();
                            (inner_ty, tcx.types.err)
                        }
                    }
                    ty::TySlice(inner_ty) => (inner_ty, expected_ty),
                    _ => {
                        if !expected_ty.references_error() {
                            let mut err = struct_span_err!(
                                tcx.sess, pat.span, E0529,
                                "expected an array or slice, found `{}`",
                                expected_ty);
                            if let ty::TyRef(_, ty::TypeAndMut { mutbl: _, ty }) = expected_ty.sty {
                                match ty.sty {
                                    ty::TyArray(..) | ty::TySlice(..) => {
                                        err.help("the semantics of slice patterns changed \
                                                  recently; see issue #23121");
                                    }
                                    _ => {}
                                }
                            }

                            err.span_label( pat.span,
                                format!("pattern cannot match with input type `{}`", expected_ty)
                            ).emit();
                        }
                        (tcx.types.err, tcx.types.err)
                    }
                };

                for elt in before {
                    self.check_pat_walk(&elt, inner_ty, def_bm, true);
                }
                if let Some(ref slice) = *slice {
                    self.check_pat_walk(&slice, slice_ty, def_bm, true);
                }
                for elt in after {
                    self.check_pat_walk(&elt, inner_ty, def_bm, true);
                }
                expected_ty
            }
        };

        self.write_ty(pat.hir_id, ty);

        // (*) In most of the cases above (literals and constants being
        // the exception), we relate types using strict equality, even
        // though subtyping would be sufficient. There are a few reasons
        // for this, some of which are fairly subtle and which cost me
        // (nmatsakis) an hour or two debugging to remember, so I thought
        // I'd write them down this time.
        //
        // 1. There is no loss of expressiveness here, though it does
        // cause some inconvenience. What we are saying is that the type
        // of `x` becomes *exactly* what is expected. This can cause unnecessary
        // errors in some cases, such as this one:
        //
        // ```
        // fn foo<'x>(x: &'x int) {
        //    let a = 1;
        //    let mut z = x;
        //    z = &a;
        // }
        // ```
        //
        // The reason we might get an error is that `z` might be
        // assigned a type like `&'x int`, and then we would have
        // a problem when we try to assign `&a` to `z`, because
        // the lifetime of `&a` (i.e., the enclosing block) is
        // shorter than `'x`.
        //
        // HOWEVER, this code works fine. The reason is that the
        // expected type here is whatever type the user wrote, not
        // the initializer's type. In this case the user wrote
        // nothing, so we are going to create a type variable `Z`.
        // Then we will assign the type of the initializer (`&'x
        // int`) as a subtype of `Z`: `&'x int <: Z`. And hence we
        // will instantiate `Z` as a type `&'0 int` where `'0` is
        // a fresh region variable, with the constraint that `'x :
        // '0`.  So basically we're all set.
        //
        // Note that there are two tests to check that this remains true
        // (`regions-reassign-{match,let}-bound-pointer.rs`).
        //
        // 2. Things go horribly wrong if we use subtype. The reason for
        // THIS is a fairly subtle case involving bound regions. See the
        // `givens` field in `region_constraints`, as well as the test
        // `regions-relate-bound-regions-on-closures-to-inference-variables.rs`,
        // for details. Short version is that we must sometimes detect
        // relationships between specific region variables and regions
        // bound in a closure signature, and that detection gets thrown
        // off when we substitute fresh region variables here to enable
        // subtyping.
    }

    pub fn check_dereferencable(&self, span: Span, expected: Ty<'tcx>, inner: &hir::Pat) -> bool {
        if let PatKind::Binding(..) = inner.node {
            if let Some(mt) = self.shallow_resolve(expected).builtin_deref(true) {
                if let ty::TyDynamic(..) = mt.ty.sty {
                    // This is "x = SomeTrait" being reduced from
                    // "let &x = &SomeTrait" or "let box x = Box<SomeTrait>", an error.
                    let type_str = self.ty_to_string(expected);
                    let mut err = struct_span_err!(
                        self.tcx.sess,
                        span,
                        E0033,
                        "type `{}` cannot be dereferenced",
                        type_str
                    );
                    err.span_label(span, format!("type `{}` cannot be dereferenced", type_str));
                    if self.tcx.sess.teach(&err.get_code().unwrap()) {
                        err.note("\
This error indicates that a pointer to a trait type cannot be implicitly dereferenced by a \
pattern. Every trait defines a type, but because the size of trait implementors isn't fixed, \
this type has no compile-time size. Therefore, all accesses to trait types must be through \
pointers. If you encounter this error you should try to avoid dereferencing the pointer.

You can read more about trait objects in the Trait Objects section of the Reference: \
https://doc.rust-lang.org/reference/types.html#trait-objects");
                    }
                    err.emit();
                    return false
                }
            }
        }
        true
    }

    pub fn check_match(&self,
                       expr: &'gcx hir::Expr,
                       discrim: &'gcx hir::Expr,
                       arms: &'gcx [hir::Arm],
                       expected: Expectation<'tcx>,
                       match_src: hir::MatchSource) -> Ty<'tcx> {
        let tcx = self.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,
                                        });
        let discrim_ty;
        if let Some(m) = contains_ref_bindings {
            discrim_ty = 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.
            discrim_ty = self.next_ty_var(ty::UniverseIndex::ROOT,
                                          TypeVariableOrigin::TypeInference(discrim.span));
            self.check_expr_has_type_or_error(discrim, discrim_ty);
        };

        // If the discriminant diverges, the match is pointless (e.g.,
        // `match (return) { }`).
        self.warn_if_unreachable(expr.id, expr.span, "expression");

        // 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;
        }

        // 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);

        // Typecheck the patterns first, so that we get types for all the
        // 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_walk(&p, discrim_ty,
                    ty::BindingMode::BindByValue(hir::Mutability::MutImmutable), true);
                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_nil() => ety,
                _ => self.next_ty_var(ty::UniverseIndex::ROOT,
                                      TypeVariableOrigin::MiscVariable(expr.span)),
            };
            CoerceMany::with_coercion_sites(coerce_first, arms)
        };

        for (i, (arm, pats_diverge)) in arms.iter().zip(all_arm_pats_diverge).enumerate() {
            if let Some(ref e) = arm.guard {
                self.diverges.set(pats_diverge);
                self.check_expr_has_type_or_error(e, tcx.types.bool);
            }

            self.diverges.set(pats_diverge);
            let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
            all_arms_diverge &= self.diverges.get();

            // Handle the fallback arm of a desugared if-let like a missing else.
            let is_if_let_fallback = match match_src {
                hir::MatchSource::IfLetDesugar { contains_else_clause: false } => {
                    i == arms.len() - 1 && arm_ty.is_nil()
                }
                _ => false
            };

            if is_if_let_fallback {
                let cause = self.cause(expr.span, ObligationCauseCode::IfExpressionWithNoElse);
                assert!(arm_ty.is_nil());
                coercion.coerce_forced_unit(self, &cause, &mut |_| (), true);
            } else {
                let cause = self.cause(expr.span, ObligationCauseCode::MatchExpressionArm {
                    arm_span: arm.body.span,
                    source: match_src
                });
                coercion.coerce(self, &cause, &arm.body, arm_ty, self.diverges.get());
            }
        }

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

        coercion.complete(self)
    }

    fn check_pat_struct(&self,
                        pat: &'gcx hir::Pat,
                        qpath: &hir::QPath,
                        fields: &'gcx [Spanned<hir::FieldPat>],
                        etc: bool,
                        expected: Ty<'tcx>,
                        def_bm: ty::BindingMode) -> Ty<'tcx>
    {
        // Resolve the path and check the definition for errors.
        let (variant, pat_ty) = if let Some(variant_ty) = self.check_struct_path(qpath, pat.id) {
            variant_ty
        } else {
            for field in fields {
                self.check_pat_walk(&field.node.pat, self.tcx.types.err, def_bm, true);
            }
            return self.tcx.types.err;
        };

        // Type check the path.
        self.demand_eqtype(pat.span, expected, pat_ty);

        // Type check subpatterns.
        self.check_struct_pat_fields(pat_ty, pat.id, pat.span, variant, fields, etc, def_bm);
        pat_ty
    }

    fn check_pat_path(&self,
                      pat: &hir::Pat,
                      qpath: &hir::QPath,
                      expected: Ty<'tcx>) -> Ty<'tcx>
    {
        let tcx = self.tcx;
        let report_unexpected_def = |def: Def| {
            span_err!(tcx.sess, pat.span, E0533,
                      "expected unit struct/variant or constant, found {} `{}`",
                      def.kind_name(),
                      hir::print::to_string(&tcx.hir, |s| s.print_qpath(qpath, false)));
        };

        // Resolve the path and check the definition for errors.
        let (def, opt_ty, segments) = self.resolve_ty_and_def_ufcs(qpath, pat.id, pat.span);
        match def {
            Def::Err => {
                self.set_tainted_by_errors();
                return tcx.types.err;
            }
            Def::Method(..) => {
                report_unexpected_def(def);
                return tcx.types.err;
            }
            Def::VariantCtor(_, CtorKind::Const) |
            Def::StructCtor(_, CtorKind::Const) |
            Def::Const(..) | Def::AssociatedConst(..) => {} // OK
            _ => bug!("unexpected pattern definition: {:?}", def)
        }

        // Type check the path.
        let pat_ty = self.instantiate_value_path(segments, opt_ty, def, pat.span, pat.id);
        self.demand_suptype(pat.span, expected, pat_ty);
        pat_ty
    }

    fn check_pat_tuple_struct(&self,
                              pat: &hir::Pat,
                              qpath: &hir::QPath,
                              subpats: &'gcx [P<hir::Pat>],
                              ddpos: Option<usize>,
                              expected: Ty<'tcx>,
                              def_bm: ty::BindingMode) -> Ty<'tcx>
    {
        let tcx = self.tcx;
        let on_error = || {
            for pat in subpats {
                self.check_pat_walk(&pat, tcx.types.err, def_bm, true);
            }
        };
        let report_unexpected_def = |def: Def| {
            let msg = format!("expected tuple struct/variant, found {} `{}`",
                              def.kind_name(),
                              hir::print::to_string(&tcx.hir, |s| s.print_qpath(qpath, false)));
            struct_span_err!(tcx.sess, pat.span, E0164, "{}", msg)
                .span_label(pat.span, "not a tuple variant or struct").emit();
            on_error();
        };

        // Resolve the path and check the definition for errors.
        let (def, opt_ty, segments) = self.resolve_ty_and_def_ufcs(qpath, pat.id, pat.span);
        let variant = match def {
            Def::Err => {
                self.set_tainted_by_errors();
                on_error();
                return tcx.types.err;
            }
            Def::AssociatedConst(..) | Def::Method(..) => {
                report_unexpected_def(def);
                return tcx.types.err;
            }
            Def::VariantCtor(_, CtorKind::Fn) |
            Def::StructCtor(_, CtorKind::Fn) => {
                tcx.expect_variant_def(def)
            }
            _ => bug!("unexpected pattern definition: {:?}", def)
        };

        // Type check the path.
        let pat_ty = self.instantiate_value_path(segments, opt_ty, def, pat.span, pat.id);
        // Replace constructor type with constructed type for tuple struct patterns.
        let pat_ty = pat_ty.fn_sig(tcx).output();
        let pat_ty = pat_ty.no_late_bound_regions().expect("expected fn type");

        self.demand_eqtype(pat.span, expected, pat_ty);

        // Type check subpatterns.
        if subpats.len() == variant.fields.len() ||
                subpats.len() < variant.fields.len() && ddpos.is_some() {
            let substs = match pat_ty.sty {
                ty::TyAdt(_, substs) => substs,
                ref ty => bug!("unexpected pattern type {:?}", ty),
            };
            for (i, subpat) in subpats.iter().enumerate_and_adjust(variant.fields.len(), ddpos) {
                let field_ty = self.field_ty(subpat.span, &variant.fields[i], substs);
                self.check_pat_walk(&subpat, field_ty, def_bm, true);

                self.tcx.check_stability(variant.fields[i].did, Some(pat.id), subpat.span);
            }
        } else {
            let subpats_ending = if subpats.len() == 1 { "" } else { "s" };
            let fields_ending = if variant.fields.len() == 1 { "" } else { "s" };
            struct_span_err!(tcx.sess, pat.span, E0023,
                             "this pattern has {} field{}, but the corresponding {} has {} field{}",
                             subpats.len(), subpats_ending, def.kind_name(),
                             variant.fields.len(),  fields_ending)
                .span_label(pat.span, format!("expected {} field{}, found {}",
                                               variant.fields.len(), fields_ending, subpats.len()))
                .emit();
            on_error();
            return tcx.types.err;
        }
        pat_ty
    }

    fn check_struct_pat_fields(&self,
                               adt_ty: Ty<'tcx>,
                               pat_id: ast::NodeId,
                               span: Span,
                               variant: &'tcx ty::VariantDef,
                               fields: &'gcx [Spanned<hir::FieldPat>],
                               etc: bool,
                               def_bm: ty::BindingMode) {
        let tcx = self.tcx;

        let (substs, adt) = match adt_ty.sty {
            ty::TyAdt(adt, substs) => (substs, adt),
            _ => span_bug!(span, "struct pattern is not an ADT")
        };
        let kind_name = adt.variant_descr();

        // Index the struct fields' types.
        let field_map = variant.fields
            .iter()
            .map(|field| (field.name, field))
            .collect::<FxHashMap<_, _>>();

        // Keep track of which fields have already appeared in the pattern.
        let mut used_fields = FxHashMap();

        let mut inexistent_fields = vec![];
        // Typecheck each field.
        for &Spanned { node: ref field, span } in fields {
            let field_ty = match used_fields.entry(field.name) {
                Occupied(occupied) => {
                    struct_span_err!(tcx.sess, span, E0025,
                                     "field `{}` bound multiple times \
                                      in the pattern",
                                     field.name)
                        .span_label(span,
                                    format!("multiple uses of `{}` in pattern", field.name))
                        .span_label(*occupied.get(), format!("first use of `{}`", field.name))
                        .emit();
                    tcx.types.err
                }
                Vacant(vacant) => {
                    vacant.insert(span);
                    field_map.get(&field.name)
                        .map(|f| {
                            self.tcx.check_stability(f.did, Some(pat_id), span);

                            self.field_ty(span, f, substs)
                        })
                        .unwrap_or_else(|| {
                            inexistent_fields.push((span, field.name));
                            tcx.types.err
                        })
                }
            };

            self.check_pat_walk(&field.pat, field_ty, def_bm, true);
        }

        if inexistent_fields.len() > 0 {
            let (field_names, t, plural) = if inexistent_fields.len() == 1 {
                (format!("a field named `{}`", inexistent_fields[0].1), "this", "")
            } else {
                (format!("fields named {}",
                         inexistent_fields.iter()
                            .map(|(_, name)| format!("`{}`", name))
                            .collect::<Vec<String>>()
                            .join(", ")), "these", "s")
            };
            let spans = inexistent_fields.iter().map(|(span, _)| *span).collect::<Vec<_>>();
            let mut err = struct_span_err!(tcx.sess,
                                           spans,
                                           E0026,
                                           "{} `{}` does not have {}",
                                           kind_name,
                                           tcx.item_path_str(variant.did),
                                           field_names);
            if let Some((span, _)) = inexistent_fields.last() {
                err.span_label(*span,
                               format!("{} `{}` does not have {} field{}",
                                       kind_name,
                                       tcx.item_path_str(variant.did),
                                       t,
                                       plural));
            }
            if tcx.sess.teach(&err.get_code().unwrap()) {
                err.note(
                    "This error indicates that a struct pattern attempted to \
                     extract a non-existent field from a struct. Struct fields \
                     are identified by the name used before the colon : so struct \
                     patterns should resemble the declaration of the struct type \
                     being matched.\n\n\
                     If you are using shorthand field patterns but want to refer \
                     to the struct field by a different name, you should rename \
                     it explicitly."
                );
            }
            err.emit();
        }

        // Require `..` if struct has non_exhaustive attribute.
        if adt.is_struct() && adt.is_non_exhaustive() && !adt.did.is_local() && !etc {
            span_err!(tcx.sess, span, E0638,
                      "`..` required with {} marked as non-exhaustive",
                      kind_name);
        }

        // Report an error if incorrect number of the fields were specified.
        if kind_name == "union" {
            if fields.len() != 1 {
                tcx.sess.span_err(span, "union patterns should have exactly one field");
            }
            if etc {
                tcx.sess.span_err(span, "`..` cannot be used in union patterns");
            }
        } else if !etc {
            let unmentioned_fields = variant.fields
                .iter()
                .map(|field| field.name)
                .filter(|field| !used_fields.contains_key(&field))
                .collect::<Vec<_>>();
            if unmentioned_fields.len() > 0 {
                let field_names = if unmentioned_fields.len() == 1 {
                    format!("field `{}`", unmentioned_fields[0])
                } else {
                    format!("fields {}",
                            unmentioned_fields.iter()
                                .map(|name| format!("`{}`", name))
                                .collect::<Vec<String>>()
                                .join(", "))
                };
                let mut diag = struct_span_err!(tcx.sess, span, E0027,
                                                "pattern does not mention {}",
                                                field_names);
                diag.span_label(span, format!("missing {}", field_names));
                if variant.ctor_kind == CtorKind::Fn {
                    diag.note("trying to match a tuple variant with a struct variant pattern");
                }
                if tcx.sess.teach(&diag.get_code().unwrap()) {
                    diag.note(
                        "This error indicates that a pattern for a struct fails to specify a \
                         sub-pattern for every one of the struct's fields. Ensure that each field \
                         from the struct's definition is mentioned in the pattern, or use `..` to \
                         ignore unwanted fields."
                    );
                }
                diag.emit();
            }
        }
    }
}