Point at the match discriminant when arm pattern has a type mismatch

src/librustc/infer/error_reporting/mod.rs

@@ -487,6 +487,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
     fn note_error_origin(&self, err: &mut DiagnosticBuilder<'tcx>, cause: &ObligationCause<'tcx>) {
         match cause.code {
+            ObligationCauseCode::MatchExpressionArmPattern { span, ty } => {
+                err.span_label(span, format!("this match expression evaluates to `{}`", ty));
+            }
             ObligationCauseCode::MatchExpressionArm { arm_span, source } => match source {
                 hir::MatchSource::IfLetDesugar { .. } => {
                     let msg = "`if let` arm with an incompatible type";

src/librustc/traits/error_reporting.rs

@@ -1444,6 +1444,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         match *cause_code {
             ObligationCauseCode::ExprAssignable |
             ObligationCauseCode::MatchExpressionArm { .. } |
+            ObligationCauseCode::MatchExpressionArmPattern { .. } |
             ObligationCauseCode::IfExpression |
             ObligationCauseCode::IfExpressionWithNoElse |
             ObligationCauseCode::MainFunctionType |
@@ -1451,8 +1452,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             ObligationCauseCode::IntrinsicType |
             ObligationCauseCode::MethodReceiver |
             ObligationCauseCode::ReturnNoExpression |
-            ObligationCauseCode::MiscObligation => {
-            }
+            ObligationCauseCode::MiscObligation => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
             }

src/librustc/traits/mod.rs

@@ -220,8 +220,13 @@ pub enum ObligationCauseCode<'tcx> {
     ExprAssignable,
 
     /// Computing common supertype in the arms of a match expression
-    MatchExpressionArm { arm_span: Span,
-                         source: hir::MatchSource },
+    MatchExpressionArm {
+        arm_span: Span,
+        source: hir::MatchSource,
+    },
+
+    /// Computing common supertype in the pattern guard for the arms of a match expression
+    MatchExpressionArmPattern { span: Span, ty: Ty<'tcx> },
 
     /// Computing common supertype in an if expression
     IfExpression,

src/librustc/traits/structural_impls.rs

@@ -517,6 +517,9 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                 arm_span,
                 source: source,
             }),
+            super::MatchExpressionArmPattern { span, ty } => {
+                tcx.lift(&ty).map(|ty| super::MatchExpressionArmPattern { span, ty })
+            }
             super::IfExpression => Some(super::IfExpression),
             super::IfExpressionWithNoElse => Some(super::IfExpressionWithNoElse),
             super::MainFunctionType => Some(super::MainFunctionType),

src/librustc_typeck/check/_match.rs

@@ -29,12 +29,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         pat: &'gcx hir::Pat,
         mut expected: Ty<'tcx>,
         mut def_bm: ty::BindingMode,
-        is_arg: bool)
-    {
+        match_discrim_span: Option<Span>,
+    ) {
         let tcx = self.tcx;
 
-        debug!("check_pat_walk(pat={:?},expected={:?},def_bm={:?},is_arg={})",
-            pat, expected, def_bm, is_arg);
+        debug!("check_pat_walk(pat={:?},expected={:?},def_bm={:?})", pat, expected, def_bm);
 
         let is_non_ref_pat = match pat.node {
             PatKind::Struct(..) |
@@ -210,8 +209,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 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);
+                self.demand_eqtype_pat(pat.span, expected, lhs_ty, match_discrim_span);
+                self.demand_eqtype_pat(pat.span, expected, rhs_ty, match_discrim_span);
                 common_type
             }
             PatKind::Binding(ba, var_id, _, ref sub) => {
@@ -240,13 +239,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                         // `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, local_ty);
+                        self.demand_eqtype_pat(pat.span, region_ty, local_ty, match_discrim_span);
                     }
                     // 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, local_ty);
+                        self.demand_eqtype_pat(pat.span, expected, local_ty, match_discrim_span);
                     }
                 }
 
@@ -254,23 +253,31 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 // what the type of the binding `x` ought to be
                 if var_id != pat.id {
                     let vt = self.local_ty(pat.span, var_id).decl_ty;
-                    self.demand_eqtype(pat.span, vt, local_ty);
+                    self.demand_eqtype_pat(pat.span, vt, local_ty, match_discrim_span);
                 }
 
                 if let Some(ref p) = *sub {
-                    self.check_pat_walk(&p, expected, def_bm, true);
+                    self.check_pat_walk(&p, expected, def_bm, match_discrim_span);
                 }
 
                 local_ty
             }
             PatKind::TupleStruct(ref qpath, ref subpats, ddpos) => {
-                self.check_pat_tuple_struct(pat, qpath, &subpats, ddpos, expected, def_bm)
+                self.check_pat_tuple_struct(
+                    pat,
+                    qpath,
+                    &subpats,
+                    ddpos,
+                    expected,
+                    def_bm,
+                    match_discrim_span,
+                )
             }
             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)
+                self.check_pat_struct(pat, qpath, fields, etc, expected, def_bm, match_discrim_span)
             }
             PatKind::Tuple(ref elements, ddpos) => {
                 let mut expected_len = elements.len();
@@ -295,12 +302,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     // further errors being emitted when using the bindings. #50333
                     let element_tys_iter = (0..max_len).map(|_| tcx.types.err);
                     for (_, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
-                        self.check_pat_walk(elem, &tcx.types.err, def_bm, true);
+                        self.check_pat_walk(elem, &tcx.types.err, def_bm, match_discrim_span);
                     }
                     tcx.mk_tup(element_tys_iter)
                 } else {
                     for (i, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
-                        self.check_pat_walk(elem, &element_tys[i], def_bm, true);
+                        self.check_pat_walk(elem, &element_tys[i], def_bm, match_discrim_span);
                     }
                     pat_ty
                 }
@@ -313,11 +320,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     // 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);
+                    self.demand_eqtype_pat(pat.span, expected, uniq_ty, match_discrim_span);
+                    self.check_pat_walk(&inner, inner_ty, def_bm, match_discrim_span);
                     uniq_ty
                 } else {
-                    self.check_pat_walk(&inner, tcx.types.err, def_bm, true);
+                    self.check_pat_walk(&inner, tcx.types.err, def_bm, match_discrim_span);
                     tcx.types.err
                 }
             }
@@ -349,15 +356,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                             // 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.source_map()
-                                                                     .span_to_snippet(pat.span)
-                                        {
-                                            err.help(&format!("did you mean `{}: &{}`?",
-                                                              &snippet[1..],
-                                                              expected));
-                                        }
+                                if let PatKind::Binding(..) = inner.node {
+                                    if let Ok(snippet) = tcx.sess.source_map()
+                                                                    .span_to_snippet(pat.span)
+                                    {
+                                        err.help(&format!("did you mean `{}: &{}`?",
+                                                            &snippet[1..],
+                                                            expected));
                                     }
                                 }
                                 err.emit();
@@ -366,10 +371,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                         }
                     };
 
-                    self.check_pat_walk(&inner, inner_ty, def_bm, true);
+                    self.check_pat_walk(&inner, inner_ty, def_bm, match_discrim_span);
                     rptr_ty
                 } else {
-                    self.check_pat_walk(&inner, tcx.types.err, def_bm, true);
+                    self.check_pat_walk(&inner, tcx.types.err, def_bm, match_discrim_span);
                     tcx.types.err
                 }
             }
@@ -427,13 +432,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 };
 
                 for elt in before {
-                    self.check_pat_walk(&elt, inner_ty, def_bm, true);
+                    self.check_pat_walk(&elt, inner_ty, def_bm, match_discrim_span);
                 }
                 if let Some(ref slice) = *slice {
-                    self.check_pat_walk(&slice, slice_ty, def_bm, true);
+                    self.check_pat_walk(&slice, slice_ty, def_bm, match_discrim_span);
                 }
                 for elt in after {
-                    self.check_pat_walk(&elt, inner_ty, def_bm, true);
+                    self.check_pat_walk(&elt, inner_ty, def_bm, match_discrim_span);
                 }
                 expected_ty
             }
@@ -524,12 +529,14 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
         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> {
+    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
@@ -624,8 +631,12 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
             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);
+                self.check_pat_walk(
+                    &p,
+                    discrim_ty,
+                    ty::BindingMode::BindByValue(hir::Mutability::MutImmutable),
+                    Some(discrim.span),
+                );
                 all_pats_diverge &= self.diverges.get();
             }
 
@@ -703,26 +714,29 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
         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>
+    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,
+        match_discrim_span: Option<Span>,
+    ) -> 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);
+                self.check_pat_walk(&field.node.pat, self.tcx.types.err, def_bm, match_discrim_span);
             }
             return self.tcx.types.err;
         };
 
         // Type-check the path.
-        self.demand_eqtype(pat.span, expected, pat_ty);
+        self.demand_eqtype_pat(pat.span, expected, pat_ty, match_discrim_span);
 
         // Type-check subpatterns.
         if self.check_struct_pat_fields(pat_ty, pat.id, pat.span, variant, fields, etc, def_bm) {
@@ -732,11 +746,12 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
         }
     }
 
-    fn check_pat_path(&self,
-                      pat: &hir::Pat,
-                      qpath: &hir::QPath,
-                      expected: Ty<'tcx>) -> Ty<'tcx>
-    {
+    fn check_pat_path(
+        &self,
+        pat: &hir::Pat,
+        qpath: &hir::QPath,
+        expected: Ty<'tcx>,
+    ) -> Ty<'tcx> {
         let tcx = self.tcx;
 
         // Resolve the path and check the definition for errors.
@@ -767,18 +782,20 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
         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>
-    {
+    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,
+        match_arm_pat_span: Option<Span>,
+    ) -> Ty<'tcx> {
         let tcx = self.tcx;
         let on_error = || {
             for pat in subpats {
-                self.check_pat_walk(&pat, tcx.types.err, def_bm, true);
+                self.check_pat_walk(&pat, tcx.types.err, def_bm, match_arm_pat_span);
             }
         };
         let report_unexpected_def = |def: Def| {
@@ -826,7 +843,7 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
         let pat_ty = pat_ty.fn_sig(tcx).output();
         let pat_ty = pat_ty.no_bound_vars().expect("expected fn type");
 
-        self.demand_eqtype(pat.span, expected, pat_ty);
+        self.demand_eqtype_pat(pat.span, expected, pat_ty, match_arm_pat_span);
 
         // Type-check subpatterns.
         if subpats.len() == variant.fields.len() ||
@@ -837,7 +854,7 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
             };
             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.check_pat_walk(&subpat, field_ty, def_bm, match_arm_pat_span);
 
                 self.tcx.check_stability(variant.fields[i].did, Some(pat.id), subpat.span);
             }
@@ -917,7 +934,7 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
                 }
             };
 
-            self.check_pat_walk(&field.pat, field_ty, def_bm, true);
+            self.check_pat_walk(&field.pat, field_ty, def_bm, None);
         }
         let mut unmentioned_fields = variant.fields
                 .iter()

src/librustc_typeck/check/demand.rs

@@ -1,6 +1,6 @@
 use check::FnCtxt;
 use rustc::infer::InferOk;
-use rustc::traits::ObligationCause;
+use rustc::traits::{ObligationCause, ObligationCauseCode};
 
 use syntax::ast;
 use syntax::util::parser::PREC_POSTFIX;
@@ -66,6 +66,25 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
+    pub fn demand_eqtype_pat(
+        &self,
+        cause_span: Span,
+        expected: Ty<'tcx>,
+        actual: Ty<'tcx>,
+        match_expr_span: Option<Span>,
+    ) {
+        let cause = if let Some(span) = match_expr_span {
+            self.cause(
+                cause_span,
+                ObligationCauseCode::MatchExpressionArmPattern { span, ty: expected },
+            )
+        } else {
+            self.misc(cause_span)
+        };
+        self.demand_eqtype_with_origin(&cause, expected, actual).map(|mut err| err.emit());
+    }
+
+
     pub fn demand_coerce(&self,
                          expr: &hir::Expr,
                          checked_ty: Ty<'tcx>,

src/librustc_typeck/check/mod.rs

@@ -1084,8 +1084,12 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     // Add formal parameters.
     for (arg_ty, arg) in fn_sig.inputs().iter().zip(&body.arguments) {
         // Check the pattern.
-        fcx.check_pat_walk(&arg.pat, arg_ty,
-            ty::BindingMode::BindByValue(hir::Mutability::MutImmutable), true);
+        fcx.check_pat_walk(
+            &arg.pat,
+            arg_ty,
+            ty::BindingMode::BindByValue(hir::Mutability::MutImmutable),
+            None,
+        );
 
         // Check that argument is Sized.
         // The check for a non-trivial pattern is a hack to avoid duplicate warnings
@@ -4723,9 +4727,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             }
         }
 
-        self.check_pat_walk(&local.pat, t,
-                            ty::BindingMode::BindByValue(hir::Mutability::MutImmutable),
-                            true);
+        self.check_pat_walk(
+            &local.pat,
+            t,
+            ty::BindingMode::BindByValue(hir::Mutability::MutImmutable),
+            None,
+        );
         let pat_ty = self.node_ty(local.pat.hir_id);
         if pat_ty.references_error() {
             self.write_ty(local.hir_id, pat_ty);

src/test/ui/block-result/issue-13624.stderr

@@ -12,6 +12,8 @@ LL |     Enum::EnumStructVariant { x: 1, y: 2, z: 3 }
 error[E0308]: mismatched types
   --> $DIR/issue-13624.rs:22:9
    |
+LL |       match enum_struct_variant {
+   |             ------------------- this match expression evaluates to `()`
 LL |         a::Enum::EnumStructVariant { x, y, z } => {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `a::Enum`
    |

src/test/ui/error-codes/E0308-4.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/E0308-4.rs:4:9
    |
+LL |     match x {
+   |           - this match expression evaluates to `u8`
 LL |         0u8..=3i8 => (), //~ ERROR E0308
    |         ^^^^^^^^^ expected u8, found i8
 

src/test/ui/issues/issue-11844.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-11844.rs:6:9
    |
+LL |     match a {
+   |           - this match expression evaluates to `std::option::Option<std::boxed::Box<{integer}>>`
 LL |         Ok(a) => //~ ERROR: mismatched types
    |         ^^^^^ expected enum `std::option::Option`, found enum `std::result::Result`
    |

src/test/ui/issues/issue-12552.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-12552.rs:6:5
    |
+LL |   match t {
+   |         - this match expression evaluates to `std::result::Result<_, {integer}>`
 LL |     Some(k) => match k { //~ ERROR mismatched types
    |     ^^^^^^^ expected enum `std::result::Result`, found enum `std::option::Option`
    |

src/test/ui/issues/issue-13466.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-13466.rs:8:9
    |
+LL |     let _x: usize = match Some(1) {
+   |                           ------- this match expression evaluates to `std::option::Option<{integer}>`
 LL |         Ok(u) => u,
    |         ^^^^^ expected enum `std::option::Option`, found enum `std::result::Result`
    |
@@ -10,6 +12,9 @@ LL |         Ok(u) => u,
 error[E0308]: mismatched types
   --> $DIR/issue-13466.rs:14:9
    |
+LL |     let _x: usize = match Some(1) {
+   |                           ------- this match expression evaluates to `std::option::Option<{integer}>`
+...
 LL |         Err(e) => panic!(e)
    |         ^^^^^^ expected enum `std::option::Option`, found enum `std::result::Result`
    |

src/test/ui/issues/issue-15896.stderr

@@ -1,6 +1,9 @@
 error[E0308]: mismatched types
   --> $DIR/issue-15896.rs:11:11
    |
+LL |     let u = match e {
+   |                   - this match expression evaluates to `main::R`
+LL |         E::B(
 LL |           Tau{t: x},
    |           ^^^^^^^^^ expected enum `main::R`, found struct `main::Tau`
    |

src/test/ui/issues/issue-16401.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-16401.rs:8:9
    |
+LL |     match () {
+   |           -- this match expression evaluates to `()`
 LL |         Slice { data: data, len: len } => (),
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found struct `Slice`
    |

src/test/ui/issues/issue-3680.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-3680.rs:3:9
    |
+LL |     match None {
+   |           ---- this match expression evaluates to `std::option::Option<_>`
 LL |         Err(_) => ()
    |         ^^^^^^ expected enum `std::option::Option`, found enum `std::result::Result`
    |

src/test/ui/issues/issue-5100.stderr

@@ -28,6 +28,8 @@ LL |         (true, false, false) => ()
 error[E0308]: mismatched types
   --> $DIR/issue-5100.rs:33:9
    |
+LL |     match (true, false) {
+   |           ------------- this match expression evaluates to `(bool, bool)`
 LL |         box (true, false) => ()
    |         ^^^^^^^^^^^^^^^^^ expected tuple, found struct `std::boxed::Box`
    |

src/test/ui/issues/issue-5358-1.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-5358-1.rs:6:9
    |
+LL |     match S(Either::Left(5)) {
+   |           ------------------ this match expression evaluates to `S`
 LL |         Either::Right(_) => {}
    |         ^^^^^^^^^^^^^^^^ expected struct `S`, found enum `Either`
    |

src/test/ui/issues/issue-7092.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-7092.rs:6:9
    |
+LL |     match x {
+   |           - this match expression evaluates to `Whatever`
 LL |         Some(field) =>
    |         ^^^^^^^^^^^ expected enum `Whatever`, found enum `std::option::Option`
    |

src/test/ui/match/match-range-fail.stderr

@@ -19,6 +19,8 @@ LL |         10 ..= "what" => ()
 error[E0308]: mismatched types
   --> $DIR/match-range-fail.rs:17:9
    |
+LL |     match 5 {
+   |           - this match expression evaluates to `{integer}`
 LL |         'c' ..= 100 => { }
    |         ^^^^^^^^^^^ expected integer, found char
    |

src/test/ui/match/match-struct.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/match-struct.rs:6:9
    |
+LL |     match (S { a: 1 }) {
+   |           ------------ this match expression evaluates to `S`
 LL |         E::C(_) => (),
    |         ^^^^^^^ expected struct `S`, found enum `E`
    |

src/test/ui/match/match-tag-unary.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/match-tag-unary.rs:4:43
    |
 LL | fn main() { let x: A = A::A(0); match x { B::B(y) => { } } } //~ ERROR mismatched types
-   |                                           ^^^^^^^ expected enum `A`, found enum `B`
+   |                                       -   ^^^^^^^ expected enum `A`, found enum `B`
+   |                                       |
+   |                                       this match expression evaluates to `A`
    |
    = note: expected type `A`
               found type `B`

src/test/ui/mismatched_types/E0409.stderr

@@ -9,6 +9,8 @@ LL |         (0, ref y) | (y, 0) => {} //~ ERROR E0409
 error[E0308]: mismatched types
   --> $DIR/E0409.rs:5:23
    |
+LL |     match x {
+   |           - this match expression evaluates to `_`
 LL |         (0, ref y) | (y, 0) => {} //~ ERROR E0409
    |                       ^ expected &{integer}, found integer
    |

src/test/ui/or-pattern-mismatch.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/or-pattern-mismatch.rs:3:68
    |
 LL | fn main() { match Blah::A(1, 1, 2) { Blah::A(_, x, y) | Blah::B(x, y) => { } } }
-   |                                                                    ^ expected usize, found isize
+   |                   ----------------                                 ^ expected usize, found isize
+   |                   |
+   |                   this match expression evaluates to `_`
    |
    = note: expected type `usize`
               found type `isize`

src/test/ui/pattern/pattern-error-continue.stderr

@@ -21,6 +21,8 @@ LL |         A::B(_, _, _) => (), //~ ERROR this pattern has 3 fields, but
 error[E0308]: mismatched types
   --> $DIR/pattern-error-continue.rs:22:9
    |
+LL |     match 'c' {
+   |           --- this match expression evaluates to `char`
 LL |         S { .. } => (),
    |         ^^^^^^^^ expected char, found struct `S`
    |

src/test/ui/pattern/pattern-tyvar.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/pattern-tyvar.rs:5:18
    |
+LL |     match t {
+   |           - this match expression evaluates to `std::option::Option<std::vec::Vec<isize>>`
 LL |       Bar::T1(_, Some::<isize>(x)) => { //~ ERROR mismatched types
    |                  ^^^^^^^^^^^^^^^^ expected struct `std::vec::Vec`, found isize
    |

src/test/ui/resolve/resolve-inconsistent-binding-mode.stderr

@@ -23,6 +23,8 @@ LL |       Opts::A(ref mut i) | Opts::B(ref i) => {}
 error[E0308]: mismatched types
   --> $DIR/resolve-inconsistent-binding-mode.rs:7:32
    |
+LL |     match x {
+   |           - this match expression evaluates to `_`
 LL |       Opts::A(ref i) | Opts::B(i) => {}
    |                                ^ expected &isize, found isize
    |
@@ -32,6 +34,8 @@ LL |       Opts::A(ref i) | Opts::B(i) => {}
 error[E0308]: mismatched types
   --> $DIR/resolve-inconsistent-binding-mode.rs:16:32
    |
+LL |     match x {
+   |           - this match expression evaluates to `_`
 LL |       Opts::A(ref i) | Opts::B(i) => {}
    |                                ^ expected &isize, found isize
    |
@@ -41,6 +45,8 @@ LL |       Opts::A(ref i) | Opts::B(i) => {}
 error[E0308]: mismatched types
   --> $DIR/resolve-inconsistent-binding-mode.rs:25:36
    |
+LL |     match x {
+   |           - this match expression evaluates to `_`
 LL |       Opts::A(ref mut i) | Opts::B(ref i) => {}
    |                                    ^^^^^ types differ in mutability
    |

src/test/ui/structs/structure-constructor-type-mismatch.stderr

@@ -109,6 +109,8 @@ LL |         PointF::<u32> { .. } => {} //~ ERROR wrong number of type arguments
 error[E0308]: mismatched types
   --> $DIR/structure-constructor-type-mismatch.rs:54:9
    |
+LL |     match (Point { x: 1, y: 2 }) {
+   |           ---------------------- this match expression evaluates to `Point<{integer}>`
 LL |         PointF::<u32> { .. } => {} //~ ERROR wrong number of type arguments
    |         ^^^^^^^^^^^^^^^^^^^^ expected integer, found f32
    |
@@ -118,6 +120,8 @@ LL |         PointF::<u32> { .. } => {} //~ ERROR wrong number of type arguments
 error[E0308]: mismatched types
   --> $DIR/structure-constructor-type-mismatch.rs:59:9
    |
+LL |     match (Point { x: 1, y: 2 }) {
+   |           ---------------------- this match expression evaluates to `Point<{integer}>`
 LL |         PointF { .. } => {} //~ ERROR mismatched types
    |         ^^^^^^^^^^^^^ expected integer, found f32
    |
@@ -127,6 +131,8 @@ LL |         PointF { .. } => {} //~ ERROR mismatched types
 error[E0308]: mismatched types
   --> $DIR/structure-constructor-type-mismatch.rs:67:9
    |
+LL |     match (Pair { x: 1, y: 2 }) {
+   |           --------------------- this match expression evaluates to `Pair<{integer}, {integer}>`
 LL |         PairF::<u32> { .. } => {} //~ ERROR mismatched types
    |         ^^^^^^^^^^^^^^^^^^^ expected integer, found f32
    |