Tweak mismatched types error

src/librustc/hir/map/mod.rs

@@ -287,7 +287,7 @@ impl<'hir> Map<'hir> {
         self.definitions.def_index_to_hir_id(def_id.to_def_id().index)
     }
 
-    fn def_kind(&self, hir_id: HirId) -> Option<DefKind> {
+    pub fn def_kind(&self, hir_id: HirId) -> Option<DefKind> {
         let node = if let Some(node) = self.find(hir_id) {
             node
         } else {

src/librustc/infer/error_reporting/mod.rs

@@ -662,19 +662,22 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     }
                 }
                 _ => {
+                    // `last_ty` can be `!`, `expected` will have better info when present.
+                    let t = self.resolve_vars_if_possible(&match exp_found {
+                        Some(ty::error::ExpectedFound { expected, .. }) => expected,
+                        _ => last_ty,
+                    });
                     let msg = "`match` arms have incompatible types";
                     err.span_label(cause.span, msg);
                     if prior_arms.len() <= 4 {
                         for sp in prior_arms {
-                            err.span_label(*sp, format!(
-                                "this is found to be of type `{}`",
-                                self.resolve_vars_if_possible(&last_ty),
-                            ));
+                            err.span_label( *sp, format!("this is found to be of type `{}`", t));
                         }
                     } else if let Some(sp) = prior_arms.last() {
-                        err.span_label(*sp, format!(
-                            "this and all prior arms are found to be of type `{}`", last_ty,
-                        ));
+                        err.span_label(
+                            *sp,
+                            format!("this and all prior arms are found to be of type `{}`", t),
+                        );
                     }
                 }
             },
@@ -1143,27 +1146,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 }
                 (_, false, _) => {
                     if let Some(exp_found) = exp_found {
-                        let (def_id, ret_ty) = match exp_found.found.sty {
-                            ty::FnDef(def, _) => {
-                                (Some(def), Some(self.tcx.fn_sig(def).output()))
-                            }
-                            _ => (None, None),
-                        };
-
-                        let exp_is_struct = match exp_found.expected.sty {
-                            ty::Adt(def, _) => def.is_struct(),
-                            _ => false,
-                        };
-
-                        if let (Some(def_id), Some(ret_ty)) = (def_id, ret_ty) {
-                            if exp_is_struct && &exp_found.expected == ret_ty.skip_binder() {
-                                let message = format!(
-                                    "did you mean `{}(/* fields */)`?",
-                                    self.tcx.def_path_str(def_id)
-                                );
-                                diag.span_label(span, message);
-                            }
-                        }
                         self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
                     }
 

src/librustc_typeck/check/expr.rs

@@ -548,7 +548,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     coerce.coerce(self, &cause, e, e_ty);
                 } else {
                     assert!(e_ty.is_unit());
-                    coerce.coerce_forced_unit(self, &cause, &mut |_| (), true);
+                    let ty = coerce.expected_ty();
+                    coerce.coerce_forced_unit(self, &cause, &mut |err| {
+                        let val = match ty.sty {
+                            ty::Bool => "true",
+                            ty::Char => "'a'",
+                            ty::Int(_) | ty::Uint(_) => "42",
+                            ty::Float(_) => "3.14159",
+                            ty::Error | ty::Never => return,
+                            _ => "value",
+                        };
+                        let msg = "give it a value of the expected type";
+                        let label = destination.label
+                            .map(|l| format!(" {}", l.ident))
+                            .unwrap_or_else(String::new);
+                        let sugg = format!("break{} {}", label, val);
+                        err.span_suggestion(expr.span, msg, sugg, Applicability::HasPlaceholders);
+                    }, false);
                 }
             } else {
                 // If `ctxt.coerce` is `None`, we can just ignore

src/librustc_typeck/check/mod.rs

@@ -3709,7 +3709,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             self.consider_hint_about_removing_semicolon(blk, expected_ty, err);
                         }
                         if let Some(fn_span) = fn_span {
-                            err.span_label(fn_span, "this function's body doesn't return");
+                            err.span_label(
+                                fn_span,
+                                "implicitly returns `()` as its body has no tail or `return` \
+                                 expression",
+                            );
                         }
                     }, false);
                 }
@@ -3819,6 +3823,101 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         pointing_at_return_type
     }
 
+    /// When encountering an fn-like ctor that needs to unify with a value, check whether calling
+    /// the ctor would successfully solve the type mismatch and if so, suggest it:
+    /// ```
+    /// fn foo(x: usize) -> usize { x }
+    /// let x: usize = foo;  // suggest calling the `foo` function: `foo(42)`
+    /// ```
+    fn suggest_fn_call(
+        &self,
+        err: &mut DiagnosticBuilder<'tcx>,
+        expr: &hir::Expr,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+    ) -> bool {
+        match found.sty {
+            ty::FnDef(..) | ty::FnPtr(_) => {}
+            _ => return false,
+        }
+        let hir = self.tcx.hir();
+
+        let sig = found.fn_sig(self.tcx);
+        let sig = self
+            .replace_bound_vars_with_fresh_vars(expr.span, infer::FnCall, &sig)
+            .0;
+        let sig = self.normalize_associated_types_in(expr.span, &sig);
+        if let Ok(_) = self.try_coerce(expr, sig.output(), expected, AllowTwoPhase::No) {
+            let (mut sugg_call, applicability) = if sig.inputs().is_empty() {
+                (String::new(), Applicability::MachineApplicable)
+            } else {
+                ("...".to_string(), Applicability::HasPlaceholders)
+            };
+            let mut msg = "call this function";
+            if let ty::FnDef(def_id, ..) = found.sty {
+                match hir.get_if_local(def_id) {
+                    Some(Node::Item(hir::Item {
+                        node: ItemKind::Fn(.., body_id),
+                        ..
+                    })) |
+                    Some(Node::ImplItem(hir::ImplItem {
+                        node: hir::ImplItemKind::Method(_, body_id),
+                        ..
+                    })) |
+                    Some(Node::TraitItem(hir::TraitItem {
+                        node: hir::TraitItemKind::Method(.., hir::TraitMethod::Provided(body_id)),
+                        ..
+                    })) => {
+                        let body = hir.body(*body_id);
+                        sugg_call = body.arguments.iter()
+                            .map(|arg| match &arg.pat.node {
+                                hir::PatKind::Binding(_, _, ident, None)
+                                if ident.name != kw::SelfLower => ident.to_string(),
+                                _ => "_".to_string(),
+                            }).collect::<Vec<_>>().join(", ");
+                    }
+                    Some(Node::Ctor(hir::VariantData::Tuple(fields, _))) => {
+                        sugg_call = fields.iter().map(|_| "_").collect::<Vec<_>>().join(", ");
+                        match hir.as_local_hir_id(def_id).and_then(|hir_id| hir.def_kind(hir_id)) {
+                            Some(hir::def::DefKind::Ctor(hir::def::CtorOf::Variant, _)) => {
+                                msg = "instantiate this tuple variant";
+                            }
+                            Some(hir::def::DefKind::Ctor(hir::def::CtorOf::Struct, _)) => {
+                                msg = "instantiate this tuple struct";
+                            }
+                            _ => {}
+                        }
+                    }
+                    Some(Node::ForeignItem(hir::ForeignItem {
+                        node: hir::ForeignItemKind::Fn(_, idents, _),
+                        ..
+                    })) |
+                    Some(Node::TraitItem(hir::TraitItem {
+                        node: hir::TraitItemKind::Method(.., hir::TraitMethod::Required(idents)),
+                        ..
+                    })) => sugg_call = idents.iter()
+                            .map(|ident| if ident.name != kw::SelfLower {
+                                ident.to_string()
+                            } else {
+                                "_".to_string()
+                            }).collect::<Vec<_>>()
+                            .join(", "),
+                    _ => {}
+                }
+            };
+            if let Ok(code) = self.sess().source_map().span_to_snippet(expr.span) {
+                err.span_suggestion(
+                    expr.span,
+                    &format!("use parentheses to {}", msg),
+                    format!("{}({})", code, sugg_call),
+                    applicability,
+                );
+                return true;
+            }
+        }
+        false
+    }
+
     pub fn suggest_ref_or_into(
         &self,
         err: &mut DiagnosticBuilder<'tcx>,
@@ -3833,6 +3932,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 suggestion,
                 Applicability::MachineApplicable,
             );
+        } else if let (ty::FnDef(def_id, ..), true) = (
+            &found.sty,
+            self.suggest_fn_call(err, expr, expected, found),
+        ) {
+            if let Some(sp) = self.tcx.hir().span_if_local(*def_id) {
+                let sp = self.sess().source_map().def_span(sp);
+                err.span_label(sp, &format!("{} defined here", found));
+            }
         } else if !self.check_for_cast(err, expr, found, expected) {
             let is_struct_pat_shorthand_field = self.is_hir_id_from_struct_pattern_shorthand_field(
                 expr.hir_id,

src/libsyntax/parse/parser.rs

@@ -2052,9 +2052,23 @@ impl<'a> Parser<'a> {
                 while self.token != token::CloseDelim(token::Paren) {
                     es.push(match self.parse_expr() {
                         Ok(es) => es,
-                        Err(err) => {
+                        Err(mut err) => {
                             // recover from parse error in tuple list
-                            return Ok(self.recover_seq_parse_error(token::Paren, lo, Err(err)));
+                            match self.token.kind {
+                                token::Ident(name, false)
+                                if name == kw::Underscore && self.look_ahead(1, |t| {
+                                    t == &token::Comma
+                                }) => {
+                                    // Special-case handling of `Foo<(_, _, _)>`
+                                    err.emit();
+                                    let sp = self.token.span;
+                                    self.bump();
+                                    self.mk_expr(sp, ExprKind::Err, ThinVec::new())
+                                }
+                                _ => return Ok(
+                                    self.recover_seq_parse_error(token::Paren, lo, Err(err)),
+                                ),
+                            }
                         }
                     });
                     recovered = self.expect_one_of(
@@ -2456,37 +2470,35 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses `a.b` or `a(13)` or `a[4]` or just `a`.
-    fn parse_dot_or_call_expr(&mut self,
-                                  already_parsed_attrs: Option<ThinVec<Attribute>>)
-                                  -> PResult<'a, P<Expr>> {
+    fn parse_dot_or_call_expr(
+        &mut self,
+        already_parsed_attrs: Option<ThinVec<Attribute>>,
+    ) -> PResult<'a, P<Expr>> {
         let attrs = self.parse_or_use_outer_attributes(already_parsed_attrs)?;
 
         let b = self.parse_bottom_expr();
         let (span, b) = self.interpolated_or_expr_span(b)?;
         self.parse_dot_or_call_expr_with(b, span, attrs)
     }
 
-    fn parse_dot_or_call_expr_with(&mut self,
-                                       e0: P<Expr>,
-                                       lo: Span,
-                                       mut attrs: ThinVec<Attribute>)
-                                       -> PResult<'a, P<Expr>> {
+    fn parse_dot_or_call_expr_with(
+        &mut self,
+        e0: P<Expr>,
+        lo: Span,
+        mut attrs: ThinVec<Attribute>,
+    ) -> PResult<'a, P<Expr>> {
         // Stitch the list of outer attributes onto the return value.
         // A little bit ugly, but the best way given the current code
         // structure
-        self.parse_dot_or_call_expr_with_(e0, lo)
-        .map(|expr|
+        self.parse_dot_or_call_expr_with_(e0, lo).map(|expr|
             expr.map(|mut expr| {
                 attrs.extend::<Vec<_>>(expr.attrs.into());
                 expr.attrs = attrs;
                 match expr.node {
                     ExprKind::If(..) if !expr.attrs.is_empty() => {
                         // Just point to the first attribute in there...
                         let span = expr.attrs[0].span;
-
-                        self.span_err(span,
-                            "attributes are not yet allowed on `if` \
-                            expressions");
+                        self.span_err(span, "attributes are not yet allowed on `if` expressions");
                     }
                     _ => {}
                 }
@@ -2624,7 +2636,24 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_paren_expr_seq(&mut self) -> PResult<'a, Vec<P<Expr>>> {
-        self.parse_paren_comma_seq(|p| p.parse_expr()).map(|(r, _)| r)
+        self.parse_paren_comma_seq(|p| {
+            match p.parse_expr() {
+                Ok(expr) => Ok(expr),
+                Err(mut err) => match p.token.kind {
+                    token::Ident(name, false)
+                    if name == kw::Underscore && p.look_ahead(1, |t| {
+                        t == &token::Comma
+                    }) => {
+                        // Special-case handling of `foo(_, _, _)`
+                        err.emit();
+                        let sp = p.token.span;
+                        p.bump();
+                        Ok(p.mk_expr(sp, ExprKind::Err, ThinVec::new()))
+                    }
+                    _ => Err(err),
+                },
+            }
+        }).map(|(r, _)| r)
     }
 
     crate fn process_potential_macro_variable(&mut self) {
@@ -2806,9 +2835,10 @@ impl<'a> Parser<'a> {
     /// This parses an expression accounting for associativity and precedence of the operators in
     /// the expression.
     #[inline]
-    fn parse_assoc_expr(&mut self,
-                            already_parsed_attrs: Option<ThinVec<Attribute>>)
-                            -> PResult<'a, P<Expr>> {
+    fn parse_assoc_expr(
+        &mut self,
+        already_parsed_attrs: Option<ThinVec<Attribute>>,
+    ) -> PResult<'a, P<Expr>> {
         self.parse_assoc_expr_with(0, already_parsed_attrs.into())
     }
 

src/test/ui/async-await/async-block-control-flow-static-semantics.stderr

@@ -16,7 +16,7 @@ error[E0308]: mismatched types
 LL | fn return_targets_async_block_not_fn() -> u8 {
    |    ---------------------------------      ^^ expected u8, found ()
    |    |
-   |    this function's body doesn't return
+   |    implicitly returns `()` as its body has no tail or `return` expression
    |
    = note: expected type `u8`
               found type `()`
@@ -57,7 +57,7 @@ error[E0308]: mismatched types
 LL | fn rethrow_targets_async_block_not_fn() -> Result<u8, MyErr> {
    |    ----------------------------------      ^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found ()
    |    |
-   |    this function's body doesn't return
+   |    implicitly returns `()` as its body has no tail or `return` expression
    |
    = note: expected type `std::result::Result<u8, MyErr>`
               found type `()`
@@ -68,7 +68,7 @@ error[E0308]: mismatched types
 LL | fn rethrow_targets_async_block_not_async_fn() -> Result<u8, MyErr> {
    |    ----------------------------------------      ^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found ()
    |    |
-   |    this function's body doesn't return
+   |    implicitly returns `()` as its body has no tail or `return` expression
    |
    = note: expected type `std::result::Result<u8, MyErr>`
               found type `()`

src/test/ui/block-result/consider-removing-last-semi.stderr

@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL | fn f() -> String {
    |    -      ^^^^^^ expected struct `std::string::String`, found ()
    |    |
-   |    this function's body doesn't return
+   |    implicitly returns `()` as its body has no tail or `return` expression
 LL |     0u8;
 LL |     "bla".to_string();
    |                      - help: consider removing this semicolon
@@ -18,7 +18,7 @@ error[E0308]: mismatched types
 LL | fn g() -> String {
    |    -      ^^^^^^ expected struct `std::string::String`, found ()
    |    |
-   |    this function's body doesn't return
+   |    implicitly returns `()` as its body has no tail or `return` expression
 LL |     "this won't work".to_string();
 LL |     "removeme".to_string();
    |                           - help: consider removing this semicolon

src/test/ui/block-result/issue-11714.stderr

@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL | fn blah() -> i32 {
    |    ----      ^^^ expected i32, found ()
    |    |
-   |    this function's body doesn't return
+   |    implicitly returns `()` as its body has no tail or `return` expression
 ...
 LL |     ;
    |     - help: consider removing this semicolon