Suggest type param when encountering `_` in fn defs

When encountering `_` type placeholder in fn arguments and return type,
suggest using generic type parameters.
Expand what counts as an inferable return type to slice, array and
tuples of `_`.

8 files changed, 376 insertions, 80 deletions

diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index f3ac159a3aa..6a681be6f3e 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -7,6 +7,7 @@ use crate::hir::def_id::DefId;
 use crate::hir::print;
 use crate::hir::ptr::P;
 use crate::hir::{self, ExprKind, GenericArg, GenericArgs, HirVec};
+use crate::hir::intravisit::{NestedVisitorMap, Visitor};
 use crate::lint;
 use crate::middle::lang_items::SizedTraitLangItem;
 use crate::middle::resolve_lifetime as rl;
@@ -66,6 +67,8 @@ pub trait AstConv<'tcx> {
     /// Returns the type to use when a type is omitted.
     fn ty_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx>;
 
+    fn allow_ty_infer(&self) -> bool;
+
     /// Returns the const to use when a const is omitted.
     fn ct_infer(
         &self,
@@ -2593,7 +2596,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
             hir::TyKind::BareFn(ref bf) => {
                 require_c_abi_if_c_variadic(tcx, &bf.decl, bf.abi, ast_ty.span);
-                tcx.mk_fn_ptr(self.ty_of_fn(bf.unsafety, bf.abi, &bf.decl))
+                tcx.mk_fn_ptr(self.ty_of_fn(bf.unsafety, bf.abi, &bf.decl, &[], None))
             }
             hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
                 self.conv_object_ty_poly_trait_ref(ast_ty.span, bounds, lifetime)
@@ -2758,14 +2761,55 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         unsafety: hir::Unsafety,
         abi: abi::Abi,
         decl: &hir::FnDecl<'_>,
+        generic_params: &[hir::GenericParam<'_>],
+        ident_span: Option<Span>,
     ) -> ty::PolyFnSig<'tcx> {
         debug!("ty_of_fn");
 
         let tcx = self.tcx();
-        let input_tys = decl.inputs.iter().map(|a| self.ty_of_arg(a, None));
 
+        // We proactively collect all the infered type params to emit a single error per fn def.
+        struct PlaceholderHirTyCollector(Vec<Span>);
+        impl<'v> Visitor<'v> for PlaceholderHirTyCollector {
+            fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
+                NestedVisitorMap::None
+            }
+            fn visit_ty(&mut self, t: &'v hir::Ty<'v>) {
+                if let hir::TyKind::Infer = t.kind {
+                    self.0.push(t.span);
+                }
+                hir::intravisit::walk_ty(self, t)
+            }
+        }
+        let mut placeholder_types = vec![];
+        let mut output_placeholder_types = vec![];
+
+        let input_tys = decl.inputs.iter().map(|a| {
+            let mut visitor = PlaceholderHirTyCollector(vec![]);
+            visitor.visit_ty(&a);
+            if visitor.0.is_empty() || self.allow_ty_infer() {
+                self.ty_of_arg(a, None)
+            } else {
+                placeholder_types.extend(visitor.0);
+                tcx.types.err
+            }
+        });
         let output_ty = match decl.output {
-            hir::Return(ref output) => self.ast_ty_to_ty(output),
+            hir::Return(ref output) => {
+                let mut visitor = PlaceholderHirTyCollector(vec![]);
+                visitor.visit_ty(output);
+                let is_infer = if let hir::TyKind::Infer = output.kind {
+                    true
+                } else {
+                    false
+                };
+                if (is_infer || !visitor.0.is_empty()) && !self.allow_ty_infer() {
+                    output_placeholder_types.extend(visitor.0);
+                    tcx.types.err
+                } else {
+                    self.ast_ty_to_ty(output)
+                }
+            }
             hir::DefaultReturn(..) => tcx.mk_unit(),
         };
 
@@ -2774,6 +2818,39 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let bare_fn_ty =
             ty::Binder::bind(tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi));
 
+        placeholder_types.extend(output_placeholder_types);
+
+        if !placeholder_types.is_empty() {
+            let mut sugg = placeholder_types.iter().cloned()
+                .map(|sp| (sp, "T".to_owned()))
+                .collect::<Vec<_>>();
+            if let Some(span) = ident_span {
+                if generic_params.is_empty() {
+                    sugg.push((span.shrink_to_hi(), "<T>".to_string()));
+                } else {
+                    sugg.push((
+                        generic_params.iter().last().unwrap().span.shrink_to_hi(),
+                        ", T".to_string(),
+                    ));
+                }
+            }
+            let mut err = struct_span_err!(
+                tcx.sess,
+                placeholder_types,
+                E0121,
+                "the type placeholder `_` is not allowed within types on item signatures",
+            );
+            if ident_span.is_some() {
+                err.multipart_suggestion(
+                    "use type parameters instead",
+                    sugg,
+                    Applicability::HasPlaceholders,
+                );
+            }
+            err.emit();
+        }
+
+
         // Find any late-bound regions declared in return type that do
         // not appear in the arguments. These are not well-formed.
         //
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index c6c3ada49e3..d9beb8ee3bd 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -963,7 +963,7 @@ fn typeck_tables_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::TypeckTables<'_> {
         let fcx = if let (Some(header), Some(decl)) = (fn_header, fn_decl) {
             let fn_sig = if crate::collect::get_infer_ret_ty(&decl.output).is_some() {
                 let fcx = FnCtxt::new(&inh, param_env, body.value.hir_id);
-                AstConv::ty_of_fn(&fcx, header.unsafety, header.abi, decl)
+                AstConv::ty_of_fn(&fcx, header.unsafety, header.abi, decl, &[], None)
             } else {
                 tcx.fn_sig(def_id)
             };
@@ -1069,6 +1069,7 @@ fn typeck_tables_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::TypeckTables<'_> {
             let ty = fcx.normalize_ty(span, ty);
             fcx.require_type_is_sized(ty, span, code);
         }
+
         fcx.select_all_obligations_or_error();
 
         if fn_decl.is_some() {
@@ -2563,6 +2564,10 @@ impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
         Some(self.next_region_var(v))
     }
 
+    fn allow_ty_infer(&self) -> bool {
+        true
+    }
+
     fn ty_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx> {
         if let Some(param) = param {
             if let GenericArgKind::Type(ty) = self.var_for_def(span, param).unpack() {
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index 9604a9ade92..6ab30c4b7e7 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -195,6 +195,10 @@ impl AstConv<'tcx> for ItemCtxt<'tcx> {
         None
     }
 
+    fn allow_ty_infer(&self) -> bool {
+        false
+    }
+
     fn ty_infer(&self, _: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx> {
         bad_placeholder_type(self.tcx(), span).emit();
 
@@ -1699,9 +1703,26 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
     }
 }
 
+fn is_infer_ty(ty: &hir::Ty<'_>) -> bool {
+    match &ty.kind {
+        hir::TyKind::Infer => true,
+        hir::TyKind::Slice(ty) | hir::TyKind::Array(ty, _) => is_infer_ty(ty),
+        hir::TyKind::Tup(tys)
+            if !tys.is_empty()
+                && tys.iter().all(|ty| match ty.kind {
+                    hir::TyKind::Infer => true,
+                    _ => false,
+                }) =>
+        {
+            true
+        }
+        _ => false,
+    }
+}
+
 pub fn get_infer_ret_ty(output: &'hir hir::FunctionRetTy<'hir>) -> Option<&'hir hir::Ty<'hir>> {
     if let hir::FunctionRetTy::Return(ref ty) = output {
-        if let hir::TyKind::Infer = ty.kind {
+        if is_infer_ty(ty) {
             return Some(&**ty);
         }
     }
@@ -1719,10 +1740,12 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
     match tcx.hir().get(hir_id) {
         TraitItem(hir::TraitItem {
             kind: TraitItemKind::Method(sig, TraitMethod::Provided(_)),
+            ident,
+            generics,
             ..
         })
-        | ImplItem(hir::ImplItem { kind: ImplItemKind::Method(sig, _), .. })
-        | Item(hir::Item { kind: ItemKind::Fn(sig, _, _), .. }) => {
+        | ImplItem(hir::ImplItem { kind: ImplItemKind::Method(sig, _), ident, generics, .. })
+        | Item(hir::Item { kind: ItemKind::Fn(sig, generics, _), ident, .. }) => {
             match get_infer_ret_ty(&sig.decl.output) {
                 Some(ty) => {
                     let fn_sig = tcx.typeck_tables_of(def_id).liberated_fn_sigs()[hir_id];
@@ -1731,7 +1754,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                     if ret_ty != tcx.types.err {
                         diag.span_suggestion(
                             ty.span,
-                            "replace `_` with the correct return type",
+                            "replace this with the correct return type",
                             ret_ty.to_string(),
                             Applicability::MaybeIncorrect,
                         );
@@ -1739,14 +1762,30 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                     diag.emit();
                     ty::Binder::bind(fn_sig)
                 }
-                None => AstConv::ty_of_fn(&icx, sig.header.unsafety, sig.header.abi, &sig.decl),
+                None => AstConv::ty_of_fn(
+                    &icx,
+                    sig.header.unsafety,
+                    sig.header.abi,
+                    &sig.decl,
+                    &generics.params[..],
+                    Some(ident.span),
+                ),
             }
         }
 
         TraitItem(hir::TraitItem {
             kind: TraitItemKind::Method(FnSig { header, decl }, _),
+            ident,
+            generics,
             ..
-        }) => AstConv::ty_of_fn(&icx, header.unsafety, header.abi, decl),
+        }) => AstConv::ty_of_fn(
+            &icx,
+            header.unsafety,
+            header.abi,
+            decl,
+            &generics.params[..],
+            Some(ident.span),
+        ),
 
         ForeignItem(&hir::ForeignItem { kind: ForeignItemKind::Fn(ref fn_decl, _, _), .. }) => {
             let abi = tcx.hir().get_foreign_abi(hir_id);
@@ -2351,7 +2390,7 @@ fn compute_sig_of_foreign_fn_decl<'tcx>(
     } else {
         hir::Unsafety::Unsafe
     };
-    let fty = AstConv::ty_of_fn(&ItemCtxt::new(tcx, def_id), unsafety, abi, decl);
+    let fty = AstConv::ty_of_fn(&ItemCtxt::new(tcx, def_id), unsafety, abi, decl, &[], None);
 
     // Feature gate SIMD types in FFI, since I am not sure that the
     // ABIs are handled at all correctly. -huonw
diff --git a/src/test/ui/error-codes/E0121.stderr b/src/test/ui/error-codes/E0121.stderr
index beb8941320b..5da9a4c080f 100644
--- a/src/test/ui/error-codes/E0121.stderr
+++ b/src/test/ui/error-codes/E0121.stderr
@@ -5,7 +5,7 @@ LL | fn foo() -> _ { 5 }
    |             ^
    |             |
    |             not allowed in type signatures
-   |             help: replace `_` with the correct return type: `i32`
+   |             help: replace this with the correct return type: `i32`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/E0121.rs:3:13
diff --git a/src/test/ui/self/self-infer.stderr b/src/test/ui/self/self-infer.stderr
index f91cfe5eb62..b064928d8a3 100644
--- a/src/test/ui/self/self-infer.stderr
+++ b/src/test/ui/self/self-infer.stderr
@@ -2,13 +2,23 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
   --> $DIR/self-infer.rs:4:16
    |
 LL |     fn f(self: _) {}
-   |                ^ not allowed in type signatures
+   |                ^
+   |
+help: use type parameters instead
+   |
+LL |     fn f<T>(self: T) {}
+   |         ^^^       ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/self-infer.rs:5:17
    |
 LL |     fn g(self: &_) {}
-   |                 ^ not allowed in type signatures
+   |                 ^
+   |
+help: use type parameters instead
+   |
+LL |     fn g<T>(self: &T) {}
+   |         ^^^        ^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/typeck/typeck_type_placeholder_item.rs b/src/test/ui/typeck/typeck_type_placeholder_item.rs
index 46a5b8580dc..03ee61486c6 100644
--- a/src/test/ui/typeck/typeck_type_placeholder_item.rs
+++ b/src/test/ui/typeck/typeck_type_placeholder_item.rs
@@ -6,7 +6,6 @@ fn test() -> _ { 5 }
 
 fn test2() -> (_, _) { (5, 5) }
 //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
-//~^^ ERROR the type placeholder `_` is not allowed within types on item signatures
 
 static TEST3: _ = "test";
 //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
@@ -59,7 +58,6 @@ pub fn main() {
 
     fn fn_test2() -> (_, _) { (5, 5) }
     //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
-    //~^^ ERROR the type placeholder `_` is not allowed within types on item signatures
 
     static FN_TEST3: _ = "test";
     //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
@@ -106,4 +104,28 @@ pub fn main() {
         //~^^ ERROR the type placeholder `_` is not allowed within types on item signatures
     }
 
+    fn fn_test11(_: _) -> (_, _) { panic!() }
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    //~| ERROR type annotations needed
+
+    fn fn_test12(x: i32) -> (_, _) { (x, x) }
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+
+    fn fn_test13(x: _) -> (i32, _) { (x, x) }
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
 }
+
+trait T {
+    fn method_test1(&self, x: _);
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    fn method_test2(&self, x: _) -> _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    fn method_test3(&self) -> _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    fn assoc_fn_test1(x: _);
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    fn assoc_fn_test2(x: _) -> _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    fn assoc_fn_test3() -> _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+}
\ No newline at end of file
diff --git a/src/test/ui/typeck/typeck_type_placeholder_item.stderr b/src/test/ui/typeck/typeck_type_placeholder_item.stderr
index 2b4d9966c3d..0edfa07a656 100644
--- a/src/test/ui/typeck/typeck_type_placeholder_item.stderr
+++ b/src/test/ui/typeck/typeck_type_placeholder_item.stderr
@@ -5,22 +5,19 @@ LL | fn test() -> _ { 5 }
    |              ^
    |              |
    |              not allowed in type signatures
-   |              help: replace `_` with the correct return type: `i32`
+   |              help: replace this with the correct return type: `i32`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:7:16
+  --> $DIR/typeck_type_placeholder_item.rs:7:15
    |
 LL | fn test2() -> (_, _) { (5, 5) }
-   |                ^ not allowed in type signatures
-
-error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:7:19
-   |
-LL | fn test2() -> (_, _) { (5, 5) }
-   |                   ^ not allowed in type signatures
+   |               ^^^^^^
+   |               |
+   |               not allowed in type signatures
+   |               help: replace this with the correct return type: `(i32, i32)`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:11:15
+  --> $DIR/typeck_type_placeholder_item.rs:10:15
    |
 LL | static TEST3: _ = "test";
    |               ^
@@ -29,7 +26,7 @@ LL | static TEST3: _ = "test";
    |               help: replace `_` with the correct type: `&'static str`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:14:15
+  --> $DIR/typeck_type_placeholder_item.rs:13:15
    |
 LL | static TEST4: _ = 145;
    |               ^
@@ -38,94 +35,106 @@ LL | static TEST4: _ = 145;
    |               help: replace `_` with the correct type: `i32`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:17:16
+  --> $DIR/typeck_type_placeholder_item.rs:16:16
    |
 LL | static TEST5: (_, _) = (1, 2);
    |                ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:17:19
+  --> $DIR/typeck_type_placeholder_item.rs:16:19
    |
 LL | static TEST5: (_, _) = (1, 2);
    |                   ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:21:13
+  --> $DIR/typeck_type_placeholder_item.rs:20:13
    |
 LL | fn test6(_: _) { }
-   |             ^ not allowed in type signatures
+   |             ^
+   |
+help: use type parameters instead
+   |
+LL | fn test6<T>(_: T) { }
+   |         ^^^    ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:24:13
+  --> $DIR/typeck_type_placeholder_item.rs:23:13
    |
 LL | fn test7(x: _) { let _x: usize = x; }
-   |             ^ not allowed in type signatures
+   |             ^
+   |
+help: use type parameters instead
+   |
+LL | fn test7<T>(x: T) { let _x: usize = x; }
+   |         ^^^    ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:27:22
+  --> $DIR/typeck_type_placeholder_item.rs:26:22
    |
 LL | fn test8(_f: fn() -> _) { }
-   |                      ^ not allowed in type signatures
+   |                      ^
+   |
+help: use type parameters instead
+   |
+LL | fn test8<T>(_f: fn() -> T) { }
+   |         ^^^             ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:49:8
+  --> $DIR/typeck_type_placeholder_item.rs:48:8
    |
 LL |     a: _,
    |        ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:51:9
+  --> $DIR/typeck_type_placeholder_item.rs:50:9
    |
 LL |     b: (_, _),
    |         ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:51:12
+  --> $DIR/typeck_type_placeholder_item.rs:50:12
    |
 LL |     b: (_, _),
    |            ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:102:12
+  --> $DIR/typeck_type_placeholder_item.rs:100:12
    |
 LL |         a: _,
    |            ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:104:13
+  --> $DIR/typeck_type_placeholder_item.rs:102:13
    |
 LL |         b: (_, _),
    |             ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:104:16
+  --> $DIR/typeck_type_placeholder_item.rs:102:16
    |
 LL |         b: (_, _),
    |                ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:57:21
+  --> $DIR/typeck_type_placeholder_item.rs:56:21
    |
 LL |     fn fn_test() -> _ { 5 }
    |                     ^
    |                     |
    |                     not allowed in type signatures
-   |                     help: replace `_` with the correct return type: `i32`
-
-error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:60:23
-   |
-LL |     fn fn_test2() -> (_, _) { (5, 5) }
-   |                       ^ not allowed in type signatures
+   |                     help: replace this with the correct return type: `i32`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:60:26
+  --> $DIR/typeck_type_placeholder_item.rs:59:22
    |
 LL |     fn fn_test2() -> (_, _) { (5, 5) }
-   |                          ^ not allowed in type signatures
+   |                      ^^^^^^
+   |                      |
+   |                      not allowed in type signatures
+   |                      help: replace this with the correct return type: `(i32, i32)`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:64:22
+  --> $DIR/typeck_type_placeholder_item.rs:62:22
    |
 LL |     static FN_TEST3: _ = "test";
    |                      ^
@@ -134,7 +143,7 @@ LL |     static FN_TEST3: _ = "test";
    |                      help: replace `_` with the correct type: `&'static str`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:67:22
+  --> $DIR/typeck_type_placeholder_item.rs:65:22
    |
 LL |     static FN_TEST4: _ = 145;
    |                      ^
@@ -143,95 +152,229 @@ LL |     static FN_TEST4: _ = 145;
    |                      help: replace `_` with the correct type: `i32`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:70:23
+  --> $DIR/typeck_type_placeholder_item.rs:68:23
    |
 LL |     static FN_TEST5: (_, _) = (1, 2);
    |                       ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:70:26
+  --> $DIR/typeck_type_placeholder_item.rs:68:26
    |
 LL |     static FN_TEST5: (_, _) = (1, 2);
    |                          ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:74:20
+  --> $DIR/typeck_type_placeholder_item.rs:72:20
    |
 LL |     fn fn_test6(_: _) { }
-   |                    ^ not allowed in type signatures
+   |                    ^
+   |
+help: use type parameters instead
+   |
+LL |     fn fn_test6<T>(_: T) { }
+   |                ^^^    ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:77:20
+  --> $DIR/typeck_type_placeholder_item.rs:75:20
    |
 LL |     fn fn_test7(x: _) { let _x: usize = x; }
-   |                    ^ not allowed in type signatures
+   |                    ^
+   |
+help: use type parameters instead
+   |
+LL |     fn fn_test7<T>(x: T) { let _x: usize = x; }
+   |                ^^^    ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:80:29
+  --> $DIR/typeck_type_placeholder_item.rs:78:29
    |
 LL |     fn fn_test8(_f: fn() -> _) { }
-   |                             ^ not allowed in type signatures
+   |                             ^
+   |
+help: use type parameters instead
+   |
+LL |     fn fn_test8<T>(_f: fn() -> T) { }
+   |                ^^^             ^
+
+error[E0282]: type annotations needed
+  --> $DIR/typeck_type_placeholder_item.rs:107:27
+   |
+LL |     fn fn_test11(_: _) -> (_, _) { panic!() }
+   |                           ^^^^^^ cannot infer type
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:33:24
+  --> $DIR/typeck_type_placeholder_item.rs:107:27
+   |
+LL |     fn fn_test11(_: _) -> (_, _) { panic!() }
+   |                           ^^^^^^ not allowed in type signatures
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:111:29
+   |
+LL |     fn fn_test12(x: i32) -> (_, _) { (x, x) }
+   |                             ^^^^^^
+   |                             |
+   |                             not allowed in type signatures
+   |                             help: replace this with the correct return type: `(i32, i32)`
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:114:21
+   |
+LL |     fn fn_test13(x: _) -> (i32, _) { (x, x) }
+   |                     ^           ^
+   |
+help: use type parameters instead
+   |
+LL |     fn fn_test13<T>(x: T) -> (i32, T) { (x, x) }
+   |                 ^^^    ^           ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:119:31
+   |
+LL |     fn method_test1(&self, x: _);
+   |                               ^
+   |
+help: use type parameters instead
+   |
+LL |     fn method_test1<T>(&self, x: T);
+   |                    ^^^           ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:121:31
+   |
+LL |     fn method_test2(&self, x: _) -> _;
+   |                               ^     ^
+   |
+help: use type parameters instead
+   |
+LL |     fn method_test2<T>(&self, x: T) -> T;
+   |                    ^^^           ^     ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:123:31
+   |
+LL |     fn method_test3(&self) -> _;
+   |                               ^
+   |
+help: use type parameters instead
+   |
+LL |     fn method_test3<T>(&self) -> T;
+   |                    ^^^           ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:125:26
+   |
+LL |     fn assoc_fn_test1(x: _);
+   |                          ^
+   |
+help: use type parameters instead
+   |
+LL |     fn assoc_fn_test1<T>(x: T);
+   |                      ^^^    ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:127:26
+   |
+LL |     fn assoc_fn_test2(x: _) -> _;
+   |                          ^     ^
+   |
+help: use type parameters instead
+   |
+LL |     fn assoc_fn_test2<T>(x: T) -> T;
+   |                      ^^^    ^     ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:129:28
+   |
+LL |     fn assoc_fn_test3() -> _;
+   |                            ^
+   |
+help: use type parameters instead
+   |
+LL |     fn assoc_fn_test3<T>() -> T;
+   |                      ^^^      ^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:32:24
    |
 LL |     fn test9(&self) -> _ { () }
    |                        ^
    |                        |
    |                        not allowed in type signatures
-   |                        help: replace `_` with the correct return type: `()`
+   |                        help: replace this with the correct return type: `()`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:36:27
+  --> $DIR/typeck_type_placeholder_item.rs:35:27
    |
 LL |     fn test10(&self, _x : _) { }
-   |                           ^ not allowed in type signatures
+   |                           ^
+   |
+help: use type parameters instead
+   |
+LL |     fn test10<T>(&self, _x : T) { }
+   |              ^^^             ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:41:24
+  --> $DIR/typeck_type_placeholder_item.rs:40:24
    |
 LL |     fn clone(&self) -> _ { Test9 }
    |                        ^
    |                        |
    |                        not allowed in type signatures
-   |                        help: replace `_` with the correct return type: `Test9`
+   |                        help: replace this with the correct return type: `Test9`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:44:37
+  --> $DIR/typeck_type_placeholder_item.rs:43:37
    |
 LL |     fn clone_from(&mut self, other: _) { *self = Test9; }
-   |                                     ^ not allowed in type signatures
+   |                                     ^
+   |
+help: use type parameters instead
+   |
+LL |     fn clone_from<T>(&mut self, other: T) { *self = Test9; }
+   |                  ^^^                   ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:86:31
+  --> $DIR/typeck_type_placeholder_item.rs:84:31
    |
 LL |         fn fn_test9(&self) -> _ { () }
    |                               ^
    |                               |
    |                               not allowed in type signatures
-   |                               help: replace `_` with the correct return type: `()`
+   |                               help: replace this with the correct return type: `()`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:89:34
+  --> $DIR/typeck_type_placeholder_item.rs:87:34
    |
 LL |         fn fn_test10(&self, _x : _) { }
-   |                                  ^ not allowed in type signatures
+   |                                  ^
+   |
+help: use type parameters instead
+   |
+LL |         fn fn_test10<T>(&self, _x : T) { }
+   |                     ^^^             ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:94:28
+  --> $DIR/typeck_type_placeholder_item.rs:92:28
    |
 LL |         fn clone(&self) -> _ { FnTest9 }
    |                            ^
    |                            |
    |                            not allowed in type signatures
-   |                            help: replace `_` with the correct return type: `main::FnTest9`
+   |                            help: replace this with the correct return type: `main::FnTest9`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:97:41
+  --> $DIR/typeck_type_placeholder_item.rs:95:41
    |
 LL |         fn clone_from(&mut self, other: _) { *self = FnTest9; }
-   |                                         ^ not allowed in type signatures
+   |                                         ^
+   |
+help: use type parameters instead
+   |
+LL |         fn clone_from<T>(&mut self, other: T) { *self = FnTest9; }
+   |                      ^^^                   ^
 
-error: aborting due to 34 previous errors
+error: aborting due to 42 previous errors
 
-For more information about this error, try `rustc --explain E0121`.
+Some errors have detailed explanations: E0121, E0282.
+For more information about an error, try `rustc --explain E0121`.
diff --git a/src/test/ui/typeck/typeck_type_placeholder_item_help.stderr b/src/test/ui/typeck/typeck_type_placeholder_item_help.stderr
index c5b9566290c..ab002381b1f 100644
--- a/src/test/ui/typeck/typeck_type_placeholder_item_help.stderr
+++ b/src/test/ui/typeck/typeck_type_placeholder_item_help.stderr
@@ -5,7 +5,7 @@ LL | fn test1() -> _ { Some(42) }
    |               ^
    |               |
    |               not allowed in type signatures
-   |               help: replace `_` with the correct return type: `std::option::Option<i32>`
+   |               help: replace this with the correct return type: `std::option::Option<i32>`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/typeck_type_placeholder_item_help.rs:7:14