Require a trait in the bounds of existential types

src/librustc_passes/ast_validation.rs

@@ -20,7 +20,7 @@ use syntax::ptr::P;
 use syntax::visit::{self, Visitor};
 use syntax::{span_err, struct_span_err, walk_list};
 use syntax_ext::proc_macro_decls::is_proc_macro_attr;
-use syntax_pos::Span;
+use syntax_pos::{Span, MultiSpan};
 use errors::Applicability;
 use log::debug;
 
@@ -679,6 +679,14 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                                                 "unions cannot have zero fields");
                 }
             }
+            ItemKind::Existential(ref bounds, _) => {
+                if !bounds.iter()
+                          .any(|b| if let GenericBound::Trait(..) = *b { true } else { false }) {
+                    let msp = MultiSpan::from_spans(bounds.iter()
+                        .map(|bound| bound.span()).collect());
+                    self.err_handler().span_err(msp, "at least one trait must be specified");
+                }
+            }
             _ => {}
         }
 

src/test/run-pass/existential_type.rs

@@ -68,14 +68,14 @@ fn my_other_iter<U>(u: U) -> MyOtherIter<U> {
 }
 
 trait Trait {}
-existential type GenericBound<'a, T: Trait>: 'a;
+existential type GenericBound<'a, T: Trait>: Sized + 'a;
 
 fn generic_bound<'a, T: Trait + 'a>(t: T) -> GenericBound<'a, T> {
     t
 }
 
 mod pass_through {
-    pub existential type Passthrough<T>: 'static;
+    pub existential type Passthrough<T>: Sized + 'static;
 
     fn define_passthrough<T: 'static>(t: T) -> Passthrough<T> {
         t

src/test/ui/existential_types/existential-types-with-no-traits.rs

@@ -0,0 +1,14 @@
+#![feature(existential_type)]
+
+existential type Foo: 'static;
+//~^ ERROR: at least one trait must be specified
+
+fn foo() -> Foo {
+    "foo"
+}
+
+fn bar() -> impl 'static { //~ ERROR: at least one trait must be specified
+    "foo"
+}
+
+fn main() {}

src/test/ui/existential_types/existential-types-with-no-traits.stderr

@@ -0,0 +1,14 @@
+error: at least one trait must be specified
+  --> $DIR/existential-types-with-no-traits.rs:3:23
+   |
+LL | existential type Foo: 'static;
+   |                       ^^^^^^^
+
+error: at least one trait must be specified
+  --> $DIR/existential-types-with-no-traits.rs:10:13
+   |
+LL | fn bar() -> impl 'static {
+   |             ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/existential_types/generic_nondefining_use.rs

@@ -4,6 +4,8 @@ fn main() {}
 
 existential type Cmp<T>: 'static;
 //~^ ERROR could not find defining uses
+//~^^ ERROR: at least one trait must be specified
+
 
 // not a defining use, because it doesn't define *all* possible generics
 fn cmp() -> Cmp<u32> { //~ ERROR defining existential type use does not fully define

src/test/ui/existential_types/generic_nondefining_use.stderr

@@ -1,5 +1,11 @@
+error: at least one trait must be specified
+  --> $DIR/generic_nondefining_use.rs:5:26
+   |
+LL | existential type Cmp<T>: 'static;
+   |                          ^^^^^^^
+
 error: defining existential type use does not fully define existential type
-  --> $DIR/generic_nondefining_use.rs:9:1
+  --> $DIR/generic_nondefining_use.rs:11:1
    |
 LL | / fn cmp() -> Cmp<u32> {
 LL | |     5u32
@@ -12,5 +18,5 @@ error: could not find defining uses
 LL | existential type Cmp<T>: 'static;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 

src/test/ui/existential_types/generic_not_used.rs

@@ -3,6 +3,7 @@
 fn main() {}
 
 existential type WrongGeneric<T: 'static>: 'static;
+//~^ ERROR: at least one trait must be specified
 
 fn wrong_generic<U: 'static, V: 'static>(_: U, v: V) -> WrongGeneric<U> {
 //~^ ERROR type parameter `V` is part of concrete type but not used in parameter list

src/test/ui/existential_types/generic_not_used.stderr

@@ -1,5 +1,11 @@
+error: at least one trait must be specified
+  --> $DIR/generic_not_used.rs:5:44
+   |
+LL | existential type WrongGeneric<T: 'static>: 'static;
+   |                                            ^^^^^^^
+
 error: type parameter `V` is part of concrete type but not used in parameter list for existential type
-  --> $DIR/generic_not_used.rs:7:73
+  --> $DIR/generic_not_used.rs:8:73
    |
 LL |   fn wrong_generic<U: 'static, V: 'static>(_: U, v: V) -> WrongGeneric<U> {
    |  _________________________________________________________________________^
@@ -8,5 +14,5 @@ LL | |     v
 LL | | }
    | |_^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 

src/test/ui/existential_types/generic_type_does_not_live_long_enough.rs

@@ -8,6 +8,7 @@ fn main() {
 
 existential type WrongGeneric<T>: 'static;
 //~^ ERROR the parameter type `T` may not live long enough
+//~^^ ERROR: at least one trait must be specified
 
 fn wrong_generic<T>(t: T) -> WrongGeneric<T> {
     t

src/test/ui/existential_types/generic_type_does_not_live_long_enough.stderr

@@ -1,3 +1,9 @@
+error: at least one trait must be specified
+  --> $DIR/generic_type_does_not_live_long_enough.rs:9:35
+   |
+LL | existential type WrongGeneric<T>: 'static;
+   |                                   ^^^^^^^
+
 error[E0308]: mismatched types
   --> $DIR/generic_type_does_not_live_long_enough.rs:6:18
    |
@@ -22,7 +28,7 @@ note: ...so that the type `T` will meet its required lifetime bounds
 LL | existential type WrongGeneric<T>: 'static;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0308, E0310.
 For more information about an error, try `rustc --explain E0308`.

src/test/ui/existential_types/generic_underconstrained.rs

@@ -4,6 +4,7 @@ fn main() {}
 
 trait Trait {}
 existential type Underconstrained<T: Trait>: 'static; //~ ERROR the trait bound `T: Trait`
+//~^ ERROR: at least one trait must be specified
 
 // no `Trait` bound
 fn underconstrain<T>(_: T) -> Underconstrained<T> {

src/test/ui/existential_types/generic_underconstrained.stderr

@@ -1,3 +1,9 @@
+error: at least one trait must be specified
+  --> $DIR/generic_underconstrained.rs:6:46
+   |
+LL | existential type Underconstrained<T: Trait>: 'static;
+   |                                              ^^^^^^^
+
 error[E0277]: the trait bound `T: Trait` is not satisfied
   --> $DIR/generic_underconstrained.rs:6:1
    |
@@ -7,6 +13,6 @@ LL | existential type Underconstrained<T: Trait>: 'static;
    = help: consider adding a `where T: Trait` bound
    = note: the return type of a function must have a statically known size
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/existential_types/generic_underconstrained2.rs

@@ -4,6 +4,7 @@ fn main() {}
 
 existential type Underconstrained<T: std::fmt::Debug>: 'static;
 //~^ ERROR `U` doesn't implement `std::fmt::Debug`
+//~^^ ERROR: at least one trait must be specified
 
 // not a defining use, because it doesn't define *all* possible generics
 fn underconstrained<U>(_: U) -> Underconstrained<U> {
@@ -12,6 +13,7 @@ fn underconstrained<U>(_: U) -> Underconstrained<U> {
 
 existential type Underconstrained2<T: std::fmt::Debug>: 'static;
 //~^ ERROR `V` doesn't implement `std::fmt::Debug`
+//~^^ ERROR: at least one trait must be specified
 
 // not a defining use, because it doesn't define *all* possible generics
 fn underconstrained2<U, V>(_: U, _: V) -> Underconstrained2<V> {

src/test/ui/existential_types/generic_underconstrained2.stderr

@@ -1,3 +1,15 @@
+error: at least one trait must be specified
+  --> $DIR/generic_underconstrained2.rs:5:56
+   |
+LL | existential type Underconstrained<T: std::fmt::Debug>: 'static;
+   |                                                        ^^^^^^^
+
+error: at least one trait must be specified
+  --> $DIR/generic_underconstrained2.rs:14:57
+   |
+LL | existential type Underconstrained2<T: std::fmt::Debug>: 'static;
+   |                                                         ^^^^^^^
+
 error[E0277]: `U` doesn't implement `std::fmt::Debug`
   --> $DIR/generic_underconstrained2.rs:5:1
    |
@@ -9,7 +21,7 @@ LL | existential type Underconstrained<T: std::fmt::Debug>: 'static;
    = note: the return type of a function must have a statically known size
 
 error[E0277]: `V` doesn't implement `std::fmt::Debug`
-  --> $DIR/generic_underconstrained2.rs:13:1
+  --> $DIR/generic_underconstrained2.rs:14:1
    |
 LL | existential type Underconstrained2<T: std::fmt::Debug>: 'static;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `V` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
@@ -18,6 +30,6 @@ LL | existential type Underconstrained2<T: std::fmt::Debug>: 'static;
    = help: consider adding a `where V: std::fmt::Debug` bound
    = note: the return type of a function must have a statically known size
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/existential_types/unused_generic_param.rs

@@ -1,18 +1,17 @@
-// compile-pass
 #![feature(existential_type)]
 
 fn main() {
 }
 
-// test that unused generic parameters are ok
 existential type PartiallyDefined<T>: 'static;
+//~^ ERROR: at least one trait must be specified
 
 fn partially_defined<T: std::fmt::Debug>(_: T) -> PartiallyDefined<T> {
     4u32
 }
 
-// test that unused generic parameters are ok
 existential type PartiallyDefined2<T>: 'static;
+//~^ ERROR: at least one trait must be specified
 
 fn partially_defined2<T: std::fmt::Debug>(_: T) -> PartiallyDefined2<T> {
     4u32

src/test/ui/existential_types/unused_generic_param.stderr

@@ -0,0 +1,14 @@
+error: at least one trait must be specified
+  --> $DIR/unused_generic_param.rs:6:39
+   |
+LL | existential type PartiallyDefined<T>: 'static;
+   |                                       ^^^^^^^
+
+error: at least one trait must be specified
+  --> $DIR/unused_generic_param.rs:13:40
+   |
+LL | existential type PartiallyDefined2<T>: 'static;
+   |                                        ^^^^^^^
+
+error: aborting due to 2 previous errors
+