Implement existential types

src/test/parse-fail/issue-20711-2.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // compile-flags: -Z parse-only
+// ignore-tidy-linelength
 
 struct Foo;
 
@@ -16,6 +17,6 @@ impl Foo {
     fn foo() {}
 
     #[stable(feature = "rust1", since = "1.0.0")]
-} //~ ERROR expected one of `async`, `const`, `crate`, `default`, `extern`, `fn`, `pub`, `type`, or
+} //~ ERROR expected one of `async`, `const`, `crate`, `default`, `existential`, `extern`, `fn`, `pub`, `type`, or
 
 fn main() {}

src/test/parse-fail/issue-20711.rs

@@ -9,11 +9,12 @@
 // except according to those terms.
 
 // compile-flags: -Z parse-only
+// ignore-tidy-linelength
 
 struct Foo;
 
 impl Foo {
     #[stable(feature = "rust1", since = "1.0.0")]
-} //~ ERROR expected one of `async`, `const`, `crate`, `default`, `extern`, `fn`, `pub`, `type`, or
+} //~ ERROR expected one of `async`, `const`, `crate`, `default`, `existential`, `extern`, `fn`, `pub`, `type`, or
 
 fn main() {}

src/test/parse-fail/removed-syntax-static-fn.rs

@@ -9,10 +9,11 @@
 // except according to those terms.
 
 // compile-flags: -Z parse-only
+// ignore-tidy-linelength
 
 struct S;
 
 impl S {
     static fn f() {}
 }
-//~^^ ERROR expected one of `async`, `const`, `crate`, `default`, `extern`, `fn`, `pub`, `type`,
+//~^^ ERROR expected one of `async`, `const`, `crate`, `default`, `existential`, `extern`, `fn`, `pub`, `type`,

src/test/run-pass/existential_type.rs

@@ -0,0 +1,113 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(existential_type)]
+
+fn main() {
+    assert_eq!(foo().to_string(), "foo");
+    assert_eq!(bar1().to_string(), "bar1");
+    assert_eq!(bar2().to_string(), "bar2");
+    let mut x = bar1();
+    x = bar2();
+    assert_eq!(boo::boo().to_string(), "boo");
+    assert_eq!(my_iter(42u8).collect::<Vec<u8>>(), vec![42u8]);
+}
+
+// single definition
+existential type Foo: std::fmt::Display;
+
+fn foo() -> Foo {
+    "foo"
+}
+
+// two definitions
+existential type Bar: std::fmt::Display;
+
+fn bar1() -> Bar {
+    "bar1"
+}
+
+fn bar2() -> Bar {
+    "bar2"
+}
+
+// definition in submodule
+existential type Boo: std::fmt::Display;
+
+mod boo {
+    pub fn boo() -> super::Boo {
+        "boo"
+    }
+}
+
+existential type MyIter<T>: Iterator<Item = T>;
+
+fn my_iter<T>(t: T) -> MyIter<T> {
+    std::iter::once(t)
+}
+
+fn my_iter2<T>(t: T) -> MyIter<T> {
+    std::iter::once(t)
+}
+
+// param names should not have an effect!
+fn my_iter3<U>(u: U) -> MyIter<U> {
+    std::iter::once(u)
+}
+
+// param position should not have an effect!
+fn my_iter4<U, V>(_: U, v: V) -> MyIter<V> {
+    std::iter::once(v)
+}
+
+// param names should not have an effect!
+existential type MyOtherIter<T>: Iterator<Item = T>;
+
+fn my_other_iter<U>(u: U) -> MyOtherIter<U> {
+    std::iter::once(u)
+}
+
+trait Trait {}
+existential type GenericBound<T: Trait>: 'static;
+
+fn generic_bound<T: Trait>(_: T) -> GenericBound<T> {
+    unimplemented!()
+}
+
+mod pass_through {
+    pub existential type Passthrough<T>: 'static;
+
+    fn define_passthrough<T: 'static>(t: T) -> Passthrough<T> {
+        t
+    }
+}
+
+fn use_passthrough(x: pass_through::Passthrough<u32>) -> pass_through::Passthrough<u32> {
+    x
+}
+
+existential type PartiallyDefined<T>: 'static;
+
+// doesn't declare all PartiallyDefined for all possible `T`, but since it's the only
+// function producing the value, noone can ever get a value that is problematic
+fn partially_defined<T: std::fmt::Debug>(_: T) -> PartiallyDefined<T> {
+    4u32
+}
+
+existential type PartiallyDefined2<T>: 'static;
+
+fn partially_defined2<T: std::fmt::Debug>(_: T) -> PartiallyDefined2<T> {
+    4u32
+}
+
+// fully defines PartiallyDefine2
+fn partially_defined22<T>(_: T) -> PartiallyDefined2<T> {
+    4u32
+}

src/test/ui/existential_type.nll.stderr

@@ -0,0 +1,111 @@
+error: defining existential type use differs from previous
+  --> $DIR/existential_type.rs:23:1
+   |
+LL | / fn bar() -> Foo { //~ ERROR defining existential type use differs from previous
+LL | |     42i32
+LL | | }
+   | |_^
+   |
+note: previous use here
+  --> $DIR/existential_type.rs:19:1
+   |
+LL | / fn foo() -> Foo {
+LL | |     ""
+LL | | }
+   | |_^
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:36:5
+   |
+LL | fn bomp() -> boo::Boo {
+   |              -------- expected `Boo` because of return type
+LL |     "" //~ ERROR mismatched types
+   |     ^^ expected anonymized type, found reference
+   |
+   = note: expected type `Boo`
+              found type `&'static str`
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:50:23
+   |
+LL |         let _: &str = bomp(); //~ ERROR mismatched types
+   |                       ^^^^^^ expected &str, found anonymized type
+   |
+   = note: expected type `&str`
+              found type `Boo`
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:54:9
+   |
+LL |     fn bomp() -> boo::Boo {
+   |                  -------- expected `Boo` because of return type
+LL |         "" //~ ERROR mismatched types
+   |         ^^ expected anonymized type, found reference
+   |
+   = note: expected type `Boo`
+              found type `&'static str`
+
+error[E0277]: the trait bound `T: Trait` is not satisfied
+  --> $DIR/existential_type.rs:61:1
+   |
+LL | existential type Underconstrained<T: Trait>: 'static; //~ ERROR the trait bound `T: Trait`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
+   |
+   = help: consider adding a `where T: Trait` bound
+   = note: the return type of a function must have a statically known size
+
+warning: not reporting region error due to nll
+  --> $DIR/existential_type.rs:78:1
+   |
+LL | existential type WrongGeneric<T>: 'static;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:93:27
+   |
+LL |     let _: &'static str = x; //~ mismatched types
+   |                           ^ expected reference, found anonymized type
+   |
+   = note: expected type `&'static str`
+              found type `NoReveal`
+
+error[E0605]: non-primitive cast: `NoReveal` as `&'static str`
+  --> $DIR/existential_type.rs:94:13
+   |
+LL |     let _ = x as &'static str; //~ non-primitive cast
+   |             ^^^^^^^^^^^^^^^^^
+   |
+   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+
+error: could not find defining uses
+  --> $DIR/existential_type.rs:28:1
+   |
+LL | existential type Bar: std::fmt::Debug; //~ ERROR could not find defining uses
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: could not find defining uses
+  --> $DIR/existential_type.rs:32:5
+   |
+LL |     pub existential type Boo: ::std::fmt::Debug; //~ ERROR could not find defining uses
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: defining existential type use differs from previous
+  --> $DIR/existential_type.rs:74:1
+   |
+LL | / fn my_iter2<T>(t: T) -> MyIter<T> { //~ ERROR defining existential type use differs from previous
+LL | |     Some(t).into_iter()
+LL | | }
+   | |_^
+   |
+note: previous use here
+  --> $DIR/existential_type.rs:70:1
+   |
+LL | / fn my_iter<T>(t: T) -> MyIter<T> {
+LL | |     std::iter::once(t)
+LL | | }
+   | |_^
+
+error: aborting due to 10 previous errors
+
+Some errors occurred: E0277, E0308, E0605.
+For more information about an error, try `rustc --explain E0277`.

src/test/ui/existential_type.rs

@@ -0,0 +1,95 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+#![feature(existential_type)]
+
+fn main() {}
+
+// two definitions with different types
+existential type Foo: std::fmt::Debug;
+
+fn foo() -> Foo {
+    ""
+}
+
+fn bar() -> Foo { //~ ERROR defining existential type use differs from previous
+    42i32
+}
+
+// declared but never defined
+existential type Bar: std::fmt::Debug; //~ ERROR could not find defining uses
+
+mod boo {
+    // declared in module but not defined inside of it
+    pub existential type Boo: ::std::fmt::Debug; //~ ERROR could not find defining uses
+}
+
+fn bomp() -> boo::Boo {
+    "" //~ ERROR mismatched types
+}
+
+mod boo2 {
+    mod boo {
+        pub existential type Boo: ::std::fmt::Debug;
+        fn bomp() -> Boo {
+            ""
+        }
+    }
+
+    // don't actually know the type here
+
+    fn bomp2() {
+        let _: &str = bomp(); //~ ERROR mismatched types
+    }
+
+    fn bomp() -> boo::Boo {
+        "" //~ ERROR mismatched types
+    }
+}
+
+// generics
+
+trait Trait {}
+existential type Underconstrained<T: Trait>: 'static; //~ ERROR the trait bound `T: Trait`
+
+// no `Trait` bound
+fn underconstrain<T>(_: T) -> Underconstrained<T> {
+    unimplemented!()
+}
+
+existential type MyIter<T>: Iterator<Item = T>;
+
+fn my_iter<T>(t: T) -> MyIter<T> {
+    std::iter::once(t)
+}
+
+fn my_iter2<T>(t: T) -> MyIter<T> { //~ ERROR defining existential type use differs from previous
+    Some(t).into_iter()
+}
+
+existential type WrongGeneric<T>: 'static;
+//~^ ERROR the parameter type `T` may not live long enough
+
+fn wrong_generic<T>(t: T) -> WrongGeneric<T> {
+    t
+}
+
+// don't reveal the concrete type
+existential type NoReveal: std::fmt::Debug;
+
+fn define_no_reveal() -> NoReveal {
+    ""
+}
+
+fn no_reveal(x: NoReveal) {
+    let _: &'static str = x; //~ mismatched types
+    let _ = x as &'static str; //~ non-primitive cast
+}

src/test/ui/existential_type.stderr

@@ -0,0 +1,120 @@
+error: defining existential type use differs from previous
+  --> $DIR/existential_type.rs:23:1
+   |
+LL | / fn bar() -> Foo { //~ ERROR defining existential type use differs from previous
+LL | |     42i32
+LL | | }
+   | |_^
+   |
+note: previous use here
+  --> $DIR/existential_type.rs:19:1
+   |
+LL | / fn foo() -> Foo {
+LL | |     ""
+LL | | }
+   | |_^
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:36:5
+   |
+LL | fn bomp() -> boo::Boo {
+   |              -------- expected `Boo` because of return type
+LL |     "" //~ ERROR mismatched types
+   |     ^^ expected anonymized type, found reference
+   |
+   = note: expected type `Boo`
+              found type `&'static str`
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:50:23
+   |
+LL |         let _: &str = bomp(); //~ ERROR mismatched types
+   |                       ^^^^^^ expected &str, found anonymized type
+   |
+   = note: expected type `&str`
+              found type `Boo`
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:54:9
+   |
+LL |     fn bomp() -> boo::Boo {
+   |                  -------- expected `Boo` because of return type
+LL |         "" //~ ERROR mismatched types
+   |         ^^ expected anonymized type, found reference
+   |
+   = note: expected type `Boo`
+              found type `&'static str`
+
+error[E0277]: the trait bound `T: Trait` is not satisfied
+  --> $DIR/existential_type.rs:61:1
+   |
+LL | existential type Underconstrained<T: Trait>: 'static; //~ ERROR the trait bound `T: Trait`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
+   |
+   = help: consider adding a `where T: Trait` bound
+   = note: the return type of a function must have a statically known size
+
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/existential_type.rs:78:1
+   |
+LL | existential type WrongGeneric<T>: 'static;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | fn wrong_generic<T>(t: T) -> WrongGeneric<T> {
+   |                  - help: consider adding an explicit lifetime bound `T: 'static`...
+   |
+note: ...so that the type `T` will meet its required lifetime bounds
+  --> $DIR/existential_type.rs:78:1
+   |
+LL | existential type WrongGeneric<T>: 'static;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/existential_type.rs:93:27
+   |
+LL |     let _: &'static str = x; //~ mismatched types
+   |                           ^ expected reference, found anonymized type
+   |
+   = note: expected type `&'static str`
+              found type `NoReveal`
+
+error[E0605]: non-primitive cast: `NoReveal` as `&'static str`
+  --> $DIR/existential_type.rs:94:13
+   |
+LL |     let _ = x as &'static str; //~ non-primitive cast
+   |             ^^^^^^^^^^^^^^^^^
+   |
+   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+
+error: could not find defining uses
+  --> $DIR/existential_type.rs:28:1
+   |
+LL | existential type Bar: std::fmt::Debug; //~ ERROR could not find defining uses
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: could not find defining uses
+  --> $DIR/existential_type.rs:32:5
+   |
+LL |     pub existential type Boo: ::std::fmt::Debug; //~ ERROR could not find defining uses
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: defining existential type use differs from previous
+  --> $DIR/existential_type.rs:74:1
+   |
+LL | / fn my_iter2<T>(t: T) -> MyIter<T> { //~ ERROR defining existential type use differs from previous
+LL | |     Some(t).into_iter()
+LL | | }
+   | |_^
+   |
+note: previous use here
+  --> $DIR/existential_type.rs:70:1
+   |
+LL | / fn my_iter<T>(t: T) -> MyIter<T> {
+LL | |     std::iter::once(t)
+LL | | }
+   | |_^
+
+error: aborting due to 11 previous errors
+
+Some errors occurred: E0277, E0308, E0310, E0605.
+For more information about an error, try `rustc --explain E0277`.

src/test/ui/existential_type2.rs

@@ -0,0 +1,30 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+#![feature(existential_type)]
+
+fn main() {}
+
+existential type Underconstrained<T: std::fmt::Debug>: 'static;
+//~^ ERROR `U` doesn't implement `std::fmt::Debug`
+
+// not a defining use, because it doesn't define *all* possible generics
+fn underconstrained<U>(_: U) -> Underconstrained<U> {
+    5u32
+}
+
+existential type Underconstrained2<T: std::fmt::Debug>: 'static;
+//~^ ERROR `V` doesn't implement `std::fmt::Debug`
+
+// not a defining use, because it doesn't define *all* possible generics
+fn underconstrained2<U, V>(_: U, _: V) -> Underconstrained2<V> {
+    5u32
+}

src/test/ui/existential_type2.stderr

@@ -0,0 +1,23 @@
+error[E0277]: `U` doesn't implement `std::fmt::Debug`
+  --> $DIR/existential_type2.rs:16:1
+   |
+LL | existential type Underconstrained<T: std::fmt::Debug>: 'static;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `U` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `U`
+   = help: consider adding a `where U: std::fmt::Debug` bound
+   = note: the return type of a function must have a statically known size
+
+error[E0277]: `V` doesn't implement `std::fmt::Debug`
+  --> $DIR/existential_type2.rs:24:1
+   |
+LL | existential type Underconstrained2<T: std::fmt::Debug>: 'static;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `V` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `V`
+   = 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
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/existential_type3.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+#![feature(existential_type)]
+
+fn main() {}
+
+existential type WrongGeneric<T: 'static>: 'static;
+
+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
+    v
+}

src/test/ui/existential_type3.stderr

@@ -0,0 +1,12 @@
+error: type parameter `V` is part of concrete type but not used in parameter list for existential type
+  --> $DIR/existential_type3.rs:18:73
+   |
+LL |   fn wrong_generic<U: 'static, V: 'static>(_: U, v: V) -> WrongGeneric<U> {
+   |  _________________________________________________________________________^
+LL | | //~^ ERROR type parameter `V` is part of concrete type but not used in parameter list
+LL | |     v
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+

src/test/ui/existential_type4.rs

@@ -1,4 +1,4 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -8,15 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// run-rustfix
 
-// Point at the captured immutable outer variable
+#![feature(existential_type)]
 
-fn foo(mut f: Box<FnMut()>) {
-    f();
-}
-
-fn main() {
-    let y = true;
-    foo(Box::new(move || y = false) as Box<_>); //~ ERROR cannot assign to captured outer variable
+fn main() {}
+
+existential type Cmp<T>: 'static;
+
+// not a defining use, because it doesn't define *all* possible generics
+fn cmp() -> Cmp<u32> { //~ ERROR non-defining existential type use in defining scope
+    5u32
 }

src/test/ui/existential_type4.stderr

@@ -0,0 +1,16 @@
+error: non-defining existential type use in defining scope
+  --> $DIR/existential_type4.rs:19:1
+   |
+LL | / fn cmp() -> Cmp<u32> { //~ ERROR non-defining existential type use in defining scope
+LL | |     5u32
+LL | | }
+   | |_^
+   |
+note: used non-generic type u32 for generic parameter
+  --> $DIR/existential_type4.rs:16:22
+   |
+LL | existential type Cmp<T>: 'static;
+   |                      ^
+
+error: aborting due to previous error
+

src/test/ui/feature-gate-existential-type.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that existential types must be ungated to use the `existential` keyword
+
+
+
+existential type Foo: std::fmt::Debug; //~ ERROR existential types are unstable
+
+trait Bar {
+    type Baa: std::fmt::Debug;
+}
+
+impl Bar for () {
+    existential type Baa: std::fmt::Debug; //~ ERROR existential types are unstable
+}
+
+fn main() {}

src/test/ui/feature-gate-existential-type.stderr

@@ -0,0 +1,19 @@
+error[E0658]: existential types are unstable (see issue #34511)
+  --> $DIR/feature-gate-existential-type.rs:15:1
+   |
+LL | existential type Foo: std::fmt::Debug; //~ ERROR existential types are unstable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(existential_type)] to the crate attributes to enable
+
+error[E0658]: existential types are unstable (see issue #34511)
+  --> $DIR/feature-gate-existential-type.rs:22:5
+   |
+LL |     existential type Baa: std::fmt::Debug; //~ ERROR existential types are unstable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(existential_type)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.