Rollup merge of #55687 - alexreg:fix-24010, r=scalexm

Take supertraits into account when calculating associated types

Fixes #24010 and #23856. Applies to trait aliases too.

As a by-product, this PR also makes repeated bindings of the same associated item in the same definition a hard error. This was previously a warning with a note about it becoming a hard error in the future. See #50589 for more info.

I talked about this with @nikomatsakis recently, but only very superficially, so this shouldn't stop anyone from assigning it to themself to review and r+.

N.B. The "WIP" commits represent imperfect attempts to solve the problem just for trait objects, but I've left them in for reference for the sake of whomever is reviewing this.

CC @carllerche @theemathas @durka @mbrubeck

src/test/ui/associated-types/associated-types-overridden-binding-2.rs

@@ -0,0 +1,17 @@
+// 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(trait_alias)]
+
+trait I32Iterator = Iterator<Item = i32>;
+
+fn main() {
+    let _: &I32Iterator<Item = u32> = &vec![42].into_iter();
+}

src/test/ui/associated-types/associated-types-overridden-binding-2.stderr

@@ -0,0 +1,13 @@
+error[E0271]: type mismatch resolving `<std::vec::IntoIter<u32> as std::iter::Iterator>::Item == i32`
+  --> $DIR/associated-types-overridden-binding-2.rs:16:39
+   |
+LL |     let _: &I32Iterator<Item = u32> = &vec![42].into_iter();
+   |                                       ^^^^^^^^^^^^^^^^^^^^^ expected u32, found i32
+   |
+   = note: expected type `u32`
+              found type `i32`
+   = note: required for the cast to the object type `dyn I32Iterator<Item=u32, Item=i32>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.

src/test/ui/associated-types/associated-types-overridden-binding.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(trait_alias)]
+
+trait Foo: Iterator<Item = i32> {}
+trait Bar: Foo<Item = u32> {}
+
+trait I32Iterator = Iterator<Item = i32>;
+trait U32Iterator = I32Iterator<Item = u32>;
+
+fn main() {
+    let _: &I32Iterator<Item = u32>;
+}

src/test/ui/associated-types/associated-types-overridden-binding.stderr

@@ -0,0 +1,15 @@
+error[E0284]: type annotations required: cannot resolve `<Self as std::iter::Iterator>::Item == i32`
+  --> $DIR/associated-types-overridden-binding.rs:14:1
+   |
+LL | trait Bar: Foo<Item = u32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: required by `Foo`
+  --> $DIR/associated-types-overridden-binding.rs:13:1
+   |
+LL | trait Foo: Iterator<Item = i32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0284`.

src/test/ui/error-codes/E0191.rs

@@ -14,5 +14,4 @@ trait Trait {
 
 type Foo = Trait; //~ ERROR E0191
 
-fn main() {
-}
+fn main() {}

src/test/ui/error-codes/E0719.rs

@@ -8,16 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-pass
-
-use std::iter::Iterator;
+trait Foo: Iterator<Item = i32, Item = i32> {}
 
 type Unit = ();
 
-fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
+fn test() -> Box<Iterator<Item = (), Item = Unit>> {
     Box::new(None.into_iter())
 }
 
 fn main() {
+    let _: &Iterator<Item = i32, Item = i32>;
     test();
 }

src/test/ui/error-codes/E0719.stderr

@@ -0,0 +1,19 @@
+error[E0719]: the value of the associated type `Item` (from the trait `std::iter::Iterator`) is already specified
+  --> $DIR/E0719.rs:11:33
+   |
+LL | trait Foo: Iterator<Item = i32, Item = i32> {}
+   |                     ----------  ^^^^^^^^^^ re-bound here
+   |                     |
+   |                     `Item` bound here first
+
+error[E0719]: the value of the associated type `Item` (from the trait `std::iter::Iterator`) is already specified
+  --> $DIR/E0719.rs:15:38
+   |
+LL | fn test() -> Box<Iterator<Item = (), Item = Unit>> {
+   |                           ---------  ^^^^^^^^^^^ re-bound here
+   |                           |
+   |                           `Item` bound here first
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0719`.

src/test/ui/issue-51947.rs

@@ -1,17 +0,0 @@
-// compile-pass
-
-#![crate_type = "lib"]
-#![feature(linkage)]
-
-// MergeFunctions will merge these via an anonymous internal
-// backing function, which must be named if ThinLTO buffers are used
-
-#[linkage = "weak"]
-pub fn fn1(a: u32, b: u32, c: u32) -> u32 {
-    a + b + c
-}
-
-#[linkage = "weak"]
-pub fn fn2(a: u32, b: u32, c: u32) -> u32 {
-    a + b + c
-}

src/test/ui/issues/issue-51947.rs

@@ -0,0 +1,27 @@
+// 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.
+
+// compile-pass
+
+#![crate_type = "lib"]
+#![feature(linkage)]
+
+// MergeFunctions will merge these via an anonymous internal
+// backing function, which must be named if ThinLTO buffers are used
+
+#[linkage = "weak"]
+pub fn fn1(a: u32, b: u32, c: u32) -> u32 {
+    a + b + c
+}
+
+#[linkage = "weak"]
+pub fn fn2(a: u32, b: u32, c: u32) -> u32 {
+    a + b + c
+}

src/test/ui/lint/issue-50589-multiple-associated-types.stderr

@@ -1,23 +0,0 @@
-warning: associated type binding `Item` specified more than once
-  --> $DIR/issue-50589-multiple-associated-types.rs:17:39
-   |
-LL | fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
-   |                            ---------  ^^^^^^^^^^^ used more than once
-   |                            |
-   |                            first use of `Item`
-   |
-   = note: #[warn(duplicate_associated_type_bindings)] on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #50589 <https://github.com/rust-lang/rust/issues/50589>
-
-warning: associated type binding `Item` specified more than once
-  --> $DIR/issue-50589-multiple-associated-types.rs:17:39
-   |
-LL | fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
-   |                            ---------  ^^^^^^^^^^^ used more than once
-   |                            |
-   |                            first use of `Item`
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #50589 <https://github.com/rust-lang/rust/issues/50589>
-

src/test/ui/traits/trait-alias-object.stderr

@@ -1,5 +1,5 @@
 error[E0038]: the trait `EqAlias` cannot be made into an object
-  --> $DIR/trait-alias-objects.rs:17:13
+  --> $DIR/trait-alias-object.rs:17:13
    |
 LL |     let _: &dyn EqAlias = &123;
    |             ^^^^^^^^^^^ the trait `EqAlias` cannot be made into an object
@@ -7,7 +7,7 @@ LL |     let _: &dyn EqAlias = &123;
    = note: the trait cannot use `Self` as a type parameter in the supertraits or where-clauses
 
 error[E0191]: the value of the associated type `Item` (from the trait `std::iter::Iterator`) must be specified
-  --> $DIR/trait-alias-objects.rs:18:13
+  --> $DIR/trait-alias-object.rs:18:13
    |
 LL |     let _: &dyn IteratorAlias = &vec![123].into_iter();
    |             ^^^^^^^^^^^^^^^^^ missing associated type `Item` value