Rollup merge of #57502 - nikomatsakis:fix-trait-alias-1b, r=nikomatsakis

make trait-aliases work across crates

This is rebase of a small part of @alexreg's PR #55994. It focuses just on the changes that integrate trait aliases properly into crate metadata, excluding the stylistic edits and the trait objects.

The stylistic edits I also rebased and can open a separate PR.

The trait object stuff I found challenging and decided it basically needed to be reimplemented. For now I've excluded it.

Since this is really @alexreg's work (I really just rebased) I am going to make it r=me once it is working.

Fixes #56488.
Fixes #57023.

src/test/ui/issues/issue-56488.rs

@@ -0,0 +1,13 @@
+// run-pass
+
+#![feature(trait_alias)]
+
+mod alpha {
+    pub trait A {}
+    pub trait C = A;
+}
+
+#[allow(unused_imports)]
+use alpha::C;
+
+fn main() {}

src/test/ui/lint/lint-incoherent-auto-trait-objects.rs

@@ -0,0 +1,21 @@
+// ignore-tidy-linelength
+
+trait Foo {}
+
+impl Foo for dyn Send {}
+
+impl Foo for dyn Send + Send {}
+//~^ ERROR conflicting implementations
+//~| hard error
+
+impl Foo for dyn Send + Sync {}
+
+impl Foo for dyn Sync + Send {}
+//~^ ERROR conflicting implementations
+//~| hard error
+
+impl Foo for dyn Send + Sync + Send {}
+//~^ ERROR conflicting implementations
+//~| hard error
+
+fn main() {}

src/test/ui/lint/lint-incoherent-auto-trait-objects.stderr

@@ -0,0 +1,39 @@
+error: conflicting implementations of trait `Foo` for type `(dyn std::marker::Send + 'static)`: (E0119)
+  --> $DIR/lint-incoherent-auto-trait-objects.rs:7:1
+   |
+LL | impl Foo for dyn Send {}
+   | --------------------- first implementation here
+LL | 
+LL | impl Foo for dyn Send + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + 'static)`
+   |
+   = note: #[deny(order_dependent_trait_objects)] 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 #56484 <https://github.com/rust-lang/rust/issues/56484>
+
+error: conflicting implementations of trait `Foo` for type `(dyn std::marker::Send + std::marker::Sync + 'static)`: (E0119)
+  --> $DIR/lint-incoherent-auto-trait-objects.rs:13:1
+   |
+LL | impl Foo for dyn Send + Sync {}
+   | ---------------------------- first implementation here
+LL | 
+LL | impl Foo for dyn Sync + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + std::marker::Sync + 'static)`
+   |
+   = 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 #56484 <https://github.com/rust-lang/rust/issues/56484>
+
+error: conflicting implementations of trait `Foo` for type `(dyn std::marker::Send + std::marker::Sync + 'static)`: (E0119)
+  --> $DIR/lint-incoherent-auto-trait-objects.rs:17:1
+   |
+LL | impl Foo for dyn Sync + Send {}
+   | ---------------------------- first implementation here
+...
+LL | impl Foo for dyn Send + Sync + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + std::marker::Sync + 'static)`
+   |
+   = 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 #56484 <https://github.com/rust-lang/rust/issues/56484>
+
+error: aborting due to 3 previous errors
+

src/test/ui/traits/auxiliary/trait_alias.rs

@@ -0,0 +1,3 @@
+#![feature(trait_alias)]
+
+pub trait SendSync = Send + Sync;

src/test/ui/traits/trait-alias-cross-crate.rs

@@ -0,0 +1,17 @@
+// aux-build:trait_alias.rs
+
+#![feature(trait_alias)]
+
+extern crate trait_alias;
+
+use std::rc::Rc;
+use trait_alias::SendSync;
+
+fn use_alias<T: SendSync>() {}
+
+fn main() {
+    use_alias::<u32>();
+    use_alias::<Rc<u32>>();
+    //~^ ERROR `std::rc::Rc<u32>` cannot be sent between threads safely [E0277]
+    //~^^ ERROR `std::rc::Rc<u32>` cannot be shared between threads safely [E0277]
+}

src/test/ui/traits/trait-alias-cross-crate.stderr

@@ -0,0 +1,29 @@
+error[E0277]: `std::rc::Rc<u32>` cannot be sent between threads safely
+  --> $DIR/trait-alias-cross-crate.rs:14:5
+   |
+LL |     use_alias::<Rc<u32>>();
+   |     ^^^^^^^^^^^^^^^^^^^^ `std::rc::Rc<u32>` cannot be sent between threads safely
+   |
+   = help: the trait `std::marker::Send` is not implemented for `std::rc::Rc<u32>`
+note: required by `use_alias`
+  --> $DIR/trait-alias-cross-crate.rs:10:1
+   |
+LL | fn use_alias<T: SendSync>() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: `std::rc::Rc<u32>` cannot be shared between threads safely
+  --> $DIR/trait-alias-cross-crate.rs:14:5
+   |
+LL |     use_alias::<Rc<u32>>();
+   |     ^^^^^^^^^^^^^^^^^^^^ `std::rc::Rc<u32>` cannot be shared between threads safely
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::rc::Rc<u32>`
+note: required by `use_alias`
+  --> $DIR/trait-alias-cross-crate.rs:10:1
+   |
+LL | fn use_alias<T: SendSync>() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.