Rollup merge of #59166 - seanmonstar:trait-alias-import, r=alexreg

resolve: collect trait aliases along with traits

It seems trait aliases weren't being collected as `TraitCandidates` in resolve, this should change that. (I can't compile the full compiler locally, so relying on CI...)

Fixes https://github.com/rust-lang/rust/issues/56485

r? @alexreg

src/test/run-pass/traits/auxiliary/trait_alias.rs

@@ -0,0 +1,13 @@
+#![feature(trait_alias)]
+
+pub trait Hello {
+    fn hello(&self);
+}
+
+pub struct Hi;
+
+impl Hello for Hi {
+    fn hello(&self) {}
+}
+
+pub trait Greet = Hello;

src/test/run-pass/traits/trait-alias-import-cross-crate.rs

@@ -0,0 +1,14 @@
+// run-pass
+// aux-build:trait_alias.rs
+
+#![feature(trait_alias)]
+
+extern crate trait_alias;
+
+// Import only the alias, not the real trait.
+use trait_alias::{Greet, Hi};
+
+fn main() {
+    let hi = Hi;
+    hi.hello(); // From `Hello`, via `Greet` alias.
+}

src/test/run-pass/traits/trait-alias-import.rs

@@ -0,0 +1,38 @@
+#![feature(trait_alias)]
+
+mod inner {
+    pub trait Foo {
+        fn foo(&self);
+    }
+
+    pub struct Qux;
+
+    impl Foo for Qux {
+        fn foo(&self) {}
+    }
+
+    pub trait Bar = Foo;
+}
+
+mod two {
+    pub trait A {
+        fn foo();
+    }
+
+    impl A for u8 {
+        fn foo() {}
+    }
+}
+
+// Import only the alias, not the `Foo` trait.
+use inner::{Bar, Qux};
+
+// Declaring an alias also brings in aliased methods.
+trait Two = two::A;
+
+fn main() {
+    let q = Qux;
+    q.foo(); // From Bar.
+
+    u8::foo(); // From A.
+}

src/test/ui/traits/trait-alias-ambiguous.rs

@@ -0,0 +1,24 @@
+#![feature(trait_alias)]
+
+mod inner {
+    pub trait A { fn foo(&self); }
+    pub trait B { fn foo(&self); }
+
+    impl A for u8 {
+        fn foo(&self) {}
+    }
+    impl B for u8 {
+        fn foo(&self) {}
+    }
+
+    pub trait C = A + B;
+}
+
+use inner::C;
+
+fn main() {
+    let t = 1u8;
+    t.foo(); //~ ERROR E0034
+
+    inner::A::foo(&t); // ok
+}

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

@@ -0,0 +1,20 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/trait-alias-ambiguous.rs:21:7
+   |
+LL |     t.foo();
+   |       ^^^ multiple `foo` found
+   |
+note: candidate #1 is defined in an impl of the trait `inner::A` for the type `u8`
+  --> $DIR/trait-alias-ambiguous.rs:8:9
+   |
+LL |         fn foo(&self) {}
+   |         ^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl of the trait `inner::B` for the type `u8`
+  --> $DIR/trait-alias-ambiguous.rs:11:9
+   |
+LL |         fn foo(&self) {}
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0034`.