Rollup merge of #100247 - cjgillot:verify-dyn-trait-alias-defaults, r=lcnr

Generalize trait object generic param check to aliases.

The current algorithm only checks that `Self` does not appear in defaults for traits.  This is not sufficient for trait aliases.
This PR moves the check to trait object elaboration, which sees through trait aliases.

Fixes https://github.com/rust-lang/rust/issues/82927.
Fixes https://github.com/rust-lang/rust/issues/84789.

10 files changed, 101 insertions, 53 deletions

diff --git a/src/test/ui/associated-types/issue-22560.stderr b/src/test/ui/associated-types/issue-22560.stderr
index 700923c1b3f..2b88cf0b441 100644
--- a/src/test/ui/associated-types/issue-22560.stderr
+++ b/src/test/ui/associated-types/issue-22560.stderr
@@ -1,25 +1,3 @@
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/issue-22560.rs:9:23
-   |
-LL | trait Sub<Rhs=Self> {
-   | ------------------- type parameter `Rhs` must be specified for this
-...
-LL | type Test = dyn Add + Sub;
-   |                       ^^^ help: set the type parameter to the desired type: `Sub<Rhs>`
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/issue-22560.rs:9:17
-   |
-LL | trait Add<Rhs=Self> {
-   | ------------------- type parameter `Rhs` must be specified for this
-...
-LL | type Test = dyn Add + Sub;
-   |                 ^^^ help: set the type parameter to the desired type: `Add<Rhs>`
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-
 error[E0225]: only auto traits can be used as additional traits in a trait object
   --> $DIR/issue-22560.rs:9:23
    |
@@ -28,7 +6,7 @@ LL | type Test = dyn Add + Sub;
    |                 |
    |                 first non-auto trait
    |
-   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Add<[type error]> + Sub<[type error]> {}`
+   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Add + Sub {}`
    = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `Output` (from trait `Add`), `Output` (from trait `Sub`) must be specified
@@ -50,6 +28,28 @@ help: specify the associated types
 LL | type Test = dyn Add<Output = Type> + Sub<Output = Type>;
    |                 ~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~
 
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/issue-22560.rs:9:17
+   |
+LL | trait Add<Rhs=Self> {
+   | ------------------- type parameter `Rhs` must be specified for this
+...
+LL | type Test = dyn Add + Sub;
+   |                 ^^^ help: set the type parameter to the desired type: `Add<Rhs>`
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/issue-22560.rs:9:23
+   |
+LL | trait Sub<Rhs=Self> {
+   | ------------------- type parameter `Rhs` must be specified for this
+...
+LL | type Test = dyn Add + Sub;
+   |                       ^^^ help: set the type parameter to the desired type: `Sub<Rhs>`
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
 error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0191, E0225, E0393.
diff --git a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs
index b2edc1a1f66..6175b7df110 100644
--- a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs
+++ b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs
@@ -3,7 +3,6 @@
 
 trait Foo<X = Box<dyn Foo>> {
     //~^ ERROR cycle detected
-    //~| ERROR cycle detected
 }
 
 fn main() { }
diff --git a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr
index d4976a0f9c9..9d715f49471 100644
--- a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr
+++ b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr
@@ -10,30 +10,11 @@ note: cycle used when collecting item types in top-level module
    |
 LL | / trait Foo<X = Box<dyn Foo>> {
 LL | |
-LL | |
-LL | | }
-LL | |
-LL | | fn main() { }
-   | |_____________^
-
-error[E0391]: cycle detected when computing type of `Foo::X`
-  --> $DIR/cycle-trait-default-type-trait.rs:4:23
-   |
-LL | trait Foo<X = Box<dyn Foo>> {
-   |                       ^^^
-   |
-   = note: ...which immediately requires computing type of `Foo::X` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/cycle-trait-default-type-trait.rs:4:1
-   |
-LL | / trait Foo<X = Box<dyn Foo>> {
-LL | |
-LL | |
 LL | | }
 LL | |
 LL | | fn main() { }
    | |_____________^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0391`.
diff --git a/src/test/ui/issues/issue-21950.stderr b/src/test/ui/issues/issue-21950.stderr
index 4909398bb84..731615a6bd8 100644
--- a/src/test/ui/issues/issue-21950.stderr
+++ b/src/test/ui/issues/issue-21950.stderr
@@ -1,3 +1,12 @@
+error[E0191]: the value of the associated type `Output` (from trait `Add`) must be specified
+  --> $DIR/issue-21950.rs:10:25
+   |
+LL |     type Output;
+   |     ----------- `Output` defined here
+...
+LL |     let x = &10 as &dyn Add;
+   |                         ^^^ help: specify the associated type: `Add<Output = Type>`
+
 error[E0393]: the type parameter `Rhs` must be explicitly specified
   --> $DIR/issue-21950.rs:10:25
    |
@@ -9,15 +18,6 @@ LL |     let x = &10 as &dyn Add;
    |
    = note: because of the default `Self` reference, type parameters must be specified on object types
 
-error[E0191]: the value of the associated type `Output` (from trait `Add`) must be specified
-  --> $DIR/issue-21950.rs:10:25
-   |
-LL |     type Output;
-   |     ----------- `Output` defined here
-...
-LL |     let x = &10 as &dyn Add;
-   |                         ^^^ help: specify the associated type: `Add<Output = Type>`
-
 error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0191, E0393.
diff --git a/src/test/ui/traits/alias/generic-default-in-dyn.rs b/src/test/ui/traits/alias/generic-default-in-dyn.rs
new file mode 100644
index 00000000000..d44e1c2a975
--- /dev/null
+++ b/src/test/ui/traits/alias/generic-default-in-dyn.rs
@@ -0,0 +1,10 @@
+trait SendEqAlias<T> = PartialEq;
+//~^ ERROR trait aliases are experimental
+
+struct Foo<T>(dyn SendEqAlias<T>);
+//~^ ERROR the type parameter `Rhs` must be explicitly specified [E0393]
+
+struct Bar<T>(dyn SendEqAlias<T>, T);
+//~^ ERROR the type parameter `Rhs` must be explicitly specified [E0393]
+
+fn main() {}
diff --git a/src/test/ui/traits/alias/generic-default-in-dyn.stderr b/src/test/ui/traits/alias/generic-default-in-dyn.stderr
new file mode 100644
index 00000000000..76a068e864a
--- /dev/null
+++ b/src/test/ui/traits/alias/generic-default-in-dyn.stderr
@@ -0,0 +1,39 @@
+error[E0658]: trait aliases are experimental
+  --> $DIR/generic-default-in-dyn.rs:1:1
+   |
+LL | trait SendEqAlias<T> = PartialEq;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #41517 <https://github.com/rust-lang/rust/issues/41517> for more information
+   = help: add `#![feature(trait_alias)]` to the crate attributes to enable
+
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/generic-default-in-dyn.rs:4:19
+   |
+LL | struct Foo<T>(dyn SendEqAlias<T>);
+   |                   ^^^^^^^^^^^^^^ missing reference to `Rhs`
+   |
+  ::: $SRC_DIR/core/src/cmp.rs:LL:COL
+   |
+LL | pub trait PartialEq<Rhs: ?Sized = Self> {
+   | --------------------------------------- type parameter `Rhs` must be specified for this
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/generic-default-in-dyn.rs:7:19
+   |
+LL | struct Bar<T>(dyn SendEqAlias<T>, T);
+   |                   ^^^^^^^^^^^^^^ missing reference to `Rhs`
+   |
+  ::: $SRC_DIR/core/src/cmp.rs:LL:COL
+   |
+LL | pub trait PartialEq<Rhs: ?Sized = Self> {
+   | --------------------------------------- type parameter `Rhs` must be specified for this
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0393, E0658.
+For more information about an error, try `rustc --explain E0393`.
diff --git a/src/test/ui/traits/alias/self-in-generics.rs b/src/test/ui/traits/alias/self-in-generics.rs
new file mode 100644
index 00000000000..6b99431f5bb
--- /dev/null
+++ b/src/test/ui/traits/alias/self-in-generics.rs
@@ -0,0 +1,8 @@
+#![feature(trait_alias)]
+
+pub trait SelfInput = Fn(&mut Self);
+
+pub fn f(_f: &dyn SelfInput) {}
+//~^ ERROR the trait alias `SelfInput` cannot be made into an object [E0038]
+
+fn main() {}
diff --git a/src/test/ui/traits/alias/self-in-generics.stderr b/src/test/ui/traits/alias/self-in-generics.stderr
new file mode 100644
index 00000000000..a1056872ea6
--- /dev/null
+++ b/src/test/ui/traits/alias/self-in-generics.stderr
@@ -0,0 +1,11 @@
+error[E0038]: the trait alias `SelfInput` cannot be made into an object
+  --> $DIR/self-in-generics.rs:5:19
+   |
+LL | pub fn f(_f: &dyn SelfInput) {}
+   |                   ^^^^^^^^^
+   |
+   = note: it cannot use `Self` as a type parameter in a supertrait or `where`-clause
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/unspecified-self-in-trait-ref.rs b/src/test/ui/traits/unspecified-self-in-trait-ref.rs
index 158b5a98557..158b5a98557 100644
--- a/src/test/ui/unspecified-self-in-trait-ref.rs
+++ b/src/test/ui/traits/unspecified-self-in-trait-ref.rs
diff --git a/src/test/ui/unspecified-self-in-trait-ref.stderr b/src/test/ui/traits/unspecified-self-in-trait-ref.stderr
index 7869176bb3a..7869176bb3a 100644
--- a/src/test/ui/unspecified-self-in-trait-ref.stderr
+++ b/src/test/ui/traits/unspecified-self-in-trait-ref.stderr