support as _ and add tests

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -431,7 +431,9 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
                 }
             }
             // These are not actually relevant for us here, so we can ignore them.
-            StatementKind::StorageLive(_) | StatementKind::StorageDead(_) => Ok(()),
+            StatementKind::AscribeUserType(..)
+            | StatementKind::StorageLive(_)
+            | StatementKind::StorageDead(_) => Ok(()),
             _ => self.error(Some(stmt.source_info.span), "unsupported statement")?,
         }
     }

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-1.rs

@@ -6,6 +6,7 @@ struct Foo<const N: u8>([u8; N as usize])
 where
     [(); N as usize]:;
 
+struct Bar<const N: u8>([u8; (N + 2) as usize]) where [(); (N + 2) as usize]:;
 
 // unifying with subtrees
 struct Evaluatable<const N: u16>;
@@ -13,6 +14,5 @@ fn foo<const N: u8>() where Evaluatable<{N as usize as u16 }>: {
     let _ = Foo::<N>([1; N as usize]);
 }
 
-struct Bar<const N: u8>([u8; (N + 2) as usize]) where [(); (N + 2) as usize]:;
 
 fn main() {}

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-2.rs

@@ -14,7 +14,7 @@ struct Foo2<const N: u8>(Evaluatable::<{N as u128}>) where Evaluatable<{N as usi
 //~| help: try adding a `where` bound using this expression: `where [(); {N as u128}]:`
 
 struct Bar<const N: u8>([u8; (N + 2) as usize]) where [(); (N + 1) as usize]:;
-//~^ unconstrained generic constant
+//~^ Error: unconstrained generic constant
 //~| help: try adding a `where` bound using this expression: `where [(); (N + 2) as usize]:`
 
 fn main() {}

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-3.rs

@@ -0,0 +1,47 @@
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+trait Trait {}
+pub struct EvaluatableU128<const N: u128>;
+
+struct HasCastInTraitImpl<const N: usize, const M: u128>;
+impl<const O: usize> Trait for HasCastInTraitImpl<O, { O as u128 }> {}
+
+pub fn use_trait_impl<const N: usize>()
+where
+    [(); { N + 1}]:,
+    EvaluatableU128<{N as u128}>:, {
+    fn assert_impl<T: Trait>() {}
+
+    // errors are bad but seems to be pre-existing issue #86198
+    assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
+    //~^ Error: mismatched types
+    //~^^ Error: unconstrained generic constant
+    assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
+    //~^ Error: mismatched types
+    //~^^ Error: unconstrained generic constant
+    assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
+    //~^ Error: mismatched types
+    assert_impl::<HasCastInTraitImpl<14, 13>>();
+    //~^ Error: mismatched types
+}
+pub fn use_trait_impl_2<const N: usize>()
+where
+    [(); { N + 1}]:,
+    EvaluatableU128<{N as _}>:, {
+    fn assert_impl<T: Trait>() {}
+
+    // errors are bad but seems to be pre-existing issue #86198
+    assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
+    //~^ Error: mismatched types
+    //~^^ Error: unconstrained generic constant
+    assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
+    //~^ Error: mismatched types
+    //~^^ Error: unconstrained generic constant
+    assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
+    //~^ Error: mismatched types
+    assert_impl::<HasCastInTraitImpl<14, 13>>();
+    //~^ Error: mismatched types
+}
+
+fn main() {}

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-3.stderr

@@ -0,0 +1,139 @@
+error: unconstrained generic constant
+  --> $DIR/abstract-const-as-cast-3.rs:17:5
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ----- required by this bound in `use_trait_impl::assert_impl`
+...
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); { O as u128 }]:`
+note: required because of the requirements on the impl of `Trait` for `HasCastInTraitImpl<{ N + 1 }, { N as u128 }>`
+  --> $DIR/abstract-const-as-cast-3.rs:8:22
+   |
+LL | impl<const O: usize> Trait for HasCastInTraitImpl<O, { O as u128 }> {}
+   |                      ^^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:17:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
+   |
+   = note: expected type `{ N as u128 }`
+              found type `{ O as u128 }`
+
+error: unconstrained generic constant
+  --> $DIR/abstract-const-as-cast-3.rs:20:5
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ----- required by this bound in `use_trait_impl::assert_impl`
+...
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); { O as u128 }]:`
+note: required because of the requirements on the impl of `Trait` for `HasCastInTraitImpl<{ N + 1 }, { N as _ }>`
+  --> $DIR/abstract-const-as-cast-3.rs:8:22
+   |
+LL | impl<const O: usize> Trait for HasCastInTraitImpl<O, { O as u128 }> {}
+   |                      ^^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:20:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
+   |
+   = note: expected type `{ N as _ }`
+              found type `{ O as u128 }`
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:23:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12_u128`, found `13_u128`
+   |
+   = note: expected type `12_u128`
+              found type `13_u128`
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:25:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13_u128`, found `14_u128`
+   |
+   = note: expected type `13_u128`
+              found type `14_u128`
+
+error: unconstrained generic constant
+  --> $DIR/abstract-const-as-cast-3.rs:35:5
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ----- required by this bound in `use_trait_impl_2::assert_impl`
+...
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); { O as u128 }]:`
+note: required because of the requirements on the impl of `Trait` for `HasCastInTraitImpl<{ N + 1 }, { N as u128 }>`
+  --> $DIR/abstract-const-as-cast-3.rs:8:22
+   |
+LL | impl<const O: usize> Trait for HasCastInTraitImpl<O, { O as u128 }> {}
+   |                      ^^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:35:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
+   |
+   = note: expected type `{ N as u128 }`
+              found type `{ O as u128 }`
+
+error: unconstrained generic constant
+  --> $DIR/abstract-const-as-cast-3.rs:38:5
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ----- required by this bound in `use_trait_impl_2::assert_impl`
+...
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); { O as u128 }]:`
+note: required because of the requirements on the impl of `Trait` for `HasCastInTraitImpl<{ N + 1 }, { N as _ }>`
+  --> $DIR/abstract-const-as-cast-3.rs:8:22
+   |
+LL | impl<const O: usize> Trait for HasCastInTraitImpl<O, { O as u128 }> {}
+   |                      ^^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:38:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
+   |
+   = note: expected type `{ N as _ }`
+              found type `{ O as u128 }`
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:41:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12_u128`, found `13_u128`
+   |
+   = note: expected type `12_u128`
+              found type `13_u128`
+
+error[E0308]: mismatched types
+  --> $DIR/abstract-const-as-cast-3.rs:43:5
+   |
+LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13_u128`, found `14_u128`
+   |
+   = note: expected type `13_u128`
+              found type `14_u128`
+
+error: aborting due to 12 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-4.rs

@@ -0,0 +1,29 @@
+// check-pass
+#![feature(const_evaluatable_checked, const_generics)]
+#![allow(incomplete_features)]
+
+trait Trait {}
+pub struct EvaluatableU128<const N: u128>;
+
+struct HasCastInTraitImpl<const N: usize, const M: u128>;
+impl<const O: usize> Trait for HasCastInTraitImpl<O, { O as u128 }> {}
+
+pub fn use_trait_impl<const N: usize>() where EvaluatableU128<{N as u128}>:, {
+    fn assert_impl<T: Trait>() {}
+
+    assert_impl::<HasCastInTraitImpl<N, { N as u128 }>>();
+    assert_impl::<HasCastInTraitImpl<N, { N as _ }>>();
+    assert_impl::<HasCastInTraitImpl<12, { 12 as u128 }>>();
+    assert_impl::<HasCastInTraitImpl<13, 13>>();
+}
+pub fn use_trait_impl_2<const N: usize>() where EvaluatableU128<{N as _}>:, {
+    fn assert_impl<T: Trait>() {}
+
+    assert_impl::<HasCastInTraitImpl<N, { N as u128 }>>();
+    assert_impl::<HasCastInTraitImpl<N, { N as _ }>>();
+    assert_impl::<HasCastInTraitImpl<12, { 12 as u128 }>>();
+    assert_impl::<HasCastInTraitImpl<13, 13>>();
+}
+
+
+fn main() {}