Rollup merge of #60284 - varkor:const-param-of-type-param, r=cramertj

Do not allow const generics to depend on type parameters

Fixes https://github.com/rust-lang/rust/issues/60264. In https://github.com/rust-lang/rust/pull/58191/files/b534cf992d0189032207f395c27ed092c89b40c7#diff-aeb0880081a991f34aef2ab889e1fb7a, it was suggested that there might be a better place for this error, but as this bug already affects stable, it's probably worth merging this now, and refactoring afterwards (I can open an issue for this).

4 files changed, 62 insertions, 0 deletions

diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs b/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs
new file mode 100644
index 00000000000..af5e8f49754
--- /dev/null
+++ b/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs
@@ -0,0 +1,6 @@
+use std::marker::PhantomData;
+
+struct B<T, const N: T>(PhantomData<[T; N]>); //~ ERROR const generics are unstable
+//~^ ERROR const parameters cannot depend on type parameters
+
+fn main() {}
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr b/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr
new file mode 100644
index 00000000000..e3adbcfe602
--- /dev/null
+++ b/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr
@@ -0,0 +1,19 @@
+error[E0671]: const parameters cannot depend on type parameters
+  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:22
+   |
+LL | struct B<T, const N: T>(PhantomData<[T; N]>);
+   |                      ^ const parameter depends on type parameter
+
+error[E0658]: const generics are unstable
+  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:19
+   |
+LL | struct B<T, const N: T>(PhantomData<[T; N]>);
+   |                   ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/44580
+   = help: add #![feature(const_generics)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0658, E0671.
+For more information about an error, try `rustc --explain E0658`.
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param.rs b/src/test/ui/const-generics/const-param-type-depends-on-type-param.rs
new file mode 100644
index 00000000000..28e0d6c2bb7
--- /dev/null
+++ b/src/test/ui/const-generics/const-param-type-depends-on-type-param.rs
@@ -0,0 +1,13 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+// Currently, const parameters cannot depend on type parameters, because there is no way to
+// enforce the `structural_match` property on an arbitrary type parameter. This restriction
+// may be relaxed in the future. See https://github.com/rust-lang/rfcs/pull/2000 for more
+// details.
+
+pub struct Dependent<T, const X: T>([(); X]);
+//~^ ERROR const parameters cannot depend on type parameters
+//~^^ ERROR parameter `T` is never used
+
+fn main() {}
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr b/src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr
new file mode 100644
index 00000000000..c7dcbe13542
--- /dev/null
+++ b/src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr
@@ -0,0 +1,24 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/const-param-type-depends-on-type-param.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error[E0671]: const parameters cannot depend on type parameters
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:34
+   |
+LL | pub struct Dependent<T, const X: T>([(); X]);
+   |                                  ^ const parameter depends on type parameter
+
+error[E0392]: parameter `T` is never used
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:22
+   |
+LL | pub struct Dependent<T, const X: T>([(); X]);
+   |                      ^ unused parameter
+   |
+   = help: consider removing `T` or using a marker such as `std::marker::PhantomData`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0392, E0671.
+For more information about an error, try `rustc --explain E0392`.