Add a rule that enum discriminants may not use generic parameters

This is implemented in the compiler with the diagnostic
[`ParamInEnumDiscriminant`](https://github.com/rust-lang/rust/blob/3b1b0ef4d80d3117924d91352c8b6ca528708b3c/compiler/rustc_resolve/src/errors.rs#L651-L661)
and
[`NoConstantGenericsReason::IsEnumDiscriminant`](https://github.com/rust-lang/rust/blob/3b1b0ef4d80d3117924d91352c8b6ca528708b3c/compiler/rustc_resolve/src/late.rs#L160-L168).

Author: ehuss

src/items/enumerations.md

@@ -196,6 +196,25 @@ enum OverflowingDiscriminantError2 {
 }
 ```
 
+r[items.enum.discriminant.restrictions.generics]
+Explicit enum discriminants may not use generic parameters from the enclosing enum.
+
+> [!EXAMPLE]
+> ```rust,compile_fail
+> #[repr(u32)]
+> enum E<'a, T, const N: u32> {
+>     Lifetime(&'a T) = {
+>         let a: &'a (); // ERROR: generic parameters may not be used in enum discriminant
+>         1
+>     },
+>     Type(T) = {
+>         let x: T; // ERROR: generic parameters may not be used in enum discriminant
+>         2
+>     },
+>     Const = N, // ERROR: generic parameters may not be used in enum discriminant
+> }
+> ```
+
 ### Accessing discriminant
 
 #### Via `mem::discriminant`