add regression test for rust-lang#79495

Author: TaKO8Ki

src/test/rustdoc-ui/issue-79495.rs

@@ -0,0 +1,16 @@
+#![feature(arbitrary_enum_discriminant, core_intrinsics)]
+
+extern crate core;
+use core::intrinsics::discriminant_value;
+
+#[repr(usize)]
+enum MyWeirdOption<T> {
+    None = 0,
+    Some(T) = std::mem::size_of::<T>(),
+    //~^ ERROR generic parameters may not be used in const operations
+}
+
+fn main() {
+    assert_eq!(discriminant_value(&MyWeirdOption::<u8>::None), 0);
+    assert_eq!(discriminant_value(&MyWeirdOption::Some(0u8)), 1);
+}

src/test/rustdoc-ui/issue-79495.stderr

@@ -0,0 +1,13 @@
+error: generic parameters may not be used in const operations
+  --> $DIR/issue-79495.rs:9:35
+   |
+LL |     Some(T) = std::mem::size_of::<T>(),
+   |                                   ^ cannot perform const operation using `T`
+   |
+   = note: type parameters may not be used in const expressions
+   = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
+
+error: Compilation failed, aborting rustdoc
+
+error: aborting due to 2 previous errors
+