diagnostics: add test case for bogus T:Sized suggestion

Closes #69228

src/test/ui/consts/const-eval/size-of-t.rs

@@ -0,0 +1,13 @@
+// https://github.com/rust-lang/rust/issues/69228
+// Used to give bogus suggestion about T not being Sized.
+
+use std::mem::size_of;
+
+fn foo<T>() {
+    let _arr: [u8; size_of::<T>()];
+    //~^ ERROR generic parameters may not be used in const operations
+    //~| NOTE cannot perform const operation
+    //~| NOTE type parameters may not be used in const expressions
+}
+
+fn main() {}

src/test/ui/consts/const-eval/size-of-t.stderr

@@ -0,0 +1,11 @@
+error: generic parameters may not be used in const operations
+  --> $DIR/size-of-t.rs:7:30
+   |
+LL |     let _arr: [u8; 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: aborting due to previous error
+