Mark `std::mem::size_of` and `std::mem::size_of_val` as const function

[mzji]

This allows using size_of and size_of_val in constants contexts. Does this need an RFC?

[abonander]

IIRC from the last time this came up, the data used by these intrinsics isn't available until the translation pass, long after const-expansion, and because of things like arrays, where you can use a const fn for the size, it becomes a chicken-or-egg problem. Imagine if you had something like this:

type MyArray = [i32; size_of::<MyArray>];

Of course, you could paper over this by making it an error to do something like this, but that would require an RFC.

[mzji]

@abonander You are right. If we want to do this, we need to add a constrain to size_of and size_of_val to make them only accept "concrete" types (i.e. types with known size). If it's related to the compilation passes then I think we need to listen to some rustc gurus.

[nagisa]

Duplicate of #34078.