Uniformative error message when casting a thin raw pointer to a fat one

[SimonSapin]

This is fine:

fn cast_thin_to_thin(x: *const ()) {
    x as *const u8;
}

This does not compile, but the error message is not particularly helpful:

fn cast_thin_to_fat(x: *const ()) {
    x as *const [u8];
}

a.rs:8:5: 8:21 error: casting `*const ()` as `*const [u8]` is invalid
a.rs:8     x as *const [u8];
           ^~~~~~~~~~~~~~~~
error: aborting due to previous error

Invalid how? The error message should mention thin vs fat pointers or statically-sized vs dynamically-sized types, and maybe link to https://doc.rust-lang.org/book/unsized-types.html

(To make this a bit less artificial, this came up in FFI code casting *mut c_void to a struct containing an unboxed FnMut closure. The fix was to box the closure to make that struct statically-sized.)

CC @julienw