"only `u8` can be cast as `char`, not `i32`" needs further information

[GKFX]

Given the following code: link

0x61i32 as char

The current output is:

only `u8` can be cast as `char`, not `i32`

This is not very clear. There are two pieces of background information the user needs here:

• chars are Unicode scalar values, which are any integer value from 0 to 0xD7FF and 0xE000 to 0x10FFFF. Only u8 can be statically guaranteed to meet this requirement so only u8 can be cast to char.
• char::from_u32 is probably what the user wants, if they are interested in non-ASCII values.

The user is directed to further information available at rustc --explain E0604 and https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions, neither of which explains either of those two points.

[jyn514]

I think a good improvement would be to
a) Add more detail about this to https://github.com/rust-lang/rust/blob/master/compiler/rustc_error_codes/src/error_codes/E0604.md
b) Add a help to this error that suggests from_u32

@GKFX are you interested in working on one or both of those? :)

[GKFX]

@rustbot claim

[GKFX]

While I was reading the docs I noticed a minor error in char::MAX:

The highest valid code point a char can have.

A [`char`] is a Unicode Scalar Value, which means that it is a Code
Point, but only ones within a certain range. MAX is the highest valid
code point that's a valid Unicode Scalar Value.

This isn't true; a USV is a Code Point within either of two ranges, not one. It's also a bit cagey about what the ranges are numerically; I don't know if this is intentional. Personally I would re-write it to the below:

The highest valid code point a char can have, 0x10FFFF.

A Code Point is any value between zero and char::MAX. A [`char`] is a
Unicode Scalar Value, which is any Code Point that is not in the range
0xD800..=0xDFFF.

Would I put that in the same PR (as they're both char documentation) or a new issue?

[camelid]

@GKFX A new PR would be better.