Would you like signs with those chars? [LWN.net]

Would you like signs with those chars?

Posted Oct 24, 2022 21:19 UTC (Mon) by joib (subscriber, #8541)
In reply to: Would you like signs with those chars? by cesarb
Parent article: Would you like signs with those chars?

Not sure why that would matter?

When passing strings between C and Rust, the pointers can/must be cast to the correct type anyway. And when working in Rust-land, the Rust compiler handles strings per its own rules, and in C-land the C compiler handles chars per its rules.

(Adding to the potential confusion, Rust also has a 'char' type, which is a 4-byte value capable of storing a single UTF-32 codepoint, presumably under the covers it's considered unsigned. But indeed a string (str/String) is an u8 array containing UTF-8.)

to post comments

Would you like signs with those chars?

Posted Oct 25, 2022 3:30 UTC (Tue) by tialaramex (subscriber, #21167) [Link]

> Rust also has a 'char' type, which is a 4-byte value capable of storing a single UTF-32 codepoint, presumably under the covers it's considered unsigned.

Rust defines char as a Unicode scalar value, Unicode only has one set of codepoints, and some of them aren't valid scalar values because they were used to make UTF-16 work, but UTF-32 maps all that ones which are scalar values to single code *units* and the rest are invalid.

Thus char::from_u32(0xDE01).unwrap(); will panic because this claims 0xDE01 is a Unicode scalar value, but it isn't because code point U+DE01 is a surrogate used for UTF-16 and has no meaning on its own.

char is not "considered" to be signed or unsigned. You can't do arithmetic on Rust types unless they implement the (Traits signifying) arithmetic operators, which char does not, so in Rust neither 'A' + 'B' nor false + true (bools only implement some logical and bitwise boolean operators, not general arithmetic) work.