idna: Support `no_alloc` idna validator

[al8n]

Hi, the current idna crate supports no_std, but it cannot compile without alloc feature. Sometimes, users may only want to check if a &str is a valid international domain name and do not need to convert it to either unicode or ascii. For the validator, it should be possible to be implemented without alloc.

[hsivonen]

This would require the core of icu_normalizer to have a no-alloc mode. The issue is that the input may have an unbounded sequence of characters whose canonical combining class is non-zero, so there's no stack-allocated buffer size that would be long enough for arbitrary input.

Even if the stack-allocated buffer size was 32 Rust chars based on the "stream-safe" definition, there would need to be an answer for what happens when the input isn't "stream-safe". Making the current interface that transforms an iterator over char into another iterator over char produce an iterator over Result<char> instead and having possible return points all over the place would be disruptive for a relative specialized use case.

How common is it to have code that doesn't have alloc but that deals with domain names?

[nikki-aranya]

How common is it to have code that doesn't have alloc but that deals with domain names?

Extremely common. Embedded often uses the Embassy ecosystem, and includes embassy-net which is both no_std and no_alloc and would benefit greatly from being able to add urls to their custom IpAddress to use for e.g. TcpSocket, as well as something like embedded-nal. I was just looking into whether I could create a FixedUrl<const N: usize> using something like heapless and came across this issue.

[hsivonen]

includes embassy-net which is both no_std and no_alloc and would benefit greatly from being able to add urls to their custom IpAddress to use for e.g. TcpSocket, as well as something like embedded-nal.

The OP wanted just validation without an allocator (use case not explained), but this seems to imply running the "ToASCII" operation without an allocator, since that's needed when actually opening connections. Do I understand correctly?