Add no_std support

[pawelchcki]

Make the crate usable in no_std + alloc environments. Follow the serde_json pattern: a custom io module abstracts over std::io in std mode and provides a minimal Write trait with Vec impl for no_std.

This allows embedding serde yaml in a lot more contexts. e.g. as a low size overhead shared library.
no_std compilation means that artifacts of few tens of kilobyte size can be produced.

Also this could be useful for some embedded projects, e.g. allow a uC parsing yaml config (I know yaml is a bit unorthodox there, but someone could find it useful.

I need it to reduce the binary size of a shared library we use in one of our projects.

This PR:

• Add std feature (default) that gates from_reader and std::io
• Add src/io/ module: re-exports std::io in std mode, provides custom Write trait + fallback Error in no_std mode
• Replace all std:: imports with core::/alloc:: equivalents
• Replace DefaultHasher with a simple FNV hasher for order-independent Mapping hashing (no_std compatible, no extra dependencies)
• Use FnvBuildHasher for IndexMap in no_std mode (avoids pulling in hashbrown)

[pawelchcki]

Hi, sorry for this unprompted PR. I hope you'll find it useful.

I needed to be able to implement certain feature as no_std, as this allows for a smaller shared library to be generated. Currently there are mature enough no_std yaml deserializers, and since this library now exsist and libyaml-rs is already no-std.

Then a simple change would be required to allow no_std operation. I don't expect Embedded developers to parse a lot of yaml, although maybe?

This is quite straightforward PR. I've run all tests locally and they pass.

LMK if you'd like to see anything changed in this PR

[pawelchcki]

Instead of pulling something like hashbrown and/or complicating feature resolution. For no_std I added this super simple hasher.

It's not cryptographic, but neither is DefaultHasher/SipHash in a security sense for HashMaps — both just need good distribution to avoid collisions.

[Copilot]

Pull request overview

This PR adapts yaml_serde to support no_std + alloc builds by introducing a std feature (default-on) and refactoring the crate to use core/alloc equivalents while abstracting I/O behind a crate io module.

Changes:

• Add std feature (default) and gate reader-based APIs (from_reader) behind it.
• Introduce src/io.rs abstraction to replace direct std::io usage and update serializer/emitter to use it.
• Replace std imports with core/alloc, and introduce a no_std-compatible hashing strategy for Mapping.

Reviewed changes

Copilot reviewed 25 out of 26 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/lib.rs	Switch crate to #![no_std], add alloc, gate from_reader, add io module
Cargo.toml	Add std feature (default), disable default features for deps to support no_std
src/io.rs	New std/no_std I/O abstraction layer (re-export std::io or custom Write)
src/de.rs	Gate from_reader and Progress::Read behind std, move to core/alloc imports
src/loader.rs	Gate Progress::Read handling behind std, move to alloc containers
src/ser.rs	Switch to crate::io and core/alloc imports for serialization path
src/error.rs	Replace std types with alloc/core equivalents; use crate::io::Error
src/mapping.rs	Use core/alloc; add no_std hasher + hasher-aware IndexMap type alias
src/number.rs	Replace std imports with core equivalents
src/path.rs	Replace std fmt import with core fmt
src/value/mod.rs	Replace std imports with core/alloc equivalents
src/value/de.rs	Replace std imports with core/alloc equivalents
src/value/ser.rs	Replace std imports with core/alloc equivalents; use core::result::Result
src/value/tagged.rs	Replace std imports with core/alloc equivalents
src/value/index.rs	Replace std imports with core/alloc equivalents
src/value/from.rs	Replace std Cow import with alloc::borrow::Cow; add alloc imports
src/value/debug.rs	Replace std fmt import with core fmt
src/value/partial_eq.rs	Add alloc::string::String import for no_std build
src/with.rs	Replace std fmt import with core fmt; add alloc imports in modules
src/libyaml/emitter.rs	Replace std usage with core/alloc; route output through crate::io
src/libyaml/parser.rs	Replace std usage with core/alloc equivalents
src/libyaml/error.rs	Replace std usage with core equivalents; use core::result::Result
src/libyaml/cstr.rs	Replace std usage with core equivalents
src/libyaml/tag.rs	Replace std usage with core/alloc equivalents
src/libyaml/util.rs	Replace std usage with core/alloc equivalents; allocate via Box
.gitignore	Ignore rust_out

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[pawelchcki]

I hope it wasn't my settings which pulled in copilot. But it posted some interesting points, I've updated the PR. and await further feedback