Add Wasm support for AsyncMediaSource

[Enduriel]

Closes #52, option A as discussed in that issue.

[mindeng]

Code review

Found 2 issues:

1. cargo test --no-default-features --features tokio no longer compiles. The PR re-gates AsyncMediaSource::open, read_exif_async, read_exif_iter_async, read_metadata_async, and read_track_async to feature = "tokio-fs", but several #[cfg(feature = "tokio")] call sites still reference these now-tokio-fs-only symbols. Reproduced locally — 18+ E0599 / E0425 errors. Sites needing the gate widened to tokio-fs (or per-fn cfgs):

• Test mod at

  nom-exif/src/parser_async.rs

  Lines 272 to 290 in ba2a417

  	#[cfg(test)]
  	mod tests {
  	use std::path::Path;
  	use super::*;
  	use crate::{ExifIter, TrackInfo};
  	use test_case::case;
  	enum TrackExif {
  	Track,
  	Exif,
  	NoData,
  	Invalid,
  	}
  	use tokio::fs::File;
  	use TrackExif::*;
  	#[tokio::test(flavor = "multi_thread", worker_threads = 1)]

  uses tokio::fs::File and AsyncMediaSource::open under #[cfg(test)].

• nom-exif/src/lib.rs

  Lines 395 to 408 in ba2a417

  	#[cfg(feature = "tokio")]
  	#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
  	async fn read_exif_async_jpg() {
  	let exif = read_exif_async("testdata/exif.jpg").await.unwrap();
  	assert!(exif.get(ExifTag::Make).is_some());
  	}
  	#[cfg(feature = "tokio")]
  	#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
  	async fn read_track_async_mov() {
  	let info = read_track_async("testdata/meta.mov").await.unwrap();
  	assert!(info.get(TrackInfoTag::Make).is_some());
  	}

  — async test helpers gated only on tokio.

• nom-exif/tests/migration_guide.rs

  Lines 204 to 219 in ba2a417

  	#[cfg(feature = "tokio")]
  	#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
  	async fn s5_8_async_top_level_helper() {
  	let exif = read_exif_async("./testdata/exif.jpg").await.unwrap();
  	assert!(exif.get(ExifTag::Make).is_some());
  	}
  	#[cfg(feature = "tokio")]
  	#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
  	async fn s5_8_async_media_parser_method() {
  	let mut parser = MediaParser::new();
  	let ms = AsyncMediaSource::open("./testdata/exif.jpg").await.unwrap();
  	let _iter = parser.parse_exif_async(ms).await.unwrap();
  	}

• nom-exif/tests/png.rs

  Lines 60 to 67 in ba2a417

  	#[cfg(feature = "tokio")]
  	#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
  	async fn parse_image_metadata_async_exif_png() {
  	use nom_exif::AsyncMediaSource;
  	let mut parser = MediaParser::new();
  	let ms = AsyncMediaSource::open("testdata/exif.png").await.unwrap();
  	let img = parser.parse_image_metadata_async(ms).await.unwrap();

2. Silent breaking change for current features = ["tokio"] users, with crate docs and README left advertising the old gate. Any downstream consumer calling read_exif_async / AsyncMediaSource::open under features = ["tokio"] will get unresolved-symbol errors on upgrade. The breakage is acknowledged in Wasm support for AsyncMediaSource #52 as "mildly breaking" — please pair the split with a CHANGELOG entry and update the rustdoc + README that still claim the old gate:

nom-exif/src/lib.rs

Lines 55 to 59 in ba2a417

	//!
	//! Async variants live behind `feature = "tokio"`:
	//! [`read_exif_async`], [`read_track_async`], [`read_metadata_async`],
	//! plus [`MediaParser::parse_exif_async`] / [`MediaParser::parse_track_async`].
	//!

nom-exif/src/lib.rs

Lines 150 to 156 in ba2a417

	//!
	//! # Cargo features
	//!
	//! - `tokio` — async API via tokio (`AsyncMediaSource`, `read_*_async`,
	//! `MediaParser::parse_*_async`).
	//! - `serde` — derives `Serialize`/`Deserialize` on the public types.

nom-exif/README.md

Lines 216 to 244 in ba2a417

	```
	## Async API
	Enable the `tokio` feature in your `Cargo.toml`:
	```toml
	[dependencies]
	nom-exif = { version = "3", features = ["tokio"] }
	```
	Then use the `_async` helpers, or call `parse_exif_async` /
	`parse_track_async` on a `MediaParser` directly:
	```rust,no_run
	# #[cfg(feature = "tokio")]
	# async fn demo() -> nom_exif::Result<()> {
	use nom_exif::{read_exif_async, MediaParser, AsyncMediaSource};
	// One-shot:
	let exif = read_exif_async("./testdata/exif.jpg").await?;
	// Reusable:
	let mut parser = MediaParser::new();
	let ms = AsyncMediaSource::open("./testdata/exif.jpg").await?;
	let iter = parser.parse_exif_async(ms).await?;
	# let _ = (exif, iter); Ok(())
	# }
	```

Cargo.toml still on 3.4.1 (already released) — bump before publishing:

nom-exif/Cargo.toml

Lines 1 to 7 in ba2a417

	[package]
	name = "nom-exif"
	rust-version = "1.83"
	version = "3.4.1"
	edition = "2021"
	license-file = "LICENSE"
	description = "Exif/metadata parsing library written in pure Rust, both image (jpeg/heif/heic/jpg/tiff etc.) and video/audio (mov/mp4/3gp/webm/mkv/mka, etc.) files are supported."

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[Enduriel]

Yeah that's my bad, this was pretty quickly put together just to get it working on my side (which it does btw, ty for all your help in that regard).

I'll clean all this up and request another review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.84%. Comparing base (dad49e0) to head (f9b6480).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   89.84%   89.84%           
=======================================
  Files          38       38           
  Lines        8399     8399           
=======================================
  Hits         7546     7546           
  Misses        853      853           

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[Enduriel]

Should have everything you previously mentioned fixed. Let me know if you have any other concerns/suggestions.

[mindeng]

Thanks for both contributions! The tokio/tokio-fs split (#53) and the slice coercion fix (#58) are now included in nom-exif v3.6.0, just published to crates.io. Appreciate the help!

[Enduriel]

@mindeng Thanks for your quick responses! One thing that might be an issue is that if my understanding is correct this PR should be a SemVer breaking change, and people updating from 3.5 to 3.6 may have issues. I added the ! in the conventional commit to signal that, but I now realize that may not be obvious.

Not sure how you want to resolve this but going by strictly semver rules this should probably be a 4.0.0 release.