embed-doc-image

embed-doc-image is proc macro crate that facilitates the embedding of images in Rust documentation.

Please see the documentation for motivation, usage instructions and more.

Contributing

I'm happy to accept contributions in the form of pull requests, bug reports or feature requests.

For major contributions I'd prefer that you first open an issue so that we can discuss whether the proposed changes are appropriate.

Acknowledgements

As an inexperienced proc macro hacker, I would not have managed to arrive at this solution without the help of several individuals on the Rust Programming Language Community Discord server, most notably:

• Yandros (github.com/danielhenrymantilla)
• Nemo157 (github.com/Nemo157)

License

This crate is licensed under the MIT license. See LICENSE for details.

lib.rs:

Embed images in documentation.

This crate enables the portable embedding of images in rustdoc-generated documentation. Standard web-compatible image formats should be supported. Please file an issue if you have problems. Read on to learn how it works.

Showcase

See the showcase documentation for an example with embedded images.

Please also check out the source code for the showcase crate for a fleshed out example.

Motivation

A picture is worth a thousand words. This oft quoted adage is no less true for technical documentation. A carefully crafted diagram lets a new user immediately grasp the high-level architecture of a complex library. Illustrations of geometric conventions can vastly reduce confusion among users of scientific libraries. Despite the central role of images in technical documentation, embedding images in Rust documentation in a way that portably works correctly across local installations and docs.rs has been a longstanding issue of rustdoc.

This crate represents a carefully crafted solution based on procedural macros that works around the current limitations of rustdoc and enables a practically workable approach to embedding images in a portable manner.

How to embed images in documentation

First, you'll need to depend on this crate. In cargo.toml:

[dependencies]
// Replace x.x with the latest version
embed-doc-image = "x.x"

What the next step is depends on whether you want to embed images into inner attribute documentation or outer attribute documentation. Inner attribute documentation is usually used to document crate-level or module-level documentation, and typically starts each line with //!. Outer attribute docs are used for most other forms of documentation, such as function and struct documentation. Outer attribute documentation typically starts each line with ///.

In both cases all image paths are relative to the crate root.

Embedding images in outer attribute documentation

Outer attribute documentation is typically used for documenting functions, structs, traits, macros and so on. Let's consider documenting a function and embedding an image into its documentation:

// Import the attribute macro
use embed_doc_image::embed_doc_image;

/// Foos the bar.
///
/// Let's drop an image below this text.
///
/// ![Alt text goes here][myimagelabel]
///
/// And another one.
///
/// ![A Foobaring][foobaring]
///
/// We can include any number of images in the above fashion. The important part is that
/// you match the label ("myimagelabel" or "foobaring" in this case) with the label in the
/// below attribute macro.
// Paths are always relative to the **crate root**
#[embed_doc_image("myimagelabel", "images/foo.png")]
#[embed_doc_image("foobaring", "assets/foobaring.jpg")]
fn foobar() {}

And that's it! If you run cargo doc, you should hopefully be able to see your images in the documentation for foobar, and it should also work on docs.rs without trouble.

Embedding images in inner attribute documentation

The ability for macros to do anything with inner attributes is very limited. In fact, before Rust 1.54 (which at the time of writing has not yet been released), it is for all intents and purposes non-existent. This also means that we can not directly use our approach to embed images in documentation for Rust < 1.54. However, we can make our code compile with Rust < 1.54 and instead inject a prominent message that some images are missing. docs.rs, which always uses a nightly compiler, will be able to show the images. We'll also locally be able to properly embed the images as long as we're using Rust >= 1.54 (or nightly). Here's how you can embed images in crate-level or module-level documentation:

//! My awesome crate for fast foobaring in latent space.
//!
// Important: note the blank line of documentation on each side of the image lookup table.
// The "image lookup table" can be placed anywhere, but we place it here together with the
// warning if the `doc-images` feature is not enabled.
#![cfg_attr(feature = "doc-images",
cfg_attr(all(),
doc = ::embed_doc_image::embed_image!("myimagelabel", "images/foo.png"),
doc = ::embed_doc_image::embed_image!("foobaring", "assets/foobaring.png")))]
#![cfg_attr(
not(feature = "doc-images"),
doc = "**Doc images not enabled**. Compile with feature `doc-images` and Rust version >= 1.54 \
           to enable."
)]
//!
//! Let's use our images:
//! ![Alt text goes here][myimagelabel] ![A Foobaring][foobaring]

Sadly there is currently no way to detect Rust versions in cfg_attr. Therefore we must rely on a feature flag for toggling proper image embedding. We'll need the following in our Cargo.toml:

[features]
doc-images = []

[package.metadata.docs.rs]
# docs.rs uses a nightly compiler, so by instructing it to use our `doc-images` feature we
# ensure that it will render any images that we may have in inner attribute documentation.
features = ["doc-images"]

Let's summarize:

• docs.rs will correctly render our documentation with images.
• Locally:
  • for Rust >= 1.54 with --features doc-images, the local documentation will correctly render images.
  • for Rust < 1.54: the local documentation will be missing some images, and will contain a warning with instructions on how to enable proper image embedding.
  • we can also use e.g. cargo +nightly doc --features doc-images to produce correct documentation with a nightly compiler.

How it works

The crux of the issue is that rustdoc does not have a mechanism for tracking locally stored images referenced by documentation and carry them over to the final documentation. Therefore currently images on docs.rs can only be included if you host the image somewhere on the internet and include the image with its URL. However, this has a number of issues:

• You need to host the image, which incurs considerable additional effort on the part of crate authors.
• The image is only available for as long as the image is hosted.
• Images in local documentation will not work without internet access.
• Images are not versioned, unless carefully done so manually by the crate author. That is, the author must carefully provide all versions of the image across all versions of the crate with a consistent naming convention in order to ensure that documentation of older versions of the crate display the image consistent with that particular version.

The solution employed by this crate is based on a remark made in an old reddit comment from 2017. In short, Rustdoc allows images to be provided inline in the Markdown as base64 encoded binary blobs in the following way:

![Alt text][myimagelabel]

[myimagelabel]: data:image/png;base64,BaSe64EnCoDeDdAtA

Basically we can use the "reference" feature of Markdown links/images to provide the URL of the image in a different location than the image itself, but instead of providing an URL we can directly provide the binary data of the image in the Markdown documentation.

However, doing this manually with images would terribly clutter the documentation, which seems less than ideal. Instead, we do this programmatically. The macros available in this crate essentially follow this idea:

• Take a label and image path relative to the crate root as input.
• Determine the MIME type (based on extension) and base64 encoding of the image.
• Produce an appropriate doc string and inject it into the Markdown documentation for the crate/function/struct/etc.

Clearly, this is still quite hacky, but it seems like a workable solution until proper support in rustdoc arrives, at which point we may rejoice and abandon this crate to the annals of history.

Acknowledgements

As an inexperienced proc macro hacker, I would not have managed to arrive at this solution without the help of several individuals on the Rust Programming Language Community Discord server, most notably:

• Yandros (github.com/danielhenrymantilla)
• Nemo157 (github.com/Nemo157)