Rustdoc JSON in 2022

It’s that time of year again, when everyone is writing excellent summary articles about what happened this year, and goals for the next one. I figured I should do the same for rustdoc JSON.

What is rustdoc JSON?

If you haven’t heard of it yet, rustdoc JSON is an unstable feature for rustdoc that allows rustdoc to output a JSON description of a crates API, as opposed to the stable HTML output.

This allows tools to be written that reason about an API without them having to interface directly with the (even more unstable) rustc APIs, and that benefit from all the processing and cleanup that rustdoc does.

Format Changes

The biggest user-facing change has been the number of changes to the JSON Format itself. Version 1 11 to 23 were released this year 2.

  1. #94137: Clean up the Header struct by:
    • Making ABI an enum, instead of being stringly typed.
    • Replace HashSet<Qualifiers> with 3 bools (const_, unsafe_, and async_).
    • Merge ABI field into Header, as they always occur together.
  2. #94009: Support GATs.
  3. #94150: Report whether a generic type bound is synthetic (generated from impl Trait in argument position).
  4. #94921: Make names more consistent:
    • Consistently use type_ over ty.
    • Have Trait call it implementations, (not implementors), because the Id’s are the impl blocks, not the types that impl the trait.
  5. #96647: Report HRTBs on where predicates.
  6. #99287: This is a larger structural change, that will be discussed next. It’s also a format change because it added the is_stripped field to Module.
  7. #99787: Add dyn Trait as a separate variant to the Type enum. This allows HRTBs to be reported, and is also more principled as it doesn’t use Type::ResolvedPath for both concrete types (struct, enum, union) and dyn traits.
  8. #100335: Refer to traits directly with a Path struct, instead of using Type::ResolvedPath.
  9. #101386: Report the discriminant of enum variants. 3
  10. #101462: Change how fields in enum variants are stored, to better support ordering and #[doc(hidden)].
  11. #101521: Store a Struct’s fields inside the StructKind enum, to better support ordering and #[doc(hidden)].
  12. #102321: Add impls to Primitives.
  13. #104499: Use Function everywhere and remove Method

While doing this many changes (on average about 2 a month), may seem disruptive, there are many things that make it less of a burden for users:

  1. Version numbering: Because each version increase changes a constant. This makes error reporting much friendlier. For example cargo-check-external-types first attempts to deserialize just the format version, and bails if that doesn’t match. This means the user receives an error about the version of nightly being wrong, which is much more useful and actionable than an error about a missing or unknown JSON field.
  2. JSON’s inherent flexibility: Because of how serde_json works, adding a field won’t break old code, nor will removing an enum variant. This means that many of the smaller changes may not actually require users to update.
  3. Automatically notifying users: Maintainers of tools that consume rustdoc JSON can be automatically notified when a change to the format is proposed. In some cases, this has led to fixes to tools being written before the PR to rust has landed (eg Enselic/cargo-public-api#95 and #100335). If you want to get an early warning for changes, add yourself to this list, and the magic of rust’s bots will keep you up to date.

Big Change: Don’t inline

Format version 16, introduced in #99287 merits its own discussion, as it was a much deeper change to how the format represents rust code, and fixed a lot more bugs.

The root of the problem is that each new file in rust is its own module 4. This means that if each type went in its own file (which was a pub mod), then the type name is duplicated with the module name.

Eg if there’s a library called collections that’s laid out like

├── Cargo.lock
├── Cargo.toml
└── src

And written like

// collections/src/
pub mod list;
pub mod set;
pub mod map;
// collections/src/
pub struct List;
// collections/src/
pub struct Set;
// collections/src/
pub struct Map;

Then users of the module see the paths like collections::list::List, which needlessly duplicates “list”. To avoid this, code like this tends to get written as

// collections/src/
mod list;
mod set;
mod map;
pub use list::List;
pub use set::Set;
pub use map::Map;
// collections/src/
pub struct List;
// collections/src/
pub struct Set;
// collections/src/
pub struct Map;

And the user now sees List as collections::List, which is much nicer. It is as if the library author instead just wrote

// collections/src/
pub struct List;
pub struct Set;
pub struct Map;

But also allows the separate types to be in their own files, which is much nicer for the library author.

Rustdoc goes to a lot of effort to make the code with private mods and pub uses look like it was all written in one file. In particular it sometimes “inline"s items into the locations that they are used, by replacing a pub use of an item with the item being used.

While this is great for the HTML output, it caused boundless problems for JSON. The most canonical example is

mod style {
    pub struct Color;

pub use style::Color;
pub use style::Color as Colour;

In HTML Output, both Color and Colour are created as separate pages, with no indication that they are the same item. In fact, it is the same result as if

pub struct Color;
pub struct Colour;

was written.

In JSON this would crash, as two different items were created with the same ID, triggering an assertion failure.

The fix for this in JSON is to not inline, and instead report the root module as having two items, both of which are imports of the same struct item. The struct item isn’t a member of any module, and is only accessible via the imports. While this would be an unacceptable UI issue for HTML, in JSON it’s better to report the true nature of the code than to try to clean it up with inlines.

Changing this fixed a major source of issues for rustdoc JSON, and make the output far less likely to ICE.

Package std docs

Another nice user-facing change this year was including the docs for std (and friends) as a rustup component. Because std is special in that it isn’t built like normal dependencies, but is magically made available by cargo and rustc, its JSON 5 docs can’t be produced by cargo like they can for normal dependencies. Therefore they need to be shipped by rustup.

Making this work took several attempts, but now that this is all done, anyone can run rustup component add --toolchain nightly rust-docs-json to get the docs for std, alloc, core, test, and proc_macro in the share/doc/rust/json/ directory of the rustup toolchain directory, and automatically kept up to date with the nightly toolchain by rustup.

Test tool improvements

While these were the big user-facing improvements, there were also many internal improvements, particularly around the test tooling.

Rustdoc JSON is currently tested with two tools. The first, jsondocck reads comments from the files which contain assertions about the JSON output, and checks that the output matches the assertions. The assertions are written in JsonPath, and let you check that the output has (and doesn’t have) the values that you expect.

Eg src/test/rustdoc-json/reexport/ currently looks like

// @set impl_S = "$.index[*][?('impl S')].id"
// @has "$.index[*][?('S')].inner.impls[*]" $impl_S
// @set is_present = "$.index[*][?('is_present')].id"
// @is "$.index[*][?('impl S')].inner.items[*]" $is_present
// @!has "$.index[*][?('hidden_impl')]"
// @!has "$.index[*][?('hidden_fn')]"

mod private_mod {
    pub struct S;

    /// impl S
    impl S {
        pub fn is_present() {}
        pub fn hidden_fn() {}

    impl S {
        pub fn hidden_impl() {}

pub use private_mod::*;

It checks that the struct S has an impl block whose only method is is_present, and that hidden_impl and hidden_fn aren’t mentioned.

Over this year, two major changes were landed to jsondocck that make writing these tests much nicer.

  1. #99474: Add @ismany to jsondocck to do a setwise comparison.
  2. #100678: Don’t require specifying file in jsondocck.

Between them, they mean a test like:

struct S
/// the impl
impl S {
    pub fn foo() {}
    pub fn bar() {}
// @set foo = "$.index[*][?('foo')].id"
// @set bar = - "$.index[*][?('foo')].id"
// @count - "$.index[*][?('the impl')].inner.items[*]" 2
// @has   - "$.index[*][?('the impl')].inner.items[*]" $foo
// @has   - "$.index[*][?('the impl')].inner.items[*]" $bar

can be rewritten to be

// @set foo = "$.index[*][?('foo')].id"
// @set bar = "$.index[*][?('foo')].id"
// @ismany "$.index[*][?('the impl')].inner.items[*]" $foo $bar

which is much nicer.

The other tool that’s used is one that checks that all Ids mentioned are present in the index (or paths). Originally this was a python script called, but in #101809, it was replaced with jsondoclint, a rust rewrite. This had many advantages, such as being able to use rustdoc-json-types to keep up with format changes, and exhaustively matching on kinds, leading to more bugs being caught.

Interestingly, these bugs all had to be fixed before the tool could be landed, and in doing so, was fixed so it could catch them if they regressed before jsondoclint landed. Despite this, it was still great to get rid of it, and replace it with a much more maintainable tool.

However, with any big rewrite, there were bound to be bugs, and this was no exception. In particular, a number of false positives were introduced for code patterns not covered by the test suite. They were only unearthed when jsondoclint was run on core.json, which isn’t currently done in CI, but should be. 6. These were fixed, and tests were added.

More Tests

Another longstanding issue that was partially addressed this year is the relative lack of tests. This year the rustdoc-json suite has grown from 26 to 98 tests 7. (For what it’s worth, in the same time period, the main rustdoc suite 8 went from 484 to 586 tests.)

This was addressed in part with dedicated test adding PRs 9, but mainly due to good habits of always adding tests when changing behaviour that we were lucky to inherit from the wider rust project.

Correctness Fixes

The final change for 2022 was the vast, vast number of bug fixes 10. The fact that we were able to make so many fixes is a testament to how many users are reporting issues. This is mainly driven by tools that make use rustdoc JSON, and in paticular cargo-public-api and cargo-semver-checks have driven a lot more eyes towards the code.

The other major source of bug reports was running with crater, which while it can only find assertion failures, makes up for this with sheer volume. One thing I want to look into next year is running the jsondoclint tool in crater, so it can catch missing IDs, instead of just internal assertions failing.


2022 was a good year for rustdoc JSON. The format is better; The code is more reliable; The tests are more numerous and easier to write; Their are more users depending on it. All this was made possible by many people working on and around the format. In particular, I’d like to thank Alex Kladov, Didrik Nordström, Guillaume Gomez, Jacob Hoffman-Andrews, Joseph Ryan, Joshua Nelson, León Orell Valerian Liehr, Luca Palmieri, Martin Nordholts, Matthias Krüger , Michael Goulet, Michael Howell, Noah Lev, Predrag Gruevski, QuietMisdreavus, Rune Tynan, Tyler Mandry, and Urgau for their invaluable contributions.

Hopefully next year we can continue to improve at this solid pace. My main goal is to impove the way cross-crate ID lookup works, but there’s also more work to be done to fix more bugs, further flesh out the test suite, and increase performance. I’ll write more about these in a future post.

If you want to hear about that when it comes out, or just generally want to be notified the next time I have something to share online, you can find me in the Fediverse If you have questions or comments on this post, I’d love to hear them on github.

  1. At the root level of the output, there’s a field called format_version, which gets increased by 1 every time we change the definition of the types that get serialized. ↩︎

  2. Assuming nothing gets released on New Year’s Eve. ↩︎

  3. It turns out this support isn’t great. While writing this post, I realised that we only support discriminants on unit variants. This restriction has been lifted, and I’ve filled an issue and intend to fix it in the new year. ↩︎

  4. Barring the include! macro, which isn’t relevant here. ↩︎

  5. Or HTML for that matter. Rustup has long distributed html docs for std as a component. ↩︎

  6. Hopefully I’ll talk more about this in an upcoming post about my goals for rustdoc JSON next year. ↩︎

  7. Measured on bbdca4c (most recent commit as of the time of writing) and 1e6ced3 (last change to src/test/rustdoc-json in 2021). Number of tests measured with fd -e rs | rg -v "auxiliary" | wc -l↩︎

  8. This is only for the src/test/rustdoc/ suite, and doesn’t include ui, gui and std-json. But these are much smaller, and I’m trying to make a point about the rate of growth and the size of a mature test suite, not provide exact numbers. ↩︎

  9. #93660 , #94861 , #98166 , #98548 , #99479 , #101634 , #101701 , #103065 , #105027 , #105063 ↩︎

  10. #92860 , #93132 , #93954 , #97599 , #98053 , #98195 , #98390 , #98577 , #98611 , #98681 , #100299 , #100325 , #100582 , #100630 , #101106 , #101204 , #101633 , #101722 , #101770 , #101914 , #103653 , #105182 ↩︎