Rustdoc Internals

This page describes rustdoc's passes and modes. For an overview of rustdoc, see the "Rustdoc overview" chapter.

From Crate to Clean

In are two central items: the rustdoc::core::DocContext struct, and the rustdoc::core::run_global_ctxt function. The latter is where rustdoc calls out to rustc to compile a crate to the point where rustdoc can take over. The former is a state container used when crawling through a crate to gather its documentation.

The main process of crate crawling is done in clean/ through several functions with names that start with clean_. Each function accepts an hir or ty data structure, and outputs a clean structure used by rustdoc. For example, this function for converting lifetimes:

fn clean_lifetime<'tcx>(lifetime: &hir::Lifetime, cx: &mut DocContext<'tcx>) -> Lifetime {
    if let Some(
        | rbv::ResolvedArg::LateBound(_, _, did)
        | rbv::ResolvedArg::Free(_, did),
    ) = cx.tcx.named_bound_var(lifetime.hir_id)
        && let Some(lt) = cx.args.get(&did).and_then(|arg| arg.as_lt())
        return lt.clone();

Also, clean/ defines the types for the "cleaned" Abstract Syntax Tree (AST) used later to render documentation pages. Each usually accompanies a clean_* function that takes some AST or High-Level Intermediate Representation (HIR) type from rustc and converts it into the appropriate "cleaned" type. "Big" items like modules or associated items may have some extra processing in its clean function, but for the most part these impls are straightforward conversions. The "entry point" to this module is clean::utils::krate, which is called by run_global_ctxt.

The first step in clean::utils::krate is to invoke visit_ast::RustdocVisitor to process the module tree into an intermediate visit_ast::Module. This is the step that actually crawls the rustc_hir::Crate, normalizing various aspects of name resolution, such as:

  • handling #[doc(inline)] and #[doc(no_inline)]
  • handling import globs and cycles, so there are no duplicates or infinite directory trees
  • inlining public use exports of private items, or showing a "Reexport" line in the module page
  • inlining items with #[doc(hidden)] if the base item is hidden but the
  • showing #[macro_export]-ed macros at the crate root, regardless of where they're defined reexport is not

After this step, clean::krate invokes clean_doc_module, which actually converts the HIR items to the cleaned AST. This is also the step where cross- crate inlining is performed, which requires converting rustc_middle data structures into the cleaned AST.

The other major thing that happens in clean/ is the collection of doc comments and #[doc=""] attributes into a separate field of the Attributes struct, present on anything that gets hand-written documentation. This makes it easier to collect this documentation later in the process.

The primary output of this process is a clean::types::Crate with a tree of Items which describe the publicly-documentable items in the target crate.

Passes Anything But a Gas Station (or: Hot Potato)

Before moving on to the next major step, a few important "passes" occur over the cleaned AST. Several of these passes are lints and reports, but some of them mutate or generate new items.

These are all implemented in the librustdoc/passes directory, one file per pass. By default, all of these passes are run on a crate, but the ones regarding dropping private/hidden items can be bypassed by passing --document-private-items to rustdoc. Note that unlike the previous set of AST transformations, the passes are run on the cleaned crate.

Here is the list of passes as of March 2023:

  • calculate-doc-coverage calculates information used for the --show-coverage flag.

  • check-doc-test-visibility runs doctest visibility–related lints. This pass runs before strip-private, which is why it needs to be separate from run-lints.

  • collect-intra-doc-links resolves intra-doc links.

  • collect-trait-impls collects trait impls for each item in the crate. For example, if we define a struct that implements a trait, this pass will note that the struct implements that trait.

  • propagate-doc-cfg propagates #[doc(cfg(...))] to child items.

  • run-lints runs some of rustdoc's lints, defined in passes/lint. This is the last pass to run.

    • bare_urls detects links that are not linkified, e.g., in Markdown such as Go to It suggests wrapping the link with angle brackets: Go to <>. to linkify it. This is the code behind the rustdoc::bare_urls lint.

    • check_code_block_syntax validates syntax inside Rust code blocks (```rust)

    • html_tags detects invalid HTML (like an unclosed <span>) in doc comments.

  • strip-hidden and strip-private strip all doc(hidden) and private items from the output. strip-private implies strip-priv-imports. Basically, the goal is to remove items that are not relevant for public documentation. This pass is skipped when --document-hidden-items is passed.

  • strip-priv-imports strips all private import statements (use, extern crate) from a crate. This is necessary because rustdoc will handle public imports by either inlining the item's documentation to the module or creating a "Reexports" section with the import in it. The pass ensures that all of these imports are actually relevant to documentation. It is technically only run when --document-private-items is passed, but strip-private accomplishes the same thing.

  • strip-private strips all private items from a crate which cannot be seen externally. This pass is skipped when --document-private-items is passed.

There is also a stripper module in librustdoc/passes, but it is a collection of utility functions for the strip-* passes and is not a pass itself.

From Clean To HTML

This is where the "second phase" in rustdoc begins. This phase primarily lives in the librustdoc/formats and librustdoc/html folders, and it all starts with formats::renderer::run_format. This code is responsible for setting up a type that impl FormatRenderer, which for HTML is Context.

This structure contains methods that get called by run_format to drive the doc rendering, which includes:

  • init generates static.files, as well as search index and src/
  • item generates the item HTML files themselves
  • after_krate generates other global resources like all.html

In item, the "page rendering" occurs, via a mixture of Askama templates and manual write!() calls, starting in html/ The parts that have not been converted to templates occur within a series of std::fmt::Display implementations and functions that pass around a &mut std::fmt::Formatter.

The parts that actually generate HTML from the items and documentation start with print_item defined in html/render/, which switches out to one of several item_* functions based on kind of Item being rendered.

Depending on what kind of rendering code you're looking for, you'll probably find it either in html/render/ for major items like "what sections should I print for a struct page" or html/ for smaller component pieces like "how should I print a where clause as part of some other item".

Whenever rustdoc comes across an item that should print hand-written documentation alongside, it calls out to html/ which interfaces with the Markdown parser. This is exposed as a series of types that wrap a string of Markdown, and implement fmt::Display to emit HTML text. It takes special care to enable certain features like footnotes and tables and add syntax highlighting to Rust code blocks (via html/ before running the Markdown parser. There's also a function find_codes which is called by find_testable_codes that specifically scans for Rust code blocks so the test-runner code can find all the doctests in the crate.

From Soup to Nuts (or: "An Unbroken Thread Stretches From Those First Cells To Us")

It's important to note that rustdoc can ask the compiler for type information directly, even during HTML generation. This didn't used to be the case, and a lot of rustdoc's architecture was designed around not doing that, but a TyCtxt is now passed to formats::renderer::run_format, which is used to run generation for both HTML and the (unstable as of March 2023) JSON format.

This change has allowed other changes to remove data from the "clean" AST that can be easily derived from TyCtxt queries, and we'll usually accept PRs that remove fields from "clean" (it's been soft-deprecated), but this is complicated from two other constraints that rustdoc runs under:

  • Docs can be generated for crates that don't actually pass type checking. This is used for generating docs that cover mutually-exclusive platform configurations, such as libstd having a single package of docs that cover all supported operating systems. This means rustdoc has to be able to generate docs from HIR.
  • Docs can inline across crates. Since crate metadata doesn't contain HIR, it must be possible to generate inlined docs from the rustc_middle data.

The "clean" AST acts as a common output format for both input formats. There is also some data in clean that doesn't correspond directly to HIR, such as synthetic impls for auto traits and blanket impls generated by the collect-trait-impls pass.

Some additional data is stored in html::render::context::{Context, SharedContext}. These two types serve as ways to segregate rustdoc's data for an eventual future with multithreaded doc generation, as well as just keeping things organized:

  • Context stores data used for generating the current page, such as its path, a list of HTML IDs that have been used (to avoid duplicate id=""), and the pointer to SharedContext.
  • SharedContext stores data that does not vary by page, such as the tcx pointer, and a list of all types.

Other Tricks Up Its Sleeve

All this describes the process for generating HTML documentation from a Rust crate, but there are couple other major modes that rustdoc runs in. It can also be run on a standalone Markdown file, or it can run doctests on Rust code or standalone Markdown files. For the former, it shortcuts straight to html/, optionally including a mode which inserts a Table of Contents to the output HTML.

For the latter, rustdoc runs a similar partial-compilation to get relevant documentation in, but instead of going through the full clean and render process, it runs a much simpler crate walk to grab just the hand-written documentation. Combined with the aforementioned "find_testable_code" in html/, it builds up a collection of tests to run before handing them off to the test runner. One notable location in is the function make_test, which is where hand-written doctests get transformed into something that can be executed.

Some extra reading about make_test can be found here.

Dotting i's And Crossing t's

So that's rustdoc's code in a nutshell, but there's more things in the compiler that deal with it. Since we have the full compiletest suite at hand, there's a set of tests in tests/rustdoc that make sure the final HTML is what we expect in various situations. These tests also use a supplementary script, src/etc/, that allows it to look through the final HTML using XPath notation to get a precise look at the output. The full description of all the commands available to rustdoc tests (e.g. @has and @matches) is in

To use multiple crates in a rustdoc test, add // to the top of the test file. should be placed in an auxiliary directory relative to the test file with the comment. If you need to build docs for the auxiliary file, use // build-aux-docs.

In addition, there are separate tests for the search index and rustdoc's ability to query it. The files in tests/rustdoc-js each contain a different search query and the expected results, broken out by search tab. These files are processed by a script in src/tools/rustdoc-js and the Node.js runtime. These tests don't have as thorough of a writeup, but a broad example that features results in all tabs can be found in basic.js. The basic idea is that you match a given QUERY with a set of EXPECTED results, complete with the full item path of each item.

Testing Locally

Some features of the generated HTML documentation might require local storage to be used across pages, which doesn't work well without an HTTP server. To test these features locally, you can run a local HTTP server, like this:

$ ./x doc library
# The documentation has been generated into `build/[YOUR ARCH]/doc`.
$ python3 -m http.server -d build/[YOUR ARCH]/doc

Now you can browse your documentation just like you would if it was hosted on the internet. For example, the url for std will be rust/std/.

See Also