# Rustdoc Internals

<!-- toc -->

This page describes [`rustdoc`]'s passes and modes. For an overview of `rustdoc`,
see the ["Rustdoc overview" chapter](./rustdoc.md).

[`rustdoc`]: https://github.com/rust-lang/rust/tree/master/src/tools/rustdoc

## From Crate to Clean

In [`core.rs`] 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/mod.rs`] 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]:

```rust,ignore
fn clean_lifetime<'tcx>(lifetime: &hir::Lifetime, cx: &mut DocContext<'tcx>) -> Lifetime {
    if let Some(
        rbv::ResolvedArg::EarlyBound(did)
        | 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();
    }
    Lifetime(lifetime.ident.name)
}
```

Also, `clean/mod.rs` defines the types for the "cleaned" [Abstract Syntax Tree
(`AST`)][ast] used later to render documentation pages. Each usually accompanies a
`clean_*` function that takes some [`AST`][ast] or [High-Level Intermediate
Representation (`HIR`)][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
`impl`s are straightforward conversions. The "entry point" to this module is
[`clean::utils::krate`][ck0], which is called by [`run_global_ctxt`].

The first step in [`clean::utils::krate`][ck1] 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 whether
    they're defined as a reexport or not

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

The other major thing that happens in `clean/mod.rs` 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 [`Item`]s
which describe the publicly-documentable items in the target crate.

[`Attributes`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/types/struct.Attributes.html
[`clean_doc_module`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/fn.clean_doc_module.html
[`clean::types::Crate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/types/struct.Crate.html
[`clean/mod.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/clean/mod.rs
[`core.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/core.rs
[`Item`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/types/struct.Item.html
[`run_global_ctxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/core/fn.run_global_ctxt.html
[`rustc_hir::Crate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Crate.html
[`rustdoc::core::DocContext`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/core/struct.DocContext.html
[`rustdoc::core::run_global_ctxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/core/fn.run_global_ctxt.html
[`visit_ast::Module`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/visit_ast/struct.Module.html
[`visit_ast::RustdocVisitor`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/visit_ast/struct.RustdocVisitor.html
[ast]: ./ast-validation.md
[ck0]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/utils/fn.krate.html#
[ck1]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustdoc/clean/utils.rs.html#31-77
[hir]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/index.html
[this function for converting lifetimes]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustdoc/clean/mod.rs.html#256-267

### Passes Anything But a Gas Station (or: [Hot Potato](https://www.youtube.com/watch?v=WNFBIt5HxdY))

Before moving on to the next major step, a few important "passes" occur over
the cleaned [`AST`][ast]. Several of these passes are `lint`s 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`][ast]
transformations, the passes are run on the _cleaned_ crate.

Here is the list of passes as of <!-- date-check --> March 2023:

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

- `check-doc-test-visibility` runs `doctest` visibility–related `lint`s. 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](https://doc.rust-lang.org/nightly/rustdoc/write-documentation/linking-to-items-by-name.html).

- `collect-trait-impls` collects `trait` `impl`s 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 `lint`s, 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 https://example.com/.` It suggests wrapping the link with angle brackets:
    `Go to <https://example.com/>.` to linkify it. This is the code behind the <!--
    date-check: may 2022 --> `rustdoc::bare_urls` `lint`.

  - `check_code_block_syntax` validates syntax inside Rust code blocks
    (<code>```rust</code>)

  - `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.

[`librustdoc/passes`]: https://github.com/rust-lang/rust/tree/master/src/librustdoc/passes
[`stripper`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/passes/stripper/index.html

## 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/layout.rs`]. 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/print_item.rs`], 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/mod.rs`] for major items like "what sections
should I print for a `struct` page" or [`html/format.rs`] 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/markdown.rs`] 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/highlight.rs`) 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 `doctest`s in the crate.

[`find_codes`]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustdoc/html/markdown.rs.html#749-818
[`formats::renderer::run_format`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/formats/renderer/fn.run_format.html
[`html/format.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/format.rs
[`html/layout.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/layout.rs
[`html/markdown.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/markdown.rs
[`html/render/mod.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/render/mod.rs
[`html/render/print_item.rs`]: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/render/print_item.rs
[`librustdoc/formats`]: https://github.com/rust-lang/rust/tree/master/src/librustdoc/formats
[`librustdoc/html`]: https://github.com/rust-lang/rust/tree/master/src/librustdoc/html
[`print_item`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/html/render/print_item/fn.print_item.html
[Askama]: https://docs.rs/askama/latest/askama/

### From Soup to Nuts (or: ["An Unbroken Thread Stretches From Those First `Cell`s To Us"][video])

[video]: https://www.youtube.com/watch?v=hOLAGYmUQV0

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 <!-- date-check --> March 2023) JSON format.

This change has allowed other changes to remove data from the "clean" [`AST`][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`][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 `impl`s for auto traits and blanket `impl`s 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.

[`Context`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/html/render/context/struct.Context.html
[didn't used to be the case]: https://github.com/rust-lang/rust/pull/80090
[`SharedContext`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/html/render/context/struct.SharedContext.html

## 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 `doctest`s on Rust code or
standalone Markdown files. For the former, it shortcuts straight to
`html/markdown.rs`, 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 `test.rs`, 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/markdown.rs`, it builds up a collection of
tests to run before handing them off to the test runner. One notable location
in `test.rs` is the function `make_test`, which is where hand-written
`doctest`s get transformed into something that can be executed.

Some extra reading about `make_test` can be found
[here](https://quietmisdreavus.net/code/2018/02/23/how-the-doctests-get-made/).

## 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:

```bash
$ ./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

- The [`rustdoc` api docs]
- [An overview of `rustdoc`](./rustdoc.md)
- [The rustdoc user guide]

[`rustdoc` api docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/
[The rustdoc user guide]: https://doc.rust-lang.org/nightly/rustdoc/
