Split up rustdoc page

Author: jyn514

src/SUMMARY.md

@@ -51,6 +51,7 @@
         - [Salsa](./salsa.md)
     - [Memory Management in Rustc](./memory.md)
     - [Parallel Compilation](./parallel-rustc.md)
+    - [Rustdoc](./rustdoc-internals.md)
 
 - [Part 3: Source Code Representations](./part-3-intro.md)
     - [Command-line arguments](./cli.md)

src/rustdoc-internals.md

@@ -0,0 +1,197 @@
+# Rustdoc internals
+
+This page describes rustdoc's passes and modes. For an overview of rustdoc,
+see [`rustdoc`](./rustdoc.md).
+
+## From crate to clean
+
+In `core.rs` are two central items: the `DocContext` struct, and the `run_core`
+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
+implementations of the `Clean` trait defined within. This is a conversion
+trait, which defines one method:
+
+```rust,ignore
+pub trait Clean<T> {
+    fn clean(&self, cx: &DocContext) -> T;
+}
+```
+
+`clean/mod.rs` also defines the types for the "cleaned" AST used later on to
+render documentation pages. Each usually accompanies an implementation of
+`Clean` that takes some AST or 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` implementation, but for the most part
+these impls are straightforward conversions. The "entry point" to this module
+is the `impl Clean<Crate> for visit_ast::RustdocVisitor`, which is called by
+`run_core` above.
+
+You see, I actually lied a little earlier: There's another AST transformation
+that happens before the events in `clean/mod.rs`.  In `visit_ast.rs` is the
+type `RustdocVisitor`, which *actually* crawls a `rustc_hir::Crate` to get the first
+intermediate representation, defined in `doctree.rs`. This pass is mainly to
+get a few intermediate wrappers around the HIR types and to process visibility
+and inlining. This is where `#[doc(inline)]`, `#[doc(no_inline)]`, and
+`#[doc(hidden)]` are processed, as well as the logic for whether a `pub use`
+should get the full page or a "Reexport" line in the module page.
+
+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::Crate` with a tree of Items
+which describe the publicly-documentable items in the target crate.
+
+### Hot potato
+
+Before moving on to the next major step, a few important "passes" occur over
+the documentation.  These do things like combine the separate "attributes" into
+a single string and strip leading whitespace to make the document easier on the
+markdown parser, or drop items that are not public or deliberately hidden with
+`#[doc(hidden)]`. These are all implemented in the `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 happen on the _cleaned_ crate.
+
+(Strictly speaking, you can fine-tune the passes run and even add your own, but
+[we're trying to deprecate that][44136]. If you need finer-grain control over
+these passes, please let us know!)
+
+[44136]: https://github.com/rust-lang/rust/issues/44136
+
+Here is current (as of this writing) list of passes:
+
+- `propagate-doc-cfg` - propagates `#[doc(cfg(...))]` to child items.
+- `collapse-docs` concatenates all document attributes into one document
+  attribute. This is necessary because each line of a doc comment is given as a
+  separate doc attribute, and this will combine them into a single string with
+  line breaks between each attribute.
+- `unindent-comments` removes excess indentation on comments in order for
+  markdown to like it. This is necessary because the convention for writing
+  documentation is to provide a space between the `///` or `//!` marker and the
+  text, and stripping that leading space will make the text easier to parse by
+  the Markdown parser. (In the past, the markdown parser used was not
+  Commonmark- compliant, which caused annoyances with extra whitespace but this
+  seems to be less of an issue today.)
+- `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.
+- `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.
+
+## From clean to crate
+
+This is where the "second phase" in rustdoc begins. This phase primarily lives
+in the `html/` folder, and it all starts with `run()` in `html/render.rs`. This
+code is responsible for setting up the `Context`, `SharedContext`, and `Cache`
+which are used during rendering, copying out the static files which live in
+every rendered set of documentation (things like the fonts, CSS, and JavaScript
+that live in `html/static/`), creating the search index, and printing out the
+source code rendering, before beginning the process of rendering all the
+documentation for the crate.
+
+Several functions implemented directly on `Context` take the `clean::Crate` and
+set up some state between rendering items or recursing on a module's child
+items. From here the "page rendering" begins, via an enormous `write!()` call
+in `html/layout.rs`. The parts that actually generate HTML from the items and
+documentation occurs within a series of `std::fmt::Display` implementations and
+functions that pass around a `&mut std::fmt::Formatter`. The top-level
+implementation that writes out the page body is the `impl<'a> fmt::Display for
+Item<'a>` in `html/render.rs`, which switches out to one of several `item_*`
+functions based on the 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.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 in here
+(`find_testable_code`) that specifically scans for Rust code blocks so the
+test-runner code can find all the doctests in the crate.
+
+### From soup to nuts
+
+(alternate title: ["An unbroken thread that stretches from those first `Cell`s
+to us"][video])
+
+[video]: https://www.youtube.com/watch?v=hOLAGYmUQV0
+
+It's important to note that the AST cleaning can ask the compiler for
+information (crucially, `DocContext` contains a `TyCtxt`), but page rendering
+cannot. The `clean::Crate` created within `run_core` is passed outside the
+compiler context before being handed to `html::render::run`. This means that a
+lot of the "supplementary data" that isn't immediately available inside an
+item's definition, like which trait is the `Deref` trait used by the language,
+needs to be collected during cleaning, stored in the `DocContext`, and passed
+along to the `SharedContext` during HTML rendering.  This manifests as a bunch
+of shared state, context variables, and `RefCell`s.
+
+Also of note is that some items that come from "asking the compiler" don't go
+directly into the `DocContext` - for example, when loading items from a foreign
+crate, rustdoc will ask about trait implementations and generate new `Item`s
+for the impls based on that information. This goes directly into the returned
+`Crate` rather than roundabout through the `DocContext`. This way, these
+implementations can be collected alongside the others, right before rendering
+the 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 doctests 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 libtest test runner. One notable
+location in `test.rs` 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](https://quietmisdreavus.net/code/2018/02/23/how-the-doctests-get-made/).
+
+## Dotting i's and crossing t's
+
+So that's rustdoc's code in a nutshell, but there's more things in the repo
+that deal with it. Since we have the full `compiletest` suite at hand, there's
+a set of tests in `src/test/rustdoc` that make sure the final HTML is what we
+expect in various situations. These tests also use a supplementary script,
+`src/etc/htmldocck.py`, 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 is in `htmldocck.py`.
+
+To use multiple crates in a rustdoc test, add `// aux-build:filename.rs`
+to the top of the test file. `filename.rs` 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 `src/test/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.
+
+You can run tests using the name of the folder. For example,
+`x.py test --stage 1 src/test/rustdoc` will run the output tests using a stage1 rustdoc.