Rollup merge of #48283 - QuietMisdreavus:rustdoc-readme, r=@GuillaumeGomez

add readme for librustdoc

In the same vein as the other compiler-library readmes, here's one for rustdoc! It's mainly a "how does rustdoc even" blog-post-style writeup, but i wanted to have something in-repo so people could get a sense of what bits did what.

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+# The walking tour of rustdoc
+
+Rustdoc is implemented entirely within the crate `librustdoc`. After partially compiling a crate to
+get its AST (technically the HIR map) from rustc, librustdoc performs two major steps past that to
+render a set of documentation:
+
+* "Clean" the AST into a form that's more suited to creating documentation (and slightly more
+  resistant to churn in the compiler).
+* Use this cleaned AST to render a crate's documentation, one page at a time.
+
+Naturally, there's more than just this, and those descriptions simplify out lots of details, but
+that's the high-level overview.
+
+(Side note: this is a library crate! The `rustdoc` binary is crated using the project in
+`src/tools/rustdoc`. Note that literally all that does is call the `main()` that's in this crate's
+`lib.rs`, though.)
+
+## Cheat sheet
+
+* Use `x.py build --stage 1 src/libstd src/tools/rustdoc` to make a useable rustdoc you can run on
+  other projects.
+  * Add `src/libtest` to be able to use `rustdoc --test`.
+  * If you've used `rustup toolchain link local /path/to/build/$TARGET/stage1` previously, then
+    after the previous build command, `cargo +local doc` will Just Work.
+* Use `x.py doc --stage 1 src/libstd` to use this rustdoc to generate the standard library docs.
+  * The completed docs will be available in `build/$TARGET/doc/std`, though the bundle is meant to
+    be used as though you would copy out the `doc` folder to a web server, since that's where the
+    CSS/JS and landing page are.
+* Most of the HTML printing code is in `html/format.rs` and `html/render.rs`. It's in a bunch of
+  `fmt::Display` implementations and supplementary functions.
+* The types that got `Display` impls above are defined in `clean/mod.rs`, right next to the custom
+  `Clean` trait used to process them out of the rustc HIR.
+* The bits specific to using rustdoc as a test harness are in `test.rs`.
+* The Markdown renderer is loaded up in `html/markdown.rs`, including functions for extracting
+  doctests from a given block of Markdown.
+* The tests on rustdoc *output* are located in `src/test/rustdoc`, where they're handled by the test
+  runner of rustbuild and the supplementary script `src/etc/htmldocck.py`.
+* Tests on search index generation are located in `src/test/rustdoc-js`, as a series of JavaScript
+  files that encode queries on the standard library search index and expected results.
+
+## 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
+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 `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.
+
+(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
+
+## 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.
+
+## 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`.
+
+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.