Start documenting tests/rustdoc-json

src/compiler-src.md

@@ -153,7 +153,8 @@ The bulk of [`rustdoc`] is in [`librustdoc`]. However, the [`rustdoc`] binary
 itself is [`src/tools/rustdoc`], which does nothing except call [`rustdoc::main`].
 
 There is also `JavaScript` and `CSS` for the docs in [`src/tools/rustdoc-js`]
-and [`src/tools/rustdoc-themes`].
+and [`src/tools/rustdoc-themes`]. The type definitions for `--output-format=json`
+are in a separate crate in [`src/rustdoc-json-types`].
 
 You can read more about [`rustdoc`] in [this chapter][rustdoc-chapter].
 
@@ -162,6 +163,7 @@ You can read more about [`rustdoc`] in [this chapter][rustdoc-chapter].
 [`src/tools/rustdoc-js`]: https://github.com/rust-lang/rust/tree/master/src/tools/rustdoc-js
 [`src/tools/rustdoc-themes`]: https://github.com/rust-lang/rust/tree/master/src/tools/rustdoc-themes
 [`src/tools/rustdoc`]:  https://github.com/rust-lang/rust/tree/master/src/tools/rustdoc
+[`src/rustdoc-json-types`]: https://github.com/rust-lang/rust/tree/master/src/rustdoc-json-types
 [rustdoc-chapter]: ./rustdoc.md
 
 ## Tests

src/rustdoc-internals/rustdoc-json-test-suite.md

@@ -1,3 +1,83 @@
 # The `rustdoc-json` test suite
 
-> **FIXME**: This section is a stub. It will be populated by [PR #2422](https://github.com/rust-lang/rustc-dev-guide/pull/2422/).
+This page is specifically about the test suite named `rustdoc-json`, which tests rustdoc's [json output].
+For other test suites used for testing rustdoc, see [§Rustdoc test suites](../tests/compiletest.md#rustdoc-test-suites).
+
+Tests are run with compiletest, and have access to the usual set of [directives](../tests/directives.md).
+Frequenly used directives here are:
+
+- [`//@ aux-build`][aux-build] to have dependencies.
+- `//@ edition: 2021` (or some other edition).
+- `//@ compile-flags: --document-hidden-items` to enable [document private items].
+
+Each crate's json output is checked by 2 programs: [jsondoclint](#jsondocck) and [jsondocck](#jsondocck).
+
+## jsondoclint
+
+[jsondoclint] checks that all [`Id`]s exist in the `index` (or `paths`).
+This makes sure there are no dangling [`Id`]s.
+
+<!-- TODO: It does some more things too?
+Also, talk about how it works
+ -->
+
+## jsondocck
+
+[jsondocck] processes direcives given in comments, to assert that the values in the output are expected.
+It's alot like [htmldocck](./rustdoc-test-suite.md) in that way.
+
+It uses [JSONPath] as a query language, which takes a path, and returns a *list* of values that that path is said to match to.
+
+### Directives
+
+- `//@ has <path>`: Checks `<path>` exists, i.e. matches at least 1 value.
+- `//@ !has <path>`: Checks `<path>` doesn't exist, i.e. matches 0 values.
+- `//@ has <path> <value>`: Check `<path>` exists, and at least 1 of the matches is equal to the given `<value>` 
+- `//@ !has <path> <value>`: Checks `<path>` exists, but none of the matches equal the given `<value>`.
+- `//@ is <path> <value>`: Check `<path>` matches exactly one value, and it's equal to the given `<value>`.
+- `//@ is <path> <value> <value>...`: Check that `<path>` matches to exactly every given `<value>`. 
+   Ordering doesn't matter here.
+- `//@ !is <path> <value>`: Check `<path>` matches exactly one value, and that value is not equal to the given `<value>`.
+- `//@ count <path> <number>`: Check that `<path>` matches to `<number>` of values.
+- `//@ set <name> = <path>`: Check that `<path>` matches exactly one value, and store that value to the variable called `<name>`.
+
+These are defined in [`directive.rs`].
+
+### Values
+
+Values can be either JSON values, or variables.
+
+- JSON values are JSON literals, e.g. `true`, `"string"`, `{"key": "value"}`. 
+  These often need to be quoted using `'`, to be processed as 1 value. See [§Argument spliting](#argument-spliting)
+- Variables can be used to store the value in one path, and use it in later queries.
+  They are set with the `//@ set <name> = <path>` directive, and accessed with `$<name>`
+
+  ```rust
+  //@ set foo = $some.path
+  //@ is $.some.other.path $foo
+  ```
+
+### Argument spliting
+
+Arguments to directives are split using the [shlex] crate, which implements POSIX shell escaping.
+This is because both `<path>` and `<value>` arguments to [directives](#directives) frequently have both
+whitespace and quote marks.
+
+To use the `@ is` with a `<path>` of `$.index[?(@.docs == "foo")].some.field` and a value of `"bar"` [^why_quote], you'd write:
+
+```rust
+//@ is '$.is[?(@.docs == "foo")].some.field' '"bar"'
+```
+
+[^why_quote]: The value needs to be `"bar"` *after* shlex splitting, because we
+    it needs to be a JSON string value.
+
+[json output]: https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#json-output
+[jsondocck]: https://github.com/rust-lang/rust/tree/master/src/tools/jsondocck
+[jsondoclint]: https://github.com/rust-lang/rust/tree/master/src/tools/jsondoclint
+[aux-build]: ../tests/compiletest.md#building-auxiliary-crates
+[`Id`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc_json_types/struct.Id.html
+[document private items]: https://doc.rust-lang.org/nightly/rustdoc/command-line-arguments.html#--document-private-items-show-items-that-are-not-public
+[`directive.rs`]: https://github.com/rust-lang/rust/blob/master/src/tools/jsondocck/src/directive.rs
+[shlex]: https://docs.rs/shlex/1.3.0/shlex/index.html
+[JSONPath]: https://www.rfc-editor.org/rfc/rfc9535.html