Rollup merge of #89596 - GuillaumeGomez:implicit-doc-cfg, r=jyn514

Make cfg imply doc(cfg)

This is a reopening of #79341, rebased and modified a bit (we made a lot of refactoring in rustdoc's types so they needed to be reflected in this PR as well):

 * `hidden_cfg` is now in the `Cache` instead of `DocContext` because `cfg` information isn't stored anymore on `clean::Attributes` type but instead computed on-demand, so we need this information in later parts of rustdoc.
 * I removed the `bool_to_options` feature (which makes the code a bit simpler to read for `SingleExt` trait implementation.
 * I updated the version for the feature.

There is only one thing I couldn't figure out: [this comment](https://github.com/rust-lang/rust/pull/79341#discussion_r561855624)

> I think I'll likely scrap the whole `SingleExt` extension trait as the diagnostics for 0 and >1 items should be different.

How/why should they differ?

EDIT: this part has been solved, the current code was fine, just needed a little simplification.

cc `@Nemo157`
r? `@jyn514`

Original PR description:

This is only active when the `doc_cfg` feature is active.

The implicit cfg can be overridden via `#[doc(cfg(...))]`, so e.g. to hide a `#[cfg]` you can use something like:

```rust
#[cfg(unix)]
#[doc(cfg(all()))]
pub struct Unix;
```

By adding `#![doc(cfg_hide(foobar))]` to the crate attributes the cfg `#[cfg(foobar)]` (and _only_ that _exact_ cfg) will not be implicitly treated as a `doc(cfg)` to render a message in the documentation.

3 files changed, 70 insertions, 0 deletions

diff --git a/src/test/rustdoc/doc-cfg-hide.rs b/src/test/rustdoc/doc-cfg-hide.rs
new file mode 100644
index 00000000000..b9d0d323137
--- /dev/null
+++ b/src/test/rustdoc/doc-cfg-hide.rs
@@ -0,0 +1,32 @@
+#![crate_name = "oud"]
+#![feature(doc_cfg, doc_cfg_hide)]
+
+#![doc(cfg_hide(feature = "solecism"))]
+
+// @has 'oud/struct.Solecism.html'
+// @count   - '//*[@class="stab portability"]' 0
+// compile-flags:--cfg feature="solecism"
+#[cfg(feature = "solecism")]
+pub struct Solecism;
+
+// @has 'oud/struct.Scribacious.html'
+// @count   - '//*[@class="stab portability"]' 1
+// @matches - '//*[@class="stab portability"]' 'crate feature solecism'
+#[cfg(feature = "solecism")]
+#[doc(cfg(feature = "solecism"))]
+pub struct Scribacious;
+
+// @has 'oud/struct.Hyperdulia.html'
+// @count   - '//*[@class="stab portability"]' 1
+// @matches - '//*[@class="stab portability"]' 'crate feature hyperdulia'
+// compile-flags:--cfg feature="hyperdulia"
+#[cfg(feature = "solecism")]
+#[cfg(feature = "hyperdulia")]
+pub struct Hyperdulia;
+
+// @has 'oud/struct.Oystercatcher.html'
+// @count   - '//*[@class="stab portability"]' 1
+// @matches - '//*[@class="stab portability"]' 'crate features solecism and oystercatcher'
+// compile-flags:--cfg feature="oystercatcher"
+#[cfg(all(feature = "solecism", feature = "oystercatcher"))]
+pub struct Oystercatcher;
diff --git a/src/test/rustdoc/doc-cfg-implicit-gate.rs b/src/test/rustdoc/doc-cfg-implicit-gate.rs
new file mode 100644
index 00000000000..92804d3729b
--- /dev/null
+++ b/src/test/rustdoc/doc-cfg-implicit-gate.rs
@@ -0,0 +1,7 @@
+// compile-flags:--cfg feature="worricow"
+#![crate_name = "xenogenous"]
+
+// @has 'xenogenous/struct.Worricow.html'
+// @count   - '//*[@class="stab portability"]' 0
+#[cfg(feature = "worricow")]
+pub struct Worricow;
diff --git a/src/test/rustdoc/doc-cfg-implicit.rs b/src/test/rustdoc/doc-cfg-implicit.rs
new file mode 100644
index 00000000000..36c2025785d
--- /dev/null
+++ b/src/test/rustdoc/doc-cfg-implicit.rs
@@ -0,0 +1,31 @@
+#![crate_name = "funambulism"]
+#![feature(doc_cfg)]
+
+// @has 'funambulism/struct.Disorbed.html'
+// @count   - '//*[@class="stab portability"]' 1
+// @matches - '//*[@class="stab portability"]' 'crate feature disorbed'
+// compile-flags:--cfg feature="disorbed"
+#[cfg(feature = "disorbed")]
+pub struct Disorbed;
+
+// @has 'funambulism/struct.Aesthesia.html'
+// @count   - '//*[@class="stab portability"]' 1
+// @matches - '//*[@class="stab portability"]' 'crate feature aesthesia'
+// compile-flags:--cfg feature="aesthesia"
+#[doc(cfg(feature = "aesthesia"))]
+pub struct Aesthesia;
+
+// @has 'funambulism/struct.Pliothermic.html'
+// @count   - '//*[@class="stab portability"]' 1
+// @matches - '//*[@class="stab portability"]' 'crate feature pliothermic'
+// compile-flags:--cfg feature="epopoeist"
+#[cfg(feature = "epopoeist")]
+#[doc(cfg(feature = "pliothermic"))]
+pub struct Pliothermic;
+
+// @has 'funambulism/struct.Simillimum.html'
+// @count   - '//*[@class="stab portability"]' 0
+// compile-flags:--cfg feature="simillimum"
+#[cfg(feature = "simillimum")]
+#[doc(cfg(all()))]
+pub struct Simillimum;