Rollup merge of #75330 - Nemo157:improve-doc-cfg-features, r=GuillaumeGomez

Improve rendering of crate features via doc(cfg)

The current rendering of crate features with `doc(cfg(feature = ".."))` is verbose and unwieldy for users, `doc(cfg(target_feature = ".."))` is special-cased to make it render nicely, and a similar rendering can be applied to `doc(cfg(feature))` to make it easier for users to read.

I also added special casing of `all`/`any` cfgs consisting of just `feature`/`target-feature` to remove the repetitive "target/crate feature" prefix.

The downside of this current rendering is that there is no distinction between `feature` and `target_feature` in the shorthand display. IMO this is ok, or if anything `target_feature` should have a more verbose shorthand, because `doc(cfg(feature = ".."))` usage is going to vastly outstrip `doc(cfg(target_feature = ".."))` usage in non-stdlib crates when it eventually stabilizes (or even before that given the number of crates using `cfg_attr(docsrs)` like constructs).

## Previously

<img width="259" alt="Screenshot 2020-08-09 at 13 32 42" src="https://user-images.githubusercontent.com/81079/89731110-d090c000-da44-11ea-96fa-56adc6339123.png">
<img width="438" alt="image" src="https://user-images.githubusercontent.com/81079/89731116-d7b7ce00-da44-11ea-87c6-022d192d6eca.png">
<img width="765" alt="image" src="https://user-images.githubusercontent.com/81079/89731152-24030e00-da45-11ea-9552-1c270bff2729.png">
<img width="671" alt="image" src="https://user-images.githubusercontent.com/81079/89731158-28c7c200-da45-11ea-8acb-97d8a4ce00eb.png">

## Now

<img width="216" alt="image" src="https://user-images.githubusercontent.com/81079/89731123-e1d9cc80-da44-11ea-82a8-5900bd9448a5.png">
<img width="433" alt="image" src="https://user-images.githubusercontent.com/81079/89731127-e8684400-da44-11ea-9d18-572fd810f19f.png">
<img width="606" alt="image" src="https://user-images.githubusercontent.com/81079/89731162-2feed000-da45-11ea-98d2-8a88c364d903.png">
<img width="669" alt="image" src="https://user-images.githubusercontent.com/81079/89731991-ccb46c00-da4b-11ea-9416-cd20a3193826.png">

cc #43781

1 files changed, 21 insertions, 5 deletions

diff --git a/src/test/rustdoc/duplicate-cfg.rs b/src/test/rustdoc/duplicate-cfg.rs
index 9ccc5d7882e..47ba362c977 100644
--- a/src/test/rustdoc/duplicate-cfg.rs
+++ b/src/test/rustdoc/duplicate-cfg.rs
@@ -3,22 +3,34 @@
 #![crate_name = "foo"]
 #![feature(doc_cfg)]
 
+// @has 'foo/index.html'
+// @matches '-' '//*[@class="module-item"]//*[@class="stab portability"]' '^sync$'
+// @has '-' '//*[@class="module-item"]//*[@class="stab portability"]/@title' 'This is supported on crate feature `sync` only'
+
 // @has 'foo/struct.Foo.html'
-// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" only.'
+// @has '-' '//*[@class="stab portability"]' 'sync'
 #[doc(cfg(feature = "sync"))]
 #[doc(cfg(feature = "sync"))]
 pub struct Foo;
 
+// @has 'foo/bar/index.html'
+// @matches '-' '//*[@class="module-item"]//*[@class="stab portability"]' '^sync$'
+// @has '-' '//*[@class="module-item"]//*[@class="stab portability"]/@title' 'This is supported on crate feature `sync` only'
+
 // @has 'foo/bar/struct.Bar.html'
-// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" only.'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on crate feature sync only.'
 #[doc(cfg(feature = "sync"))]
 pub mod bar {
     #[doc(cfg(feature = "sync"))]
     pub struct Bar;
 }
 
+// @has 'foo/baz/index.html'
+// @matches '-' '//*[@class="module-item"]//*[@class="stab portability"]' '^sync and send$'
+// @has '-' '//*[@class="module-item"]//*[@class="stab portability"]/@title' 'This is supported on crate features `sync` and `send` only'
+
 // @has 'foo/baz/struct.Baz.html'
-// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" and feature="send" only.'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on crate features sync and send only.'
 #[doc(cfg(all(feature = "sync", feature = "send")))]
 pub mod baz {
     #[doc(cfg(feature = "sync"))]
@@ -26,15 +38,19 @@ pub mod baz {
 }
 
 // @has 'foo/qux/struct.Qux.html'
-// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" and feature="send" only.'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on crate features sync and send only.'
 #[doc(cfg(feature = "sync"))]
 pub mod qux {
     #[doc(cfg(all(feature = "sync", feature = "send")))]
     pub struct Qux;
 }
 
+// @has 'foo/quux/index.html'
+// @matches '-' '//*[@class="module-item"]//*[@class="stab portability"]' '^sync and send and foo and bar$'
+// @has '-' '//*[@class="module-item"]//*[@class="stab portability"]/@title' 'This is supported on crate feature `sync` and crate feature `send` and `foo` and `bar` only'
+
 // @has 'foo/quux/struct.Quux.html'
-// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" and feature="send" and foo and bar only.'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on crate feature sync and crate feature send and foo and bar only.'
 #[doc(cfg(all(feature = "sync", feature = "send", foo)))]
 pub mod quux {
     #[doc(cfg(all(feature = "send", feature = "sync", bar)))]