Rollup merge of rust-lang#84283 - jsha:de-emphasize-attributes, r=GuillaumeGomez

rustdoc: Reduce visual weight of attributes.

Followup from rust-lang#83337. As part of that PR, we stopped hiding attributes behind a toggle, because most things have just zero or one attributes. However, this made clear that the current rendering of attributes emphasizes them a lot, which distracts from function signatures. This PR changes their color of attributes to be the same as the toggles, and reduces their font weight.

This also removes `#[lang]` from the list of ALLOWED_ATTRIBUTES. This attribute is an implementation detail rather than part of the public-facing documentation.

![image](https://user-images.githubusercontent.com/220205/115131061-cc407d80-9fa9-11eb-9a77-ad3f3217f391.png)

Demo at https://hoffman-andrews.com/rust/de-emph-attr/std/string/struct.String.html#method.trim

Author: GuillaumeGomez

src/librustdoc/html/render/mod.rs

@@ -989,7 +989,6 @@ fn render_assoc_item(
 
 const ALLOWED_ATTRIBUTES: &[Symbol] = &[
     sym::export_name,
-    sym::lang,
     sym::link_section,
     sym::must_use,
     sym::no_mangle,

src/librustdoc/html/static/rustdoc.css

@@ -967,6 +967,10 @@ a.test-arrow:hover{
 	color: inherit;
 }
 
+.code-attribute {
+	font-weight: 300;
+}
+
 .collapse-toggle {
 	font-weight: 300;
 	position: absolute;

src/librustdoc/html/static/themes/ayu.css

@@ -329,7 +329,8 @@ a.test-arrow:hover {
 	color: #c5c5c5;
 }
 
-.toggle-label {
+.toggle-label,
+.code-attribute {
 	color: #999;
 }
 

src/librustdoc/html/static/themes/dark.css

@@ -274,7 +274,8 @@ a.test-arrow:hover{
 	background-color: #4e8bca;
 }
 
-.toggle-label {
+.toggle-label,
+.code-attribute {
 	color: #999;
 }
 

src/librustdoc/html/static/themes/light.css

@@ -267,7 +267,8 @@ a.test-arrow:hover{
 	background-color: #4e8bca;
 }
 
-.toggle-label {
+.toggle-label,
+.code-attribute {
 	color: #999;
 }