Rollup merge of #99235 - WaffleLapkin:rustdoc_implement_support_for_must_implement, r=GuillaumeGomez

rustdoc: Add support for `#[rustc_must_implement_one_of]`

This PR adds support for `#[rustc_must_implement_one_of]` attribute added in #92164. There is a desire to eventually use this attribute of `Read`, so making it show up in docs is a good thing.

I "stole" the styling from cfg notes, not sure what would be a proper styling. Currently it looks like this:
![2022-07-14_15-00](https://user-images.githubusercontent.com/38225716/178968170-913c1dd5-8875-4a95-9848-b075a0bb8998.png)

<details><summary>Code to reproduce</summary>
<p>

```rust
#![feature(rustc_attrs)]

#[rustc_must_implement_one_of(a, b)]
pub trait Trait {
    fn req();
    fn a(){ Self::b() }
    fn b(){ Self::a() }
}
```

</p>
</details>

6 files changed, 32 insertions, 6 deletions

diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index c9ef4748a48..07237438a0d 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -1993,6 +1993,7 @@ fn clean_maybe_renamed_item<'tcx>(
             ItemKind::Trait(_, _, generics, bounds, item_ids) => {
                 let items =
                     item_ids.iter().map(|ti| cx.tcx.hir().trait_item(ti.id).clean(cx)).collect();
+
                 TraitItem(Trait {
                     def_id,
                     items,
diff --git a/src/librustdoc/html/render/print_item.rs b/src/librustdoc/html/render/print_item.rs
index 81cc12c9d55..dcd2eaac7ea 100644
--- a/src/librustdoc/html/render/print_item.rs
+++ b/src/librustdoc/html/render/print_item.rs
@@ -1,9 +1,5 @@
 use clean::AttributesExt;
 
-use std::cmp::Ordering;
-use std::fmt;
-use std::rc::Rc;
-
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir as hir;
 use rustc_hir::def::CtorKind;
@@ -15,6 +11,9 @@ use rustc_middle::ty::{Adt, TyCtxt};
 use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_target::abi::{Layout, Primitive, TagEncoding, Variants};
+use std::cmp::Ordering;
+use std::fmt;
+use std::rc::Rc;
 
 use super::{
     collect_paths_for_type, document, ensure_trailing_slash, item_ty_to_section,
@@ -37,6 +36,7 @@ use crate::html::markdown::{HeadingOffset, MarkdownSummaryLine};
 use crate::html::url_parts_builder::UrlPartsBuilder;
 
 use askama::Template;
+use itertools::Itertools;
 
 const ITEM_TABLE_OPEN: &str = "<div class=\"item-table\">";
 const ITEM_TABLE_CLOSE: &str = "</div>";
@@ -539,6 +539,8 @@ fn item_trait(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean:
     let count_types = required_types.len() + provided_types.len();
     let count_consts = required_consts.len() + provided_consts.len();
     let count_methods = required_methods.len() + provided_methods.len();
+    let must_implement_one_of_functions =
+        cx.tcx().trait_def(t.def_id).must_implement_one_of.clone();
 
     // Output the trait definition
     wrap_into_docblock(w, |w| {
@@ -784,13 +786,22 @@ fn item_trait(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean:
     }
 
     // Output the documentation for each function individually
-    if !required_methods.is_empty() {
+    if !required_methods.is_empty() || must_implement_one_of_functions.is_some() {
         write_small_section_header(
             w,
             "required-methods",
             "Required Methods",
             "<div class=\"methods\">",
         );
+
+        if let Some(list) = must_implement_one_of_functions.as_deref() {
+            write!(
+                w,
+                "<div class=\"stab must_implement\">At least one of the `{}` methods is required.</div>",
+                list.iter().join("`, `")
+            );
+        }
+
         for m in required_methods {
             trait_item(w, cx, m, it);
         }
diff --git a/src/librustdoc/html/static/css/themes/ayu.css b/src/librustdoc/html/static/css/themes/ayu.css
index 7ff8063904a..c42cac59bd6 100644
--- a/src/librustdoc/html/static/css/themes/ayu.css
+++ b/src/librustdoc/html/static/css/themes/ayu.css
@@ -205,11 +205,13 @@ details.rustdoc-toggle > summary::before {
 
 /* Created this empty rule to satisfy the theme checks. */
 .stab.empty-impl {}
+.stab.must_implement {}
 
 .stab.unstable,
 .stab.deprecated,
 .stab.portability,
-.stab.empty-impl {
+.stab.empty-impl,
+.stab.must_implement {
 	color: #c5c5c5;
 	background: #314559 !important;
 	border-style: none !important;
diff --git a/src/librustdoc/html/static/css/themes/dark.css b/src/librustdoc/html/static/css/themes/dark.css
index 8e753f57682..62d9eaa02e6 100644
--- a/src/librustdoc/html/static/css/themes/dark.css
+++ b/src/librustdoc/html/static/css/themes/dark.css
@@ -180,6 +180,7 @@ details.rustdoc-toggle > summary::before {
 .stab.empty-impl { background: #FFF5D6; border-color: #FFC600; color: #2f2f2f; }
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; color: #2f2f2f; }
 .stab.deprecated { background: #ffc4c4; border-color: #db7b7b; color: #2f2f2f; }
+.stab.must_implement { background: #F3DFFF; border-color: #b07bdb; color: #2f2f2f; }
 .stab.portability { background: #F3DFFF; border-color: #b07bdb; color: #2f2f2f; }
 .stab.portability > code { background: none; }
 
diff --git a/src/librustdoc/html/static/css/themes/light.css b/src/librustdoc/html/static/css/themes/light.css
index 40d965c39c3..b751acff152 100644
--- a/src/librustdoc/html/static/css/themes/light.css
+++ b/src/librustdoc/html/static/css/themes/light.css
@@ -163,6 +163,7 @@ details.rustdoc-toggle > summary::before {
 .stab.empty-impl { background: #FFF5D6; border-color: #FFC600; }
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; }
 .stab.deprecated { background: #ffc4c4; border-color: #db7b7b; }
+.stab.must_implement { background: #F3DFFF; border-color: #b07bdb; }
 .stab.portability { background: #F3DFFF; border-color: #b07bdb; }
 .stab.portability > code { background: none; }
 
diff --git a/src/test/rustdoc/must_implement_one_of.rs b/src/test/rustdoc/must_implement_one_of.rs
new file mode 100644
index 00000000000..1f1dd5d5796
--- /dev/null
+++ b/src/test/rustdoc/must_implement_one_of.rs
@@ -0,0 +1,10 @@
+#![crate_name = "c"]
+#![feature(rustc_attrs)]
+
+#[rustc_must_implement_one_of(a, b)]
+// @matches c/trait.Trait.html '//*[@class="stab must_implement"]' \
+//      'At least one of the `a`, `b` methods is required.$'
+pub trait Trait {
+    fn a() {}
+    fn b() {}
+}