rustdoc: show tuple impls as `impl Trait for (T, ...)`

This commit adds a new unstable attribute, `#[doc(tuple_varadic)]`, that
shows a 1-tuple as `(T, ...)` instead of just `(T,)`, and links to a section
in the tuple primitive docs that talks about these.

3 files changed, 15 insertions, 2 deletions

diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index a82abe66926..d8f6a9c3ff0 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -500,7 +500,11 @@ pub(crate) fn build_impl(
             for_,
             items: trait_items,
             polarity,
-            kind: ImplKind::Normal,
+            kind: if utils::has_doc_flag(tcx, did, sym::tuple_varadic) {
+                ImplKind::TupleVaradic
+            } else {
+                ImplKind::Normal
+            },
         }),
         box merged_attrs,
         cx,
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index f3070fb35f1..b15ef424cb6 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -1999,7 +1999,11 @@ fn clean_impl<'tcx>(
             for_,
             items,
             polarity: tcx.impl_polarity(def_id),
-            kind: ImplKind::Normal,
+            kind: if utils::has_doc_flag(tcx, def_id.to_def_id(), sym::tuple_varadic) {
+                ImplKind::TupleVaradic
+            } else {
+                ImplKind::Normal
+            },
         });
         Item::from_hir_id_and_parts(hir_id, None, kind, cx)
     };
diff --git a/src/librustdoc/clean/types.rs b/src/librustdoc/clean/types.rs
index 4605793d0df..95c84e68b83 100644
--- a/src/librustdoc/clean/types.rs
+++ b/src/librustdoc/clean/types.rs
@@ -2394,6 +2394,7 @@ impl Impl {
 pub(crate) enum ImplKind {
     Normal,
     Auto,
+    TupleVaradic,
     Blanket(Box<Type>),
 }
 
@@ -2406,6 +2407,10 @@ impl ImplKind {
         matches!(self, ImplKind::Blanket(_))
     }
 
+    pub(crate) fn is_tuple_varadic(&self) -> bool {
+        matches!(self, ImplKind::TupleVaradic)
+    }
+
     pub(crate) fn as_blanket_ty(&self) -> Option<&Type> {
         match self {
             ImplKind::Blanket(ty) => Some(ty),