Improve the tuple and unit trait docs

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -401,6 +401,11 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                     let msg = "`#[doc(keyword)]` is meant for internal use only";
                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
                 }
+
+                if nested_meta.has_name(sym::tuple_varadic) {
+                    let msg = "`#[doc(tuple_varadic)]` is meant for internal use only";
+                    gate_feature_post!(self, rustdoc_internals, attr.span, msg);
+                }
             }
         }
 

compiler/rustc_passes/src/check_attr.rs

@@ -804,6 +804,37 @@ impl CheckAttrVisitor<'_> {
         true
     }
 
+    fn check_doc_tuple_varadic(&self, meta: &NestedMetaItem, hir_id: HirId) -> bool {
+        match self.tcx.hir().find(hir_id).and_then(|node| match node {
+            hir::Node::Item(item) => Some(&item.kind),
+            _ => None,
+        }) {
+            Some(ItemKind::Impl(ref i)) => {
+                if !matches!(&i.self_ty.kind, hir::TyKind::Tup([_])) {
+                    self.tcx
+                        .sess
+                        .struct_span_err(
+                            meta.span(),
+                            "`#[doc(tuple_varadic)]` can only be used on unary tuples",
+                        )
+                        .emit();
+                    return false;
+                }
+            }
+            _ => {
+                self.tcx
+                    .sess
+                    .struct_span_err(
+                        meta.span(),
+                        "`#[doc(keyword = \"...\")]` can only be used on impl blocks",
+                    )
+                    .emit();
+                return false;
+            }
+        }
+        true
+    }
+
     /// Checks `#[doc(inline)]`/`#[doc(no_inline)]` attributes. Returns `true` if valid.
     ///
     /// A doc inlining attribute is invalid if it is applied to a non-`use` item, or
@@ -1064,6 +1095,13 @@ impl CheckAttrVisitor<'_> {
                             is_valid = false
                         }
 
+                        sym::tuple_varadic
+                            if !self.check_attr_not_crate_level(meta, hir_id, "tuple_varadic")
+                                || !self.check_doc_tuple_varadic(meta, hir_id) =>
+                        {
+                            is_valid = false
+                        }
+
                         sym::html_favicon_url
                         | sym::html_logo_url
                         | sym::html_playground_url
@@ -1117,7 +1155,8 @@ impl CheckAttrVisitor<'_> {
                         | sym::no_inline
                         | sym::notable_trait
                         | sym::passes
-                        | sym::plugins => {}
+                        | sym::plugins
+                        | sym::tuple_varadic => {}
 
                         sym::test => {
                             if !self.check_test_attr(meta, hir_id) {

compiler/rustc_span/src/symbol.rs

@@ -1439,6 +1439,7 @@ symbols! {
         tuple,
         tuple_from_req,
         tuple_indexing,
+        tuple_varadic,
         two_phase,
         ty,
         type_alias_enum_variants,

library/core/src/clone.rs

@@ -95,7 +95,6 @@ use crate::marker::Destruct;
 ///
 /// * Function item types (i.e., the distinct types defined for each function)
 /// * Function pointer types (e.g., `fn() -> i32`)
-/// * Tuple types, if each component also implements `Clone` (e.g., `()`, `(i32, bool)`)
 /// * Closure types, if they capture no value from the environment
 ///   or if all such captured values implement `Clone` themselves.
 ///   Note that variables captured by shared reference always implement `Clone`

library/core/src/fmt/mod.rs

@@ -2313,23 +2313,40 @@ macro_rules! peel {
 macro_rules! tuple {
     () => ();
     ( $($name:ident,)+ ) => (
-        #[stable(feature = "rust1", since = "1.0.0")]
-        impl<$($name:Debug),+> Debug for ($($name,)+) where last_type!($($name,)+): ?Sized {
-            #[allow(non_snake_case, unused_assignments)]
-            fn fmt(&self, f: &mut Formatter<'_>) -> Result {
-                let mut builder = f.debug_tuple("");
-                let ($(ref $name,)+) = *self;
-                $(
-                    builder.field(&$name);
-                )+
-
-                builder.finish()
+        maybe_tuple_doc! {
+            $($name)+ @
+            #[stable(feature = "rust1", since = "1.0.0")]
+            impl<$($name:Debug),+> Debug for ($($name,)+) where last_type!($($name,)+): ?Sized {
+                #[allow(non_snake_case, unused_assignments)]
+                fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+                    let mut builder = f.debug_tuple("");
+                    let ($(ref $name,)+) = *self;
+                    $(
+                        builder.field(&$name);
+                    )+
+
+                    builder.finish()
+                }
             }
         }
         peel! { $($name,)+ }
     )
 }
 
+macro_rules! maybe_tuple_doc {
+    ($a:ident @ #[$meta:meta] $item:item) => {
+        #[cfg_attr(not(bootstrap), doc(tuple_varadic))]
+        #[doc = "This trait is implemented for tuples up to twelve items long."]
+        #[$meta]
+        $item
+    };
+    ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
+        #[doc(hidden)]
+        #[$meta]
+        $item
+    };
+}
+
 macro_rules! last_type {
     ($a:ident,) => { $a };
     ($a:ident, $($rest_a:ident,)+) => { last_type!($($rest_a,)+) };

library/core/src/hash/mod.rs

@@ -883,18 +883,35 @@ mod impls {
         );
 
         ( $($name:ident)+) => (
-            #[stable(feature = "rust1", since = "1.0.0")]
-            impl<$($name: Hash),+> Hash for ($($name,)+) where last_type!($($name,)+): ?Sized {
-                #[allow(non_snake_case)]
-                #[inline]
-                fn hash<S: Hasher>(&self, state: &mut S) {
-                    let ($(ref $name,)+) = *self;
-                    $($name.hash(state);)+
+            maybe_tuple_doc! {
+                $($name)+ @
+                #[stable(feature = "rust1", since = "1.0.0")]
+                impl<$($name: Hash),+> Hash for ($($name,)+) where last_type!($($name,)+): ?Sized {
+                    #[allow(non_snake_case)]
+                    #[inline]
+                    fn hash<S: Hasher>(&self, state: &mut S) {
+                        let ($(ref $name,)+) = *self;
+                        $($name.hash(state);)+
+                    }
                 }
             }
         );
     }
 
+    macro_rules! maybe_tuple_doc {
+        ($a:ident @ #[$meta:meta] $item:item) => {
+            #[cfg_attr(not(bootstrap), doc(tuple_varadic))]
+            #[doc = "This trait is implemented for tuples up to twelve items long."]
+            #[$meta]
+            $item
+        };
+        ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
+            #[doc(hidden)]
+            #[$meta]
+            $item
+        };
+    }
+
     macro_rules! last_type {
         ($a:ident,) => { $a };
         ($a:ident, $($rest_a:ident,)+) => { last_type!($($rest_a,)+) };

library/core/src/marker.rs

@@ -359,7 +359,6 @@ pub trait StructuralEq {
 ///
 /// * Function item types (i.e., the distinct types defined for each function)
 /// * Function pointer types (e.g., `fn() -> i32`)
-/// * Tuple types, if each component also implements `Copy` (e.g., `()`, `(i32, bool)`)
 /// * Closure types, if they capture no value from the environment
 ///   or if all such captured values implement `Copy` themselves.
 ///   Note that variables captured by shared reference always implement `Copy`

library/core/src/primitive_docs.rs

@@ -439,6 +439,27 @@ mod prim_char {}
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_unit {}
 
+// Required to make auto trait impls render.
+// See /src/librustdoc/passes/collect_trait_impls.rs:collect_trait_impls
+#[doc(hidden)]
+impl () {}
+
+// Fake impl that's only really used for docs.
+#[cfg(doc)]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Clone for () {
+    fn clone(&self) -> Self {
+        loop {}
+    }
+}
+
+// Fake impl that's only really used for docs.
+#[cfg(doc)]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Copy for () {
+    // empty
+}
+
 #[doc(primitive = "pointer")]
 #[doc(alias = "ptr")]
 #[doc(alias = "*")]
@@ -895,24 +916,11 @@ mod prim_str {}
 ///
 /// # Trait implementations
 ///
-/// If every type inside a tuple implements one of the following traits, then a
-/// tuple itself also implements it.
-///
-/// * [`Clone`]
-/// * [`Copy`]
-/// * [`PartialEq`]
-/// * [`Eq`]
-/// * [`PartialOrd`]
-/// * [`Ord`]
-/// * [`Debug`]
-/// * [`Default`]
-/// * [`Hash`]
-///
-/// [`Debug`]: fmt::Debug
-/// [`Hash`]: hash::Hash
-///
-/// Due to a temporary restriction in Rust's type system, these traits are only
-/// implemented on tuples of arity 12 or less. In the future, this may change.
+/// In this documentation the shorthand `(T, ...)` is used to represent all
+/// tuples up to length twelve. When that is used, any trait bounds expressed
+/// on `T` applies to each field of the tuple independently. Note that this is
+/// a convenience notation to avoid repetitive documentation, not valid
+/// Rust syntax.
 ///
 /// # Examples
 ///
@@ -949,6 +957,31 @@ mod prim_str {}
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_tuple {}
 
+// Required to make auto trait impls render.
+// See /src/librustdoc/passes/collect_trait_impls.rs:collect_trait_impls
+#[doc(hidden)]
+impl<T> (T,) {}
+
+// Fake impl that's only really used for docs.
+#[cfg(doc)]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), doc(tuple_varadic))]
+/// This trait is implemented on arbitrary-length tuples.
+impl<T: Clone> Clone for (T,) {
+    fn clone(&self) -> Self {
+        loop {}
+    }
+}
+
+// Fake impl that's only really used for docs.
+#[cfg(doc)]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), doc(tuple_varadic))]
+/// This trait is implemented on arbitrary-length tuples.
+impl<T: Copy> Copy for (T,) {
+    // empty
+}
+
 #[doc(primitive = "f32")]
 /// A 32-bit floating point type (specifically, the "binary32" type defined in IEEE 754-2008).
 ///