Improve the function pointer docs

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -402,8 +402,8 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
                 }
 
-                if nested_meta.has_name(sym::tuple_variadic) {
-                    let msg = "`#[doc(tuple_variadic)]` is meant for internal use only";
+                if nested_meta.has_name(sym::fake_variadic) {
+                    let msg = "`#[doc(fake_variadic)]` is meant for internal use only";
                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
                 }
             }

compiler/rustc_error_codes/src/error_codes/E0118.md

@@ -4,7 +4,7 @@ enum, union, or trait object.
 Erroneous code example:
 
 ```compile_fail,E0118
-impl fn(u8) { // error: no nominal type found for inherent implementation
+impl<T> T { // error: no nominal type found for inherent implementation
     fn get_state(&self) -> String {
         // ...
     }
@@ -20,8 +20,8 @@ trait LiveLongAndProsper {
     fn get_state(&self) -> String;
 }
 
-// and now you can implement it on fn(u8)
-impl LiveLongAndProsper for fn(u8) {
+// and now you can implement it on T
+impl<T> LiveLongAndProsper for T {
     fn get_state(&self) -> String {
         "He's dead, Jim!".to_owned()
     }
@@ -33,9 +33,9 @@ For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
 Example:
 
 ```
-struct TypeWrapper(fn(u8));
+struct TypeWrapper<T>(T);
 
-impl TypeWrapper {
+impl<T> TypeWrapper<T> {
     fn get_state(&self) -> String {
         "Fascinating!".to_owned()
     }

compiler/rustc_error_messages/locales/en-US/passes.ftl

@@ -81,8 +81,8 @@ passes-doc-keyword-not-mod = `#[doc(keyword = "...")]` should be used on modules
 
 passes-doc-keyword-invalid-ident = `{$doc_keyword}` is not a valid identifier
 
-passes-doc-tuple-variadic-not-first =
-    `#[doc(tuple_variadic)]` must be used on the first of a set of tuple trait impls with varying arity
+passes-doc-fake-variadic-not-valid =
+    `#[doc(fake_variadic)]` must be used on the first of a set of tuple or fn pointer trait impls with varying arity
 
 passes-doc-keyword-only-impl = `#[doc(keyword = "...")]` should be used on impl blocks
 

compiler/rustc_passes/src/check_attr.rs

@@ -706,14 +706,20 @@ impl CheckAttrVisitor<'_> {
         true
     }
 
-    fn check_doc_tuple_variadic(&self, meta: &NestedMetaItem, hir_id: HirId) -> bool {
+    fn check_doc_fake_variadic(&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.emit_err(errors::DocTupleVariadicNotFirst { span: meta.span() });
+                let is_valid = matches!(&i.self_ty.kind, hir::TyKind::Tup([_]))
+                    || if let hir::TyKind::BareFn(bare_fn_ty) = &i.self_ty.kind {
+                        bare_fn_ty.decl.inputs.len() == 1
+                    } else {
+                        false
+                    };
+                if !is_valid {
+                    self.tcx.sess.emit_err(errors::DocFakeVariadicNotValid { span: meta.span() });
                     return false;
                 }
             }
@@ -887,9 +893,9 @@ impl CheckAttrVisitor<'_> {
                             is_valid = false
                         }
 
-                        sym::tuple_variadic
-                            if !self.check_attr_not_crate_level(meta, hir_id, "tuple_variadic")
-                                || !self.check_doc_tuple_variadic(meta, hir_id) =>
+                        sym::fake_variadic
+                            if !self.check_attr_not_crate_level(meta, hir_id, "fake_variadic")
+                                || !self.check_doc_fake_variadic(meta, hir_id) =>
                         {
                             is_valid = false
                         }
@@ -939,7 +945,7 @@ impl CheckAttrVisitor<'_> {
                         | sym::notable_trait
                         | sym::passes
                         | sym::plugins
-                        | sym::tuple_variadic => {}
+                        | sym::fake_variadic => {}
 
                         sym::test => {
                             if !self.check_test_attr(meta, hir_id) {

compiler/rustc_passes/src/errors.rs

@@ -212,8 +212,8 @@ pub struct DocKeywordInvalidIdent {
 }
 
 #[derive(SessionDiagnostic)]
-#[error(passes::doc_tuple_variadic_not_first)]
-pub struct DocTupleVariadicNotFirst {
+#[error(passes::doc_fake_variadic_not_valid)]
+pub struct DocFakeVariadicNotValid {
     #[primary_span]
     pub span: Span,
 }

compiler/rustc_span/src/symbol.rs

@@ -685,6 +685,7 @@ symbols! {
         fabsf32,
         fabsf64,
         fadd_fast,
+        fake_variadic,
         fdiv_fast,
         feature,
         fence,
@@ -1460,7 +1461,6 @@ symbols! {
         tuple,
         tuple_from_req,
         tuple_indexing,
-        tuple_variadic,
         two_phase,
         ty,
         type_alias_enum_variants,

compiler/rustc_typeck/src/coherence/inherent_impls.rs

@@ -219,8 +219,9 @@ impl<'tcx> InherentCollect<'tcx> {
             | ty::RawPtr(_)
             | ty::Ref(..)
             | ty::Never
+            | ty::FnPtr(_)
             | ty::Tuple(..) => self.check_primitive_impl(item.def_id, self_ty, items, ty.span),
-            ty::FnPtr(_) | ty::Projection(..) | ty::Opaque(..) | ty::Param(_) => {
+            ty::Projection(..) | ty::Opaque(..) | ty::Param(_) => {
                 let mut err = struct_span_err!(
                     self.tcx.sess,
                     ty.span,

library/core/src/fmt/mod.rs

@@ -2562,7 +2562,7 @@ macro_rules! tuple {
 
 macro_rules! maybe_tuple_doc {
     ($a:ident @ #[$meta:meta] $item:item) => {
-        #[doc(tuple_variadic)]
+        #[cfg_attr(not(bootstrap), doc(fake_variadic))]
         #[doc = "This trait is implemented for tuples up to twelve items long."]
         #[$meta]
         $item

library/core/src/hash/mod.rs

@@ -900,7 +900,7 @@ mod impls {
 
     macro_rules! maybe_tuple_doc {
         ($a:ident @ #[$meta:meta] $item:item) => {
-            #[doc(tuple_variadic)]
+            #[cfg_attr(not(bootstrap), doc(fake_variadic))]
             #[doc = "This trait is implemented for tuples up to twelve items long."]
             #[$meta]
             $item

library/core/src/primitive_docs.rs

@@ -996,7 +996,7 @@ impl<T> (T,) {}
 // Fake impl that's only really used for docs.
 #[cfg(doc)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[doc(tuple_variadic)]
+#[cfg_attr(not(bootstrap), doc(fake_variadic))]
 /// This trait is implemented on arbitrary-length tuples.
 impl<T: Clone> Clone for (T,) {
     fn clone(&self) -> Self {
@@ -1007,7 +1007,7 @@ impl<T: Clone> Clone for (T,) {
 // Fake impl that's only really used for docs.
 #[cfg(doc)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[doc(tuple_variadic)]
+#[cfg_attr(not(bootstrap), doc(fake_variadic))]
 /// This trait is implemented on arbitrary-length tuples.
 impl<T: Copy> Copy for (T,) {
     // empty
@@ -1441,11 +1441,16 @@ mod prim_ref {}
 /// Note that all of this is not portable to platforms where function pointers and data pointers
 /// have different sizes.
 ///
-/// ### Traits
+/// ### Trait implementations
 ///
-/// Function pointers implement the following traits:
+/// In this documentation the shorthand `fn (T₁, T₂, …, Tₙ)` is used to represent non-variadic
+/// function pointers of varying length. Note that this is a convenience notation to avoid
+/// repetitive documentation, not valid Rust syntax.
+///
+/// Due to a temporary restriction in Rust's type system, these traits are only implemented on
+/// functions that take 12 arguments or less, with the `"Rust"` and `"C"` ABIs. In the future, this
+/// may change:
 ///
-/// * [`Clone`]
 /// * [`PartialEq`]
 /// * [`Eq`]
 /// * [`PartialOrd`]
@@ -1454,15 +1459,50 @@ mod prim_ref {}
 /// * [`Pointer`]
 /// * [`Debug`]
 ///
+/// The following traits are implemented for function pointers with any number of arguments and
+/// any ABI. These traits have implementations that are automatically generated by the compiler,
+/// so are not limited by missing language features:
+///
+/// * [`Clone`]
+/// * [`Copy`]
+/// * [`Send`]
+/// * [`Sync`]
+/// * [`Unpin`]
+/// * [`UnwindSafe`]
+/// * [`RefUnwindSafe`]
+///
 /// [`Hash`]: hash::Hash
 /// [`Pointer`]: fmt::Pointer
+/// [`UnwindSafe`]: panic::UnwindSafe
+/// [`RefUnwindSafe`]: panic::RefUnwindSafe
 ///
-/// Due to a temporary restriction in Rust's type system, these traits are only implemented on
-/// functions that take 12 arguments or less, with the `"Rust"` and `"C"` ABIs. In the future, this
-/// may change.
-///
-/// In addition, function pointers of *any* signature, ABI, or safety are [`Copy`], and all *safe*
-/// function pointers implement [`Fn`], [`FnMut`], and [`FnOnce`]. This works because these traits
-/// are specially known to the compiler.
+/// In addition, all *safe* function pointers implement [`Fn`], [`FnMut`], and [`FnOnce`], because
+/// these traits are specially known to the compiler.
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_fn {}
+
+// Required to make auto trait impls render.
+// See src/librustdoc/passes/collect_trait_impls.rs:collect_trait_impls
+#[doc(hidden)]
+#[cfg(not(bootstrap))]
+impl<Ret, T> fn(T) -> Ret {}
+
+// Fake impl that's only really used for docs.
+#[cfg(doc)]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), doc(fake_variadic))]
+/// This trait is implemented on function pointers with any number of arguments.
+impl<Ret, T> Clone for fn(T) -> Ret {
+    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(fake_variadic))]
+/// This trait is implemented on function pointers with any number of arguments.
+impl<Ret, T> Copy for fn(T) -> Ret {
+    // empty
+}

library/core/src/ptr/mod.rs

@@ -1819,6 +1819,27 @@ pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
     hashee.hash(into);
 }
 
+// If this is a unary fn pointer, it adds a doc comment.
+// Otherwise, it hides the docs entirely.
+macro_rules! maybe_fnptr_doc {
+    (@ #[$meta:meta] $item:item) => {
+        #[doc(hidden)]
+        #[$meta]
+        $item
+    };
+    ($a:ident @ #[$meta:meta] $item:item) => {
+        #[cfg_attr(not(bootstrap), doc(fake_variadic))]
+        #[doc = "This trait is implemented for function pointers with up to twelve arguments."]
+        #[$meta]
+        $item
+    };
+    ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
+        #[doc(hidden)]
+        #[$meta]
+        $item
+    };
+}
+
 // FIXME(strict_provenance_magic): function pointers have buggy codegen that
 // necessitates casting to a usize to get the backend to do the right thing.
 // for now I will break AVR to silence *a billion* lints. We should probably
@@ -1827,51 +1848,72 @@ pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
 // Impls for function pointers
 macro_rules! fnptr_impls_safety_abi {
     ($FnTy: ty, $($Arg: ident),*) => {
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> PartialEq for $FnTy {
-            #[inline]
-            fn eq(&self, other: &Self) -> bool {
-                *self as usize == *other as usize
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> PartialEq for $FnTy {
+                #[inline]
+                fn eq(&self, other: &Self) -> bool {
+                    *self as usize == *other as usize
+                }
             }
         }
 
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> Eq for $FnTy {}
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> Eq for $FnTy {}
+        }
 
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> PartialOrd for $FnTy {
-            #[inline]
-            fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-                (*self as usize).partial_cmp(&(*other as usize))
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> PartialOrd for $FnTy {
+                #[inline]
+                fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+                    (*self as usize).partial_cmp(&(*other as usize))
+                }
             }
         }
 
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> Ord for $FnTy {
-            #[inline]
-            fn cmp(&self, other: &Self) -> Ordering {
-                (*self as usize).cmp(&(*other as usize))
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> Ord for $FnTy {
+                #[inline]
+                fn cmp(&self, other: &Self) -> Ordering {
+                    (*self as usize).cmp(&(*other as usize))
+                }
             }
         }
 
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> hash::Hash for $FnTy {
-            fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
-                state.write_usize(*self as usize)
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> hash::Hash for $FnTy {
+                fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
+                    state.write_usize(*self as usize)
+                }
             }
         }
 
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
-            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                fmt::pointer_fmt_inner(*self as usize, f)
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
+                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                    fmt::pointer_fmt_inner(*self as usize, f)
+                }
             }
         }
 
-        #[stable(feature = "fnptr_impls", since = "1.4.0")]
-        impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
-            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                fmt::pointer_fmt_inner(*self as usize, f)
+        maybe_fnptr_doc! {
+            $($Arg)* @
+            #[stable(feature = "fnptr_impls", since = "1.4.0")]
+            impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
+                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                    fmt::pointer_fmt_inner(*self as usize, f)
+                }
             }
         }
     }
@@ -1896,7 +1938,7 @@ macro_rules! fnptr_impls_args {
 }
 
 fnptr_impls_args! {}
-fnptr_impls_args! { A }
+fnptr_impls_args! { T }
 fnptr_impls_args! { A, B }
 fnptr_impls_args! { A, B, C }
 fnptr_impls_args! { A, B, C, D }

library/core/src/tuple.rs

@@ -107,7 +107,7 @@ macro_rules! tuple_impls {
 // Otherwise, it hides the docs entirely.
 macro_rules! maybe_tuple_doc {
     ($a:ident @ #[$meta:meta] $item:item) => {
-        #[doc(tuple_variadic)]
+        #[cfg_attr(not(bootstrap), doc(fake_variadic))]
         #[doc = "This trait is implemented for tuples up to twelve items long."]
         #[$meta]
         $item