Add reference for attributes in function parameters

Author: c410-f3r

src/attributes.md

@@ -50,6 +50,9 @@ Attributes may be applied to many things in the language:
 * [Generic lifetime or type parameter][generics] accept outer attributes.
 * Expressions accept outer attributes in limited situations, see [Expression
   Attributes] for details.
+* [Function][function], [closure][closure] and [function pointer][function pointer]
+  parameters accept outer attributes. This includes attributes on variadic parameters
+  denoted with `...` in function pointers and [external blocks][variadic functions].
 
 Some examples of attributes:
 
@@ -306,3 +309,6 @@ The following is an index of all built-in attributes.
 [statements]: statements.md
 [struct]: items/structs.md
 [union]: items/unions.md
+[closure]: expressions/closure-expr.md
+[function pointer]: types/function-pointer.md
+[variadic functions]: items/external-blocks.html#variadic-functions

src/expressions/closure-expr.md

@@ -10,7 +10,7 @@
 > &nbsp;&nbsp; _ClosureParam_ (`,` _ClosureParam_)<sup>\*</sup> `,`<sup>?</sup>
 >
 > _ClosureParam_ :\
-> &nbsp;&nbsp; [_Pattern_]&nbsp;( `:` [_Type_] )<sup>?</sup>
+> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> [_Pattern_]&nbsp;( `:` [_Type_] )<sup>?</sup>
 
 A _closure expression_ defines a closure and denotes it as a value, in a single
 expression. A closure expression is a pipe-symbol-delimited (`|`) list of
@@ -77,3 +77,4 @@ ten_times(move |j| println!("{}, {}", word, j));
 [_Pattern_]: ../patterns.md
 [_Type_]: ../types.md#type-expressions
 [`let` binding]: ../statements.md#let-statements
+[_OuterAttribute_]: ../attributes.md

src/items/external-blocks.md

@@ -23,11 +23,11 @@
 > _NamedFunctionParameters_ :\
 > &nbsp;&nbsp; _NamedFunctionParam_ ( `,` _NamedFunctionParam_ )<sup>\*</sup> `,`<sup>?</sup>
 >
-> _NamedFunctionParam_ :\
-> &nbsp;&nbsp; ( [IDENTIFIER] | `_` ) `:` [_Type_]
->
 > _NamedFunctionParametersWithVariadics_ :\
-> &nbsp;&nbsp; ( _NamedFunctionParam_ `,` )<sup>\*</sup> _NamedFunctionParam_ `,` `...`
+> &nbsp;&nbsp; ( _NamedFunctionParam_ `,` )<sup>\*</sup> _NamedFunctionParam_ `,` [_OuterAttribute_]<sup>\*</sup> `...`
+>
+> _NamedFunctionParam_ :\
+> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> ( [IDENTIFIER] | `_` ) `:` [_Type_]
 
 External blocks provide _declarations_ of items that are not _defined_ in the
 current crate and are the basis of Rust's foreign function interface. These are

src/items/functions.md

@@ -17,7 +17,7 @@
 > &nbsp;&nbsp; _FunctionParam_ (`,` _FunctionParam_)<sup>\*</sup> `,`<sup>?</sup>
 >
 > _FunctionParam_ :\
-> &nbsp;&nbsp; [_Pattern_] `:` [_Type_]
+> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup>\ [_Pattern_] `:` [_Type_]
 >
 > _FunctionReturnType_ :\
 > &nbsp;&nbsp; `->` [_Type_]
@@ -250,6 +250,21 @@ attributes], [`must_use`], [the procedural macro attributes], [the testing
 attributes], and [the optimization hint attributes]. Functions also accept
 attributes macros.
 
+## Attributes on function parameters
+
+[Outer attributes][attributes] are allowed on function parameters and the
+permitted [built-in attributes] are restricted to `cfg`, `cfg_attr`, `allow`,
+`warn`, `deny`, and `forbid`. For example:
+
+```rust
+fn len(
+    #[cfg(windows)] slice: &[u16],
+    #[cfg(not(windows))] slice: &[u8],
+) -> usize {
+    slice.len()
+}
+```
+
 [IDENTIFIER]: ../identifiers.md
 [RAW_STRING_LITERAL]: ../tokens.md#raw-string-literals
 [STRING_LITERAL]: ../tokens.md#string-literals
@@ -282,3 +297,5 @@ attributes macros.
 [`link_section`]: ../abi.md#the-link_section-attribute
 [`no_mangle`]: ../abi.md#the-no_mangle-attribute
 [external_block_abi]: external-blocks.md#abi
+[_OuterAttribute_]: ../attributes.md
+[built-in attributes]: ../attributes.html#built-in-attributes-index

src/types/function-pointer.md

@@ -14,11 +14,11 @@
 > _MaybeNamedFunctionParameters_ :\
 > &nbsp;&nbsp; _MaybeNamedParam_ ( `,` _MaybeNamedParam_ )<sup>\*</sup> `,`<sup>?</sup>
 >
-> _MaybeNamedParam_ :\
-> &nbsp;&nbsp; ( ( [IDENTIFIER] | `_` ) `:` )<sup>?</sup> [_Type_]
->
 > _MaybeNamedFunctionParametersVariadic_ :\
-> &nbsp;&nbsp; ( _MaybeNamedParam_ `,` )<sup>\*</sup> _MaybeNamedParam_ `,` `...`
+> &nbsp;&nbsp; ( _MaybeNamedParam_ `,` )<sup>\*</sup> _MaybeNamedParam_ `,` [_OuterAttribute_]<sup>\*</sup> `...`
+>
+> _MaybeNamedParam_ :\
+> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> ( ( [IDENTIFIER] | `_` ) `:` )<sup>?</sup> [_Type_]
 
 Function pointer types, written using the `fn` keyword, refer to a function
 whose identity is not necessarily known at compile-time. They can be created
@@ -54,3 +54,4 @@ x = bo(5,7);
 [extern function]: ../items/functions.md#extern-function-qualifier
 [function items]: function-item.md
 [unsafe function]: ../unsafe-functions.md
+[_OuterAttribute_]: ../attributes.md