Add to unsafety section for unsafe attributes

Let's describe unsafe attributes in the chapter on unsafety.

Author: traviscross

src/unsafe-keyword.md

@@ -1,10 +1,10 @@
 # The `unsafe` keyword
 
 The `unsafe` keyword can occur in several different contexts:
-unsafe functions (`unsafe fn`), unsafe blocks (`unsafe {}`), unsafe traits (`unsafe trait`), and unsafe trait implementations (`unsafe impl`).
+unsafe functions (`unsafe fn`), unsafe blocks (`unsafe {}`), unsafe traits (`unsafe trait`), unsafe trait implementations (`unsafe impl`), and unsafe attributes (`#[unsafe(attr)]`).
 It plays several different roles, depending on where it is used and whether the `unsafe_op_in_unsafe_fn` lint is enabled:
 - it is used to mark code that *defines* extra safety conditions (`unsafe fn`, `unsafe trait`)
-- it is used to mark code that needs to *satisfy* extra safety conditions (`unsafe {}`, `unsafe impl`, `unsafe fn` without [`unsafe_op_in_unsafe_fn`])
+- it is used to mark code that needs to *satisfy* extra safety conditions (`unsafe {}`, `unsafe impl`, `unsafe fn` without [`unsafe_op_in_unsafe_fn`], `#[unsafe(attr)]`)
 
 The following discusses each of these cases.
 See the [keyword documentation][keyword] for some illustrative examples.
@@ -56,3 +56,9 @@ Unsafe trait implementations are the logical dual to unsafe traits: where unsafe
 [keyword]: ../std/keyword.unsafe.html
 [`get_unchecked`]: ../std/primitive.slice.html#method.get_unchecked
 [`unsafe_op_in_unsafe_fn`]: ../rustc/lints/listing/allowed-by-default.html#unsafe-op-in-unsafe-fn
+
+## Unsafe attributes (`#[unsafe(attr)]`)
+
+An [unsafe attribute] is one that has extra safety conditions that must be upheld when using the attribute. The compiler cannot check whether these conditions have been upheld.  To assert that they have been, these attributes must be wrapped in `unsafe(..)`, e.g. `#[unsafe(no_mangle)]`.
+
+[unsafe attribute]: attributes.md

src/unsafety.md

@@ -11,9 +11,11 @@ Rust:
 - Accessing a field of a [`union`], other than to assign to it.
 - Calling an unsafe function (including an intrinsic or foreign function).
 - Implementing an [unsafe trait].
+- Applying an [unsafe attribute] to an item.
 
 [`union`]: items/unions.md
 [mutable]: items/static-items.md#mutable-statics
 [external]: items/external-blocks.md
 [raw pointer]: types/pointer.md
 [unsafe trait]: items/traits.md#unsafe-traits
+[unsafe attribute]: attributes.md