const expression can borrow static items

Author: dingxiangfei2009

src/const_eval.md

@@ -42,7 +42,10 @@ r[const-eval.const-expr.path-item]
   Recursively defining constants is not allowed.
 
 r[const-eval.const-expr.path-static]
-* Paths to [statics]. These are only allowed within the initializer of a static.
+* Paths to [statics] with these restrictions and observations.
+    * In particular, reads and writes to any `static`, `static mut` or [`extern` statics] is not allowed.
+    * Immutable borrows and pointers into immutable part of a `static` are allowed and observes the same restriction
+      on all other forms of [borrow]s as mentioned below.
 
 r[const-eval.const-expr.tuple]
 * [Tuple expressions].
@@ -177,6 +180,7 @@ of whether you are building on a `64` bit or a `32` bit system.
 [enum discriminants]:   items/enumerations.md#discriminants
 [expression statements]: statements.md#expression-statements
 [expressions]:          expressions.md
+[`extern` statics]:     items/external-blocks.md#statics
 [field]:                expressions/field-expr.md
 [functions]:            items/functions.md
 [grouped]:              expressions/grouped-expr.md

src/items/constant-items.md

@@ -39,6 +39,8 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
 };
 ```
 
+The final value of a `const` item cannot contain references to anything mutable.
+
 The constant expression may only be omitted in a [trait definition].
 
 ## Constants with Destructors

src/items/static-items.md

@@ -22,11 +22,7 @@ Static initializers may refer to other statics.
 Non-`mut` static items that contain a type that is not [interior mutable] may
 be placed in read-only memory.
 
-All access to a static is safe, but there are a number of restrictions on
-statics:
-
-* The type must have the `Sync` trait bound to allow thread-safe access.
-* Constants cannot refer to statics.
+The type of an immutable static must implement the [`Sync`](std::marker::Sync) trait.
 
 The initializer expression must be omitted in an [external block], and must be
 provided for free static items.
@@ -131,7 +127,7 @@ It can be confusing whether or not you should use a constant item or a static
 item. Constants should, in general, be preferred over statics unless one of the
 following are true:
 
-* Large amounts of data are being stored
+* Large amounts of data are being stored.
 * The single-address property of statics is required.
 * Interior mutability is required.