Const expression can borrow static items

Co-authored-by: Eric Huss <eric@huss.org>

src/const_eval.md

@@ -42,7 +42,12 @@ 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:
+  * Writes to `static` items are not allowed in any constant evaluation context.
+  * Reads from `extern` statics are not allowed in any constant evaluation context.
+  * If the evaluation is *not* carried out in an initializer of a `static` item, then reads from any mutable `static` are not allowed. A mutable `static` is a `static mut` item, or a `static` item with an interior-mutable type.
+
+These requirements are checked only when the constant is evaluated. In other words, having such accesses syntactically occur in const contexts is allowed as long as they never get executed.
 
 r[const-eval.const-expr.tuple]
 * [Tuple expressions].
@@ -177,6 +182,7 @@ 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

@@ -48,6 +48,9 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
 };
 ```
 
+r[items.const.final-value-immutable]
+The final value of a `const` item cannot contain references to anything mutable.
+
 r[items.const.expr-omission]
 The constant expression may only be omitted in a [trait definition].
 

src/items/static-items.md

@@ -37,10 +37,7 @@ All access to a static is safe, but there are a number of restrictions on
 statics:
 
 r[items.static.sync]
-* The type must have the `Sync` trait bound to allow thread-safe access.
-
-r[items.static.const]
-* Constants cannot refer to statics.
+* The type must have the [`Sync`](std::marker::Sync) trait bound to allow thread-safe access.
 
 r[items.static.init.omission]
 The initializer expression must be omitted in an [external block], and must be
@@ -159,7 +156,7 @@ 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.