Clarify mut vs non-mut statics

Author: jacob-greenfield

src/items/external-blocks.md

@@ -66,7 +66,9 @@ of initializing the static.
 
 Extern statics can be either immutable or mutable just like [statics] outside of external blocks.
 An immutable static *must* be initialized before any Rust code is executed. It is not enough for
-the static to be initialized before Rust code reads from it.
+the static to be initialized before Rust code reads from it. Mutating a non-`mut`,
+non-interior-`mut` external static is undefined behavior, even if that mutation happens by non-Rust
+code.
 
 ## ABI
 

src/items/static-items.md

@@ -8,12 +8,15 @@
 A *static item* is similar to a [constant], except that it represents a precise
 memory location in the program. All references to the static refer to the same
 memory location. Static items have the `static` lifetime, which outlives all
-other lifetimes in a Rust program. Non-`mut` static items that contain a type
-that is not [interior mutable] may be placed in read-only memory. Static items
-do not call [`drop`] at the end of the program.
+other lifetimes in a Rust program. Static items do not call [`drop`] at the end
+of the program. Note that [external statics] have additional restrictions.
 
-All access to a static is safe, but there are a number of restrictions on
-statics:
+## Non-`mut` 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 non-`mut` static is safe, but there are a number of restrictions:
 
 * The type must have the `Sync` trait bound to allow thread-safe access.
 * Statics allow using paths to statics in the [constant expression] used to
@@ -71,6 +74,7 @@ following are true:
 [constant]: constant-items.md
 [`drop`]: ../destructors.md
 [constant expression]: ../const_eval.md#constant-expressions
+[external statics]: external-blocks.md#statics
 [interior mutable]: ../interior-mutability.md
 [IDENTIFIER]: ../identifiers.md
 [_Type_]: ../types.md#type-expressions