focus on the cases rejected by const eval

Author: dingxiangfei2009

src/const_eval.md

@@ -43,9 +43,11 @@ r[const-eval.const-expr.path-item]
 
 r[const-eval.const-expr.path-static]
 * Paths to [statics] with these restrictions and observations.
-  * Reads and writes to `extern` statics are not allowed in any constant evaluation context.
-  * If the evaluation is carried out in an initializer of a `static` item, then *writes* into mutable `static` are not allowed, such as `static mut`s and `static`s containing an interior-mutable type.
-  * If the evaluation is *not* carried out in an initializer of a `static` item, then reads and writes to any mutable `static`, such as `static mut`s and `static`s containing an interior-mutable type.
+  * 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.
+  By mutable `static` it includes `static mut`s and `static`s containing an interior-mutable type.
 
 r[const-eval.const-expr.tuple]
 * [Tuple expressions].