Create const_eval.md

Author: oli-obk

src/const_eval.md

@@ -0,0 +1,21 @@
+# Constant evaluation
+
+Constant evaluation is the process of computing the result of
+[expressions] during compilation.
+
+## Const context
+
+A _const context_ is one of the following:
+
+* [array type length expressions]
+* repeat expression length expessions
+* the initializer of
+  * [constants]
+  * [statics]
+  * [enum discriminants]
+
+[array type length expressions]: types.html#array-and-slice-types
+[enum discriminants]: items/enumerations.html#custom-discriminant-values-for-field-less-enumerations
+[constants]: items/constant-items.html
+[statics]: items/static-items.html
+[expressions]: expressions.html#constant-expressions