document guarantee about evaluation of associated consts

Author: RalfJung

src/expressions/path-expr.md

@@ -23,6 +23,14 @@ let push_integer = Vec::<i32>::push;
 let slice_reverse = <[i32]>::reverse;
 ```
 
+If the path denotes an associated constant, and if the function containing the expression is executed at runtime,
+then the associated constant is guaranteed to be evaluated. This is true even if the path expression occurs in dead code.
+In particular, unsafe code inside a function can rely on the fact that all associated constants syntactically
+occurring in the function have successfully evaluated (and not panicked).
+Note that this is only guaranteed for all currently existing syntactic constructs; in the future, a new
+kind of block may be introduced such that constants inside that block are *not* unconditionally guaranteed to be evaluated.
+If the function is not executed at runtime, associated constants inside of it may or may not be evaluated.
+
 [_PathInExpression_]: ../paths.md#paths-in-expressions
 [_QualifiedPathInExpression_]: ../paths.md#qualified-paths
 [place expressions]: ../expressions.md#place-expressions-and-value-expressions