Merge #502

502: add while loop syntax fix r=Veykril a=nefelitav

According to the Rust Reference, "while loop expressions", as well as "if expressions" should accept any expression except struct expressions (SubjectExpression). So, this must be more accurate.

Co-authored-by: nefelitav <[email protected]>

Author: bors-ferrocene[bot]

src/expressions.rst

@@ -3870,7 +3870,7 @@ While Loops
        $$while$$ IterationExpression LoopBody
 
    IterationExpression ::=
-       Expression
+       SubjectExpression
 
 .. rubric:: Legality Rules