Add let_chains references

Author: c410-f3r

src/expressions/if-expr.md

@@ -4,11 +4,21 @@
 
 > **<sup>Syntax</sup>**\
 > _IfExpression_ :\
-> &nbsp;&nbsp; `if` [_Expression_]<sub>_except struct expression_</sub> [_BlockExpression_]\
-> &nbsp;&nbsp; (`else` (
+> &nbsp;&nbsp; `if` _inIfList_ [_BlockExpression_]\
+> &nbsp;&nbsp; ( `else` (
 >   [_BlockExpression_]
 > | _IfExpression_
 > | _IfLetExpression_ ) )<sup>\?</sup>
+>
+> _inIf_ :\
+> &nbsp;&nbsp; ( `let` [_Pattern_] `=` [_Expression_]<sub>_except struct expression_</sub>
+> | [_Expression_]<sub>_except struct expression_</sub>
+> | _inIf_ )
+>
+> _inIfList_ :\
+> &nbsp;&nbsp; _inIf_ ( && _inIf_ )*
+
+[_Expression_]<sub>_except struct expression_</sub>
 
 An `if` expression is a conditional branch in program control.
 The syntax of an `if` expression is a condition operand, followed by a consequent block, any number of `else if` conditions and blocks, and an optional trailing `else` block.
@@ -126,7 +136,6 @@ if let E::X(n) | E::Y(n) = v {
 ```
 
 The expression cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_].
-Use of a lazy boolean operator is ambiguous with a planned feature change of the language (the implementation of if-let chains - see [eRFC 2947][_eRFCIfLetChain_]).
 When lazy boolean operator expression is desired, this can be achieved by using parenthesis as below:
 
 <!-- ignore: psuedo code -->
@@ -149,7 +158,6 @@ if let PAT = ( EXPR || EXPR ) { .. }
 [_LazyBooleanOperatorExpression_]: operator-expr.md#lazy-boolean-operators
 [_Pattern_]: ../patterns.md
 [_Scrutinee_]: match-expr.md
-[_eRFCIfLetChain_]: https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md#rollout-plan-and-transitioning-to-rust-2018
 [`match` expression]: match-expr.md
 [boolean type]: ../types/boolean.md
 [scrutinee]: ../glossary.md#scrutinee