Add begin0-begin-extraction rule (#407)

Closes #404.

default-recommendations/definition-shortcuts-test.rkt

@@ -220,3 +220,19 @@ test: "begin0 in right hand side of definition can be removed"
   (displayln "foo")
   (* x 2))
 --------------------
+
+
+test: "begin inside begin0 in definition context should be extractable"
+--------------------
+(define (f x)
+  (displayln "starting")
+  (begin0 (begin (displayln "before") (* x 2))
+    (displayln "after")))
+--------------------
+--------------------
+(define (f x)
+  (displayln "starting")
+  (displayln "before")
+  (begin0 (* x 2)
+    (displayln "after")))
+--------------------

default-recommendations/definition-shortcuts.rkt

@@ -68,8 +68,22 @@
   (body-before ... replacement ... body-after ...))
 
 
+(define-definition-context-refactoring-rule begin0-begin-extraction
+  #:description
+  "The `begin` form inside this `begin0` form can be extracted into the surrounding definition\
+ context."
+  #:literals (begin0 begin)
+  (~seq body-before ...
+        (~and outer-form (begin0 (begin pre-body ... expr) post-body ...)))
+  #:with (replacement ...)
+  #'(~focus-replacement-on
+     (~splicing-replacement (pre-body ... (begin0 expr post-body ...)) #:original outer-form))
+  (body-before ... replacement ...))
+
+
 (define-refactoring-suite definition-shortcuts
-  #:rules (define-begin-extraction
+  #:rules (begin0-begin-extraction
+            define-begin-extraction
             define-begin0-extraction
             define-values-values-to-define
             inline-unnecessary-define))