Refine parserStatement grammar

[jaehyun1ee]

This is a follow-up from #1375.

Previously, parserStatment grammar was defined as:

parserStatement
    : assignmentOrMethodCallStatement
    | directApplication
    | emptyStatement
    | variableDeclaration
    | constantDeclaration
    | parserBlockStatement
    | conditionalStatement
    ;

conditionalStatement
    : IF "(" expression ")" statement
    | IF "(" expression ")" statement ELSE statement
    ;

where the conditionalStatement actually led to ordinary statements (which allow more production than parserStatements) being syntactically included within parserStatement.

This PR amends the parserStatement grammar such that it is closed under itself, without a path to statement. In short, the grammar is refined as follows:

 // tag::parserState[]
 parserState
     : optAnnotations STATE name
-      "{" parserStatements transitionStatement "}"
+      "{" parserStatOrDeclList transitionStatement "}"
     ;
 // end::parserState[]

-// tag::parserStatements[]
-parserStatements
+// tag::parserStatementOrDeclaration[]
+parserStatementOrDeclaration
+    : variableDeclaration
+    | constantDeclaration
+    | parserStatement
+    ;
+// end::parserStatementOrDeclaration[]
+
+// tag::parserStatOrDeclList[]
+parserStatOrDeclList
     : /* empty */
-    | parserStatements parserStatement
+    | parserStatOrDeclList parserStatementOrDeclaration
     ;
-// end::parserStatements[]
+// end::parserStatOrDeclList[]

 // tag::parserStatement[]
 parserStatement
     : assignmentOrMethodCallStatement
     | directApplication
     | emptyStatement
-    | variableDeclaration
-    | constantDeclaration
     | parserBlockStatement
-    | conditionalStatement
+    | parserConditionalStatement
     ;
 // end::parserStatement[]

 // tag::parserBlockStatement[]
 parserBlockStatement
-    : optAnnotations "{" parserStatements "}"
+    : optAnnotations "{" parserStatOrDeclList "}"
     ;
 // end::parserBlockStatement[]

+// tag::parserConditionalStatement[]
+parserConditionalStatement
+    : IF "(" expression ")" parserStatement
+    | IF "(" expression ")" parserStatement ELSE parserStatement
+    ;
+// end::parserConditionalStatement[]
+

Similar to how statement is defined,

• this adds parserStatementOrDeclaration, parserStatOrDeclList to the grammar, where parserBlockStatement and parserState has parserStatOrDeclList as its field, and
• parserStatement has a new parserConditionalStatement variant with explicit then- and else- branches that are parserStatements.

A mirror of this PR in p4c is also made: p4lang/p4c#5638. I've checked with p4-16/spec/scriptscompare-grammar.sh that the grammar consistent with the p4c PR.

[jafingerhut]

LGTM